Sunday, August 12, 2012
Dalliances in Human Evolution
Recently a collection of hunter gatherer tribes in Africa were whole-genome sequenced. (See here.) Africa is the source of more human genetic diversity than anywhere else. This isn't surprising. We've been there for longer than we've been anywhere else.
The research was led by Penn State and in the report from them (See here and here.) some of the curious and odd sequences suggested interbreeding between Homo sapiens and perhaps as many as three other distinct species. (My wife, the biochemist, pointed out if there was interbreeding how could they be distinct species? But I digress.)
There is also evidence that humans interbred with Neanderthals and Denisovans (See here.) Again with the wife's raised eyebrow from across the room. The African genetic evidence is at odds with the fossil evidence since there haven't been three other groups that look different enough to constitute anything like a species. No so, of course, regarding the Neanderthals and Denisovans.
The lack of fossil evidence is interesting but not necessarily problematic. There are a number of species who would fossilize as identical but are reproductively isolated. The Hyla versicolor and Hyla chrysoscelis are both tree frogs and almost indistinguishable from one another. Yet they do not interbreed. H. versicolor has a faster mating call than H. chrysocelis. It is also double the number of chromosomes. A paleontologist looking at the fossils of either species would not see them as different. A geneticist would.
In addition, we can only analyze the fossils we find and most corpses do not become fossils.
Another interesting piece of news is the new species of human ancestral fossils in Kenya. (See here and here.) These new fossils are astonishingly flat faced. This is more of Leakey's work at Lake Turkana.With A. sediba, there may have been several species, some at the same time.
Another article showed that there were different diets between the different groups. (See here.) Australopithecus had a more varied diet than early Homo. More varied than another human relative, Paranthropus.
I keep coming back to my wife's comment: how can they be different species if they interbred?
I collect old jokes. Bad jokes. Really nasty ones-- jokes that are so embarrassingly horrible you want to erase the last five minutes of your life just to get rid of them. These are not funny jokes. I rarely tell them. They're for looking at the twisted bits of human beings and figure out how they tick. These are not for the squeamish. Several of them involve human males having sex with non-human primates. There have been several SF stories about this-- at least one has been nominated for the Nebula. Humans are really different from other animals in this way. Most animals are really selective on who they have sex with. Humans, not so much.
I can't help it. Humans and Neanderthals have significant differences in their bone structure. Yet the DNA shows the two groups interbred. When did this non-specific lust develop? Was it with us in the beginning? We see fossil differences between groups and interpret that fossil difference to mean species difference. Probably it is. The fossil differences between Homo sapiens and chimpanzees are more profound than between Neanderthal and Homo sapiens. We're sure that chimps and humans are different species-- though I don't know any lab unscrupulous enough to attempt interbreeding.
This loss of mate specificity happened at some point. We don't know when. But it did and it has shown an interesting mix in our genetics.
I keep having the image of groups of human ancestors getting isolated, developing adaptations to diet or behaviors and then getting brought back into the fold. Some of our relatives made it out of the human bedroom but maybe some didn't.