A new study out of Yale explains what exactly separates man from the apes.

Scenes where chimpanzees behave like humans are the stuff of science fiction and advertising cachet— like the recent marketing campaign of CareerBuilder.com, where a lone man works in an office run by chimps. Despite the high percentage of genetic material we share with our hirsute cousins—99%, in fact—the obvious physiological and behavioral differences that separate our two species has never quite been squared.

“There’s a paradox,“said Kevin White, a geneticist at Yale. “The sequences of the genomes between humans and chimps are so similar; so the genes encoded in the two genomes are so similar to one another. Yet, you have very rapid divergence in morphology and behavior and physiology between the species.”

White is a co-author of a study published in the March 9th issue of Nature that discovered that this disparity between genetic similarity and physiological appearance has to do with which genes are most quickly evolving, not necessarily the 1% of our genomes that are different.

White and his colleagues used gene-array technology, which measures the variation of gene expression over different species, to compare the genomic evolution of chimp, human, orangutan and rhesus monkey over about 70 million years. When humans split off from the chimp branch, about five million years ago, one family of genes evolved with an increased speed, which the researchers discovered quickly morphed us into a radically new sort of ape.

“When we look at the set of genes that’s evolving most rapidly in the human lineage, those are [the ones] highly enriched in these gene-encoding proteins known as transcription factors,” White said. “Transcription factors are known to be involved in regulating many other genes in the genome.”

Transcription factors affect the formation of new cellular material. Because these genes interact with the other genes in an organism’s genome, they have a disproportionately large impact on the organism’s development.

Chimps and humans may share 99% of their raw genetic material, but without sharing the factors that translate that material into brains, appendages, skin and everything else in our bodies, the two can still look and act in distinct ways.

According to White, the divergence in regulatory genes between humans and chimps is most likely due to natural selection.

“The detailed ecological factors that are involved, we don’t know,” he said. “But it could have to do with diet, or it could have to do with factors that we haven’t thought of.”

“But the bottom line is, it looks like positive natural selection is what’s responsible for these differences.”

Originally published March 8, 2006

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