Bipedalism: A Response to Climate and Other Evolutionary Pressures
Many theories regarding bipedalism in early hominids, as well as the advantages provided by bipedalism have arisen and have been debated. The theories are an attempt to reconstruct the past environs in which these early bipeds lived, to make a solid, tangible idea of how bipedalism emerged as a need of daily activity. No one theory is complete, however. As new data arises and the history of bipedalism scales back even further in history, anthropologists still have yet to find one theory to encompass all ideas.
Pete Wheeler and Pat Shipman are two anthropologist who claim to have a resolution as to why bipedalism first emerged. Wheeler, convinced that heat is the sole proprietor of bipedalism and Shipman, persuaded that bipedalism evolved as a response to freeing the hands, fail to mention that both theories have ample evidence and that both are viable explanations. To analyze either authors reasoning, we must glance into the scientific atmosphere at the time. In Shipman and Wheelers paper, Australopithecus afarensis is the oldest known hominin fossil. Consequentially, they base their research off of the environment of A. afarensis (such as savannas).
To examine the theory that Wheeler proposes, we can envision a savanna, whose climate would be much to harsh for any animal without any cooling mechanism. Most savanna mammal are fitted with the “"carotid rete, a network of fine arteries at the base of the brain, coupled with the venous circulation through the muzzle" (Wheeler, 1993). This allows the brain to cool rapidly, however interestingly, this feature is missing in these early bipeds. They would eventually “replace” this feature, as Wheeler claims, with the ability to cool the entire body and thereby cool the brain. His argument is simple; "erect posture exposes less surface area to the sun and lifts the body above the ground, which keeps the body temperature cooler and reduce the dangers of dehydration and sunstroke on the savanna." Although he did compare and contrast different quadruped and biped, hairy and naked subjects on the amount of sun exposure received, more thorough research should be conducted to refine this idea, as seen in Shipman's excerpt.
Pat Shipman begins his journey to understand the hominin atmosphere with an anecdote, explaining that he began by exploring wear marks on bones, and discovered something quite by accident. Shipman and his colleague had found cut marks of bones, as well as carnivore tooth marks, which they claimed were the first true pieces of evidence to convey the correlation between early bipedal hunters and animals. Shipman's findings are intriguing, given the assumption that early bipedal hunters were, at the time, assumed to have procured their sustenance in the same way as modern hunters do: “More than 90% of the Neolithic marks in these two categories (disarticulation or meat removal) were from disarticulation, but to my surprise, only about 45% percent of the corresponding Olduvai cut marks were from disarticulation...This finding casts serious doubt that early hominids carried their kills back to camp to share with others, since both transport and sharing are difficult unless carcasses are cut up." Furthermore, analysis on bones from Olduvai totaling 13 specimens, eight of which with signs of wear, demonstrated that carnivore tooth marks were prevalent before any other human made marks, suggesting that early bipeds were actually scavenging (Shipman, 1984). To be a scavenger, you must have the physiological ability to survey large areas of land, larger than predators survey for prey. Shipman shows that at speeds of 2.5 to 3.5mph, bipedal locomotion is more “energetically efficient;” couple that with the ability to see over the savanna grasses, the wide range of dietary needs and hands that can be used to perform a separate task such as carrying, and Shipman provides a very tantalizing argument.
What do the modern theories and studies reveal? One study compares the energy costs between humans and chimps, and concludes: “Even small increases in energy savings from slight increases in hindlimb extension or length may have provided critical selection pressure for early hominins. Our results therefore support the hypothesis that energetics played an important role in the evolution of bipedalism” (Sockol, Raichlen, and Pontze, 2007). These researchers seem to side with Wheeler and his theory that heat was the primary evolutionary stressor, in that both studies are directed by energetics studies. Additionally, because tool making arose much later than bipedalism emerged, this suggests that bipedalism was retained as a trait due to its usefulness (tool-making). “Freeing the hands for tool making was Darwin's original suggestion, but this seems unlikely in view of the later development of stone tool manufacture. More probable is that bipedalism is an energetically efficient response to the spread of nonforested environments between 10 and 5 million years ago. Apart from its locomotor efficiency in terrestrial environments, it has also been convincingly argued that it provides a number of clear thermoregulatory advantages in what would have been considerably hot environments. It is thus linked to other unique human traits such as copious sweating and loss of body hair” (Foley, 2011). These theories are in line with Wheeler's claim (albeit energetics only merits one paragraph in his excerpt): “Finally human bipedalism at low speeds uses less energy that does either true quadrupedalism or the knuckle walking used by African apes. This reduces both dietary requirements (and the time and effort spent foraging) and the rate at which heat is generated internally as a byproduct of muscular activity.” To ensure my assertion that bipedality is the response to hominin needs for energetic efficiency, a study conducted by Leslie Aiello and Jonathon Wells claims that there are massive energy requirements due to the emergence of the genus Homo. It can be assumed that as evolution continues to run it's course, energy requirements fluctuate, and humans will adapt physiologically to accommodate.
In conclusion, bipedalism emerged as the need to be energetically efficient for subsistence strategies, such as scavenging. Tool-making and freeing the hands influenced the retention of bipedalism, as did other physiological traits that evolved as hominins evolved through time.