Humanness as a numbers game

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: evidence that genomic differences between humans and other primates is far, far more complex than we originally imagined they might be,” observed Ajit Varki, who studies human/chimpanzee differences at the University of California, San Diego. “However,” he cautions, “many of the differences may or may not be relevant for explaining ‘humanness’.”

There is a silver lining to this observation. Jonathan Sebat, a geneticist at Cold Spring Harbor Laboratory in New York State, who specialises in copy number variation, is ecstatic in calling the results “quite tantalising”, but also feels that the report may have stretched things a little in its speculation on the biological effects. Unfazed, Sikela opines the paper is meant to be “hypothesis generating” and expects several genes to be investigated.

One such gene is aquaporin7, of which humans have five copies, whereas the other primates have just two, prompting researchers to suggest that the gene may be involved in exercise-induced sweating and endurance running, a feature unique among humans. Likewise, gorillas have many extra genes known to play a role in the excretion of toxins. This makes sense, too, for gorillas eat many toxin-rich plants.

These findings point to regions of chromosomes in various species that seem particularly predisposed to gene duplication or loss, and argue that these “gene nurseries” may well be hot spots for evolution. “It’s a double-edged sword,” avers Sikela. “It could easily lead to a disease, but the upside is it increases variation that evolution can use. It’s more evolutionary fuel.” Not surprisingly, the family of genes that may broaden the gorilla’s diet are restricted in humans and linked to a disease called DiGeorge syndrome.

Sunil Ahuja, who discovered in his lab at the University of Texas Health Science Center at San Antonio a copy number variation that strongly protects people from HIV, is cautiously optimistic when he says proving the link between extra or missing genes and function would warrant more work.

Heady days are here for science to peg away at genetic mysteries and the mystiques of human form and function, physiology and emotions, and that last untrammelled bastion of human behaviour—humanness!