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This text was drafted by award-winning MMW TAs Tara Carter and Beth Peterson, (Anthropology) in the summer of 2008, supported by the MMW-based research funds left in his account on the death of Prof. Donald F. Tuzin, long an instructor and supporter of MMW. The text was substantially modified by Professor David K. Jordan in summer, 2011 to fit it to the needs of MMW-11.
This text may be freely reproduced for non-commercial educational purposes.
Today we can certainly see the benefits of walking bipedally; it is a very efficient means of locomotion that allows us to carry things, run quickly over long distances, and use our hands to gesture wildly while having conversations.
But can we imagine what evolutionary pressures would have led to the evolution of bipedalism? Why was it more beneficial for one of our early ancestral forms to leave the forest canopy (if that is where it happened) to travel along the ground? We still do not know the answer to this question, and it is always rather speculative to “make up” evolutionary advantages to “explain why” a form came into being. Still, here are several ideas that have been proposed by thoughtful and knowledgeable scholars as they tried to imagine why forms ancestral to us would have become bipedal about seven to five million years ago.
1. Walking upright freed early hominid hands to carry tools, infants, or possibly food. Thereafter, an evolutionary advantage would presumably have accrued to increasingly dexterous hands.
While this certainly true, it does not suggest to us why there would have been selective pressure for this change in the first place; bipedalism appears in the fossil record well before stone tools, after all, and modern apes use occasional tools, carry food, and transport infants just fine without standing upright.2. Bipedalism evolved in response to pressures that favored hunting rather than merely scavenging. Perhaps it would have been easier for one of our ancestors to catch prey while running in an upright manner.
However, (1) hunting seems to have emerged in human evolution much later than bipedalism (by several million years) and (2) chimpanzees can be quite accomplished hunters both in the trees and on the ground.3. It may have been seed and nut gathering that drove selection pressure for the evolution of bipedalism. Perhaps it would have been more efficient for an early hominid to gather seeds and nuts from the ground or low-lying branches with free hands.
This hypothesis was drawn from a model of gelada baboons who forage for seeds and nuts while scooting around on their rear ends. If scooting worked for them, why wouldn’t it work for an ancestral hominid? It is also unlikely that bipedalism evolved in a savanna (plains) environment where foraging for seeds and nuts on the ground would have been an efficient form of food procurement.4. Bipedalism evolved to allow more efficient feeding from low-lying branches of trees or perhaps bushes.
An upright posture does make it easier to walk right up to a tree and pull off some fruit. But why would early hominids need to be able to walk up to a tree to get some fruit if they were already living in the trees?5. Bipedalism evolved in order to make it possible for males to carry resources back to females and their dependent offspring. (This is sometimes called the male provisioning hypothesis.)
This hypothesis almost requires that we assume established pair-bonds between individuals, or even monogamy. As far as we know from other indications, monogamy did not emerge until much later in pre-human evolution. (Monogamy in fossil primates is usually estimated based on the degree of sexual dimorphism —average difference in size between adult males and females. (More About Sexual Dimorphism)
Similarly, ethnographic data from living human populations does not support this hypothesis: monogamous pair-bonds may be the ideal form in some known human societies but not in most. Furthermore, in living foraging populations, male provisioning of females and dependent offspring does not in fact provide a significant portion of their calories. On the contrary, among living foragers female gathering activities supply the majority of caloric intake for all members.6. If hominids became bipedal in a savanna environment, then we can imagine that bipedalism could have emerged in response to selection pressures favoring the ability of early hominids to stand up and look over tall savanna grasses to monitor for predators. (This once popular proposal came to be called the visual surveillance hypothesis.) .
Unfortunately, although non-human primates today often stand up on their hind limbs to survey for predators (and prey), they can do this without being routinely bipedal.7. The long-distance walking hypothesis proposes that bipedalism could have resulted from selection pressures for a more efficient means of covering long distances between diminishing food sources more quickly on the savanna after climate change brought the disappearance of the forested environment to which they had been accustomed.
A major problem with this hypothesis is that it may not fit with climate data —see below— but it also makes assumptions about population size and food availability that we simply do not have data to assess.8. Bipedalism evolved in a hot environment as a mechanism for thermoregulation. This more efficient cooling mechanism would have reduced constraints on brain size and may have helped pre-adapt hominids to brain expansion.
The logic underlying this last (8th) hypothesis is particularly interesting and is worth further examination. The argument is that the vertical posture of bipeds (bipedal animals) exposes less surface area of the skin to the sun and means more of the body is further away from heat radiating off the ground, making a more efficient means of cooling the body. Why is that necessary? It obviously is not: most animals are not bipedal. However, if cooling is an effect of bipedalism, then interesting consequences can flow from bipedalism.
We have seen (in essay 6) that the bipedal human birth canal poses constraints on the head size of infants (and vice versa). Similarly, there are constraints on brain size among non-bipedal animals. Some of these stem from the energy use and temperature limitations of the brain. Brains —apparently all brains, but certainly hominid brains— use a lot of energy and produce heat, but they are also very susceptible to damage from too much heat. One of the reasons very high fevers are so dangerous in modern humans is that too high a temperature can cause brain damage. Standing up, so that less of an individual’s body faces upward, it is argued, would have constituted a “radiator” mechanism to dissipate brain-damaging heat while somewhat reducing incoming heat.
This “radiator” mechanism made it possible, the argument goes, for the expansion of the brain in human evolution by giving the body a way to let off some steam.
All of this makes great sense and is generally supported as a consequence of bipedalism but is does tell us why bipedalism emerged in the first place. Instead it requires us to assume that hominids whose anatomy and environment happened to allow them more time standing up received enough of an evolutionary advantage in brain-preservation to “reward” that behavior with more offspring.
Several of these hypotheses (and many more) imagine bipedalism to have emerged in the specific context of particular environments, either forested or savanna. Evidence is still mixed. In the late XXth century, savanna environments were assumed. By the early XXIst century, more and more fossil evidence suggested earlier bipedalism, which our models of paleo-climates suggested would therefore probably have evolved in woodland areas, not savannas. The utility of a more efficient cooling mechanism (hypothesis 8), for example, would be far less obvious in an environment shaded by trees. The need for covering more distance between food sources (hypothesis 7) assumes a climate in transition from forest to savanna. And so on.
The majority of evidence at the present time supports the emergence of bipedalism in woodland areas, and paleo-ecological data suggest that there were more “open woodland” areas in the regions where hominids likely first evolved. This means that we should probably envision neither jungle nor savanna, but rather a world where early hominids spent most of their time in woodlands (and likely in the trees themselves) but where there would have been open areas between trees and patches of forest that may have exerted selection pressure for more efficient means of moving between trees and especially between wooded areas. This slightly increases the plausibility of hypothesis 7. But only slightly.
In other words, hypotheses about the evolutionary advantages of earliest bipedalism are all troubled by challenges from new models of early climates and climate change and from new fossil finds (earlier fossils, fossils from other regions, etc.)
However, a bigger problem for most hypotheses is that they are tend to postulate prime movers. That is to say, these hypotheses generally advance one idea as the single most important factor in the evolution of bipedalism. But evolution is a very complex and dynamic process that is shaped by forces in the environment that are all interacting with each other in myriad ways. It is most likely that there is not one single factor that led to the evolution of bipedalism in our evolutionary past but rather the dynamic interplay of many related factors, possibly (probably?) including some of the ones we just discussed.
While humans are, obviously, biological animals with biological adaptations to the environment around them, we are also cultural animals with culture as our primary adaptation. We already mentioned the relationship between bipedalism and the price we pay in difficult birth and the need to take care of both new mothers and especially new infants. These exemplify the social effects and demands of our biology. Our biology and our capacity for culture and the complex social structure that it supports are not separate from one another. Instead, they interact with and depend on each other in what is coming to be called biocultural evolution.
But there’s more. More efficient food procurement means more food with less effort, which leads to time for other activities —perhaps more time spent tool making or learning by watching others make tools. As our cognitive abilities became more complex, our ability to communicate more efficiently developed and we were capable of and more dependent on social learning. All of these factors interacted with one another driving the increasing complexity of our tools and social relationships. Biologically, we easily imagine natural selection to have favored increased intelligence as cognitive abilities became more central to hominid life. Physically, this would have meant changes in the brain.
Brain size increased dramatically with the emergence of the hominid species called Homo erectus about 1.8 million years ago.(The earlier, African forms are sometimes considered a separate species and are in that case called Homo ergaster.) The archeological record provides evidence of a substantial increase in abilities. Increasingly sophisticated and efficient tools, and possibly symbolic communication emerged. Changes included more and more efficient foraging (gathering vegetable matter) and scavenging (gathering and using material from animals killed by other animals), eventually leading to active hunting (killing animals). (Foraging, scavenging, and hunting are sometimes lumped together as foraging in contrast to food production, i.e., agriculture.) The extent to which earlier hominids actively killed animals as against eating meat found at kill sites of predator animals is still unknown. In the case of Homo ergaster/ erectus, we are confident that a lot of killing went on. Homo erectus, able to hunt and hence to cope with winters in which vegetable food was hard to come by, expanded across Eurasia in addition to Africa.
We now know that bipedalism had been around for millions of years before major brain expansion, sophisticated stone tools, and increasing social complexity. This means we cannot use those later developments to try to understand how bipedalism came to exist. It also raises the question of how bipedalism affected the development of those later changes. The causal relationships among these many factors remain unknown, awaiting your research.
A review quiz is available for this essay.