Roger Pielke Jr. wonders why the speed of the fastest horses has plateaued while the fastest humans continue to break records:
One possibility, advanced by [Mark] Denny and others, is that thoroughbred race times may have leveled off because the narrow genetic diversity of racehorses limits the genetic diversity in the pool of potential thoroughbred champions. Modern thoroughbreds are descendants of a small number of horses (less than 30 in the 18th century), and 95 percent are thought to trace their ancestry to a single horse, The Darley Arabian. Today, there are fewer than 25,000 thoroughbreds born each year in the United States. Compare that with the more than 7 billion people worldwide. The size of the human population may simply lead to a greater number of potential athletes with extreme speed.
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