Each event has circumscribed specifications. For instance, the very best sprinters don't need much aerobic capacity because they rely on a preponderance of fast-twitch muscle fibers, which contract quickly and anaerobically, meaning they don't require oxygen to burn fuel. Those same athletes could not successfully run distance, because long-distance runners rely on a huge aerobic capacity and a larger percentage of slow-twitch fibers, which contract at a slower rate but can work for long periods, so long as they're being continually supplied with oxygen. These traits are largely inherited: If your muscles are made up mostly of slow-twitch fibers, you'll simply never be explosive. We might be able to do a lot to change the basic design we're born with, but not to the point of achieving a world-beating performance.

In the early days of Olympic and world competition, the athletes were probably closer in ability to the average population. Nevertheless, they came from a very small pool out of the total population, and that pool came largely from the privileged class or those who, for one odd reason or another, decided to throw the javelin, long jump, sprint, or run the marathon. Such is not the case now. First, talent is actively solicited: Individuals are identified, nurtured, and encouraged to pursue their dreams to the near-exclusion of more distracting concerns, like milking the cows or otherwise making a living. A second and perhaps much more significant phenomenon is that the pool from which the talented are selected has expanded dramatically. Since 1896, when the first modern Olympics were held, the world population has quadrupled. What's more, while Olympians were previously drawn only from Europe, Australia, and North America, now they also come from Asia, Africa, and South America. Statistically, by simply increasing the sample size, you increase the likelihood of having some individual runner who is faster than ever before in history (as well as one who is slower than ever).

The only real evolution has been in realms not directly related to biology. The most obvious factor in athletic improvement has been better technology. Running shoes are infinitely better. Vaulting poles morphed from ash to bamboo to aluminum to fiberglass, nearly doubling the record heights in the event. And of course, swimsuits have undergone all manner of makeovers, from wool trunks and tops in the early 1900s to skimpy Lycra numbers in the disco years to full-body suits debuting in Sydney called fastskins, which have a dimpled surface, much like a golf ball's, to reduce drag.

Mirroring technological breakthroughs have been changes in technique, such as Dick Fosbury's now-standard backward flop over the high-jump bar and swimmer David Berkoff's dolphin kick in the backstroke. Training methods have also evolved. Germany's Waldemar Gersheler used interval training to help his protégé, Rudolf Harbig, nab the world record of 1:46.60 in the 800-meter run in 1939. Arthur Lydiard of New Zealand helped Peter Snell take Olympic gold in the same event in 1960 and 1964 by advocating long, slow running to build endurance, and brutal hill work to build strength. And Britain's Sebastian Coe, who in 1981 set an 800-meter world record that held for 16 years, used weight lifting in addition to Gersheler's and Lydiard's methods.

Such a multitude of factors makes it nigh impossible to predict limits, but physical limits must exist. In just one century the law of diminishing returns has already set in; in certain track events, decades pass in which records improve by no more than hundredths of a second. Take the 200 meter run: In 1968, the world record stood at 19.83 seconds; in 1996 Michael Johnson lowered it to 19.32 seconds—about a half a second in 28 years.

None of this is good news for the human spirit. We need to keep desire alive. We depend on faith; records will fall only to those who believe it is possible. The heroes of my boyhood—Jim Ryun, Peter Snell, Herb Elliott, Steve Prefontaine, Billy Mills—achieved their status and success through sheer guts and work. They aspired to be gods—and to my high school cross-country mates and me, they were gods on some level. Yet the real reason we saw Pre and the others as heroes was that we secretly believed we were elementally equal. We were convinced that, if we only tried, if we did what they did, then we too could rank among the gods. To think that if they lived and ran today they would all be left in the dust by a herd of modern runners is devastating to my psyche. At our core we are endurance predators driven by dreams, spurred on by the antelope that we can't see but know is out there, somewhere, up ahead. To continue pushing, though, we must believe it's catchable—if only we apply ourselves.

Like the North American antelope's residual ability to outrun a cheetah—a cat that became extinct on the continent some 10,000 years ago—our abilities to run, throw, and jump are leftovers in our survival tool kits. As such, we use them in play because they are instinctually important to us. I'm not as athletically capable as an antelope or a bird or an Olympic athlete, but I enjoy my own capacities and I'm inspired to stretch them by seeing what others can do. I'm humbled by what is routine to the songbird or sandpiper, awed by their ability to fly unbelievably long distances to and from specific pinpoints on the globe.

Some might argue that, if I were a bird, I would not be able to enjoy my fantastic annual journeys, following the sun from perpetual daylight on the Arctic tundra to the pampas in Argentina and back again. But I think they are wrong. What makes the blackpoll warbler strike out south in the fall after a cold front is probably not fundamentally different from what motivates me to jog down a country road on a warm and sunny day. We're both responding to ancient urges. Proof that, in our case, it's impossible to extinguish our primal enthusiasm for the chase.  

Biologist Bernd Heinrich holds the masters world record for the 100k ultramarathon and is the author of 12 books, including Racing the Antelope, forthcoming from HarperCollins.