Let’s be real: The bodies of Olympic athletes are pretty weird.

Maybe you’ve seen the Instagrams of sub-5 foot gymnasts next to towering basketball players. Maybe you’ve noticed Michael Phelps’s freakishly long torso relative to his legs.

What is clear, then? Genetically speaking, Olympic athletes are much more diverse than they were a century ago. The prevailing wisdom used to be that an average, “athletic” body type was best for all sports, according to David Epstein, author of The Sports Gene. Eventually, coaches and researchers started to notice that specialized body types, or body parts, were better for specific competitions. Suddenly, being an outlier became an advantage.

What followed was not natural selection in the Darwinian sense, but instead an unnatural selection of athletes with peculiar traits believed to be advantageous for a particular athletic pursuit.

Not that evolution didn’t have a role to play. Take the Kenyan tribe of Kalenjin, for example. This small group of people, only about 5 million strong, are without peers in the world of marathon running. “There are 17 American men in history who have run under 2:10 in the marathon,” Epstein told NPR in 2013. “There were 32 Kalenjin who did it in October of 2011.”

There certainly are several factors at play in the Kalenjin dominance, but one is their particular body type. Members of the tribe tend to have long, lean legs that are particularly thin toward the extremities. This is an adaptation particular to groups of people who have lived over evolutionary time close to the equator, where staying cool is a priority. The skinnier your limbs, the more efficiently they can move heat out of the body. As a side benefit, having very little weight on the extremities means you have to expend less energy to swing your legs forward with every stride. It’s perfect for endurance running.

Allen’s rule — which presupposes that humans and other animals will become longer and leaner the closer they live to the environment — is a tidy theory, but it’s not the whole picture. A study published Friday in the Proceedings of the National Academy of Sciences challenges the idea that evolution moves forward in a neat, predictable way. The research looked at the lengths of human limb bones to try to pull apart Allen’s rule — which states that humans and animals get leaner the closer they live to the equator — and determine if natural selection was solely responsible for a tendency towards stubbiness moving away from the equator.

The authors found, surprisingly, that the upper arm would actually get longer at higher latitudes if it were acted on by natural selection alone, contrary to Allen’s rule. However, because upper arm length is correlated with other physiological traits, and does not evolve independently, Allen’s rule still stands. It’s the shortening of the forearm, which is driven by natural selection, that drives the shortening of the upper arm as you move away from the equator. This makes it what’s called a nonadaptive trait, developed not independently but in relationship to other traits acted upon through natural selection.

Jemima Sumgong of Kenya wins the Elite womens race during the Virgin Money Giving London Marathon at the finish on The Mall on April 24, 2016 in London, England.
Jemima Sumgong of Kenya wins the Elite womens race during the Virgin Money Giving London Marathon at the finish on The Mall on April 24, 2016 in London, England.

Rules about which body types are better for certain sports aren’t tidy, either. There are lots of examples of athletes who defy our expectations for how elite performers should look, including too-short high jumpers like Stefan Holm, 5’11, where the average height is 6’4.

Do these outliers succeed despite the disadvantages of their body type, or do they have unseen genetic advantages we simply fail to notice? Perhaps, like in the 20th century, athletes all look the same in a given sport because that’s what we assume they should look like – not because that form is necessarily the best. Maybe there’s a secret benefit to Holm’s particular body plan, but most of those who share it drop out of the field early on because they’re told they’ll never make it to the Olympics as a high jumper.

And there’s still a lot we don’t know about human mechanics and physiology. Olympics are essentially a big experiment in how to push these bodies higher, faster, harder. Limits will continue to be tested, and records will continue to be broken. The body types that filter to the top won’t always fit a tidy narrative, but the winners will always — always — be those with the passion and the heart to push themselves beyond what everyone said was possible.

Photos via Getty Images / Alex Morton, Getty Images / Matthew Stockman

Jacqueline Ronson is a science writer based on Vancouver Island, Canada. Before that she lived way up in Whitehorse, where she reported for the Yukon News. These days she likes to talk to smart people about the future of the planet, ride her bicycle, play her banjo, and frolic.