About Rationally Speaking
Rationally Speaking is a blog maintained by Prof. Massimo Pigliucci, a philosopher at the City University of New York. The blog reflects the Enlightenment figure Marquis de Condorcet's idea of what a public intellectual (yes, we know, that's such a bad word) ought to be: someone who devotes himself to "the tracking down of prejudices in the hiding places where priests, the schools, the government, and all long-established institutions had gathered and protected them." You're welcome. Please notice that the contents of this blog can be reprinted under the standard Creative Commons license.
Saturday, July 22, 2006
Patterson and colleagues were able to use the latest data from both the human and chimp genome projects to reconstruct the evolution of many genes across all chromosomes present in the two species. They found that the data are consistent with a more recent split from our common ancestor than indicated by the available fossil record, probably less than 6.3 million years (assuming you believe that the earth is more than 10,000 years old, of course).
Even more strikingly, the Harvard/MIT team found that the X chromosome (the one involved in sexual determination) diverged more recently than the estimated time of split of the two species. How is this possible? The most likely interpretation is that humans and chimps kept exchanging genes (i.e., hybridizing) after the separation of the two lineages. When this happens in other species, one of the results is strong natural selection on the sexual chromosomes, yielding the sort of data observed by Patterson et al. The reason the sexual chromosome is under more intense selection, especially after hybridization, is that half of the population (in primates, the males) only have one copy of that chromosome, which means that any deleterious gene has immediate effects on fitness, and is therefore promptly selected against. This is the same reason why so many human genetic defects (from color blindness to hemophilia) are much more common in men than in women (the latter have a second X chromosome, which shields them if it carries a functioning copy of the same gene).
This is a great example of how science actually provides us tantalizing clues about fundamental questions, by clever detective work that yield strong circumstantial evidence, and sometimes even permit us to arrive at conclusions beyond reasonable doubt. The “other” way of proceeding, apparently more satisfying to a large number of Americans, is to listen to the latest tall tale by the local preacher, based on the personal (and uninformed) interpretation of a book written by ignorant people thousands of years ago. Go figure.