Crunch the numbers on the animal kingdom's sizes and shapes, and humans differ from each other far less than most species. The reason why is a mystery.

"We don't have an answer. We have this interesting observation, but the explanation is an open hypothesis," said evolutionary biologist Andrew Hendry of McGill University.

Hendry and Queens University biologist Ann McKellar combed through the scientific literature on body size and length in more than 200 species, from insects to fish to birds and, of course, humans.

In terms of sheer mass, humans variation was par for the animal course. So was the height difference between populations — between, say, the average Maasai man and the average Australian aborigine. But when it came to variation within a population, such as that Maasai or aboriginal village, humans had less variation than 95 percent of all the species studied. The results were published Tuesday in Public Library of Science ONE.

Through most of human history, it seems that evolution stretched or shrunk people to fit their local environments, then rigidly enforced the size limits. People were no taller or shorter than their neighbors.

Hendry speculates reproduction strategies may explain some of the conformity. In species where males are usually large and fight for the chance to mate, small size can help a male sneak around the others. Where males tend to be small and sneaky, a big one can push right through them. Human mating habits don't work like that. Of course, the same holds for many other species, and they're not as uniform as we are. Our radical uniformity was probably useful in some way, though we've no idea how.

Now that selection pressures are less intense, human size patterns could change, said Hendry. Measurements used in the study came mostly from anthropologists studying traditionally isolated groups, who were often direct-line descendants of people with close ties to their natural environments.

"You had these differences in, say, the Arctic and the Kalahari, with strong selection for body size in those spaces. But now those selective pressures have been removed. We have heating and air conditioning," he said. "It's possible that with time, humans will come to look like other animals."

Hendry said the idea for the study came while watching bicyclists on a path outside his home, and thinking they were remarkably different in size. The study clearly contradicted that observation. Now he wonders how human variation stacks up in other ways.

"Are facial features in humans more or less variable than in mice, or in chimps?" he said. "And compared to other organisms that rely on smell or song, are we more variable in our scents, or how we sound?"

Citation: "How Humans Differ from Other Animals in Their Levels of Morphological Variation." By Ann E. McKellar and Andrew P. Hendry. PLoS ONE, September 1, 2009.

Image: McKay Savage

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