Since the time when humans and chimps evolved from our common ancestor, our species appears to have come on by leaps and bounds. We walk on two legs, we have spoken language and while there is no doubt that chimps are intelligent, there is even less doubt that our brainpower outclasses theirs.
But are our advanced abilities reflected in our genetics? After all, traits like intelligence and language give us great adaptive advantages - surely they should be mirrored by similarly large changes in our genome, compared to the chimp one?
Not so. Researchers at the University of Michigan sifted through the human and chimp genomes for signs of positive selection - the process where natural selection firmly embeds new mutations because of the advantages they provide. They found that the chimp genome contains 50% more positively-selected genes than the human one.
While earlier studies have compared individual human and chimp genes, this is the first to do a proper census. Margaret Bakewell and colleagues looked at almost 14,000 genes in both species. The project was given a valuable push by the recent publication of the fully-sequenced rhesus macaque genome. The macaque - a type of monkey - is an evolutionary cousin of both humans and chimps, and provides a useful comparison.
If the team found a difference in the human and chimp genes, the macaque version could tell them which version is closest to the ancestral one. Previously, scientists had to make do with the mouse, a much more distantly related animal. The macaque's presence gave the analysis greater accuracy.
Bakewell failed to find any noticeable differences in the function of positively-selected genes in humans and chimps. Both species even had similar proportions of positive changes among the genes that control the brain and nervous systems.
The reasons for this surprising result are unclear, but Bakewell feels that population sizes may hold the answer. For most of their evolution, chimpanzees have enjoyed a larger population size than humans have. It's only recently that our numbers have ballooned to unfeasible proportions.
According to evolutionary theory, beneficial genetic changes are more quickly established in a population if it is larger. But in smaller groups, random genetic changes can trickle down through generations without being properly weeded out. This 'genetic drift' could explain why humans have fewer positively-selected genes than chimps do.
An alternative theory is that many of the human genetic changes that provide us with the greatest advantages may be relatively new developments. It is only recently in our history that we spread around the world from our origins in Africa, and as such, new genetic innovations may not have become established in the population as a whole.
A third explanation is that the genetic changes responsible for our most human traits, may lie among stretches of DNA missed by this study, within areas that are never converted into proteins.
Clearly, we still have much to learn. Nonetheless, the study helpfully shows that evolution is not necessarily about progress. It's not an inexorable march toward some gleaming future. It's about change, regardless of direction or result. Somewhere along the line, the word 'evolved' started to gain a false value. It became an indicator of positive progress, so that claiming to be 'more evolved' than a peer is to claim superiority.
A huge number of newspapers and magazines reported this story under the headline of 'Chimps more evolved than humans'. And while that may be technically reasonable, the inferences made were anything but.
The inherent values placed upon the phrase 'more evolved' clearly emerged in the reaction to the story. Some suggested that humans were obviously 'less evolved' given for reasons ranging from pollution to capitalism. Meanwhile, creationists and ID-supporters smelled blood in the water, and claimed that such as blatantly preposterous conclusion proved that evolution was nonsense.
Of course, no such conclusions were actually made by the study itself. In the light of the proper progress-free meaning of the word 'evolution', hese results are not preposterous, but fascinating. We should use them to drive a nail in the coffin of phrases like 'more evolved', at least in its value-laden non-scientific sense.
Reference: Bakewell, Shi & Zhang. 2007. More genes underwent positive selection in chimpanzee evolution than in human evolution. PNAS 104: 7489-7494.
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Or, just possibly, the human changes reduced the selection pressure on the rest. "Quantity vs. quality" is probably how the press would put it, inappropriate as the phrase might be.
A slightly whimsical lay thought: perhaps the well fixed changes are more specifically selective for environmental constraints - Humans are much more general-purpose about where and how they live. I am reminded of a programming technique I have used - for both hardware and software - I call "defensive programming": could also be called "late binding design". Perhaps functionally non-specific adaptations are not detected by the techniques used? Just a thought.
Exactly the first thing to pop into my mind when I saw the beginning of the post. Glad you said so.
Thanks, Ed for digging this up! And, kudos for a fine performance on bhTV! Please return with a friend.
On the point, not to complicate it, but might the similarity between chimps (hey, what about bonobos for contrast?) and homo sapiens (I guess homo erectus et al would have similar variation?) support the supposition that for humans environmental factors like disease (as in William H. McNeill's theory) force evolutionary change more than genes? Might this also highlight the role of subsequent social evolution? After all, chimps and bonobos, and homo sapiens have different social structures even with all those similar genes.
Also, if chimps are the trunk of the family tree, that might have evolved more because of its population density then homo sapiens is less a refinement than a statistical possibility that got lucky. Chimps are clearly not as profligate now because of human encroachment brought about by social evolution. But perhaps climate change or some human-induced catastrophe will even the odds.
Wow I never knew we evolved from apes even though I am 20 already.