Humans VS Their Primate Relatives

Genes Don't Give Humans Edge Over Their Primate Relatives
SCIENCE JOURNAL
By SHARON BEGLEY

Ever since biologists discovered in the late 1990s that the DNA in humans and chimps is 98.7% identical, the search has been on for that magical 1.3%.

Needless to say, there aren't a lot of scientists digging for the essence of humanity in, say, the bile ducts. The real action has been in the brain. In Friday's issue of the journal Science, the first team to report findings is unveiling the results of a genetic comparison of human brains with those of their closest evolutionary relatives.

The results are a bit embarrassing for anyone trying to explain human superiority through genes.

The researchers, an international team from Germany, the Netherlands and the U.S., removed gray matter from the left prefrontal cortex of three men and three chimpanzees, all of whom had died of natural causes. In people, this part of the brain is in charge of abstract thinking.

Then the scientists took silicon chips holding about 12,000 human genetic sequences. The chips reveal which genes are "on," or active. The finding: "175 genes differ significantly in [this] area of the brain," said molecular biologist Svante Paabo of the Max-Planck-Institute for Evolutionary Anthropology in Leipzig, Germany, who led the study. That's out of some 34,000 human genes. As he had predicted, the likeness of human and chimp genomes "will be both a source of humility and a blow to the idea of human uniqueness."

Even more humbling, we differ genetically from the hairy guys in zoos in quantity more than quality. That is, our brains aren't made of very different stuff -- genes and proteins. What distinguishes ours from theirs, rather, is which genes turn on and how much. (Genes make proteins; a gene turned to "high" makes more of its protein than one turned to "low.") The kinds of proteins produced by chimp genes differ from ours by only 7.6%. The amounts of those proteins differ by 31.4%.

Take virtually the same batch of genes, dial up the "on" switch and you get a human brain; dial it down and you get a chimp brain.

"This points to gene regulation in the brain as a plausible candidate implicated in the dramatic changes in human cognition," Dr. Paabo said. The differences between chimps' brains and ours likely lie in genes that turn other genes on and off, up and down.

The paucity of differences doesn't surprise primatologist William Hopkins of the Yerkes Regional Primate Center. "It's completely consistent with our work on brain morphology, which shows that chimps have the same prefrontal trait that, in people, is associated with language," he said.

That suggests that when chimps use sign language and symbols on keyboards to communicate, they aren't just aping their trainers but truly grasping meaning and syntax.

Power of Genes

So, it seems that only a few genetic changes spell the difference between chimps and humans. That's a good argument for the power of genes: Change just a few and you get Bonzo instead of the guy sending him to bed. The human brain is about twice the size of the chimp's. A genetic program that instructed neural stem cells in the fetus to churn out just one more generation of neurons would produce that much of a difference, said geneticist David Nelson of Baylor College of Medicine. That, of course, raises the possibility that genetically engineering superhumans wouldn't take much.

The power of these so-called regulatory genes is fascinating on several levels. My favorite is this: Whether genes are on or off, and how high they're on, often depends on what your senses are taking in, what you're feeling and even what you're thinking.

Sensory Stimulation

"Immediate early genes" turn on in response to sensory stimulation. Exposure to light at certain times of day switches them on. In birds, hearing a particular song activates certain genes. Without that melody, the genes stay off. In men, thinking about *** turns on testosterone-related genes, with the result that their beards grow faster.

DNA, like neurons in a baby's brain, responds to its environment. "If [outside] events can affect hormone levels, and hormones can control genes, then the events of our daily lives can effectively control our genes," said developmental psychologist David S. Moore of Pitzer College in California, in his new book "The Dependent Gene."

Once nature figures out how to accomplish something, it doesn't reinvent the wheel. As a result, mice share around 85% of their genes with humans. Yeast shares 46%. Those tiny annoying fruit flies that descend on overripe bananas share 60%. Oh, and the banana itself shares about 50%.

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Even more humbling, we differ genetically from the hairy guys in zoos in quantity more than quality. That is, our brains aren't made of very different stuff -- genes and proteins