Go ahead, celebrate that sweaty, hairless body that made this all possible.

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At the Berlin marathon on Sunday, Eliud Kipchoge of Kenya set a new world record of 2:01:39, just four seconds per mile short of breaking two hours—a mark that once seemed unattainable. (In May 2017 he also ran a 2:00:25 in a controlled, unofficial race put on by Nike.)

How can the human body run sub-5-minute miles across 26 miles? Or, cover the 135-mile Badwater Ultramarathon in 22 hours, as Pete Kostelnick did in 2016? Scientists think they are getting closer to the answer. A break in human DNA eons ago gave us extra endurance and speed, catapulting the species to the front of the pack.

Scientists have known for some time that a gene found in most primates, CMAH, is broken in humans. A recent lab study out of University of California, San Diego found that mice implanted with the human version of the gene could run 20 percent farther and 12 percent faster than the mice with the normal version of CMAH. Upon closer examination, the mice with the human version of the gene had more blood vessels fanning through their legs and used oxygen more efficiently.

Back in the day, i.e. 3 million years ago, most animals went cheetah-style, using short bursts of intense energy and some sneakiness to capture prey. Humans, on the other hand, played the long game, following antelope and other game until they simply got too tired to keep going.

The climate was changing and African forests dried up and turned to savannas. Humans developed longer legs and other skeletal changes that benefited long distance running. Additionally, some evolutionary biologists have theorized that expanded sweat glands and less fur also helped keep humans 2.0 cool on long-distance treks. This development was crucial for eluding predators, tracking prey and, you know, becoming masters of our domain.