Still, technology isn't the only thing pushing athletes forward. While indeed we haven't evolved into a new species in a century, the gene pool within competitive sports most certainly has changed. In the early half of the 20th century, physical education instructors and coaches had the idea that the average body type was the best for all athletic endeavors: medium height, medium weight, no matter the sport. And this showed in athletes' bodies. In the 1920s, the average elite high-jumper and average elite shot-putter were the same exact size. But as that idea started to fade away, as sports scientists and coaches realized that rather than the average body type, you want highly specialized bodies that fit into certain athletic niches, a form of artificial selection took place, a self-sorting for bodies that fit certain sports, and athletes' bodies became more different from one another. Today, rather than the same size as the average elite high jumper, the average elite shot-putter is two and a half inches taller and 130 pounds heavier. And this happened throughout the sports world.
In fact, if you plot on a height versus mass graph one data point for each of two dozen sports in the first half of the 20th century, it looks like this. There's some dispersal, but it's kind of grouped around that average body type. Then that idea started to go away, and at the same time, digital technology -- first radio, then television and the Internet -- gave millions, or in some cases billions, of people a ticket to consume elite sports performance. The financial incentives and fame and glory afforded elite athletes skyrocketed, and it tipped toward the tiny upper echelon of performance. It accelerated the artificial selection for specialized bodies. And if you plot a data point for these same two dozen sports today, it looks like this. The athletes' bodies have gotten much more different from one another. And because this chart looks like the charts that show the expanding universe, with the galaxies flying away from one another, the scientists who discovered it call it "The Big Bang of Body Types."
In sports where height is prized, like basketball, the tall athletes got taller. In 1983, the National Basketball Association signed a groundbreaking agreement making players partners in the league, entitled to shares of ticket revenues and television contracts. Suddenly, anybody who could be an NBA player wanted to be, and teams started scouring the globe for the bodies that could help them win championships. Almost overnight, the proportion of men in the NBA who are at least seven feet tall doubled to 10 percent. Today, one in 10 men in the NBA is at least seven feet tall, but a seven-foot-tall man is incredibly rare in the general population -- so rare that if you know an American man between the ages of 20 and 40 who is at least seven feet tall, there's a 17 percent chance he's in the NBA right now. (Laughter) That is, find six honest seven footers, one is in the NBA right now. And that's not the only way that NBA players' bodies are unique. This is Leonardo da Vinci's "Vitruvian Man," the ideal proportions, with arm span equal to height. My arm span is exactly equal to my height. Yours is probably very nearly so. But not the average NBA player. The average NBA player is a shade under 6'7", with arms that are seven feet long. Not only are NBA players ridiculously tall, they are ludicrously long. Had Leonardo wanted to draw the Vitruvian NBA Player, he would have needed a rectangle and an ellipse, not a circle and a square.
So in sports where large size is prized, the large athletes have gotten larger. Conversely, in sports where diminutive stature is an advantage, the small athletes got smaller. The average elite female gymnast shrunk from 5'3" to 4'9" on average over the last 30 years, all the better for their power-to-weight ratio and for spinning in the air. And while the large got larger and the small got smaller, the weird got weirder. The average length of the forearm of a water polo player in relation to their total arm got longer, all the better for a forceful throwing whip. And as the large got larger, small got smaller, and the weird weirder. In swimming, the ideal body type is a long torso and short legs. It's like the long hull of a canoe for speed over the water. And the opposite is advantageous in running. You want long legs and a short torso. And this shows in athletes' bodies today. Here you see Michael Phelps, the greatest swimmer in history, standing next to Hicham El Guerrouj, the world record holder in the mile. These men are seven inches different in height, but because of the body types advantaged in their sports, they wear the same length pants. Seven inches difference in height, these men have the same length legs.
Now in some cases, the search for bodies that could push athletic performance forward ended up introducing into the competitive world populations of people that weren't previously competing at all, like Kenyan distance runners. We think of Kenyans as being great marathoners. Kenyans think of the Kalenjin tribe as being great marathoners. The Kalenjin make up just 12 percent of the Kenyan population but the vast majority of elite runners. And they happen, on average, to have a certain unique physiology: legs that are very long and very thin at their extremity, and this is because they have their ancestry at very low latitude in a very hot and dry climate, and an evolutionary adaptation to that is limbs that are very long and very thin at the extremity for cooling purposes. It's the same reason that a radiator has long coils, to increase surface area compared to volume to let heat out, and because the leg is like a pendulum, the longer and thinner it is at the extremity, the more energy-efficient it is to swing. To put Kalenjin running success in perspective, consider that 17 American men in history have run faster than two hours and 10 minutes in the marathon. That's a four-minute-and-58-second-per-mile pace. Thirty-two Kalenjin men did that last October. (Laughter) That's from a source population the size of metropolitan Atlanta.