Chimps Like Us: Baby, We Were Born to Run

Christopher Bergland

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Courtesy of amnh r mickensA model of our early human ancestors (Australopithecus) on display at the American Museum of Natural History's Hall of Human Origins. Courtesy of AMNH/R. Mickens

A new study from the American Museum of Natural History (AMNH) in New York City seems to end the "chicken-or-the-egg" debate about which came first: human's large brains, or the shape and size of our bodies. According to the latest findings, natural selection for bigger brains was the driving force that led us to grow taller than our primate cousins. 

The new study by Mark Grabowski of AMNH examined the correlated evolution of brain and body size in humans. His findings show that throughout our human evolution, natural selection appears to have been driven by the quest to increase brain size, which caused our body size to increase in tandem.

The April 2016 paper, “Bigger Brains Led to Bigger Bodies?: The Correlated Evolution of Human Brain and Body Size,” was published in Current Anthropology.

Previously, most investigations of hominin brain and body size evolution assumed that different selection pressures were primarily driven by the nutritional demands of having large brains. However, this study suggests that humans became the large-brained, large-bodied animals we are today because of natural selection to increase our brain size first and foremost. The research shows that brain size and body size are genetically linked and that selection to increase brain size will ultimately "pull along" body size.

According to the paper, this phenomenon played a large role in both brain- and body-size increases throughout human evolution and may have been solely responsible for the large increase in both traits that occurred near the origins of our genus, Homo sapiens.

In a statement, paper author Mark Grabowski, a James Arthur postdoctoral fellow in the Division of Anthropology at the American Museum of Natural History, said, 

"Over the last four million years, brain size and body size increased substantially in our human ancestors. This observation has led to numerous hypotheses attempting to explain why observed changes occurred, but these typically make the assumption that brain- and body-size evolution are the products of separate natural selection forces."

Grabowski's findings demonstrate, for the first time, that strong selection to increase brain size alone played a large role in both brain- and body-size increases throughout human evolution. "While selection no doubt played a role in refining the physical changes that came with larger body sizes, my findings suggest it was not the driving force behind body-size evolution in our lineage," Grabowski concluded. "Therefore, evolutionary models for the origins of Homo based on an adaptive increase in body size need to be reconsidered."

Daniel Lieberman and Dennis Bramble, paleontologists at Harvard, spent thirteen years pinpointing twenty-six traits that made early Homo sapiens such proficient runners. They found that endurance running is unique to human beings among all other mammals except for dogs, horses, and hyenas. It appears that our ancestors' ability to run long distances across the African savanna influenced the shape of our bodies from head to toe. 

Bramble and Lieberman established that our slender legs, shorter arms, narrower rib cage and pelvis, skulls with overheating prevention features, and the nuchal joint—which keeps our heads steady when we run—set us apart from chimpanzees. Their findings were published in a Nature paper, "Endurance Running and the Evolution of Homo."

According to this research, as hunter-gatherers, the human body evolved to run great distances in order to hunt prey and gather food. Our ability to spring through the air using our gluteus maximus muscles appears to be what most sets us apart from primate cousins. The pogo-stick ability of each leg allowed us to travel long distances and to hunt and gather a high protein diet, using relatively little fuel. As our brains grew, so did our prefrontal cortex—the seat of human intelligence—and we became smarter hunters.

The paleontologists concluded that running improved our chances of survival and reproduction. Although early humans may not have been as swift as our four-legged competitors, we could (and still can) out run and hunt over greater distances than most other predators.

Lieberman believes that endurance running most likely made it possible for humans to obtain a diet rich in fats and proteins. This nourishment fueled the unique human combination of big brains and large bodies.

Conclusion: Aerobic Activity Is An Evolutionary Cornerstone of Our Brain Size

As cliché as it is, blasting Bruce Springsteen's classic-rock anthem, "Born to Run," at top volume has always been like rocket fuel whenever I'm jogging or training for an ultra-endurance event. In light of the latest evolutionary discoveries—and other recentneuroscience-based research, which shows that daily exercise bulks up the volume of our brains—the Springsteen song takes on deeper anthropological and neurobiological meaning.

If you need motivation to stay physically active and to do aerobic exercise regularly . . . I'd recommend cueing up "Born to Run" while reminding yourself that your body and brain evolved for millions of years based on the upright striding bipedalism of walking or jogging.

Lastly, our evolutionary biology seems to have hardwired our brains to produce brain-derived neurotrophic factor (BDNF) which stimulates neurogenesis (growth of new neurons) anytime we do aerobic exercise. So, especially in modern times—with increased screen time and the epidemic of sedentarism—if you want to maintain a healthy body and a large brain throughout your lifespan, staying physically active and getting regular aerobic exercise is of timeless and paramount importance.