With few exceptions, it’s believed that humans are the only species in the animal kingdom capable of running a marathon. Horses are natural sprinters, sled dogs can be bred and diligently trained for feats such as the Iditarod, and while pronghorn antelope can maintain speeds of 40mph, it is thought they cannot do so much beyond 30 consecutive minutes. Why do we have such a rare capacity for endurance? Is it simply a lucky by-product of our ability to walk on two legs?
A landmark 2004 paper in the journal Nature, written by Daniel Lieberman (Harvard) and Dennis Bramble (University of Utah), hypothesized that our bodies evolved as they did largely because of the need to run long distances. In addition to longer legs, shorter toes, broader shoulders, and the ability to shed heat through sweat, adaptations enabling trunk and head stabilization, balance, and powerful spring-like mechanics (such as the Achilles tendon) would have all enhanced endurance running faculties. These traits may have been specifically selected for because running would have benefited hunting, foraging, and scavenging abilities. It’s even been hypothesized that early ancestors brought down large prey by relentlessly chasing them to the point of exhaustion. Greater access to food energies (like protein-rich animal sources) would have meant greater reproductive success. Thus, adept early runners would have passed along their DNA. In this way, endurance shaped our bodies.
Importantly, Lieberman and Bramble concluded that our endurance running capabilities likely began during the early evolution of the genus Homo. The first members of this genus, which originated about 2 million years ago, also stood apart from predecessors because brain size was beginning to rapidly increase (see figure at bottom). The onset of endurance capabilities and this increase in cranial capacity have long been considered two separate phenomenon. However, a new theory suggests that physical endurance may not have only shaped the human body, but also driven the growth & development of the human brain.
Proposed by anthropologists David Raichlen (University of Arizona) and John Polk (University of Illinois), this theory explains how adaptations intended to improve endurance would have secondary benefits on brain capacities. Aerobic abilities would have become increasingly advantageous as our ancestors – beginning with Homo erectus - shifted to a highly active hunting and gathering lifestyle.
Raichlen and Polk’s paper, available in this month’s publication of Proceedings of the Royal Society Biology, pulls from several lines of research. Among them were evolution, or artificial selection, experiments. Mice were systematically bred for either high aerobic capacity (VO2max) or for their inclination to run (lots of time on the wheel). Mice who naturally expressed either trait were interbred for several generations. In both cases, the resulting line of mice exhibited markedly high baseline levels of certain growth factors and neurotrophins. These compounds (i.e., VEGF, IGF-1, & BDNF) boost aerobic abilities by enhancing metabolic regulation and oxygen transport.
But, these scientists weren’t just breeding running superstars. These mice showed increased brain growth and even superior performance on cognitive tasks. The same growth factors and neurotrophins that promote cardiovascular endurance also boost the growth and health of neurons in the brain. Foremost among these is brain-derived neurotrophic factor (BDNF) – a critical link in how exercise benefits brain structure and function in humans. If the changes that took place over the course of several generations were applied on an evolutionary scale, it is not difficult to see how adapting a highly active lifestyle could have driven brain growth 2 million years ago.
None of this is to suggest that the evolution of an active lifestyle was the only factor that boosted brain size and function. Selection pressures acting specifically on cognitive abilities (rather than athletic ones) may have also played a part. In fact, this has been the prevailing theory until now. For example, improved spatial and relational memory, as well as planning abilities, would have benefited long-distance foraging and the coordination of group hunts. Social skills needed to interact with others would have also required higher-order patterns of thought. Numerous factors likely contributed to human brain evolution, but this theory is the first to suggest that an active lifestyle – in and of itself – was among them.
So, what does all of this mean for us? It suggests that if movement helped mold the structure and function of our brains, then our brains likely still require regular physical activity to function optimally. Substantial research now exists to support the idea that regular exercise leads to more robust cognitive abilities in both children and adults. But, could it be that these studies are not actually demonstrating that exercise boosts cognitive abilities, but that the absence of exercise adversely affects them? By adapting sedentary lifestyles, so far removed from that of our hunter-gatherer ancestors, it may be that more than our waistlines suffer.