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I just got my copy of this month's issue of Natural History, and wanted to mention that there are a lot of extremely interesting and easily comprehensible articles on evolution: there's one on the end of the Neandertals, one on mass extinctions in a Devonian reef, and a book review on cooperative genes. I just want to mention one:

Zimmer, C (2002) Searching for your inner chimp. Natural History 111(10):36-40.

The article is a discussion of current findings in comparative genomics, looking at the similarities and differences between us and chimpanzees. In particular, it references the <a href="http://sayer.lab.nig.ac.jp/~silver/" target="_blank">Silver Project: Ape Genome Sequencing</a>, a project to compare known sequences in the Human Genome Project with the relatively small number of ape sequences.

The author starts with a magic number, the extrapolated percent identity of the sequences of chimps and humans: 98.7%. Then he asks "how reliable is that number?" and "what does it mean?"

There are problems with the number. Only about 1% of the chimp genome has been sequenced, so it does require a fair amount of extrapolation. It also is only examining published, isolated sequences, so there could be some underestimation of the differences -- deleted genes in one or the other wouldn't show up, nor would duplicated copies. Zimmer, however, cites Roy Britten's extimate of the number of such cases, which would reduce the overall similarity to a "mere" 95%. This still does not take into account the fact that many of these differences will be genetically neutral, either causing no difference at all in the amino acid sequence of proteins, or making an insignificant substitution. The bottom line is that we're going to have to get used to the fact that people are at least 95% chimpanzee, and vice versa.

So what does it mean*? Zimmer points out:
"A lot of those altered genes could well turn out to specialize in regulating other genes. Regulatory genes code for proteins that help switch on other genes or shut them off, thereby promoting or inhibiting the production of the proteins those other genes are responsible for. Hence a single regulatory gene can "leverage" its effects, and if a regulatory gene evolves into a different form, it could alter not only whether but also when and where various proteins are transcribed. Small genetic alterations can create an avalanche of changes in the animal's anatomy and way of life."

The article then briefly discusses the work of Svante Paabo, who has found different patterns of gene expression in the neurons of humans and chimpanzees, and the history of one particular sugar, Neu5Gc, which is found on the surface of all mammals studied so far, except for humans. The enzyme to manufacture it is present in us only as a pseudogene, which has been dated to have lost its function roughly 2 million years ago. It's nice to know we're unique, and that the appearance of that unique feature is correlated with the emergence of many of our species' special morphologies, but I suspect there was much more to that process than just losing one particular ethylthioglycoside.

*A question which often means, "how can we rescue our dignity and exaggerate the differences?"