In June 1973, chestnut colt Secretariat ran the Belmont Stakes so fast that no horse has matched him since. In less than two and a half minutes, he won by 31 lengths. Fifty years later, billions have been poured into breeding, yet his record still stands. This is the paradox of the thoroughbred: breeders may have bred away the genetic diversity needed for further improvement.

The horse began as a terrier-sized, multi-toed forest dweller 55 million years ago. Over millions of years, as grasslands spread, equids developed into single-toed animals with hooves optimized for running. The evolutionary streamlining produced today's horses. Horses became extinct in the Americas 10,000 years ago but survived in Eurasia, later reintroduced by Europeans.

The first horse tamers were the Botai people of Kazakhstan 5,500 years ago, but their horses were not ancestors of modern breeds. Instead, modern domestic horses emerged 4,200 years ago in the Pontic-Caspian steppes. Genetic analysis of ancient bones has revealed this timeline. Secretariat's record stands as a testament to both the pinnacle and the fragility of selective breeding.