Bengaluru Nov 23, 2018, (Research Matters):
In 2009, a journalist named Christopher McDougall published a book called “Born to Run: A Hidden Tribe, Superathletes, and the Greatest Race the World Has Never Seen”. It is an odd combination of popular science, tirade against the modern running-shoe industry and a true story.
The true story was most readers’ introduction to the legendary native American tribe of long-distance runners, the Rarámuri. They live today in Mexico and are the titular “superathletes” who routinely run for hundreds of kilometres at a stretch to hunt and to play. Several long-distance runners from the USA, including McDougall, took part in a fifty-mile race against their Rarámuri counterparts, in a sprint through the Copper Canyons of Mexico. The story is nothing less than an uninhibited celebration of the human ability to run, its joy, its beauty, its spirituality.
The popular science, however, leaves a lot to be desired. McDougall is plainly a journalist with an eye for a story, and even more plainly, not a scientist. He explains a theory of human evolution, called the 'endurance-running theory', and strongly advocates it with little to no critical examination. As much as the book inspired me to run, it also piqued my curiosity about its critical scientific assertion—are human beings really evolved to run long distances?
From Africa to the World: The story of human evolution
Scientifically speaking, humans belong to the family Hominidae, subfamily Homininae, genus Homo and species sapiens. The origins of modern humans are regarded by consensus to lie somewhere in the Eastern and Southern part of the African continent, with one or more migrations out towards Eurasia, a theory called 'Out of Africa'. The most recent wave of emigrants left Africa about 70 to 50 thousand years ago, at a time when our close evolutionary cousins, the Neanderthals and Denisovans, both members of Homininae, still co-existed and interbred with early human species. Evidence for a third, intriguingly unknown species of Hominin named ‘Hominin X’ has been found from the DNA samples of the tribal populations of the Andaman and Nicobar islands. Approximately 2% of their DNA is unexplained in its origin, and it is hypothesised that this is accounted for by their mating with the mysterious Hominin X that has left few other traces behind.
This theory of humanity’s cradle of life has been contested by other hypotheses, though none have yet amassed enough evidence to replace the Out of Africa theory. For example, the ‘Kumari Model’ of human origins, posited by A.R Vasudevan, argues that humans first evolved on a now-submerged continent in the Indian Ocean known as Kumari Land. This argument is based on data from the National Geographic Genographic Project and asserts that humans migrated out of Kumari Land via two routes, one into Africa and another into Europe and Asia. However, whatever genetic evidence has been found for the relatedness of European and Indian populations can be explained by the OOA. Furthermore, the fact that Kumari Land was never above sea-level during this period has effectively rendered this theory pretty unlikely.
The human family tree from the blog Filthy Monkey Men by Adam Benton with references to scientific papers therein
The tropical plains of Africa are still the most likely stage on which the drama of early human evolution occurred, and many tribes of hunter-gatherers still live there today. Some of these peoples practice what could be the closest living approximation of the early human lifestyle, and one such example are the !Kung people of central and southern Africa. Many are now farmers, but a good fraction of the population still practices the nomadic lifestyle of their tradition, though the advance of modernity is continuously eroding this. Can their hunting practices throw some light on the viability of the endurance-running theory?
Running to a bigger brain?
The endurance-running theory posits that the human ability to run sustainably long distances shaped the course of our evolution over the last couple of million years, at least in part by creating a unique hunting possibility. Most large mammals easily outpace humans during short bursts of running (sprints), but almost none can keep it up for longer than a few minutes. By chasing prey over long distances without a break, humans could have forced their quarry to overheat and collapse, or at least slow them down and weaken them enough to be killed quickly. This practice of “running an animal to death”, known formally as “persistence-hunting”, would have brought a steady supply of nutrient-rich meat into the early hunter-gatherer diet, as argued by Bramble and Lieberman. It is still practised by a few hunter-gatherer societies today, such as the Kalahari bushmen of Africa and, of course, the Rarámuri of Mexico.
One implication of Bramble and Lieberman’s theory is that persistence running may also have contributed to the evolution of the human brain over this time in our species’ history. Persistence-hunting is an intrinsically social activity, requiring cooperation and communication between members of the hunting band, and this active sharing and social aspects could have shaped cognitive evolution. The skills would likely be taught to younger generations by both word of mouth and practical guidance, further increasing communication and social complexity.
In effect, tracking and anticipating the movements of prey would have required higher-level cognitive abilities and selection for such might have affected brain evolution. The aerobic activity of running itself is very conducive to the development of new nerve cells. It increases the baseline levels of proteins, such as neurotrophin and growth factors that contribute to the growth and development of new nerve cells (neurons). Finally, the steady supply of calorically dense meat could feed an energy-hungry brain, promoting better hunting skills, thereby a higher quantity of meat for still larger brains, leading to even better hunting skills, and so on.
The recent study of the !Kung people addressed a question central to the plausibility of the endurance-running hypothesis and its implications for brain evolution—is persistence hunting indeed energetically-rewarding enough to have conferred a selective advantage in human evolution?
The energetics of modern persistence-hunting: Is it all worth it?
The !Kung people live in modern-day Namibia, Botswana and Angola, in an environment likely similar to the one from which early humans emerged, making their society an excellent one to study persistence-hunting in real life. In a study, the Energy Return on Investment (EROI)—the ratio of the energy gained on completing a process to the energy invested into the same process—was calculated from the persistence-hunting practice of the !Kung people.
The Greater Kudu (Tragelaphus strepsiceros) is a species of antelope and game animal hunted by the !Kung people. The authors of the above study calculated the EROIs generated by a typical hunt for a small, average and large Kudus by comparing the energetic cost of the chase to the calorific gains from the meat of the carcass. This metric was calculated as the energy gained from the Kudu (if it were eaten) multiplied by the success rate of a hunt, and divided by the energy invested by the hunters. To calculate the number of days a single carcass could sustain a family, the energy spent in the hunt was subtracted from the total amount of energy that a Kudu could yield if eaten, and finally divided by the average daily energy expenditure of the hunters and their families.
The results were clear—the EROI was an enormous ratio, with a return of energy at 26:1 to 44:1 for a small Kudu (meaning that for every unit of energy invested in hunting it, a small Kudu yielded 26 to 44 units), 34:1 to 57:1 for an average Kudu and 41:1 to 70:1 for a large Kudu. These returns are enough to support a family for 6.7 to 11.2 days—undoubtedly worth the investment to run an antelope to death!
A body built to run?
Humans are indisputably poor sprinters. The fastest footspeed ever attained by a human is 44.72 km per hour—a speed easily surpassed by cheetahs, greyhounds, horses and other antelopes running at less than their maximum speeds. In contrast, it is just as indisputable that humans are one of the best endurance-runners on Earth. Though our speed is far below the top galloping speed of horses, we can outrun horses over very long distances.
The Rarámuri can run over 300 kilometres without stopping, and the urban humans practice their own form of recreational endurance-running through marathons. Indeed, history was made on the 16th of September this year, when Eliud Kipchoge of Kenya broke the world record for the fastest ever marathon, completing a run of 42.2 kilometres in 2 hours, 1 minute and 39 seconds in Berlin. Elite human runners even compete in ultramarathons through harsh terrains, such as the Leadville Trail 100 Run—a 160 kilometre run through the Rocky Mountains of North America, and the Hell Race—a series of gruelling runs through the Himalayas. The popularity of running as a recreational activity is behavioural evidence of our ability to run, but far stronger evidence comes from our anatomy and physiology.
Humans are bipedal—we walk on two limbs unlike quadrupeds, which walk on four. Bipedalism is an inherently unstable way to propel the body forward. When running, the body’s centre of mass (the point around which the mass of the whole body is balanced) is swung over the stride of the extended leg and the kinetic energy of the body changes. The tendons and ligaments of the legs and the arch of the foot absorb energy during the initial foot-strike and then recoil like springs to release this strain-energy during the second part of the stride, the propulsive phase. The foot-strike is the part of a stride that carries the most impact, transmitting forces up to 3-4 times the body weight through the entire skeletal system. The joints of the lower half of the human body, including the femoral head, where the thigh bone meets the pelvis, and the knee joint, have substantially larger surface areas than comparable anatomical regions in the fossil cousins and ancestors of modern humans, which could help absorb this impact.
Anatomy of the human leg (via Wikimedia Commons)
The arches of the human foot (image obtained via Wikimedia Commons and modified)
The Achilles tendon, which connects the heel and calf muscles (plantar flexors) is particularly important as a spring-mechanism, storing and releasing energy. It is notably absent in other species of great apes that can walk but not run. Indeed, the skeletal correlate of the Achilles tendon, the calcaneal tuber, is shorter in both modern and early humans compared to Neanderthals, increasing the spring’s efficiency. The plantar arch of the foot is another critical spring-like structure coupled with a medial flange projecting over the proximal cuboid bone of the foot and thus working to restrict the rotation between the anterior and posterior part of the foot, giving the foot an energy-saving bounce.
Stabilization during running is crucial as, unlike walking, at certain times no part of the body has any contact with the ground. The initial rotation of the trunk, therefore, has no counterbalancing rotational forces generated by contact with the ground. The corresponding twist comes from the body itself—a narrow waist that permits rotation of the trunk relative to the hips; broad shoulders that counterbalance the movements of the arms; and greater structural independence of the shoulders and head which permits unimpeded counter-rotations of the arms and shoulders. The structural autonomy of the pectoral girdle (shoulders) and head requires a stabilising structure. The nuchal ligament which runs between the back of the skull and the upper trapezius muscle, stretching along the upper part of the spine over the shoulder blade serves to help here. These structures are also notably absent in other species of great apes.
If persistence-hunting is to bring down prey, it is essential that the predators themselves do not overheat. Humans have very little fur on their bodies and have a dense concentration of sweat glands in the skin, a couple of hundred per cm2. Sweating is a far more efficient way of cooling than panting, which is the evaporation of water from the smaller surface area of the mouth and lungs. Panting is distinguished from mouth-breathing—another human behaviour that makes endurance running easy. The process of combined mouth- and nose-breathing and the rate at which it happens, is also decoupled from the mechanics of locomotion.
In most running quadrupeds, the synchronised movement of the diaphragm and visceral organs during running works to push air in and out of the lungs, so running speed is closely tied to the respiratory rate at a 1:1 ratio. Hence, they cannot increase their running speed without increasing their respiratory rate. Further, due to the necessity of panting through the mouth, there is a point of trade-off where greater running speed, requiring greater heat dissipation, does not allow for fast enough panting.
The metabolic Cost Of Transport (COT) is the efficiency with which energy is used to transport a body across distances. This metric increases with the speed of running in a linear fashion as a function of body mass. Though humans have a 50% higher COT than a typical mammal, an increase in human running speed has a minimal effect on COT. This favourable trade-off between speed and metabolic efficiency leading to potentially large calorific gains through greater hunting success could plausibly have given endurance-running a selective advantage.
While anatomy and physiology are the classical evidence cited in favour of the endurance-running theory, recent experiments in mice may be opening up a whole new line of genetic inquiry. Humans and chimpanzees share 98% of their DNA, and the gene CMAH (CMP-Neu5Ac Hydroxylase) was one of the first genetic differences to be identified between them, dated to about 2-3 million years ago. Chimps possess a functional copy of this gene, i.e., it codes for a protein, but the humans possess a loss-of-function version of the same gene. Now researchers at the University of California have taken a lineage of mice with this same loss-of-function version of the gene and studied their running abilities. Not only did they find greater running endurance in these mice, but a greater resistance to muscle fatigue, a greater consumption of oxygen by the muscles (necessary to maintain activity), and an increase in the presence of certain metabolic products, indicating changes in the underlying pathways. This is made all the more fascinating when one remembers that it is humans, and not chimps, that are the runners among the great apes.
Just a just-so story?
Elegant as the persistence running theory is, many remain unconvinced by it mainly because most of it, if not all, could be reduced to a ‘just-so’ story. H. sapiens are indisputably and uniquely suited to long-distance running, but other links in the theoretical chain have been questioned. Did bipedalism itself evolve as a consequence of running? Did endurance-running make persistence-hunting or scavenging possible? Did the physical traits cited as specialised for endurance-running evolve under some other selective pressure and subsequently got co-opted?
Some features such as an abundance of spring-like tendons in the leg and well-developed gluteus maximus muscles are indeed more useful during running than walking. Others, such as joints with larger cross-sectional areas, trunk rotation and a pectoral girdle decoupled from the head make carrying and transporting heavy weights possible. Efficient control of body temperature, bipedalism, proportionally longer legs and the plantar arch are just as useful for walking long distances in the sun. Hence, some researchers opine that those features may have been selected earlier for that function instead.
Paleo-archaeological studies have offered some vital evidence against the persistence-hunting hypothesis. If early hominins did indeed run their prey to death, they would disproportionately kill the animals most likely to collapse first—the young and old that could not move fast enough to escape and pregnant females more likely to overheat. When this prediction was tested against bovid fossils found in the Olduvai Gorge in Tanzania, where early humans had hunted, the opposite of what persistence-hunting would predict was found! There was a disproportionate abundance of fossils from prime adults, as opposed to those from weak, sick and old animals. Also, the early Homo is inferred to have lived in the savannah-woodlands with compact soil, ill-suited to preserve tracks and dense vegetation that does not permit prey to be spotted from a distance. Persistence-hunting works best under arid conditions on sparsely-vegetated plains, and this is supported by the comparative rarity with which modern hunter-gatherers practice it, further limiting it only to hot, open environments.
In an argument that strikes closer to the heart this theory of human brain evolution, some studies point out that tracking for persistence-hunting is an immensely sophisticated cognitive activity. While wolves and lions also hunt in packs, requiring similar cognitive skills, they do not use weapons and rely on a sense of smell that humans do not possess. Sophisticated hunting-related cognition would already have required the enormous brain that persistence-hunting is said to have made possible. Homo is unlikely to have had such capabilities early in its evolution. The persistence-hunters of today, like the Kalahari hunters, have characteristic large modern human brains, not to mention complex language. They also have far more efficient weapons, whose rudimentary precursors appear only late in the fossil record, well after endurance running is thought to have evolved.
A few tentative conclusions
The image of a band of brothers, burgeoning brains hungry for meat and effortlessly chasing down a panting antelope is undoubtedly a romantic one, but not necessarily the basis for a sound theory of human evolution. As with most complex issues, the truth probably lies somewhere between the extremes. Endurance-running may well have been the ‘unintended’ consequence of multiple traits that were initially selected for another purpose or purposes. While it seems unlikely that human intelligence is a direct consequence of persistence-hunting, it is clear from the !Kung people that the practice can be a huge advantage under the right circumstances. These data, coupled with those from paleo-archaeological studies, raise the possibility that the cause and effect relationship may be the other way around! Perhaps, it was the combination of a body selected to walk or carry weights, and growing human brain that made persistence-hunting possible, not only by improving tracking abilities but enabling hunters to seek out environments suited to endurance-running?