Human Evolution

Introduction

Humans are a young species, in geological terms. The average "lifespan" of a mammal species, measured by its duration in the fossil record, is around 10 million years. While hominids have followed a separate evolutionary path since their divergence from the ape lineage, around 7 million years ago, our own species (Homo sapiens) is much younger. Fossils classified as archaic H. sapiens appear about 400,000 years ago, and the earliest known modern humans date back only 170,000 years.

Our knowledge of human evolution is changing rapidly, as new fossils are discovered and described every year. Thirty years ago, it was generally accepted that humans and the great apes last shared a common ancestor perhaps 16-20 million years ago, and that the separate human branch was occupied by only a few species, each evolving from the one before. Now we know, through a combination of new fossil finds and molecular biology, that humans and chimpanzees diverged as little as 7 million years ago, and that our own lineage is "bushy", with many different species in existence at the same time.

Our view of our evolutionary past has changed as social attitudes have changed. Darwin was remarkably prescient when he wrote, in 1871 "The Descent of Man", that humans had evolved in Africa and were closely related to the great apes (gorilla, chimpanzee, and orang-utan). But at that time this view was anathema to many, since the majority of people still accepted the concept of special creation.

This is why the first fossil hominid material to be discovered, that of Neandertal Man, attracted even more controversy than the later discoveries of Australopithecus africanus and Homo erectus. Rather than accept the fossil as the remains of a human ancestor, the distinguished German scientist R. Virchow described it as the skeleton of a diseased Cossack cavalryman. And even once the antiquity of the remains was established, many scientists refused to accept that Neandertals could be closely related to modern humans, depicting them instead as brutish and apelike. This interpretation reflected the prevailing prejudices about human ancestry, and was supported by misinterpretation of the remains of the "Old Man of La Chapelle", whose skeleton was warped by arthritis.

Even when the idea that apes and humans shared a common ancestor became more widely accepted, the concept of an African origin was not. The scientist Ernst Haeckel, for example, was convinced that humanity's nearest common ancestor was the orang-utan, and that humans evolved in Asia. Though wrong in this, he was a persuasive writer and many people came to accept his view.

This is why Eugene Dubois sought the "missing link" between humans and apes in Indonesia (then the Dutch East Indies). However, he met with considerable disbelief - and some ridicule - when he named his Solo River fossils Pithecanthropus (now Homo) erectus and described them as belonging to a human ancestor. This rejection reflected the prevailing view that our large brain had evolved while the skeleton was still ape-like, and Dubois' suggestion that the reverse was true was sidelined.

The "large brain first" view received further support when the Piltdown fossils were presented to the world. While we now know that they are fraudulent, at the time (1911) they seemed to demonstrate quite clearly that early humans had a modern cranium atop an ape-like body. And since the Piltdown remains were found in England, they conveniently supported the prevailing idea that modern humans had evolved in Europe, rather than in Africa.

Consequently, when in 1924 Raymond Dart recognised the position of the Taung baby (Australopithecus africanus) on the human family tree, his ideas initially faced considerable opposition. Not until more australopithecine fossils were discovered did his recognition of A. australis as a hominid gain credence. However, it is now accepted that the ancestors of modern humans evolved in Africa and remained there until perhaps 1.5 million years ago, when Homo erectus populations left Africa and moved rapidly across Europe and Asia.

This diaspora was the reverse of a movement that occurred in the late Miocene, when the ancestors of the African apes migrated from Eurasia into Africa. Here they underwent another adaptive radiation, culminating in the divergence of ancestral chimp and hominid populations from their last common ancestor, 7 million years ago.

Miocene Apes

Apes evolved in Africa at least 20 million years ago, when the continent was a separate land mass. The best known of these early apes was Proconsul. Proconsul was recognisably an ape, but retained some monkey-like characteristics of the backbone, pelvis, and forelimbs that suggest it was quadrupedal, rather than a brachiator

Lowered sea levels 17 - 16.5 million years ago provided a land bridge between Africa and Eurasia, and some of these early apes used it to enter Eurasia, along with elephants, pigs and antelopes, and rodents. By this stage the apes had developed a thick coating of enamel on their teeth, which enabled them to eat the harder foods (such as nuts and tough-coated seeds) that weren't available to Proconsul. This evolutionary innovation was significant, as within 1.5 million years of apes moving into Eurasia they had diversified into at least eight different forms.

By 13 million years ago apes were found throughout Eurasia, including the lineages of Dryopithecus (in Europe) and Sivapithecus (in Asia). Both had very similar anatomy to modern great apes. These two lineages survived the major climate changes that marked the end of the Miocene, while the many other Eurasian ape species became extinct. They survived by moving into Southeast Asia (Sivapithecus) and back into Africa (Dryopithecus). The living great apes are descended from these two lineages. Phylogenetic analyses indicate that Sivapithecus is the likely ancestor of the orang-utan, while Dryopithecus is probably the forebear of African apes and humans.

The Earliest Hominids

Evidence from molecular biology strongly suggests that humans and chimpanzees last shared a common ancestor no more than 5-8 million years ago, and in recent years researchers have focused on finding fossils close to this divergence. The descriptions of Orrorin tugenensis (in 2001) and Sahelanthropus tchadensis (in 2002) have added to our knowledge of this period in our history. In addition, publication of the chimpanzee genome has allowed scientists to compare chimp and human genetic sequences, and to use the differences between them to estimate the date of divergence of the two species. The result suggests humans and chimps diverged "no more than 6.3 million years ago and perhaps even more recently than 5.4 million years ago" (Pennisi 2006). Interestingly, the data also raise the possibility that the two new species may have hybridised for some time after their initial separation (Patterson et al. 2006).

Orrorin tugenensis

Orrorin tugenensis, from Kenya, is dated at 6 million years old. Its remains are fragmentary, consisting of some limb bones, partial jaw material, and a few teeth. Its discoverers place it in the hominid family tree, and describe it as bipedal. This suggests that bipedalism in hominids evolved very early indeed. Not all researchers agree that Orrorin is a hominid, on the basis that its canine teeth are extremely ape-like. However, its lower limb bones are those of a bipedal organism. Recent examination (Galik et al. 2004) of the neck of the femur shows that the bone is thickest on the underside of the neck, as seen in humans. In chimpanzees the neck is of uniform thickness.

Sahelanthropus tchadensis

Sahelanthropus tchadensis was described in 2002 from an extremely well preserved cranium, a partial mandible and some teeth, dated at 6-7 million years old. This is very close indeed to the likely human-chimpanzee split. Michel Brunet's team describe Sahelanthropus as a hominid, for reasons including the shape and angle of the face and skull, and its dentition. Not all scientists agree with this, saying that the position of the foramen magnum suggests it was not a true biped, and that features of its dentition and skull are reminiscent of chimpanzees. However, a recent reconstruction of the cranium (Zollikofer et al., 2005) places the foramen magnum well under the skull, suggesting Sahelanthropus was indeed bipedal. Zollikofer et al. also suggest that comparisons of the reconstructed cranium with those of both modern apes and other fossil hominins demonstrate that it belongs on the hominin lineage, although other researchers disagree with this interpretation.

If Sahelanthropus is a hominid, it broadens the geographic range from which hominids are known: Lake Chad is well outside the region where previous fossil hominids have been found, and 7 million years ago the environment there would have been forested.

Ardipithecus ramidus

Ardepithecus ramidus is a third ancient hominid, which some scientists place in the genus Australopithecus. The oldest remains of this species, belonging to the subspecies kadabba, are from Ethiopian rocks dated at between 5.2 and 5.8 million years old. More recent Ardepithecus ramidus remains are dated at 4.4 million years. Most of the fossil material for this species consists of skull fragments and some teeth, and possibly a toe bone (which belonged to a bipedal organism).

Again, evidence from other fossils suggests that Ardipithecus lived in a forested environment. This is quite different from the open savannah conditions in which hominids were thought to have become bipedal.

That all these species existed so close to the origin of hominids suggests that even then our family tree could be described as bushy, rather than having the single linear progression from species to species that is so often presented in images of human evolution.

The Australopithecines

This large group of species comprises both the gracile and the robust australopithecines. Recently some scientists have suggested that some species presently assigned to the Homo clade would be better placed in Australopithecus - an example of how rapidly our understanding of our evolutionary past is changing, and of the reviews, discussion and disagreements that characterise scientific research.

Australopithecus anamensis

The earliest known australopithecine is Australopithecus anamensis, which lived between 4.2 and 3.9 million years ago. This has teeth and jaws that strongly resemble those of older fossil apes. However, they were very likely bipedal (based on the structure of the tibia) and had human-like upper limbs. Maeve Leakey's research team suggests that anamensis may be ancestral to all later hominids.

Australpithecus afarensis

The best-known member of this species is "Lucy" , discovered in 1974 by Don Johanson & Tom Gray and estimated to be around 3.2 million years old (afarensis lived from 3.9 to 3 million years ago). This is an important find as the skeleton is remarkably complete for its age (40% complete by some estimates, but this does not include the bones of the hands and feet), providing a wealth of data about her size, posture, and gait. A range of other finds, including the 13 individuals of the "First Family", give supporting information, and the famous Laetoli footprints have also been attributed to this species.

Afarensis remains indicate that the species was strongly sexually dimorphic, with males much larger than females. The remains indicate that afarensis heights ranged from 107 to 152 cm, and cranial capacity from 375 cc to 550cc (AL 444-2, a large adult male). This may give us some clues about social behaviour in anamensis, since modern apes with a high degree of sexual dimorphism are polygynous.

The face and cranium of afarensis was ape-like: a prominent brow ridge, low forehead, and a prognathous muzzle that lacked a chin. The teeth are intermediate between ape and human: the molars are large and the canines, though much smaller than those of living apes, are larger and more pointed than those of humans. The shape of the dental arcade lies between the human parabolic form and the apes' rectangular shape, and the foramen magnum, while further forward than in apes, is not directly under the cranium as in humans.

However, their postcranial skeleton is far closer to that of modern humans. The pelvic, leg, and foot bones clearly show that this species was bipedal, though not well adapted for running. While the finger & toe bones are curved and longer than in humans, a feature that most scientists consider to be evidence that afarensis still spent time in the trees, their hands are otherwise human-like. (A recent study suggests that afarensis' wrist bones still show some adaptations for knuckle-walking.)

"Lucy's child"

Our knowledge of Australopithecus afarensis has been extended with the description of a juvenile afarensis (Alemseged et al. 2006). The complete skull and partial skeleton (around 50% complete) are probably those of a female who was around 3 years old when she died.

The find is particularly exciting in that it yields information lacking in other afarensis remains: an endocranial cast of the cranium; shoulder blades and collarbones - permitting an estimate of the extent of arm movement in this species; and a hyoid bone, which shows that the voice box had an ape-like structure.

Scientists have always wondered how much time afarensis spent in the trees. The form of the shoulder blade, the fact that the arms could be swung above the head, and the presence of long, curved fingers all suggest that the species was at least partly aboreal.

Kenyanthropus platyops

Until recently our knowledge of early hominids suggested that there was a single early-middle Pliocene lineage, of which A. afarensis is the best example. However, in 2001 Maeve Leakey's research team reported the find of a well-preserved cranium that they placed in a new genus: Kenyanthropus platyops.

This fossil is described as a mosaic, with a combination of ape-like and hominid features. On the ape side, it has a small ear opening, thick enamel on its molar teeth, a cranial capacity of about 400cc (the same as a chimp's), and a flat nose. However, its face has a number of novel features not seen in the other gracile australopithecines: a flat face, vertical cheek region, and small brow ridge and cheek teeth.

Some researchers dispute the classification of this specimen into a new genus, noting that the cranium is severely distorted and that many of the features used to define this genus are simply a result of the distortion.

Australopithecus garhi

This species , Australopithecus garhi , which its discoverers felt could be intermediate between A. afarensis & early Homo, was described in 1999 from fossils found in the Awash region of Ethiopia. The 2.5 million-year-old fossils were found in association with animal bones - which had marks on them that appeared to be from stone tools. Very simple stone tools were found in a nearby site of the same age.

The fossils of Australopithecus garhi comprise a partial cranium and a fragment of another, partial skeletons and other postcranial bones, and two mandibles. (The remains may or may not be all from the same species.) Many features of the skull, including the small cranial capacity of 450cm3, are very similar to A. afarensis. There was a sagittal crest and an ape-like dental arcade. The molars and canines were extremely large - an unexpected feature if garhi is to be regarded as ancestral to Homo. The proportions of the limb bones are of interest: the femur-to-humerus ratio was like that of modern humans, but the ratio of forearm-to-humerus was like that of a chimpanzee.

Australopithecus africanus

Raymond Dart's "Taung child", consisting of teeth, jaws, a complete set of facial bones, and an endocranial cast, was the first australopithecine fossil to be found. The fact that all its milk teeth are present, and the state of its cranial sutures, suggests that this individual was about 3 years old when it died. Although his description of the Taung infant as a hominid was originally questioned, the subsequent discovery of many adult africanus fossils confirmed his classification. As a species, africanus lived between 3.3 and 2 million years ago. Like afarensis, it showed strong sexual dimorphism, and bones of the feet, legs, pelvis and spine show that it was bipedal. However, both body size and cranial capacity (420 - 500cc) were slightly larger in africanus, which also had larger molar teeth, smaller canines, and a fully parabolic dental arcade.

Australopithecus anamensis, afarensis , and africanus, and Kenyanthropus platyops are collectively known as gracile australopithecines, because of their relatively light, slender build. This is by comparison to the "robust" australopithecines: all the gracile species were still more robust than modern H. sapiens.

Australopithecus robustus

Together with A. aethiopicus and boisei, robustus is one of the "robust" australopithecines. (Some authors have placed these three species in the genus Paranthropus.) All of them had large jaws, heavily built skulls, sagittal crests, and thick enamel on their molar teeth.

Robustus lived between 2 and 1.5 million years ago. While its body size was similar to that of A. africanus, it had a larger, more robust cranium (average capacity 530cc) and very large molars in its large lower jaw. Its face was also large, with no forehead, and many specimens had sagittal crests in addition to their big brow ridges. The combination of these ridges and crests with the large jaws and molar teeth suggest that this species must have eaten coarse, tough food requiring much chewing.

Interestingly, while tool use was regarded for many years as a characteristically human feature, robustus may have been one of the first hominids to use tools. This interpretation was placed on bones found with robustus fossils as the worn ends of these bones suggest they may have been used for digging.

Australopithecus aethiopicus

This is the oldest of the three robust australopith species, living between 2.6 and 2.3 million years ago. There is one major fossil, the "Black Skull" , so named because it had been stained by minerals in the soil. Some researchers consider it an ancestor of both robustus and boisei (see Figure 1). At 410cc its cranial capacity is little more than that of a chimpanzee, and posterior parts of the skull are similar to those of A. afarensis. But it also has the very heavily built face and jaws of the other robust species, and the largest sagittal crest ever seen in a hominid.

A ustralopithecus boisei

A. boisei , originally named Zinjanthropus boisei, is the most robust of all the robust australopithecines. Louis Leakey nicknamed it "Nutcracker Man" because of its huge molar teeth - some up to 2cm across. Its cranial capacity is similar to that of robustus, around 530cc, while its face and jaws are even more massively built. The "hyper-robust" nature of this species suggests it was highly specialised to chew hard, low quality foods. It's been suggested that boisei became extinct because it was so highly adapted to a specific ecological niche, and could not evolve fast enough to adapt when the environment changed.

Homo Species

Homo habilis

Until Jane Goodall's pioneering studies of chimpanzees most palaeoanthropologists believed that tool use was a hallmark of humanity. Consequently, when Louis Leakey's team found very simple stone tools closely associated with hominid remains, in Olduvai Gorge, he named the hominid Homo habilis. The associated tools are assigned to the Oldowan tool culture.

Homo habilis lived from 2.4 until 1.5 million years ago, and closely resembles the australopithecines. In fact, recent papers have suggested that habilis would be more appropriately classified in Australopithecus. The face still projects forwards but the facial angle is less than in A. africanus. Average cranial capacity in habilis is about 650cc, and the range is 500 - 800cc - this overlaps both the australopithecines (at the lower end) and H. erectus (at the upper limit). Analysis of wear patterns on the teeth suggest that habilis was adding meat to its diet - probably as a scavenger as there is no evidence that hunting was a common practice.

Postcranial remains are fragmentary, and in fact only one set of limb bones has been securely assigned to habilis. This fragmentary skeletal material suggests that the average height of Homo habilis was around 127cm, and they were probably about 45kg in weight. They were also obviously bipedal.

There is some argument about the taxonomic status of the habilis specimens. Recent papers have made the suggestion that at least some should be reclassified with the australopithecines, on the basis of features of the limbs. For example, the "dik-dik hominid" (OH 62) has arms that are considerably longer than its legs, an australopithecine characteristic.

Homo erectus or Homo ergaster

Homo erectus lived between 1.8 million and 300,000 years ago, and was probably the first hominid species to move out of Africa and colonise Europe and Asia. This significant event must have happened early in the species' history, as fossils that may belong to erectus but which are dated at around 1.8 million years ago have been found in Dmanisi, Georgia.

Note that some authors recognise two sister species: erectus is assigned to the Eurasian specimens while Homo ergaster is reserved for those from Africa. Ergaster has a smaller cranial capacity, and the two differ in some features of the skull, such as the shape of the brow ridges.

Erectus had a long, low skull, with little forehead and a cranial capacity of between 750 and 1225cc. The smaller brain sizes are associated with older specimens. The face was prognathous, and the protruding jaws supported large molar teeth but lacked a chin. This was the first hominid to have a projecting, rather than a flattened, nose.

The post-cranial skeleton of Homo erectus was robust, suggesting they were stronger than modern humans. Members of this species, at least in Africa, were tall; some estimates place them in the upper quartile of the height range for H. sapiens. However, the few remains from China ("Peking man") are from shorter individuals. This probably reflects adaptation to the local climate. Tall, slender individuals are well adapted to lose heat in hot climates, as their bodies have a high surface area to volume ratio (SA:V). Those living in colder regions need to conserve heat, and heat loss is reduced in short, stocky individuals with a lower SA:V.

Studies of erectus pelvic bones, particularly those of the Turkana (Nariokotome) Boy, show that members of this species had a narrower pelvis and pelvic canal than ours. This implies that their babies were smaller-brained at birth; it also suggests that erectus may have been more efficient at walking than sapiens.

Erectus was a competent toolmaker, and scientists have found large numbers of their tools, which are classified in the Acheulean tool culture. There is good evidence, in the form of scratch marks on bones, that these tools were used to butcher animal carcases; meat made up a significant portion of the erectus diet. There is also evidence that this species was the first to use fire. The first support for this hypothesis came from charcoal deposits from the Choukoutien caves near Beijing, where the fossils of "Peking man" were found. At least one researcher now suggests that these "hearths" are natural deposits. However, palaeontological and experimental evidence does support the idea of widespread use of fire among African erectus populations.

Homo floresiensis

On 27 October 2004, one of the most exciting discoveries in human evolution in recent years was announced to the world: Homo floresiensis . Found on the Indonesian island of Flores, this new hominin species was named on the basis of the partial remains - including the skull, jaw, and teeth - of an adult female, and fragments from several other individuals. What makes this find unique is its age - only 18,000 years - and its tiny size: the adult female stood only a metre tall, and had a cranial capacity of just 380cm3. Up till now, scientists had understood there to be only two hominin species in Asia: our own Homo sapiens, and H.erectus. This is yet another piece of evidence for the bushy nature of the human family tree, and evidence, too, that our present status as the only living hominin was only recently acquired.

The Flores skull shows a mix of primitive and advanced features: a low cranium, and prominent brow ridges, combined with a small & relatively flat face. Interestingly, although this hominin was bipedal, the pelvis is more similar to that of australopithecines than more modern hominins. Various features of the skull, including the shape of the brain itself, suggest that floresiensis was a dwarfed species that evolved from H. erectus (erectus was present on the island of Java from as long as 1.6 million years ago). Evolution of dwarfed island forms is known in other mammals: Malta had a dwarf elephant and hippopotamus, and floresiensis was found in association with the bones of a dwarfed species of elephant.

Despite its tiny brain, Homo floresiensis was probably a toolmaker & tool-user: a number of stone tools were found with the remains. Tools found earlier, and dated at up to 800,000 years ago, are attributed to a founding population of H. erectus. In addition, since Flores is separated from the larger island of Java by a deep ocean channel, it seems likely that the ancestors of this new species must have arrived by sea, perhaps on a primitive raft.

Read Carl Zimmer's excellent article on http://www.corante.com/loom/archives/island_of_the_lost_hominids.php

The Dmanisi fossils

Between 1999 and 2001 three hominin skulls aged around 1.8 million years were discovered at Dmanisi, in Georgia. Additional material comprised jawbones and some facial bones. The cranial capacity of these skulls ranged from 600 cm3 to 780 cm3. The skull of the smallest individual was the smallest, most primitive hominin skull found outside Africa. Although the crania are small and resembled habilis in some features, the scientists who described them felt that the fossils were more closely related to H. erectus/ergaster than to H. habilis.

However, stone tools also found at Dmanisi are essentially the same as those from the Oldowan culture associated with H. habilis. This, combined with the small cranial capacity of the skulls, has led some scientists to suggest that the Dmanisi individuals belonged to H. habilis or a related species. If correct, this will require a revision of the standard view that H. erectus was the first hominin species to venture out from Africa. Some scientists have placed the Dmanisi remains in their own species, Homo georgicus.

A recent paper (Lordkipanidze et al., 2007; summarised by Gibbons, 2007) describes the post-cranial remains of both teenage and adult individuals from Dmanisi. All were short - between 145 and 166cm tall; together with the small crania, this places the Dmanisi individuals at the lower end of the size range for erectus. In addition, their shoulder and upper arm bones resemble those of australopithecines (and Homo floresiensis). However, the proportions of their bodies and legs are similar to those of erectus, and on that basis Lordkipanidze and his co-authors feel that the remains represent early Homo erectus. Lieberman (2007) comments that these Dmanisi remains add to the unusually high variability within fossils attributed to Homo erectus e.g. cranial capacities in early specimens vary from around 600cc in the Dmanisi individuals, to nearly 1100cc in 1-million-year-old specimens from Africa, and even higher in younger fossils.

Homo heidelbergensis

This is an alternative name for fossils that are also classified as "archaic Homo sapiens". Archaic H. sapiens first appears in the fossil record about half a million years ago. These fossils appear intermediate between Homo erectus and fully modern humans. Skulls attributed to archaic sapiens have an average cranial capacity of 1200cc, which is larger than erectus but less than the average value for modern sapiens. The vault of the skull is more rounded than in erectus, and many of the fossils have large brow ridges, receding foreheads, and weak chins.

The Petralona, Steinheim, and Swanscombe remains are also regarded as "archaic" Homo sapiens.

Homo neanderthalensis

Neandertals (or Neanderthals) lived between 230,000 and 30,000 years ago, during the last Ice Age, and were found only in Europe and the Middle East, where they coexisted with modern humans for the later part of their existence. This species gets its name from the Neander valley, or Tal, in Germany, where the type specimen was found in 1856.

All Neandertals are heavily built but those from Western Europe are particularly robust. Their heavy physique was probably an adaptation to the extremely cold conditions in which they lived. Neandertal men averaged only 168cm in height, but their bones are thick and heavy, and the scars of muscle attachment indicate that they were very heavily muscled.

However, the key differences between Neandertals and modern humans lie in features of the skull. The average cranial capacity is 1450cc, larger than the modern norm, although this may be a reflection of their greater bodily bulk. The skull is notably longer than that of modern humans, with a lower vault and an occipital bulge at the rear. The face was prognathous, with a receding forehead and weak chin. The cheekbones are swept back and the midfacial area protrudes (as if someone had grabbed the nose & pulled it forwards) - this feature may also be an adaptation to a cold environment as it is associated with a markedly larger nasal volume than in either erectus or modern sapiens. Two different explanations have been given for this large nose: it may have ensured that the cold air was warmed & moistened on its way to the lungs; alternatively it could have acted as a radiator, to lose heat generated by exertion while hunting.

Neandertals made a wider range of more complex tools - belonging to the Mousterian tool culture - than those used by erectus although, like erectus, they do not appear to have been particularly innovative until late in the species' existence, when Chatelperronian tools appear at some sites in France. Many researchers believe that they buried their dead, with the oldest known burial dating to about 100,000 years ago. However, not all scientists agree with this interpretation.

Recently scientists have been able to extract mitochondrial DNA from Neandertal bones. This has allowed them to compare DNA sequences from Neandertals and modern humans, an interesting experiment considering that in the past Neandertals have been viewed as direct ancestors of modern sapiens. The multiregional hypothesis of human origins also takes this stance. While there are problems with this technique, the data appear to show that Neandertals were not closely related to modern humans, but belonged to a separate species.

Homo sapiens (modern)

A recent find provides good evidence that the earliest known recognisably modern humans lived in Africa, around 160,000 years ago. These fossils come from Herto, in the Middle Awash of Ethiopia, and "are morphologically and chronologically intermediate between archaic African fossils and later anatomically modern Late Pleistocene humans" (White et al. 2003). The fact that the Herto fossils are so old supports the argument that fully modern humans first arose in Africa, later migrating into Europe & Asia to displace the other hominid species already living there (the "out of Africa" hypothesis).

The average cranial capacity of modern humans is around 1350cc, with a range for normal individuals of from 800 to close to 2000cc. The brain is enclosed in a high, vaulted skull with a high forehead, and brow ridges are absent or, if present, very small. The relatively delicate jaw has small teeth and a prominent chin, and the post-cranial skeleton is gracile.

We think of complex culture as a hallmark of humanity. However, art works, such as jewellery, carving, and cave paintings do not appear in the record until 30-40,000 years ago. This follows the development of the extremely sophisticated Aurignacian tool kits associated with Cro-Magnon culture. Some authors suggest that the use of highly sophisticated language accompanied this flowering of culture. This is not to say that earlier humans, and hominids, were not capable of speech.

Over the last 100,000 years there has been a continuation of the trend towards smaller molar teeth and a more gracile skeleton, such that the Upper Palaeolithic humans of 30,000 years are described as being 20-30% more robust than present-day people. This demonstrable trend in tooth size is probably linked to the use of food-processing techniques that reduce the need for prolonged chewing, and thus provides a good example of the results of natural selection in human populations.

Trends in Human Evolution

Examination of hominid remains indicates several trends, including changes in posture, cranial capacity (brain size), and facial angle. Such trends are often misused, e.g. in popular illustrations, to give the impression that evolution has proceeded in a linear manner, from some primitive ancestor through a series of descendants, to culminate in our own species. It's important to remember that the evolutionary history of humans, as of most organisms, is best reconstructed as a bush, where there are often several related species in existence at any one time.

Having said that, these trends do give a useful overview of the evolutionary changes that have occurred in our biological history.

Trends in cranial capacity

Early workers in the field of human evolution expected that the first hominids would have an ape-like physique with a modern cranium. This reflected the attitude that, since our intelligence and large brain size set us apart from all other species, these would be the first human characteristics to evolve. The Piltdown Man fraud exploited these expectations. It was almost 50 years before the "fossils" were recognised as a modern human braincase and orang-utan jaw.

We now know that the actual trend is the reverse of this early expectation. There has been a gradual increase in cranial capacity over the course of human evolution. Thus Sahelanthropus and the early australopithecines had cranial capacities within the range of modern chimpanzees (average around 400cc), with the later australopithecines reaching 550cc. Skulls attributed to early Homo begin at around 510cc, and there was a marked increase with Homo erectus, where later specimens had brain sizes of up to 1225cc, well within the modern range.

The average cranial capacity of the Neandertals was larger than that of modern humans (1450cc and 1350cc respectively), but this may simply reflect the larger body mass of neanderthalensis. There is a strong positive correlation between body size and brain size, even within species e.g. male humans have larger body mass than females, and correspondingly larger cranial capacity. Equally importantly, the brain size in hominids, particularly Homo species, is greater than would be predicted for animals of their body mass.

Bipedalism

Bipedalism appeared very early in our evolutionary history. Until recently there was disagreement over the posture of Sahelanthropus (c. 7 million years ago). However, a computer-assisted reconstruction of this fossil shows a foramen magnum beneath the cranium, and relatively small nuchal crest, indicating that this species was bipedal (Zollikofer et at. 2005).

This is not to say that their posture was fully erect. For example, while the pelvic, leg, and foot bones of Australopithecus afarensis clearly show that this species was bipedal, it was not well adapted for running. In addition, the position of the foramen magnum (intermediate between that of apes and humans) suggests that afarensis did not hold its head fully erect.

In contrast, Homo erectus was possibly an even more efficient biped than modern humans, due to the narrower pelvic outlet in erectus. The wider pelvic outlet in sapiens, an adaptation permitting the birth of large-brained infants, places the hip joints further apart than required for optimum locomotory efficiency. (This is a good example of how evolution produces compromise solutions, rather than perfection.)

Another feature linked to bipedalism is the Achilles tendon, which in Homo links the calf muscles to the tarsal bones of the heel. The great apes lack this structure, and the calf muscle extends right down to the tarsal bones. The presence of the Achilles tendon in humans makes us relatively good endurance runners - a necessary feature for active hunting on the open savannah. The tendon acts as a spring, storing energy when it is stretched and releasing it again as the foot pushes off for the next stride (Bramble & Lieberman, 2004). This can save up to 50% of the metabolic energy costs of rapid locomotion.

Trends in general morphology of the skull

Dental arcades: in an ape, the teeth are arranged in a rectangular dental arcade, where the left and right cheek teeth are in two parallel lines. Australopith dental arcades tend to be more rectangular than parabolic, but in Homo species the dental arcade is a full parabola, broader at the back than at the front.

There is also a strong trend in tooth size, such that the cheek teeth (in particular) of modern humans are smaller than those of australopithecines. And even within H. sapiens there has been a marked decrease in tooth size over the last 30,000 years.

Crests and ridges: both the great apes and early hominids have obvious crests and ridges on their skulls. The most obvious are the sagittal and nuchal crests and the brow ridges.

Where present, the sagittal crests provide anchorage for large chewing muscles, and are thus most prominent in species where the diet comprises hard, tough material requiring a lot of chewing. (They are also larger in males than in females, an example of sexual dimorphism.) Brow ridge development in hominids may also be related to diet, as large brow ridges help to redirect the considerable stresses placed on the skull by a diet of coarse vegetable matter.

The presence of a pronounced nuchal crest provides information about a species' posture, together with the position of the foramen magnum. The crest provides anchorage for neck muscles in those species where the skull is not balanced vertically atop the spine. Thus, modern humans have a completely upright posture: the foramen magnum is centrally placed beneath the skull and a nuchal ridge is absent. Early australopithecines such as "Lucy" (A. afarensis) have a foramen magnum position intermediate between that of humans and apes, and a small nuchal ridge. This tells us that their posture was not completely upright.

Facial angle:

There is a general trend towards a flatter facial angle with the appearance of more recent hominids, culminating in the vertical (orthognathous) face of Homo sapiens. Note, though, that there is considerable variation even among the older members of our lineage: Kenyanthropus platyops is named for its relatively flat face - its name means "flat-faced ape-man from Kenya".

Other morphological features

Reduced sexual dimorphism:

All hominoids show some differences in size between the sexes, as well as in such features as the shape of the pelvis and in crests on the skull. Thus male gorillas weigh perhaps twice as much as females. This size difference is much less in chimpanzees and even less pronounced in modern humans, where on average males are 1.2 times as heavy as females.

This trend towards a lesser degree of sexual dimorphism can be traced in hominin fossils. The skeletons of the australopithecines show a marked degree of sexual dimorphism, which is reduced in the early hominids.

Changes in size of ribcage:

"Lucy" (Australopithecus afarensis) has a funnel-shaped ribcage, which is narrow at the top and expands towards the base above wide, flaring hipbones. In comparison, Homo erectus has a barrel-shaped ribcage, indistinguishable from our own, and with a waist separating ribcage and hips.

In his excellent discussion of the significance of the Nariokotome (Turkana) boy, Allan Walker (1996) relates these changes in ribcage to a significant change in diet. Gorillas, which are herbivores, have a ribcage similar to Lucy's. This flaring shape accommodates the extensive gut needed to process the gorillas' rough diet and extract sufficient nutrients. The barrel-shaped ribcage, and the presence of a waist, in erectus suggest that significant quantities of meat were being eaten. Weight for weight, meat is much higher in calories, and easier to digest, than an all-vegetable diet. This means that an omnivore, or a carnivore, has a much shorter gut than a complete vegetarian.

An increase in the amount of meat in the diet, with its greater fat content and higher calories, could also fuel the higher energy demands of a larger brain. Significantly, average cranial capacity increases markedly in Homo erectus.

Human Cultural Evolution

| Tools and Tool Use | Fire | Domestication of Plants and Animals |

There are many examples of human cultural evolution. They include: tool making, the controlled use of fire, manufacture of shelters and clothing, appearance of art and other non-utilitarian products, development of cooperative hunting behaviour, and domestication of plant and animal species (leading to settled agricultural societies). All of these features allowed humans to have greater control of their environment, rather than responsive to it. Thus, the development of these skills would directly contribute to the survival of individuals (& groups) practising these behaviours. (Cultural evolution is also described as non-biological evolution, since what is transmitted to new generations is changes in learned behaviour patterns. However, any genetic underpinnings to these behaviours would also be passed on.)

Some aspects of cultural evolution are easier to trace than others. Examples of stone tools made by hominin species are relatively common and easily recognised. Tools made of other substances, such as wood or bone, do not survive so well in the stratigraphic record. Changes in behaviour, such as the development of cooperative hunting groups or changes in social structure, leave no direct traces at all and their presence must be inferred from other evidence.

Evidence of developing human culture appears far back in time. Homo habilis was named for its association with the crude cobble tools of the Oldowan culture, and it's possible that Australopithecus garhi and one of the robust australopithecines, A. robustus , were also tool users. What differentiates these very simple, ancient tool-making cultures from the tool manufacture and use practised by modern chimpanzees? In fact, how do we define "culture" in the evolutionary sense?

At the time that H. habilis was discovered, manufacture and use of tools was generally viewed as an exclusively human activity. The Oldowan tools marked the earliest hard evidence of culture in our ancestors. We now know that many different animals use tools. Perhaps the best-known example is that of chimpanzees. Jane Goodall first documented this in her studies of wild chimpanzees in Africa's Gombe Reserve. Not only did her animals use rocks, twigs and vegetation as simple tools, but they modified them: for example, stripping a twig of leaves, and breaking it to the right length, so that they could "fish" for termites in the insects' tunnels. Young chimps learn these skills by observing their elders, an example of cultural transmission. And chimpanzees from different areas have distinctly different tool-making cultures.

However, one of the differences between chimp and human culture is that chimps seldom carry tools, or the raw materials for tool making, for any distance. In addition, chimps make tools only immediately before using them. Tools used by early humans were typically worked and reworked at different locations

So is it the complexity of culture that sets humans apart? We think of complex culture as a hallmark of humanity. However, art works, such as jewellery, carving, and cave paintings, do not appear in the record until 30-40,000 years ago. This follows the development of the sophisticated Aurignacian tool kits associated with Cro-Magnon culture. Some authors suggest that the use of highly sophisticated language accompanied this flowering of culture, and marked the appearance of a significant capacity for abstract thought. (This is not to say that earlier humans, and hominins, were not capable of speech.)

Cultural evolution has occurred in different times in different places. This is a reflection both of the time at which different regions of the globe were settled, and also the nature of the biology & geology of an area, which poses constraints on, for example, the domestication of plants & animals. This has had far-reaching consequences on later geopolitical history. (Guns germs & steel)

Tools and tool use

At present the earliest-known evidence of the manufacture and use of tools comes from a 2.5 million-year-old site, possibly associated with Australopithecus garhi. This site contains primitive stone tools, but no hominin remains. Animal bones recovered together with garhi remains, from a nearby site of the same age, appear to show cut marks from stone tools.

These tools predate the better-known Oldowan tool culture associated with Homo habilis. These are often described as "cobble tools", and comprise two main types, core tools and flake tools. Core tools are stones with one or more flakes knocked off one end to give a jagged edge. Flake tools are the flakes removed in producing core tools, and were not modified any further before being used. It's possible that the core "tools" are simply what remained after flake tools had been produced, and that they weren't used to any great extent.

The Oldowan tool culture persisted in Africa for almost a million years. With Homo erectus came the more sophisticated Acheulean toolkit. These tools were more highly modified than the earlier cobble and flake tools, with sharper and straighter edges formed by careful removal of more and smaller flakes. Perhaps the best-known Acheulean tool is the so-called hand axe, a teardrop-shaped implement with a pointed end and sharp sides. We have no way of knowing just how these hand axes were used, but they were probably put to a wide range of uses. Other Acheulean tools included hammers, cleavers, and flake-based tools such as knives (probably held directly in the hand, rather than hafted to a handle).

Erectus was probably the first hominin to leave Africa. Acheulean tools are found in both Europe and Asia. Until recently it was believed that Chinese erectus populations continued to use Oldowan tools, but Acheulean tools dating back 1.3 million years were found in China in 2001.

Along with more sophisticated tools came a change in the foods eaten, and how these foods were obtained. While the australopithecines, and perhaps H. habilis , were essentially vegetarian, meat was a regular part of the erectus diet. Remains from many sites, including Zhoukoudian in China, show that erectus was eating meat on a large scale and from a range of animal species, in addition to a wide variety of plant foods. These animals may have been both scavenged (from other predators) and hunted.

The year-round available of calories from meat would have made it possible for erectus to move from its tropical homeland into temperate regions. Parts of China, for example, experience cold winters, when fresh plant foods are not readily available and meat would be the primary source of calories.

There is also a possible causal link between the marked increase in cranial capacity of Homo erectus - especially the rapid rate of growth of the brain after birth - compared to its predecessors, and the regular presence of meat in erectus diets. The brain is a very fatty organ, and meat is a much better source of the necessary fats than plant foods. The high calorie content of meat is also important, as the brain is a very energy-hungry organ. (And of course, breastfeeding an infant with a rapidly growing brain is energetically very expensive.)

Another leap in tool development came with the Mousterian tool culture, associated with both Neandertals and archaic Homo sapiens . In a significant advance over the Acheulean culture, a stone core was carefully shaped before flakes were struck off it: different core shapes gave different flakes. These flakes could then be further modified for a range of different tasks, and some have a tang at the end that suggested that they were hafted to a wooden or bone handle.

The appearance of modern Homo sapiens saw further innovation with a new group of tool-making styles, collectively known as the Upper Palaeolithic industry. The earliest such tools date from Africa, around 90,000 years ago. The Upper Palaeolithic industry spanned the period 40,000 to 12,000years ago and included the Aurignacian (associated with Neandertals and modern humans), Chatelperronian (used largely by declining European Neandertal groups), Gravettian, Solutrean, and Magdalenian industries. These tools are far more complex than those of the earlier Mousterian culture, and are made of a wider range of materials. They show both regional variation and adaptation to particular needs: fishhooks & harpoon points were first manufactured by Upper Palaeolithic toolmakers, as were needles of ivory and bone.

Other cultural artefacts are associated with the later Gravettian, Solutrean, and Magdalenian tool industries. Ivory beads and "Venus" figurines are associated with Gravettian sites, while necklaces, animal figurines, and symbolic art (you need to click on the title after opening the front page) appeared during the Magdalenian, 18,000 to 12,000 years ago.

This hard evidence of human cultural evolution can be used to infer something about the nature of the human societies producing these artefacts. Needles suggest the manufacture of relatively sophisticated clothing, as does the use of huge quantities of beads to decorate these clothes (found in some grave sites). Palaeoanthropologists have linked the nature of many cave paintings and carvings to the development of various rituals - and also to the development of fluent abstract language. They argue that the complex and often abstract nature of much cave art could not have been developed without an equal ability to communicate. Similarly, the appearance of complex burials at some Upper Palaeolithic sites may imply concerns or beliefs about an afterlife.

Fire

Bones found in the Swartkrans Cave in South Africa, and dating back perhaps 1.5 million years ago, provide some of the earliest evidence for the use of fire. Analysis of the bones showed that they had been heated to the high temperatures normally associated with hearths. (Bush fires reach lower temperatures and do not generate the same changes to the bone.) Two hominins were present in Swartkrans at this time: Homo erectus and Paranthropus robustus , and it's not known which species burnt the bones. However, later sites where fire was used are definitely associated with erectus . Hearth sites 790,000 years old, found in Israel, also contain the Acheulean tools produced by erectus.

Prior to these discoveries in Africa and Israel, the earliest site with evidence of regular use of fire was Zhoukoudian (Choukoudian), or "Dragon Bone Hill", near Beijing. Here researchers studying "Peking Man" ( Homo erectus ) found charcoal, charred bones, and rocks cracked by exposure to fire. Many of the bones belonged to large game animals, which may mean that the local erectus population was engaging in organised hunts.

Learning to use fire in a controlled manner was a major step for our ancestors, because it gave them greater control over their environment and also had the potential to make available a far greater range of foods. Fire would not only offer protection from predators, but would also allow its users to survive in much colder environments. In addition, the controlled use of fire is evidence of the ability to plan ahead, and would also have aided social interactions as people gathered round the hearth.

Domestication of Plants and Animals

Both plants and animals were first domesticated by humans in Europe and western Asia. Dogs may have been domesticated as early as 13000 years ago, followed by goats, sheep, pigs and cows (8-10,000 years ago), and horses around 6,000 years ago). Animals suitable for domestication had to be easy to fed, grow fast and breed easily in captivity, have a tractable nature, be unlikely to panic, and have the sort of social hierarchy where humans could slot in as the leaders of the group. A lack of large animals meeting these criteria helps to explain why widespread use of animals for food, fibres, or beasts of burden did not occur in Africa, Australia, or the Americas. (In fact, donkeys are the only domesticated mammal to come from Africa.) This in turn gave European cultures an advantage when they began to move into the other major landmasses and, ultimately, the Pacific.

Domestication of plants appears to have begun in the Fertile Crescent (the region lying between the Tigris and Euphrates rivers, in what is now Iraq and Iran), about 10,000 years ago. This saw the beginnings of agriculture, and also of settled civilisations. A hunter-gatherer lifestyle can support only a small number of people in a given area. However, the surpluses of food offered by agriculture can support a larger, settled population, and also allow a division of labour whereby individuals are freed for tasks other than food gathering. Paradoxically, while agriculture allowed more people to settle in one place, this was accompanied by a reduction in their overall health. Skeletons recovered from early cemeteries show that townsfolk were often smaller, and less-well nourished, than hunter-gatherers. This is because, while agriculture certainly provided more calories, the overall quality of the diet was less.

The Middle East was particularly conducive to the development of agriculture because of the large number of plant species with the potential for domestication. The first plants to be domesticated would have been annual plants which bore large seeds or fruits (and so were more attractive to humans), including peas and other legumes, and cereals (derived from wild grasses). Fruits such as apples and olives came later. Rice was domesticated in Asia, while squash, maize and beans were key crops in the Americas. No plants were domesticated in Australia, despite humans having lived there for perhaps 60,000 years (and the only domesticated mammal, the dingo, was brought from Asia).

A useful link: http://en.wikipedia.org/wiki/Domestication

Mitochondrial DNA

Not all of the genes in a eukaryote cell are found on its chromosomes. Both mitochondria, found in all eukaryote cells, and the chloroplasts of green plants and the algae have their own DNA. These organelles reproduce independently of the cell's nucleus and pass their genes on to their daughter organelles.

For the mitochondria and chloroplasts of multicellular organisms, In multicellular organisms the DNA of both mitochondria (mtDNA) and chloroplasts is inherited almost exclusively down the maternal line. This is because zygotes generally inherit all of their organelles from the ovum, rather than from the sperm or pollen. Just as for nuclear DNA, mtDNA can be used to examine phylogenetic relationships.

Organisms whose DNA sequence for a particular gene differs by only a few bases are likely to be closely related. This seems straightforward, but different parts of the genome mutate at different rates, and so scientists must select which region they wish to use. Ribosomal DNA (rDNA) mutates relatively slowly, and so can be used to examine the relationships between species that last shared a common ancestor hundreds of millions of years ago. However, mtDNA accumulates changes in the base sequence relatively rapidly, up to 10 times as fast as nuclear DNA. This means that it can be useful in studying the evolutionary history of species that diverged only recently, or to look at populations of the same species.

mtDNA and the "African Eve" hypothesis

mtDNA has been used to examine our own recent history. A number of studies have tried to put a date to the time when various human populations diverged. They have all found that the mtDNA of humans from a range of different geographical origins (i.e. "races") is relatively uniform. This indicates a very recent date for the populations to have separated. However, they have also shown that the mtDNA from African populations is more diverse than for any other group, suggesting that these populations have a relatively longer history. The overall conclusion is that the common origin for all modern human populations lies in Africa - the so-called "African Eve" hypothesis.

Dates for this origin range from around 60,000 to 400,000 years ago. However, more recent studies have placed the founding population at 200,000 years or less. These younger dates are supported by the discovery of modern human skulls dating back 160,000 years ago, from the Middle Awash of Ethiopia.

mtDNA and the history of Polynesian migrations

This section is based on material kindly provided by Dr Geoff Chambers and Adele Whyte of the Institute of Molecular Systematics at Victoria University, Wellington, NZ.

Studies of genomic DNA have shown that, in the NZ population as a whole, Maori and Polynesians have lower heterozygosity than the rest of the population. This fits with the idea that NZ was colonised by a series of "island-hopping" migrations, each of which would have led to a founder effect, with the result of inbreeding and decreased heterozygosity.

More detailed information on the history of these migrations can be obtained by studying mitochondrial DNA (mtDNA). This is because mtDNA is inherited almost exclusively down maternal lines. Studies of mtDNA lineages from the NZ population have confirmed earlier findings that the ancestors of NZ's indigenous population followed the Austronesian migration route, from Taiwan, into the Philippines and Indonesia, and then across the Pacific and finally to New Zealand. And like the studies of genomic DNA, they show a decrease in genetic diversity along this migration route.

The concept of a Taiwanese homeland is also supported by studies of genes coding for enzymes involved in the metabolism of alcohol in the liver. Many NZ Maori and Polynesians have a variant form of an enzyme that speeds this process up -- a trait that they share with the indigenous tribespeople of Taiwan.

The mtDNA data also make it possible to estimate the number of female colonists on the migration from Eastern Polynesia to New Zealand. There may have been more than 100 female settlers, which suggests that the migration was planned rather than the result of voyagers becoming lost between islands.

Interestingly, studies of Y chromosome DNA, which is passed exclusively from fathers to sons, indicate that male genetic lineages have different origins from female lineages. This is an outcome of gender-biased migration following contact between Austronesian and Papuan groups.

mtDNA and recent human evolution

A recent study (Ruiz-Pesini et al. 2004) of mtDNA has demonstrated that gene frequencies have changed over the last 50,000 years i.e. human populations have still been subject to evolution.

Some mutations in mtDNA may make aerobic respiration less efficient, so that the mitochondria generate more heat and less ATP. These mutations will be selected for if they are beneficial to the person carrying them - and they would certainly be advantageous for humans living in the cold climates that prevailed during the Ice Age.

Examination of the mtDNA from over 1,000 people has found that such a mutation is present in populations of Northern Europeans, East Asians, and Amerindians. Of those in the sample that live in Arctic regions, 75% had the mutation, which was also found in the 14% of the sample living in temperate zones. Some of the ancestors of these groups would have lived in Siberia, and all would have experienced the Ice Age's glacial conditions. However, the mutation is not found at all in people of African ancestry.

The study concludes that the correlation between habitat and presence of the beneficial mutation is evidence of positive selection for the changed gene sequence. That is, the mutation was selected for because those people who had them were able to generate more body heat in an extremely cold climate.

Other examples of ongoing human evolution

We can detect the influence of evolution on the present-day human gene pool just as easily as we can view the development of our species' family tree (e.g. Baulter, 2005). In the developed world, the combination of modern medicine, new agricultural and technological techniques, and cultural changes have considerably reduced the effects of natural selection. But in developing countries people are still exposed to these selection pressures, so perhaps this is where we should be looking for evidence of evolutionary change e.g. the spread of alleles giving resistance to diseases such as malaria.

In regions where malaria is endemic, anyone with a genotype giving resistance to malaria would be at an advantage in evolutionary terms, because they would be more likely to survive and reproduce, passing their advantageous combination of genes on to at least some of their children. The overlap between the geographic spread of malaria in Africa with the presence of the sickle-cell allele is an example: individuals heterozygous for this allele are at a selective advantage over unaffected individuals (and those homozygous for the allele) where malaria is present.

The "CCR5" gene is another example. This gene codes for CCR5, a surface protein on white blood cells that is also the docking site for the HIV virus. People homozygous for the 'delta 32' mutation in this gene are resistant to attack by HIV, and so they are at a selective advantage in populations where HIV infection (and AIDS) is common. But surprisingly the mutation is most common in white Europeans, and very rare in other ethnic groups, including Africans. AIDS is far more common in Africa than in Europe, so these differences in allele frequency are difficult to explain, unless they are the result of some other selective pressure that predates the AIDS epidemic. Scientists have dated the origin of the delta 32 mutation to around 700 years ago, and the current hypothesis is that it provided protection against an epidemic disease of that time, perhaps plague or smallpox.

Obesity and all its related health problems, such as adult-onset (type II) diabetes, and cardiovascular disease, are frequently in the news these days. They are most common in populations of people who have only recently taken up westernised lifestyles e.g. Nauru islanders, and the Pima Indians of North America. In both these groups, 70% of 60-year-olds have type II diabetes. In both populations, many people die before 60 of diseases related to diabetes and/or obesity.

Because there are genotypes - beneficial for hunter-gatherer populations - that can predispose to these "western" diseases (Jared Diamond, 2002), then it should be possible to see natural selection working upon them. This is especially true for populations where individuals of reproductive age are affected, such as the Nauruans and Pima Indians. In such populations we can predict strong selection against the genotypes that predispose individuals to "western" health problems. When Europeans and non-Europeans are matched for diet and lifestyle, the Europeans have a lower frequency of type II diabetes. In Diamond's words, this "suggests that natural selection had already reduced European frequencies of those genotypes in previous centuries, as the western lifestyle was developing in Europe" (Diamond, 2002: 706).

Selection for increased oxygen-carrying capacity in the blood : Tibetans living at 4,000m above sea level are exposed to lower partial pressure of oxygen, compared to people living at sea level. Researchers studying Tibetan villagers have discovered that women whose blood has a higher oxygen-carrying capacity tend to have more children that survive to adulthood. The research project combined information from family trees with data on health and lifestyle factors and a measure of the oxygen concentration of the villagers� blood.

A subgroup of villagers have a blood-oxygen concentration 10% higher than normal, and examination of their family trees indicates that this difference is controlled by a single gene. Children of women with the apparent high-oxygen genotype are more likely to survive to have their own children, which suggests that this genotype has a significant reproductive advantage and so is likely to spread through the population � an example of evolution in progress.

Diet and high levels of salivary amylase: A recent project (summarised by Shadan, 2007) examined the relationship between dietary starch and the number of copies of a gene that codes for salivary amylase, the enzyme that digests starch. People differ quite markedly in the number of copies of this gene, which is called AMY1. The researchers found that the extra copies were functional: the more copies an individual has, the more salivary amylase they produce.

They then asked whether there was any relationship between past diet and the number of copies of AMY1, and whether having multiple copies of the gene might confer a selective advantage.

To answer the first question, the research team collected copy-number data from two groups of people. The first comprised four different populations with a diet low in starch, while the second was made up of three populations with high-starch diets. Both groups contained people from different geographic areas. They found that the high-starch group contained twice as many people with at least six copies of AMY1, compared to the low-starch group. This couldn't be explained by geographical differences, and the researchers concluded that the copy-number differences were the result of natural selection for a high AMY1 copy number.

This suggests that there is a selective advantage in having a higher production of amylase if eating a diet high in starch. The team suggested that individuals with producing more salivary amylase would be better able to digest starch while chewing their food, and thus maximise the amount of glucose available for absorption.

F igure 1 Possible evolutionary relationships of the hominins, indicating the five major genera, with Kenyanthropus in red, Homo in blue, Paranthropus in green, Australopithecus in black and Ardipithecus in yellow. Question marks indicate hypothetical or conjectural relationships; horizontal bars indicate uncertainty in the species' temporal spans. With kind permission of Daniel Lieberman & Nature Publishing Group. This link takes you to the Homepage of the science journal 'Nature': http://www.nature.com/nature

References

Leslie C. Aiello & Mark Collard (2001) Our newest oldest ancestor? Nature 410: 526-527

Michael Baulter (2005) Are humans still evolving? Science 309: 234-237

David R. Begun (2003) Planet of the Apes Scientific American 289(2): 64-73

D.M. Bramble & D.E. Lieberman (2004) Endurance running and the evolution of Homo. Nature 432: 345-352

P. Brown, T. Sutikna, M. J. Morwood, R. P. Soejono, Jatmiko, E. Wayhu Saptomo & Rokus Awe Due (2004) A new small-bodied hominin from the Late Pleistocene of Flores, Indonesia Nature 431, 1055 - 1061

Michel Brunet et al. (2002) A new hominid from the Upper Miocene of Chad, Central Africa Nature 418: 145-151

Jared Diamond (1997) Guns, germs and steel: a short history of everybody for the last 13,000 years pub. Vintage

Jared Diamond (2002) Evolution, consequences and future of plant and animal domestication Nature 418: 700-706

K. Galik, B. Senut, M. Pickford, D. Gommery, J. Treil, A. J. Kuperavage, R. B. Eckhardt (2004) External and Internal Morphology of the BAR 1002'00 Orrorin tugenensis Femur Science 305 (5689):1450-1453

Henry Gee (2001) Return to the planet of the apes Nature 412:131-132

Ann Gibbons (2007) A new body of evidence fleshes out Homo erectus. Science 317: 1664

Jeff Hecht (2004) Donkey domestication began in Africa New Scientist on-line: newscientist.com accessed 17 June 2004

Meave G. Leakey, Fred Spoor, Frank H. Brown, Patrick N. Gathogo, Christopher Kiarie, Louise N. Leakey, Ian McDougall (2001) New hominin genus from eastern Africa shows diverse middle Pliocene lineages Nature 410, 433 - 440

Meave G. Leakey, Craig S. Feibel, Ian McDougall, Alan Walker (2002) New four-million-year-old hominid species from Kanapoi and Allia Bay, Kenya Nature 376: 565-571

Michael Lemonick & Andrea Dorfman (1999) Up from the apes Time August 23 1999: 36-44 (a very good overview of recent work in human palaeontology)

Daniel E. Lieberman (2001) Another face in our family tree Nature 410: 419-420

Daniel E. Lieberman (2007) Homing in on early Homo. Nature 449: 291-292

David Lordkipanidze et al. (2007) Post-cranial evidence from early Homo from Dmanisi, Georgia. Nature 449: 305-310

John McCrone (2000) Fired up New Scientist 20 May 2000 : 30-34 (use of fire by H. erectus in Africa )

Brian G. Richmond & David S. Strait (2002) Evidence that humans evolved from a knuckle-walking ancestor Nature 404: 382-385

Eduardo Ruiz-Pesini, Dan Mishmar, Martin Brandon, Vincent Procaccio, and Douglas C. Wallace (2004) Effects of Purifying and Adaptive Selection on Regional Variation in Human mtDNA Science 303(5655): 223 - 225

James Shreeve (1995) The Neandertal Enigma: solving the mystery of modern human origins , Morrow

Chris Stringer & Clive Gamble (1993) In Search of the Neanderthals: solving the puzzle of human origins , Thames & Hudson.

Shadaf Shadan (2007) You are what you ate. Nature 449: 155

Alan Walker & Pat Shipman (1996) The Wisdom of Bones: in search of human origins Weidenfeld & Nicolson , UK

Tim White, Berhane Asfaw, David Degusta, Henry Gilbert, Gary Richards, Gen Suwas & F. Clark Howell (2003) Pleistocene Homo sapiens from Middle Awash, Ethiopia Nature 423: 742-747

Bernard Wood (2002) Hominid revelations from Chad Nature 418: 133-135

Christopher Zollikofer, Marcia Ponce de Leon, Daniel Lieberman, Frank Guy, David Pilbean, Andossa Likius, Hassane Mackaye, Patrick Vignaud and Michel Brunet (2005) Virtual reconstruction of Sahelanthropus tchadensis. Nature 434: 755-759

Reference Websites

http://www.becominghuman.org - an excellent website with a wealth of information and links to other sites and sources

http://www.modernhumanorigins.com - this site provides good information on the workings of natural selection and genetic drift

http://www.rsnz.govt.nz/news/beinghuman/beinghuman.pdf - proceedings of a conference organised by RSNZ; contains an excellent article by Colin Groves on human physical & cultural evolution

http://www.stanford.edu/~harryg/protected/evolve6.htm - discusses changes in tool-making techniques, & tool-kits characteristic of various species

http://www.talkorigins.org/faqs/homs/species.html