Humans 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.
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.
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.
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|