ResearchBlogging.orgHumans are a rather self-centred bunch. From thinking an unimaginably large universe exists to benefit the inhabitants of one speck of it to, well….starting a blog called “EvoAnth.” Within science there is a significant bias towards the investigation of how we got here compared to the origins of most other living animals.

As such, we know relatively little about how many other species wound up the way they are. Even our closest living relatives, Pan troglodytes (chimpanzees) are an evolutionary enigma, with little known about their origins.

Meanwhile chimps have been studying us intently and slowly, and surely, drew their plans against us.

Luckily a new study is out that readdresses this balance somewhat, studying the evolution chimps in an effort to further our knowledge of their murky past, as well as garnering information about evolution in general.

Unluckily this study is based upon genetics and – as I’ve said before – I don’t get how genetics works so I can’t give as detailed an analysis of their methodology as I normally do (although given how long some of my recent posts have been, perhaps you think that’s a good thing).

As such, take everything I say with a grain of salt as, ultimately, I don’t have a clue what I’m saying.

This is the last thing I learnt in genetics, and all I learnt about it was “its important”

I thought the most interesting finding was that there has been ‘very little adaptive evolution on the autosomes since diverging from hominins.’ What that means, for those for whom jargon isn’t a first language, is essentially few mutations on the non-sex chromosomes (i.e. everything but X and Y) have been positively selected for since they split with the human lineage.

The implication of this is that the majority of beneficial divergence between us and chimps has been changes to us, which seems to lend support to those who try and infer what our common ancestor was like based upon chimpanzees.

But before one gets carried away and concludes our common ancestor with chimps was just a chimp, it’s worth noting that there have still been changes to chimps due to neutral and harmful mutations. Or have there? The study also found purifying selection (the removal of harmful genes because they’re, well, harmful) seems to be stronger in chimps than in humans.

Quite why that might be I can’t fathom, perhaps it’s because they’re better suited to their environment and so even slight changes reduce fitness drastically, allowing them to be easily destroyed by natural selection.

At any rate, it would seem the only real change to a chimp’s autosomes has been neutral with beneficial mutations not happening and deleterious ones being swiftly executed. Given how neutral mutations typically retain the functionality of the non-mutated gene (hence why they are neutral) this would suggest they are still quite similar to our common ancestor.

And depsite this the ol’ creationist retort “you saying I came from a monkey” is still wrong. Chimps are apes, after all.

But all that doesn’t mean there has been no recent chimp evolution. Indeed, a reduced number of variants of immune-related genes suggests that there has been selection for those variants (since something is selecting against variation of those genes, indicating they are beneficial and being selected for).

This might suggest their immune system is quite a bit different from ours, which could raise a whole number of issues regarding the practicality of animal testing. However, given the relative success of testing on something as different from us as say…mice, I doubt these changes will be significant in the scheme of things.

Also, curiously, there appears to have been significant evolution of the X chromosome, with 30% of the changes to it since divergence from humans being positively selected for (i.e. they are beneficial). Even more curiously is that the number of such beneficial changes seems to indicate adaptive evolution is occurring at a faster rate on the X chromosome than other areas.

This is what the authors suggest is the biggest find of their study and given how I always viewed the genome as pretty much the same everywhere, such significance variation in the rate of evolution is nothing less than a revelation. But given how little I know about genetics, the implications of this revelation are beyond me.

So what’s the take-home message of this article?

  • Chimps are more similar to our common ancestor than one might think, although there have been some beneficial changes
  • Most of these occurred on the X chromosome and on a few immune-related genes
  • The rate of X chromosome change suggests that it evolves faster than the autosomes. This is important. For some reason.
  • I know nothing about genetics
Hvilsom, C., Qian, Y., Bataillon, T., Li, Y., Mailund, T., Salle, B., Carlsen, F., Li, R., Zheng, H., Jiang, T., Jiang, H., Jin, X., Munch, K., Hobolth, A., Siegismund, H., Wang, J., & Schierup, M. (2012). Extensive X-linked adaptive evolution in central chimpanzees Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1106877109

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8 Comments

Jim Thomerson · 1st February 2012 at 3:04 am

Your diagram is for relationship of frequencies of of alleles A and a, and for AA, Aa, and aa individuals, under Hardy-Weinberg conditions. Under Hardy-Weinberg conditions, no evolution occurs, gene frequencies are the same generation after generation. This is why they told you it was important.

    Adam Benton · 1st February 2012 at 9:03 am

    That would explain why my notes on the issue are titled “the Hardy-Weinberg EQUILIBRIUM”

Thomas Mailund · 15th June 2012 at 2:33 pm

I’m not sure that we conclude that there was little evolution on chimps, really, just that we didn’t find significant evidence for adaptive evolution on the autosomes but did on the X chromosome. We are looking at a weak signal and looking at an average over the entire genome, so a strong signal for adaptive evolution would be surprising I guess.

If we look at the human lineage we wouldn’t see significant adaptive evolution either, is my guess (we didn’t in this study).

So the story is not really that there was little adaptive evolution in chimps — there was probably as much as in humans if not more — but that X seems to be exceptionally more directed by adaptive evolution than the autosomes. This might also explain why the divergence between human and chimpanzee X chromosomes is much lower than between their autosomes.

The reason we expect more selection in chimps is that they have a larger effective population size than humans, which all else being equal makes selection stronger, both for positive and negative selection.

Anyway, we don’t really make claims that there has been less evolution on the chimp lineage compared to the human lineage — we never looked at that — only that the X chromosome seems to be under more selection than the autosomes. Probably this is the case for humans as well, it is just harder to separate demographic effects on the X/autosome ratio for humans.

    Adam Benton · 16th June 2012 at 5:11 pm

    Now that does surprise me quite a bit. Given that the chimpanzees’ environment appears to have remained relatively stable since we diverged. Whilst it obviously has varied a bit (I believe during glacial cycles there was a ~40-60% decrease of tropical rainforest worldwide) for the most part it has remained stable. As such it makes sense that chimps haven’t had quite as much adaptive evolution since there’s been less novel pressures driving them to adapt.

    However, what you’re saying does make sense. Areas of the human genome which have evolved since our divergence are relatively small (which is what makes the HAR so special) so on average one might not find that much adaptive evolution occurring on our genome too.

    At any rate it seems I might have to give this post a bit of a rewrite now I know this. Apologies for misrepresenting your work and thank you for the correction. I’ve found this blog an invaluable learning experience and I’m grateful for anything that helps me become more accurate.

      Thomas Mailund · 16th June 2012 at 5:54 pm

      No worries about the representation of the paper, if it is unclear it is probably our own fault 🙂

      You are right that strength of selection will depend on the environmental pressure, so if the environment has been more stable for chimps, then there might have been less pressure (I’m not sure if I think this is the case – table environment or not there have been other competitors and pathogens around that they had to adapt to). The strength of selection, though, is a product of the effective population size and that “pressure”, which is what I meant by “all else being equal”. All else is never equal, of course 🙂

        Adam Benton · 16th June 2012 at 6:28 pm

        Whilst a stable environment would likely still promote some evolution due to the factors you suggest, you did find that there was some autosomal evolution occurring in relation to immunity genes. In my mind at least, that fit in with the notion of not much adaptive evolution occurring due to a stable environment. The few changes we would expect to see in such a situation we do see, whilst others are relatively absent (or you might simply have not reported them/not had the resolution to find them. HAR1, for example, is only a hundred base pairs long or so whilst those immunity genes were 6mb long).

        Also, it probably is my own lack of genetics knowledge at fault for any misunderstanding. I don’t expect a scientific paper to appeal to the lowers common denominator!

        Whilst I have taken a few genetics modules in an attempt to rectify this, most of them consisted of learning what SNPs were, calculating Hardy-Weinberg figures are drawing punnet squares. Not much use for things like this (although I do get a kick when I see something mention SNPs and I get to go “I know what that is”)

        This lack of expertise is something one of my lecturers bemoans about evolutionary anthropology in general. Since most lack major experience in the field they tend to take whatever a geneticist says on an issue as Word Of God.

Jim Thomerson · 30th June 2013 at 2:26 pm

I strongly suspect chimpanzee evolution has been constrained by their lack of ability to throw an object faster than 12 MPH. 🙂

    Adam Benton · 31st August 2013 at 11:52 am

    Once they invent the Atlatl we’re screwed

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