Speciation of humans and chimps, and evolution.

One of the most important concepts in evolution is about speciation, that is, how new species evolve. There are many ways for species to evolve, but they all boil down to one idea: reproductive isolation. Simply put, two different species cannot mate with each other, or if they somehow manage to do it, the offspring is sterile. Examples: human and chimps, the plants, etc, are all reproductively isolated; one example of species reproduce to give sterile offspring are donkeys and horses producing sterile mules.

So quick recap: evolution of new species occurs when there is reproductive isolation. Now the news…

How did the human species evolve? The traditional answer to that question is somewhere around 4.6-6 million years ago. Then, it is thought, the line that lead to humans split from the line that lead to chimpanzees from the human-chimp common ancestor. However, this was causing a bit of a problem as new fossil evidence came up, and the date was pushed back to 7 million years ago. Another way is to measure all the genetic differences between humans and chimps (or any closely-related species), and then assume a certain rate of mutation (genetic changes). Knowing the mutation rate and the number of genetic differences allows us to calculate when the two species split. However, this has its own problems as you get different answers depending on which mutations you study and what mutation rate you assume.

This week, scientists reported on a new, much more accurate, technique to measure speciation. The technique checked 20 million mutations (wow!) and measure the rate each one evolved separately. The final answer they come up with is that humans and chimps split multiple times, at one point forming a hybrid. The first split first occured more recently than we originally thought, most probably around 5.4 million years ago, and the split happened over a longer time-frame. They also report an interesting and unexpected finding, but it explains the new data.

When they looked at mutations in the X-chromosome, the DNA part of our genome that determines gender (sex), they found that human and chimp X-chromosomes are the most related chromosomes in the two species. They estimate that the X-chromosomes finally split only 1.2 million years ago. Since the X-chromosome helps determine sex and reproductive isolation, this may be why humans and chimps retained the ability to mate and form hybrids for such a long time.

So what does all this mean? Well, many things:

• We now have more evidence that humans and chimps are closely related. This adds to the debate about human evolution.
• We now have a ‘road map’ for how humans evolved. This will allow us to better place the fossil evidence, and to also figure out the stages properly. We have fossil evidence dating back to the hybrid stage, and even before the human-chimp split. The bones can now be ordered more accurately for us to see just how humans evolved.
• This is by far the most complex speciation example we have found. That doesn’t mean it hasn’t happened elsewhere; we simply haven’t been looking for it! Now that we have a technique to better study speciation, who knows what we will uncover.

If you want, the Discovery Channel and the NY Times have commentaries too.

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