Human Evolution

Determining when a fossil find is an early human

The hominid fossil record

Appearance of modern-looking humans

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The history of how the modern human species, Homo sapiens sapiens, evolved is reconstructed by evidence gathered by paleontologists, anthropologists, archeologists, anatomists, biochemists, behavioral scientists, and other professions. Evidence comes from the record left by fossils and by extrapolation from modern primates, human hunter-gatherer tribes, and (in recent years) rapidly increasing knowledge of DNA (deoxyribonucleic acid), both human and other.

Fossils are evidence of past life. In practice, human fossils are mostly bones and teeth, which are the parts of the human body least likely to decompose. Most individuals never fossilize; it is extremely unusual for bodies to be subjected to all of the conditions necessary for fossilization. Nevertheless, there is an unusually rich record of transitional fossils showing the gradual formation of human features from those of our immediate prehuman apelike ancestors. The growth of the skull, the shrinking of the jaw, and the shift to upright posture are all well documented.

Scientists date fossils by one of several techniques, including carbon-14 dating, which measures the ratio of radioactive carbon to stable carbon, and potassium-argon dating, which measures the ratio of a radioactive form of the element potassium to its breakdown product, argon. Before these methods were available, index fossils of a particular geologic period were used to give an approximate date to other fossils. More recent dating methods include thermoluminescence, electron spin resonance, and fission track dating.

Paleontologists try to recreate the entire animal from sparse bone fragments by comparing the fossil fragments with similar animals, both now living or fossil, of which more information is known. Since complete fossils are rarely found, anatomists recreate the entire skeleton by comparing it with other individuals from the same species or with closely related species. Muscles are reconstructed over the skeleton based on a knowledge of anatomy, and the animal is positioned based upon how a similar living animal would move.

Studies of the DNA of humans and the great apes indicate that the closest living relatives of humans are chimpanzees and gorillas. Humans are not thought to be direct descendants of apes, rather we have descended from a common ancestor. Initial studies comparing chimpanzee and human DNA estimated that the similarity is 98.5%. However, recent studies showed that this similarity is more likely to be lower and is estimated at 95%. The final verdict will be delivered in a few years when the chimpanzee genome project undertaken by the Riken Institute, is finished. Despite being closely related and having some things in common (number of bones) there are distinct differences between humans and chimps. These include the human’s larger brain, ability to speak due to a differently built larynx, ability to walk upright on two legs instead of swinging or knuckle-walking, and greater manual dexterity, due to the opposable thumb that enables humans to manipulate small tools with precision. The faces of humans are flattened, or reduced compared to the apes. The human skeleton is similar to that of a chimp or a gorilla, but is modified for walking upright on two legs. At some point in our development, humans began to rely more on learned behavior (which creates culture) than on genetically fixed or instinctive behavior. This cultural development might be indicated by remains other than bones or teeth, including objects such as stone tools. The first appearance of those traits in the fossil record indicate that those animals were nearly as human as us, which makes them a possible ancestor.

What is it that makes us essentially human? Our name, Homo sapiens, means “wise man.” Intelligence is the quality most widely seen as making humans unique. Fossil evidence of intelligence is based upon brain size measured in volume (cubic centimeters, cc). Human brains are three times larger than any comparable primate of a similar weight. Although they grow after birth at a rate that is average for a mammal, they continue to grow for much longer than other animals. Our brains also have different proportions than other primates. Particular areas of the human brain have developed in unique ways, especially the parts of the brain responsible for speech. The other physical traits that we have uniquely acquired include an upright posture, walking on two feet, and an opposable thumb. Finally, human young are cared for over a longer period of time than any other primate.

Other animals besides humans use tools, such as chimpanzees, who fashion twigs into devices to poke termites from termite mounds. However, humans make and preserve tools with anticipation of using them in the future. Some species of animals communicate using complex sounds or show evidence of aiding another, such as dolphins and whales. Although other primates do not use symbolic language, where the meaning of words is learned, they are capable of leaning our system of symbols; chimpanzees and gorillas have been taught to use and understand American Sign Language. Humans are unique in having developed written languages—but not all human societies have done so. In general, it is difficult to point to any single quality or ability in human beings that is not shared with some other species, especially with primates, but the degree to which human beings modify themselves and their environment through cultural behaviors has no parallel.

The first pre-human fossil to be named was Australopithecus africanus, meaning the southern ape of Africa. The fossils were found at a site called Taung in South Africa by Raymond Dart, who recognized it as being intermediate between apes and humans. The fossils are dated at three million years old. Additional fossils of A. africanus were discovered at Sterkfontein and at Makapanskat in South Africa. The bones from other animals found along with A. africanus were interpreted as meaning that our ancestors were hunters. Other scientists determined that those bones were actually the leftover meals from leopards and hyenas. It is now believed that A. africanus was primarily a vegetarian, and probably did little, if any, hunting. Teeth wear patterns indicate that A. africanus ate fruit and foliage. No stone tools were found with any of these fossils, so there is no evidence that Australopithecus made or used tools, or used fire.

In 1912, William Dawson discovered pieces of a skull and jaw along with stone tools and index fossils at Piltdown in England. The jaw was apelike, but the skull was humanlike. British anthropologists at the time judged the find to be authentic, perhaps because it appeared to support a cherished belief that humans had first developed a big brain, and then later developed other human characteristics. It was subsequently discovered that Piltdown man was a hoax, composed of a human braincase and the jaw from an orangutan, modified to look old.

Australopithecus afarensis, the southern ape of the Afar region in Ethiopia, was discovered more recently and found to be the oldest known humanlike animal to have walked upright. The most famous of these fossils, nicknamed “Lucy,” was found near Hadar, Ethiopia, by a team of anthropologists led by Dr. Donald Johanson. Lucy lived about 3.5 million years ago, and had a skull, knees and a pelvis more similar to ours than to the apes. Her brain size was about 350 cc, which was less than one third of the brain size of modern humans (1,400 cc), yet larger than any apelike ancestor to have come before. She would have stood at a height of about 3.5 ft (1 m) tall, with long arms, a v-shaped jaw, and a large projecting face.

Fossils of several male and female Australopithecus have been found together. There is some uncertainty as to whether these are A. afarensis or another closely related species. This group find gives evidence that they were social animals. Two of these early humanlike ancestors also left a trail of footprints at Laetoli in East Africa in what was then volcanic ash that later became fossilized. These were discovered by Mary Leakey (1913–1996), the wife of the pioneer paleontologist, Louis Leakey (1903–1972). The fossil footprints look very similar to modern human prints and add further proof that our ancestors walked upright.

The reasons our ancestors started to walk upright are not known. Possibly, it was a response to environmental changes; as tropical forests were beginning to shrink, walking might have been a better way to cross the grasslands to get to nearby patches of forest for food. We can get some ideas of possible advantages of upright posture to our ancestors by studying modern apes. When chimpanzees or gorillas become excited, they stand in an upright posture and shake a stick or throw an object. By standing upright, they appear bigger and more impressive in size than they normally are. This would be useful to help protect the group against predators. Also, the ability to stand up and get a wider view of the surroundings gives an animal an advantage in the tall grasses. Walking upright frees up the hands to carry objects, such as tools.

Two other species of Australopithecus are A. robustus and A. boisei. Australopithecus robustus, from South Africa, was named for its massive jaws and large flat chewing teeth. This species also had a bony ridge along the top of its skull (the sagittal crest) similar to that of an adult male gorilla, which served as a site of attachment for massive jaw muscles. Its skull had the brain capacity of 500 cc. Living about 1.9–1.5 million years ago, the diet of A. robustus probably consisted of tough gritty foods, such as plant tubers. Australopithecus robustus was probably not a direct ancestor of modern humans. The other Australopithecus species, A. boisei, was discovered by Louis Leakey at Olduvai Gorge in Tanzania, a site that has been famous for hominid fossils for more than 60 years. Sediments and fossils are exposed in the walls of the gorge that represent almost two million years of evolutionary history. Australopithecus boisei had huge flat grinding teeth, a very long face, and a large elongated cranium, with a brain capacity of 530 cc and a sagittal crest atop the skull.

The record of animals that were ancestral to Australopithecus is poor. An apelike animal (Ramapithecus ), lived in Africa some 12 million years ago and is thought to have been the first representative of the line leading to humans. Ramapithecus lived on the forest fringe, near rivers and lakes, and began to make the transition to life on the more open savanna. Very few remains of Ramapithecus have been found, only fragments of upper and lower jaws and teeth. Its dental pattern was unique among other fossil finds from that time. The canine teeth were fairly small, indicating that its diet may have included seeds and other tough plant material that required being torn apart before eaten. A five million-year gap in the fossil record between the time of Ramapithecus and Australopithecus has been recently partially filled by new finds in Africa, although it is not yet clear where exactly on the human evolution tree these fossils will be placed. Remains of a hominid from six million years ago were found at Kapsomin by a French and Kenyan anthropological group led by Martin Pickford and Brigitte Senut in 2001. It was named Orrorin tugenensis. However, in July 2002, Professor Michael Brunet with an international group of scientists found an even older (seven million-year-old) skull in Chad, called Sahelanthropus tchadensis, nicknamed Toumai. The opinions on whether it is the skull of a hominid or an ancient gorilla are divided. Independent of the final verdict, the fact that the skull was nearly intact is very important for further comparative analysis.

Australopithecus is similar enough to humans to be considered an ancestor, but different enough to be assigned to a separate genus. On the other hand, Homo habilis, which lived about 1.5–2 million years ago, is similar enough to modern humans be included in the genus Homo. The braincase of H. habilis was appreciably larger than that of Australopithecus, with a brain capacity of 750 cc. Homo habilis individuals were short and made stone tools from pebbles about 5 in (12.7 cm) long, formed from flakes of rock. The flakes had been broken off the pebble by blows from another stone and were probably used for cutting.

Homo erectus is generally thought to have been our direct ancestor. Homo erectus lived about 1.7 million years ago, and had a brain capacity of 950 cc. The first fossil of Homo erectus was found in Java; it was nicknamed Java man. Similar fossils found in China were dubbed Peking man. Recently, an entire skeleton of a closely related species, Homo ergaster, was found in Kenya. Walking with a fully upright posture, tall and slender, the fossils were found with sophisticated stone tools. They were probably hunters and also scavengers. Bones found along with the fossils have been studied closely; they carry the remains of tooth marks from predators, like leopards, as well as hominid tooth marks. Homo erectus probably scavenged from kills made by large predators, breaking bones to eat the rich marrow. The presence of charcoal provides evidence that H. erectus used fire, probably to scare off predators.

H. erectus was thought to be the first hominid to leave Africa. This notion was recently (2001) challenged by David Lordkipanidze’s group finds in the Georgian village of Dmanisi. The skulls found were much smaller (estimated brain size 600–780 cc) than those of H. erectus but had enough similarities to be classified in the same species. The fact that they were dated to 1.7–1.8 million years ago challenges the notion that long-legged, large-brained H. erectus left Africa around one million years ago. A hypothesis was made by Vekua and colleagues that Dminisi hominids might have evolved from H. habilis outside Africa. Confirmation of such a hypothesis, however, will require further fossil evidence.

Neanderthal man (Homo sapiens neanderthalensis ) was the first human fossil to be found. It was discovered in 1856 in Germany’s Neander Valley and is the source of the caveman stereotype. Neanderthals first appeared 300,000 years ago in what is now Europe, lived throughout the ice ages, and were thought to disappear about 35,000 years ago, but recently remains from 28,000 years ago were found in Croatia. Neanderthals had a large brain capacity about (1,500 cc), a strong upper body, a bulbous nose, and a prominent brow ridge. There is general agreement today that the Neanderthals were not our direct ancestors, but a cousin species. In 1997, Krings and colleagues based on the analysis of mitochondrial DNA from Neanderthal remains concluded that modern humans and Neanderthals were not related. However, just a year later in 1998 professor Erik Trinkaus discovered a skeleton of a four-year-old boy from 25,000 years ago that showed a mixture of Neanderthal and modern features suggesting that the two species interbred. But interbreedability would be consistent with a recent common ancestor species, and does not necessarily show lineal descent of humans from Neanderthals.

Recent excavations in Israel, Portugal, and Croatia show clearly that Neanderthals were contemporary with modern Homo sapiens sapiens. The two hominids apparently survived independently of each other for tens of thousands of years. Some anthropologists see this as evidence that Neanderthals were not our direct ancestors; other anthropologists speculate the two types of humans may have interbred and Neanderthal became genetically absorbed by more modern humans. We do not know why Neanderthals died out, nor what the nature of their interaction with modern Homo sapiens sapiens might have been.

Neanderthal man made a number of crafted flint tools with many different uses. Judging from the hearths found at many sites, Neanderthals had mastered the art of making fire. Fossil bones show signs of old injuries that had healed, indicating the victim had been cared for. Some Neanderthal caves contain burial sites. The Shanidar cave in Iraq held the remains of a Neanderthal buried 60,000 years, with bunches of flowers. Several of the flowers discovered were species used today as herbal medicines. It is therefore possible that Neanderthals had an elaborate culture, were aware of the medicinal properties of plants, and ritually buried their dead. One anthropologist in Israel found what he believed to be evidence that Neanderthals had the capacity for speech, a fossil bone from the throat (the hyoid), which anchors the muscles connected to the larynx and tongue, and which permit speech in modern humans.

Although all of the ancestors described thus far first evolved in Africa, there is uncertainty as to where modern humans, Homo sapiens sapiens, first appeared. There are two theories to explain this process. The first is the “multi-regional” model, which proposes that Homo sapiens sapiens evolved in Europe, Asia, and Australia from Homo erectus after the latter left Africa about one million years ago. The second model, called Out of Africa, suggests that modern humans evolved only once, in Africa, leaving there within the last 200,000 years in a rapid global expansion. They replaced other populations of older human forms in Europe and Asia, including the Neanderthals. Variations and combinations of these two theories have also been proposed.

The oldest fossils of modern human beings, Homo sapiens sapiens, are 100,000–125,000 years old, appearing at the time of the first of the great ice ages. Homo sapiens sapiens are identified by a large brain (1,400 cc), a small face in proportion to the size of the skull, a small chin, and small teeth. In addition, they were tall and relatively slender in build.

The first fossil of modern Homo sapiens sapiens was found at Cro-Magnon in France, which gave that name to all early Homo sapiens sapiens. Cro-Magnon remains have been found along with the skeletons of woolly mammoth, bison, reindeer, and with tools made from bone, antler, ivory, stone and wood, indicating that Cro-Magnon hunted game of all sizes. Cro-Magnons also cooked their food in skin-lined pots heated with stones. Pieces of amber from the Baltic found in southern Europe together with Cro-Magnon

KEY TERMS

Hominid —A primate in the family Homidae, which includes modern humans.

Primate —An animal of the order Primata, which includes lemurs, monkeys, apes, and humans.

fossils indicate these humans traded material over vast distances. Cro-Magnon humans buried their dead with body ornaments such as necklaces, beaded clothing, and bracelets.

Cro-Magnon humans lived at the mouth of caves under shelters made of skins or in huts made of sticks, saplings, stones, animal skins, or even bones. A mammoth-bone hut 15,000 years old has been excavated at a site in the Ukraine. Anthropologists interpret some of the fossil findings of early Homo sapiens sapiens, by making comparisons with present-day hunter-gatherer tribes such as the Kalahari or Kung bushmen. These nomadic people live in relatively small bands of about 25 people. A larger group of about 20 bands makes up a community of people who all speak the same dialect and occasionally gather in large groups. The groups disperse into smaller bands during the wet season, and establish clusters around permanent water holes in the dry season. The men hunt in cooperative bands when the game is plentiful. The women gather plant material; about two thirds of their diet is made up of plant food. Since only a small portion of time is spent hunting or gathering, there is plenty of time for visiting, entertaining, and sewing. The same might be true for the hunting people living in Europe before 12,000 years ago.

Ice age humans were artists, producing hauntingly beautiful cave art. Carefully rendered pictures of animals, human and mythical representations, and geometric shapes and symbols were created using charcoal and other pigments. The remains of stone lamps found deep within these caves suggested that the caves were visited often. Carvings of stone, ivory and bone have also been discovered in these caves, including female figures. We cannot know what the significance of this art was to them, other than that it was a reflection of how early humans perceived the world around them.

The end of the ice ages brought changes in climate and ecosystems. In Europe, the vast grasslands were replaced by forests, and animal populations shifted from reindeer and bison to red deer and boar. The focus of cultural innovation shifted from Europe to the Middle East, where settled cultivation began.

See also Dating techniques; Fossil and fossiliza-tion; Genetics.

BOOKS

Arsuaga, Luis, et al. Chosen Species: The Long March of Human Evolution. Malden, MA: Blackwell Publishing, 2005.

Dobzhansky, T., and E. Boesiger. Human Culture: A Moment in Evolution. New York: Columbia University Press, 1983.

Stringer, Chris and Peter Andrews. The Complete World of Human Evolution. Thames and Hudson, 2005.

PERIODICALS

Balter, Michael. “Are Human Brains Still Evolving? Brain Genes Show Signs of Selection.” Science. 309 (2005): 1662-1663.

Brunet M et al, “A New Hominid from the Upper Miocene of Chad, Central Africa.” Nature (July 2002):145–151

Ponting, Chris P. and Gerton Lunton. “Evolutionary Biology: Human Brain Gene Wins Genome Race.” Nature. 443 (2006): 149-150.

Senut, Brigitte, Pickford, Martin, Gommery, Dominique, Mein, Pierre, Cheboi, Kiptalam, Coppens, Yves, “First Hominid from the Miocene (Lukeino Formation, Kenya).” C.R.Acad.Sci.Paris, Earth Planet Sci. (January 2001) 332:137–144

Vekua, A, Lordkipanidze, D, Rightmire, G.P, Agusti, J, Ferring, R, Maisuradze, G, et al., “A New Skull of Early Homo from Dmanisi, Georgia.” Science (July 2002):85–89

OTHER

Institute of Human Origins, Arizona State University. “Becoming Human: Paleoanthropology, Evolution and Human Origins.” 2006. <www.becominghuman.org> (accessed November 2, 2006).

Marion Dresner