In particular it discusses reticular great ape evolution. The main evidential basis for the post is the observation that gorilla Y chromosomes are more similar to human Y chromosomes than they are to chimpanzee Y chromosomes. This is a widely recognized fact. It is true despite the family tree weakly predicting the opposite finding:
The Y chromosomes of great apes represent a particular puzzle: their gene content is more similar between human and gorilla than between human and chimpanzee, even though human and chimpanzee share a more recent common ancestor.
An obvious and conventional explanation for that is that the chimp lineage experienced rapid Y chromosome evolution, leaving humans and gorillas with relatively similar Y chromosomes. This could have something to do with sperm competition and the huge chimpanzee testicles. That's the conclusion drawn here:
we found that the genus Pan, which includes chimpanzee and bonobo, experienced accelerated substitution rates. Pan also exhibited elevated gene death rates. These observations are consistent with high levels of sperm competition in Pan.
Mystery solved - right? Maybe. Here, I would like to draw attention to another pair of possibilities:
Maybe a male gorilla mated with a female human ancestor - and now all human males are now descended from him. Or maybe the opposite scenario is involved: maybe a male human ancestor mated with a female gorilla and now all male gorillas are descended from him.
I think that we have to downgrade the probability of these explanations since they involve unlikely reticular evolution and hybridization. However, while hybrids are rare I think that geological time up-regulates their potential evolutionary significance. Basically, Eugene McCarthy has convinced me that we should consider hybridization much more seriously.
Can we test these theories? Yes, I think they can be tested using geonomic comparison data. In addition to a similar Y chromosome, we should expect subtle changes in the rest of the genome. Back-crossing would dilute, but perhaps not completely eliminate traces of the hybridization event.
Assuming a male chimpanzee donor, some small areas of the human genome would be less similar to chimpanzees and more similar to gorillas. That's expected anyway, but with hybridization, there would be more donor DNA involved. We could compare in both directions (human -> gorilla and gorilla -> human). If there's a big difference, hybridization becomes more likely. We could possibly even use molecular clocks to determine when shared sequences diverged between the lineages.
Which of the two scenarios I mentioned is more likely? I think that is unclear, but the scenario involving the male gorilla seems as though it would be the more dramatic finding for students of human evolution. Relationships between male gorillas and female humans have been dramatized in King Kong and its sequels. It would be a curious finding if those movies mimicked real evolutionary history.
Posts
