Jessie Riddle
PDBio 120 Dr. Jerry Johnson
November 2009
Are Humans Distance Runners by Nature?
The New York Times published an article on October 27, 2009, discussing the claim that early human beings evolved into distance runners. The theory known as “persistence hunting” suggests human evolution favored traits that allowed populations to obtain protein by chasing their prey to exhaustion and death. The article references a recent study done at the University of Utah, which points to many human traits that may have evolved in support of persistence hunting. The article also discusses the book recently published entitled Born to Run, which claims that mankind did, indeed, evolve into expert long distance runners, and that the reason running causes so many athletes injury today is our faulty methods of training (including high-tech shoes that detract from the natural use of the feet).
After a careful analysis of this news article, I have concluded that is technically accurate, and relatively specific in the evidences it cites from recent studies. The author did an excellent job of gathering research from multiple sources. However, I also found that the article fails to mention some of the more contradictory elements of the “Persistence Hunting” theory, and so paints a somewhat inaccurate picture of its probable validity.
Section One
According to a 2007 study authored by Dr. Daniel Lieberman and Dennis K. Bramble, and published in the journal Sports Medicine, human beings are remarkably well adapted to running long distances. Other recent studies have declared it likely that many modern human phenotypes involved in running are due to our early ancestors “Persistence Hunting.” Scientists from the fields of evolutionary biology, anthropology, neurophysiology, genetic morphology, sports medicine and others are currently debating whether or not Persistence Hunting could have caused the evolution of long distance running capabilities in humans.
This question comes at an important time – the list of medical problems resulting from a sedentary lifestyle is growing alongside the increasing number of exercise-related injuries. New discoveries suggesting if and how humans have evolved towards distance running could provide key insights into helping humans understand the needs and capabilities of their own bodies.
Section Two
Dr. Lieberman and Dr. Bramble’s paper describes the current theories on the origin of endurance running, which is referred to as “ER.” They state that the main difficulties of ER stem from three things: energy production and use, stabilizing the body, and regulating the body’s temperature. These three concepts are referred to as energetics, stabilization, and thermoregulation. Lieberman and Bramble account for the way in which humans have dealt with these barriers through the theory that the use of “Persistence Hunting” led to natural selection for traits that augmented ER abilities. They define Persistence Hunting as a pre-weapon technique beginning between 1.5 and 2 million years ago that involves chasing prey through the intense heat over long periods of time and killing the exhausted animal by hand.
Section Three
Energetics
The first issue discussed by Lieberman and Bramble is energetics. They explain that humans absorb the shock from the ground involved in running through tendons in the legs, converting potential energy to kinetic energy through a “mass-spring gait.” This means that the body’s Center of Mass falls after a running step is taken, pushing “elastic” energy into the tendons of the leg to be released through the leg’s upward motion. All ‘cursorial’ (walking) animals use the mass-spring gait to run. In walking there is no aerial phase and the legs have a higher friction with the ground, but running (due to its aerial phase) requires more kinetic energy to sustain, and so despite it’s minimal contact with the ground, it comes at a higher metabolic cost[i]
According to a 2009 study in the Journal of Human Evolution, humans, like other cursorial animals, minimize the costs of both running and walking at certain speeds. The study also states that running slowly is the least energetically efficient pace for humans[ii]. However, Bramble and Lieberman write that humans are remarkably well-adapted for prolonged usage of the mass-spring gait, due to “numerous tendons [in the legs], such as the Achilles…which are absent or tiny in other African Apes,” and “spring-like ligaments” in the feet. These studies appear to conflict, because one suggests that slow running is inefficient, and the other suggests that humans have evolved musculature over time specifically suited to long periods of running. This conflict may be resolved through further research on a specific type of Persistence Hunting referenced in the 2009 study, which involves extended periods of a running at a relatively faster pace alternated with periods of walking. This would allow for continued effort over long time periods and still allow humans to cover great distances
Stability
Bramble and Lieberman state that running presents more problems in terms of stability than walking. Thus, in a situation where humans are required to run for long distances, traits would likely become dominant that aid in stabilization while running. The authors suggest that this is, indeed, what humans have, in several key muscle groups. These include a narrow waist, a mobile thorax (the area from the sternum to the diaphragm) that is separated from the neck and thus allows the arms and trunk to rotate separately, sensory reflexes (specifically, the ones that allow us to absorb information and stay upright) that are suited to the rapid movements generated by running , and the gluteus maximus.
The gluteus maximus is frequently mentioned in studies concerning Persistence Hunting, because it has a markedly different structure in humans than in chimpanzees. In 2006, Lieberman, Bramble and several other scientists did a study determining the utilization of the gluteus maximus in running versus walking. They determined that this muscle’s much larger upper portion and lack of a lower portion is “unique to hominids.[iii]” Several theories could account for the divergence of this muscle for Homo sapiens, such as digging or climbing, but it clearly stabilizes the trunk and would thus have been very beneficial in ER (and thus Persistence Hunting). The lack of another human function that so significantly utilizes the gluteus maximus makes it seem likely that ER contributed to genetic selection for this muscle.
Thermoregulation
Lieberman and Bramble describe thermoregulation as the most significant physiological challenge in ER. The amount of muscle activity involved in running produces significantly more heat than walking, which creates a problem for persistence hunters, because Persistence Hunting must take place in a hot environment[iv]. However, modern persistent hunters in desert areas such as Botswana successfully outrun animals that cannot maintain speed in the heat.[v]. According to Lieberman and Bramble, this is due to humans’ use of evapo-transpiration instead of panting.
A Current Anthropology article published in 1984 explains that although panting is initially a better heat conductor than sweating, it limits an animal’s speed, because heat conduction is dependent on oxygen consumption through respiration. This article also points out that “No other species is known to sweat as much per unit surface area as man,” due to the structure and high density of sweat glands and the lack of hair[vi]. This extreme level of sweat secretion would thus enable a hairless, running human to dissipate extreme heat where other animals could not.
This article also states that further research must be done on large mammals to determine whether or not humans’ thermoregulation is unique and if it is related to bipedal ER. However, selection for such a trait suggests a lifestyle and environment that would cause early humans to benefit from high thermoregulation while running.
Section 4
Through an examination of mechanisms that assist in ER and or Persistence Hunting, there appears to be a compelling case for natural selection of Homo sapiens through such a mechanism. Many of the mechanisms supporting this theory need further exploration. Specific areas in which the debate among scientists or the lack of data clearly necessitate further study include energy use ratios at specific speeds for humans, thermoregulation capacities and mechanisms in large animals other than humans, and the different methods of Persistence Hunting. The work that has been done is ultimately inconclusive. .However, Persistence Hunting continues to gain support as a mechanism for the evolution of endurance running as the details surrounding its origin and role in human evolution become clear.
News:
Parker-Pope, T. (2009) “The human body is built for distance.” New York Times Web. October 26, 2009
http://www.nytimes.com/2009/10/27/health/27well.html
Primary:
Lieberman, D.E. Bramble, D.M. (2007) The evolution of marathon running; capabilities in humans. Sports Medicine 37 (4-5), 288-290
Review:
Steudel, K. (1996) Limb Morphology, bipedal gait, and the energetics of human locomotion. American Journal of Physical Anthropology 99 (2), 345-355
Allied:
[i] Steudel, K. (1996) Limb Morphology, bipedal gait, and the energetics of human locomotion. American Journal of Physical Anthropology 99 (2), 345-355
[ii] Steudel-Numbers, K.L., Wall-Scheffler, C.M. (2009) Optimal running speed and the evolution of hominin hunting strategies. Journal of Human Evolution 56, 355-360
[iii] Lieberman, D.E., et al. (2006) The human gluteus maximus and its role in running. The Journal of Experimental Biology 209, 2143-2155.
[iv] Carrier, D.R. et al. (1984) The energetic pardox of human running and hominid evolution. Current Anthropology, 25 (4), 483-495
[v] Liebenberg, L. (2006). Persistence hunting by modern hunter-gatherers. Current Anthropology 47 (6), 017-1025
[vi] see iv
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