Thursday, September 01, 2005

Human evolution: fascinating things that won't be taught in Kansas

In Chimpanzee DNA, Signs of Y Chromosome's Evolution - New York Times

This discussion of the evolution of the Y chromosome is fascinating, but I was more struck by some off-handed comments about current hypotheses on human and chimpanzee evolution (none of which will be taught in Kansas). Chimpanzees, are closest relatives, are monsters -- or at least the males are. They slaughter infants of other males, they kill one another frequently, they are murderous to outsiders, their transgender relationships make human males appear enlightened.

Six million years ago, what was our common ancestor like? Did chimpanzees become nastier over time, or did we become (hard to believe) "nicer" -- or is the truth more complex than we can imagine? Was neandertal gentle, and cro magnon viscious (it appears we ate the neandertals)? Six million years is a long time -- even if it is a tiny fraction of the history of large animals.

Incredibly, genetic data and studies of ancient skeletons may combine to give us some clues to a story of a 300,000 generations...
Scientists have decoded the chimp genome and compared it with that of humans, a major step toward defining what makes people human and developing a deep insight into the evolution of human sexual behavior.

The comparison pinpoints the genetic differences that have arisen in the two species since they split from a common ancestor some six million years ago....

... The chimp Y chromosome has lost the use of 5 of its 16 X-related genes. The genes are there, but have been inactivated by mutation. The explanation, in his view, lies in the chimpanzee's high-spirited sexual behavior. Female chimps mate with all males around, so as to make each refrain from killing a child that might be his.

The alpha male nonetheless scores most of the paternities, according to DNA tests. This must be because of sperm competition, primatologists believe - the alpha male produces more and better sperm, which outcompete those of rival males.

This mating system puts such intense pressure on the sperm-making genes that any improved version will be favored by natural selection. All the other genes will be dragged along with it, Dr. Page believes, even if an X-related gene has been inactivated.

If chimps have lost five of their X-related genes in the last six million years because of sperm competition, and humans have lost none, humans presumably had a much less promiscuous mating system. But experts who study fossil human remains believe that the human mating system of long-term bonds between a man and woman evolved only some 1.7 million years ago.

Males in the human lineage became much smaller at this time, a sign of reduced competition.

The new result implies that even before that time, during the first four million years after the chimp-human split, the human mating system did not rely on sperm competition.

Dr. Page said his finding did not reach to the nature of the joint chimp-human ancestor, but that "it's a reasonable inference" that the ancestor might have been gorillalike rather than chimplike, as supposed by some primatologists.

The gorilla mating system has no sperm competition because the silverback maintains exclusive access to his harem.

Frans B. M. de Waal of the Yerkes National Primate Research Center in Atlanta said he agreed with fossil experts that the human pair bonding system probably evolved 1.7 million years ago but that the joint ancestor could have resembled a chimp, a bonobo, a gorilla, or something else entirely.If ourcommon ancestor was gorilla like, then chimps have taken the nasty road ...

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