Humans and chimps share a common ancestor, which split into two species some point between 7 – 14 million years ago. We often assume that this common ancestor (and early members of our family for that matter) was very chimp like. However, there is a growing body of evidence that chimps have been evolving in a unique direction themselves, so aren’t as similar to early hominins as we might think. In fact, research indicates that more genes have undergone positive selection in chimps than in humans.
If a gene codes for a trait that benefits an individual then that guy will be more likely to survive, reproduce and pass on his genes. Thus over time that gene is spread throughout the population until most people have it and that characteristic becomes part of what defines the species. This is positive selection. The study (all the way from the mystical past that is 2007) examined around 14,000 genes from both us and humans (which is about 58% of all the genes humans have) and looked for evidence that these genes had been positively selected for using some fancy genetics tricks that go completely over my head.
All in all they found 154 genes in us which had been selected for by evolution since we split from chimps; but they found 233 genes in the chimp genome. Now, you might wonder “what if there are a bunch of positively selected genes in the 32% of the human genome they didn’t examine.” So they repeated their experiment another 2 times, taking a different sample of 14,000 genes each time. However, no matter which bits of the genome they sample they found that chimps always had more positively selected genes than humans.
The researchers then went on to examine the genes themselves. What did the positively selected genes do? Looking at the differences between chimps and humans you might think a lot of them have to do with brain development or the body plan. Yet most positively selected genes in both chimps and humans seem to have helped improve fairly unimpressive biological processes. 12 positively selected human genes, for example, influenced ion transport. 40 positively selected chimp genes were involved with protein metabolism. Not the sort of things you might expect to separate us from chimps. And in both groups most genes that were positively selected for were linked to the immune system. Still, chimps had a third more immune system related positively selected genes than humans.
Where the positively selected genes are expressed in both chimps and humans
However, in most other areas – brain, muscles, reproductive organs etc. – humans had more positively selected genes than chimps. Disease is an often underestimate evolutionary force. Yet genes which offer an advantage against disease can be strongly selected for. In humans, genes which offer resistant to Malaria are common amongst African populations, even though these genes can cause sickle cell anaemia in some cases. The benefits are so strong that this is an acceptable risk.
So perhaps chimps have to deal with more disease, or by chance more genes developed mutations that mean they are helpful against disease. Either way, the resulting evolutionary arms race inflated the number of genes which have undergone positive selection in chimpanzees. If we were to only look at the physical characteristics which define our two species, like brain size, locomotion and hairlessness, we may well find that humans have changed the most after all.
Still, we must not forget that chimps have been changing in their own right. They are not a frozen moment in time, identical to our common ancestor. Evolution waits for no man. Or ape.
Reference
Bakewell, M. A., Shi, P., & Zhang, J. (2007). More genes underwent positive selection in chimpanzee evolution than in human evolution. Proceedings of the National Academy of Sciences, 104(18), 7489-7494.