The thousands of people who will run in the Philadelphia Marathon this weekend can thank evolution for a number of distance running adaptations - not least of which is what Harvard evolutionary biologist Daniel Lieberman calls our hypertrophied gluteus maximus. That's the technical term for big butts.
Or more precisely big butt muscles. The glutes are the largest muscles in the human body. They are massive compared with those of other creatures, and research shows they're essential for running. They give runners balance that other bipedal creatures get from having a tail.
Several distinctive human features appear to be essential for running - an observation that has convinced Lieberman that running had a profound influence on shaping the human body. We would be very different animals if our ancestors hadn't needed to run after their dinners.
Even without scientists to study them, marathoners tell us something about our evolution: They disprove the popular wisdom that the growth of the brain necessitated the degeneration of the human body. It's true that we're no match for gorillas or chimpanzees in a fight, but when it comes to distance running, we kick the flat butts of all other primates and most other animals.
Dogs, cats, and many other animals can easily outsprint the fastest humans. But as the distances get longer, we start to catch up. No other primate can come close. As we branched off from other apes, "we've evolved all these metabolic mechanisms and thermoregulatory features," he said.
Lieberman had been studying the evolution of the human head, but was nudged toward the study of running by a pig.
The pig was on a treadmill for some other study when a colleague, Dennis Bramble, who was on sabbatical from the University of Utah, pointed out that the pig's head was bobbing all over the place. The researchers realized that pigs lacked what's called a nuchal ligament, which runs up and down the neck and helps some other animals keep their heads stable. Dogs and other good runners have a nuchal ligament and their heads don't bob around when they run.
Other apes don't have a nuchal ligament and we humans do, suggesting that it evolved independently in humans. Lieberman and Bramble soon found that the presence of a nuchal ligament leaves a distinctive mark on skulls, which led them to conclude that our ancestors got them about 2 million years ago.
Before that, we could walk, he said; 3.2 million-year-old Lucy was an upright walker but she was short-legged, squat, and lacking this nuchal ligament. She was probably a better tree climber than we humans, he said, but she never could have run a marathon.
He and Bramble concluded that the ability to excel at distance running coincided with the emergence of our taxonomic group: the genus Homo. Fossil evidence clearly indicates that our ancestor Homo erectus not only had a big butt, but also a nuchal ligament.
Lieberman is himself an avid distance runner and so he was able to combine his passions for science and running. He and Bramble assembled a long list of apparently running-related features that we don't share with other apes. One is the configuration of our ear canals, which gives us the kind of balance needed to run on two legs. Another is the Achilles tendon, which is vital for running.
And we're phenomenal at getting rid of excess heat, being relatively hairless and sweaty. Some of these traits might have arisen for other reasons, but they also served to help our ancestors run, Lieberman said.
In 2004, he and Bramble published a comprehensive paper on possible running-related adaptations, which ran in the journal Nature with the headline "Born to Run."
Still, Lieberman wanted to look more closely at the role of the gluteus maximus, so he started putting students on treadmills with sensors stuck to their backsides. As the students ran and walked, scientists measured the electrical activity in the gluteal muscles. From this, they discovered that we don't use our butts much at all walking on flat surfaces but we use them a lot when we're running.
What would have been the evolutionary advantage of being a good distance runner? Lieberman said he and Bramble concluded that the most plausible answer involves the pursuit of food: We can run marathons because our ancestors had to run to chase down animals.
People tend to assume early hunting was made possible by weapons, he said, but our ancestors hunted for the last 2.6 million years, and the bow and arrow was invented only in the last 100,000 years. The stone-tipped spear was invented perhaps 300,000 years ago.
Before that we hunted with nothing but sticks and clubs. Our best weapon might have been distance running, which could allow our ancestors to run down prey animals until they overheated.
Our ancestors ran barefoot, which got Lieberman interested in barefoot running. That led to a worldwide trend and the science behind the 2009 book Born to Run by Christopher McDougall.
Both sexes are good at distance running, with top women only slightly behind the top men. Lieberman attributes most of the difference to the higher percentage of fat carried by women, which acts as deadweight.
Hunter College anthropologist Herman Pontzer, who collaborated in the gluteus maximus experiment, agrees that we're born to run. Once you get to distances comparable to a marathon, he said, "we hold our own against the best animals."
It's true most people can't even run a quarter marathon today, but that likely wasn't the case for most of human existence. Pontzer studies hunter-gatherer populations, including the Hadza of Africa. They're in great aerobic shape, he said, due to their lifestyle. "They don't call it training; they call it living."
"People talk about how wimpy humans are, but these are usually out-of-shape scientists talking," said Pontzer. Scientists who run, such as Lieberman, may naturally think differently. Pontzer, being a rock climber, objects to the idea that we've traded in all our climbing ability. "Humans can climb 3,000 vertical feet in 12 hours," he said, and rock climbing is more difficult than tree climbing.
But marathon running remains one of the great demonstrations of humanity's physical and mental power. So next Sunday, cheer for the Philadelphia Marathon runners. They remind us what we're capable of doing.