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Origine

Human Lice, Human History, & Archeogenetics

Lice are ubiquitous parasites on mammals and birds. There are at least 5,000 species of lice and each host species tends to have one louse species. Lice are adapted to usually only one host species and cannot survive long off its host. Ancient nit combs have been found in Egypt and look remarkable like the ones we use today. Each of our living ape relatives have one louse species, but humans have three. Why?

Phylogeny of Human Lice

P. humanus capitus Phthirus pubis Pediculus humanus corporis

(Head louse) (Pubic louse) (Body louse)

Chimp and old world monkeys have lice that appear morphologically alike to the human head and body louse. In addition, DNA analysis confirms this similarity and upholds the justification of the current classification. Thus, the chimp louse Pediculus schaeffi and the old world monkey louse, Pedicinus sp. are more closely related to Pediculus humanus than to the human pubic louse. One can see from the morphology alone why pubic lice are called “crabs”.

Over many decades paleontologists have begun revealing the history of human evolution. It is known from hominid** fossils and the geological record that the last common ancestor of chimps and humans lived about 6 million years ago. Fossils of Homo sapiens and other studies such as DNA analysis have also demonstrated that our modern species arose about 195,000 years ago.

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Since we have little body hair except on our heads and our primate ancestors had a covering of hair, the question naturally arises as to when our ancestors lost their hair. Primate hair has not been found fossilized and so the fossil record cannot help us. We know that our lice parasites have been coevolving with us and we know from the fossil record that the last time they would have shared a common ancestor must have been 6 million years ago. Also, a comparison of human louse DNA and chimp louse DNA indicates they shared a common ancestor about 6 million years ago, confirming the fossil record. These studies used one nuclear gene and one mitochondrial gene for their analysis.

A plausible hypothesis would be that as our ancestors lost hair the ancestral single louse species became isolated into the two remaining regions of hair and the pubic louse evolved into a new species. But DNA comparisons between the human head louse and pubic louse indicate a common ancestor about 12 million years ago so this hypothesis is falsified since they diverged before hominins split from the apes.

As mentioned above, the human pubic louse does not look like the human head louse, but instead is similar to the gorilla louse, Phthirus gorillae. DNA comparisons also demonstrate that these two species are closely related and shared a common ancestor 3.3 mya. What hominins existed at that time? The Australopithecines. What probably happened is that gorilla lice jumped from gorillas to the Australopithecines at that time and then evolved into the human pubic lice we have today. There was an unoccupied niche that was present and this was filled when our ancestors perhaps used old gorilla nests or preyed on ancestral gorillas.

Other studies of head louse DNA indicate that their population suffered a bottle neck contraction about 100,000 years ago. There was a 50% reduction in their population by DNA loss of variability. This matches exactly the DNA data from human DNA markers which also independently shows a bottle neck population contraction occurring right before Homo sapiens moved out of Africa (see "Journey of Man")

Further studies of head louse DNA shows that there was a split into two distinct populations 1.18 mya on an extinct ancestor, possibly Homo erectus. One of these groups is most common in New World groups only. Reed and his associates suggest that one population stayed behind in Africa with our ancestors and another rode the waves out of Africa on H. erectus and Neanderthals. They suggest that humans must have made contact with Asian hominins during the last 100,000 years where they picked up the second population of head lice. Stoneking agrees with Reed’s reconstruction of human-louse coevolution. Even more recent studies using mitochondrial DNA indicate that there may be three clades, but the distinct clade B supports the unique population in the New World. More sampling will need to be completed before this evolutionary history becomes more clear.

But there is more. What about the third human louse, the body louse? Actually, it should be called the clothes louse since it really lives on clothing. It leaves clothing up to five times per day to feed. Its claws are adapted to clinging to clothing and it is slightly larger and can obtain a larger blood meal than the head louse. If we compare selected DNA genes of the head and body louse can we tell when they diverged? If so, we will have an idea when we began to wear clothing, or at least when the clothing was close enough for the body louse to gain access to our relatively hairless areas. Fossil sewing needles only date to about 40,000 years ago. The DNA differences between the two lice indicates an age of about 72,000 years ago in some earlier studies, but more recent studies by Stoneking give an estimate of 107,000 years ago.

Conclusions

1. Humans are infested with three types of lice, whereas modern primates are colonized by a single species. DNA analysis indicates the human head louse and chimp louse shared a common ancestor 6 million years ago. This independently agrees with the fossil record.

2. Human pubic lice look very different from head lice and most resemble gorilla lice. DNA analysis indicates human pubic lice are most closely related to gorilla lice and shared a common ancestor 3.3 mya. Thus, our ancestors had lost most of their body hair at that time and were then infected by gorilla lice, inhabiting an unoccupied “hair-niche”. We did not get our pubic lice from our ancestors but from gorillas.

3. The loss of body hair was an adaptation for persistence hunting, where hominins hunted prey during the heat of the day. They could sweat and outlast fleeing prey that could only pant. This type of hunting can still be seen in African tribal hunters today.

4. DNA analysis of head lice indicate at least two populations exist and the best explanation is infection from earlier hominins to Homo sapiens. One population is found world wide and the other is found only in North America. Thus, louse DNA studies predict that at one time H. erectus and H. sapiens came into contact with one another in Asia, picking up a second population of lice. It is unknown if this contact included interbreeding.

5. Human louse DNA studies confirm the “out of Africa” theory that Homo sapiens populations grew rapidly from a bottleneck population about 60 - 70,000 years ago when a small band left Africa. The bottleneck of Homo sapiens is supported by the louse data, which also was found independently to have suffered a severe reduction in population.

6. Studies such as these show how evolutionary theory is predictive and can be confirmed by several lines of independent evidence. For example, the fossil record of human origins is confirmed by studying parasites that coevolved with our ancestors and carry with them a history of our evolution and origins recorded in their DNA. Only by using evolution can the various observations we see in nature be explained adequately.

Illustrations: 1, 2 - "A Tale of Three Lice". 3 - "How Do You Get Crabs From A Gorilla?"

See also "Origin of Clothing Lice... (2010) Link and Link

References & ** note on next page ------->