Of lice and men...

darjeeling

OK, this paper isn't strictly palaeontology, seeing as there aren't any fossils involved. But the question it's asking is the same as that facing the bone-hunters: where did humans come from, and when?

It's not exactly 'news' either - the paper was out in 2004 - but it was news to me. And quite cool, albeit in a slightly nerdy way, rather than an impress-a-random-stranger-at-a-party way.

The basic idea is that you can chart our evolution using our parasites as a proxy. Here, they've used head lice (Pediculus spp.). Lovely. Each species of primate has lice, and the lice don't like being away from their host. In fact, they die very shortly. This means that the genetic family tree of the lice should end up as a close parallel of that of the hosts. So, if you date the speciation of chimp and human lice, the figure should approximate to the human / chimp split itself. Date variability within human lice, and you should get an idea of when the human family tree began expanding - throwing light on the recent-out-of-Africa / multi-regional human origin debate.

When these scientists did just this, they got a figure of 5.6 million years for the human / chimp louse split. And, yes, this fits very well with estimates of the host speciation date. Huzzah!

Even more interestingly, the human lice they sampled divided into two groups. One is seen all over the world, and has an estimated divergence date of half a million years. The second, smaller group is seen just in the Americas, has a much more recent divergence date of about 15,000 years, and split from the main group about 1.2 million years ago.

What does this mean? Well, the half million years echoes the estimate of the most recent common ancestor for the human genome (excluding mtDNA, Y chromosome), and supports the view that humans came out of Africa relatively recently to overrun the world. The American group, though, looks like a relic of Homo erectus lice. What seems to have happened is that African exodus Homo sapiens ran into their distant Homo erectus relatives in East Asia, swapped lice, and headed on for the New World. The dates are all remarkably congruent with the estimates from palaeontology and archaeology.

Isn't that clever?

Free PlOS paper here. Commentary here.

PS: In a paper punningly titled 'Pair of lice lost or parasites regained?' (here), the same main author later went on to show that our lineage acquired crabs from the gorilla lineage 3 million years ago, but drew a polite veil over the circumstances.

marco_polo

I think I remember reading this before, but the science involved is no less impressive the second time around.

Wibbs

darjeeling wrote:

What does this mean? Well, the half million years echoes the estimate of the most recent common ancestor for the human genome (excluding mtDNA, Y chromosome), and supports the view that humans came out of Africa relatively recently to overrun the world. The American group, though, looks like a relic of Homo erectus lice. What seems to have happened is that African exodus Homo sapiens ran into their distant Homo erectus relatives in East Asia, swapped lice, and headed on for the New World. The dates are all remarkably congruent with the estimates from palaeontology and archaeology.

Well it depends on how you look at the data too and what theory someone is looking to prove. A multiregional person could argue that we acquired the relict erectus louse in the east because of some cross breeding, rather than replacement. You have to be pretty close to pick up lice. That also could be argued in the palaeontology. One of the strange things about erectus and moderns, is that erectus and moderns found in the same areas have some common features not found elsewhere. Eye socket shapes in asian erectus are the same shape as eye sockets in asian moderns and are different to say african erectus and moderns. Europe has some commonalities, but it appears more messy. If you were just going by the bones and stone tool use and spread and didn't have access to DNA you would conclude that there was a lot of multiregional hanky panky going on. As was the general consensus not so long ago before DNA came into it. There is more going on than the DNA suggests methinks. In any case there is much of the DNA that is not looked at, where we may find evidence of interbreeding. Of course the biggest issue is the lack of ancient DNA in any number. You could sequence say Neandertal and figure, nope there's no neandertal DNA in moderns, but the neandertal you sequence may have been part of an isolated group that would have left no descendants anyway. Like Mungo man in australia. He's a modern, but with no descendants, if his was the only ancient sapiens DNA sequenced our picture may look quite different.

Plus I would be asking how they figured it was a relict erectus louse anyway. The time factor may just be a coincidence. It could have been an eastern asian louse found in monkeys in that region and predation on same could have been the vector.


The clothes louse is another interesting one. It appears to tie down when we started making and wearing clothing. 70,000 odd years ago according to the genetic clock anyway. Though I would personally suspect that happened earlier and the louse evolved into that niche more recently. Maybe even something as simple as our African ancestors didn't wear enough clothing to sustain the louse and only when we moved into colder regions was the environment good for the louse.

The gorilla crabs one is interesting alright.:D

darjeeling

The possible H. sapiens - H. erectus interbreeding angle was what I was looking up when I came across the paper - admittedly a bit late.

The story from mtDNA and the Y chromosome has been so clearly in favour of the 'recent out of Africa' model that the question has become one of whether the wave of humans coming out of Africa replaced their distant Neanderthal and H. erectus relatives with or without interbreeding.

A couple of papers looking at Neanderthal X-chromsome & autosomal DNA have suggested that the answer is no [edit]interbreeding[/edit]. However, there's some evidence from fine-scale sequence data that may indicate a small contribution from Asian H. erectus to the present-day human gene pool. This 2008 Genetics paper looked at a very small fragment of the X chromosome and found that some people living in Asia had sequences that apparently diverged from African-origin sequences over 2 million years ago. This might indicate interbreeding between H. sapiens and H. erectus in Asia. And the lice paper hints that they at least rubbed shoulders anyway.

Wibbs wrote: »

Plus I would be asking how they figured it was a relict erectus louse anyway. The time factor may just be a coincidence. It could have been an eastern asian louse found in monkeys in that region and predation on same could have been the vector.

Well, given the tight relationship between primate hosts and their lice, plus the louse phylogenetic tree, it would seem a coincidence too far. Not impossible that H. erectus cross-infested Asian monkeys and the monkeys returned the favour when H. sapiens passed by, but that would take a couple of major host species jumps.