Humans are the only mammals to habitually walk and run on two legs. One of the million-dollar questions in human origins is why did a quadrupedal ape living at about 6 Mya start to walk bipedally? Four legs are far superior over two for strength, speed, and balance. Also, what kind of selection pressure was needed to evolve adaptations for bipedalism from quadrupedalism? The other question that goes hand in hand with the question of "why" is the question of "when"—when did hominins become habitually bipedal and lose most of their arboreal adaptations?
Several hypotheses have been offered to explain the origin of bipedal-ism, which is also usually considered the wedge that split the LCA into two groups: hominins and the lineage that led to chimpanzees. Many of them explain the maintenance of selection on bipedalism once it evolved but few explain its very origin or the impetus for its evolution.
Energetics may have played a role. Human bipedalism is efficient; it takes less energy for a human to walk upright than it does for a chimpanzee to walk on all fours over the same distance. Perhaps this is because chimpanzees have anatomy that allows them to both travel terrestrially as well as climb trees for fruits, but humans are specialized solely for terrestrial movement. But when chimpanzees are trained to walk bipedally, they do not walk efficiently. They waddle, struggle for balance, and do not have a naturally smooth gait or arm swing. Presumably, this difficulty would be similar to that experienced by the first bipeds. So selection would have had to have been very strong in the early phases of bipedalism in order to overcome the difficulty of teetering on two legs. What could have driven selection to overcome the initial anatomical obstacles?
Thermoregulation, specifically the need to avoid overheating, could have been evolution's reason for bipedalism. This idea argues that it is better to walk upright in the stressful heat of the midday sun on the arid East African savannah than on all fours, because an upright posture means less skin is under the direct glare of the sun's rays. The big human brain is sensitive to heat stress, so such a strategy could have been beneficial. Plus, there are other adaptations dedicated to cooling the body, which indicate its importance, like the increase in sweat glands and the loss of body fur which aid in evaporative cooling of the skin.
Perhaps bipedal walking evolved from a vigilant upright stance used to watch for predators from the trees or while walking between tree patches. The drying of East Africa would have led to increasingly patchy woodlands and this hypothesis is linked to one of the oldest ones known as the "savannah hypothesis" that argues hominins had to move to the ground simply because there were fewer trees on the drying African savannah. However, it is clear now, based on paleoenvironmental reconstructions of the sites where Sahelanthropus, Orrorin, and Ardipitheucus fossils are found, that the earliest hominins, which presumably were somewhat bipedal, were living in forests.
Standing upright freed the hands of their locomotor role and Darwin, Owen Lovejoy, and others have emphasized the role of manipulation in their hypotheses for bipedal evolution. Chimpanzees and gorillas can carry objects in one hand, bracing it against their hip while walking in a "tripod" technique and they can also transport objects by wedging them between their mandible and their chest. But bipedal hominins would have had the use of both hands to make tools and carry them and selection was definitely strong for manipulation since by australopith times it is clear that their hands evolved precision grip capabilities that were better than chimpanzees. The thumb is longer and its joint is more mobile like a human's. Anatomical evidence for bipedalism predates the first stone tools on record by at least 2.5 million years, but the use of nonpreserving materials could have been just as significant (see the section on "Tool Use" in this chapter). Another advantage of free hands would for carrying food away from dangerous areas, like a carcass where scavengers are lurking. Carrying food to a potential mate in exchange for sex or to provision a family has also been offered as a hypothesis for bipedalism.
Bipedal walking could simply be an extension of standing bipedally to feed from small trees or a bipedal posture while feeding in the trees. Chimpanzees and gorillas stand bipedally to feed when they need to and their infants are, in fact, highly bipedal and clamor around on their mothers to keep their balance and, in turn, their mothers will sometimes walk bipedally to carry them.
No matter the cause and the driving force (it could be a combination of all of them to a degree over space and time), the freeing of the hands led to major specializations in the arm, wrist, and fingers for handling objects and the transfer of all the locomotor functions to the legs (aside from the use of the arms to swing by the side) led to major specializations in the entire body for walking and running on only two legs.
Exactly how the transition from arboreality to bipedality occurred (gradual or fast) is still unclear, but dietary and tool use hypotheses are the strongest. Once bipedalism evolved, however, all traces of arboreality did not disappear. Between 6 Mya and 2 Mya, hominins retained many features associated with arboreality like long arms, curved fingers and toes, and short legs. The question is: did the echoes of arboreality remain until 2 Mya because hominins were still arboreal to some degree or does adapting to bipedalism just take a long time?
Although the debate spans the Pliocene hominin record, the Australopithecus afarensis skeleton "Lucy" (AL 288-1) is usually the poster child for both sides of the issue, as is the rest of the species since the fossil record for it currently offers the most available evidence for a transitional hominin form.
Scientists in one camp which includes Owen Lovejoy say that Lucy was a biped just like modern humans. They point to the broad flared ilium (i.e., her basin-like pelvis), the "carrying angle" of the femur (the angle of the thigh bone at the knee), and the location of the foramen magnum under the skull. They argue that the Laetoli footprints (which were probably made by A. afarensis) clearly show a less diverged big toe, a well-developed arch (for storing energy and absorbing shock that is absent in flat-footed apes), and a clear depression for the impact ofheel strike followed by toe-off (a uniquely human gait pattern) (refer back to Figure 3.6).
But those in the opposite camp, like Jack Stern and Randall Susman, argue that A. afarensis retained a heavy reliance on the trees for survival and were therefore skilled climbers. They indicate that Lucy and her species share similarities with climbing and suspensory adaptations in great apes like a funnel-shaped thorax, long curved phalanges on the hands and feet, a climber's shoulder (scapula) anatomy, relatively short legs, and relatively long arms.
With the discovery of more fossils it will be possible to determine whether australopiths remained at least part-time tree-dwellers throughout the Pliocene or not, but it is clear, even as the fossil record stands now, that fully habitual bipedalism, and its entire suite of humanlike locomotor adaptations, arrived by 1.8 Mya with H. erectus.
Aquatic Ape Hypothesis
Every so often the Aquatic Ape Hypothesis (AAH; also called Aquatic Ape Theory) washes up in the news media, but there has only been one address of AAH published in a peer-reviewed scientific journal. The idea that humanity sprung from aquatic past was first put forth by Alister Hardy (1896-1985), a Fellow of the Royal Society and a biological oceanographer. Then author Elaine Morgan took up the idea and has since written several books in support of Hardy's hypothesis.
Supporters of the AAH point out that hominin fossils are found in close proximity to water, and many times near large bodies of water, like the lakes Turkana, Tanganyika, and Victoria. They posit that an aquatic or at least a semi-aquatic ancestry may explain major physical differences between humans and other primates, like swimming abilities, the ability of newborn babies to swim and float, and the relative hairlessness of our bodies like many other aquatic mammals. In fact, the epitomes of humanity, bipedalism, and a large brain, are also explained by the AAH: in order to wade through water, hominins became better bipeds and then feeding on fish and shellfish provided the nutrition to develop big brains.
Although the AAH gets frequent press, it is considered a just-so story by most paleoanthropologists for many reasons. Wading and swimming are not unique to the human condition. If given the opportunity, in the wild or in captivity, many other primates wade (even bipedally) and swim. Lowland gorilla silverback males splash violently with their hands in the swamps in symbolic displays and others comfortably stand and sit to feed on swamp grasses. Japanese macaques are known to relax in natural hot springs. Clearly nonhuman primates are capable of developing hydrophilic behaviors that have not resulted in bipedality, hairlessness, calculus, or poetry. However, this line of evidence can also be used by the AAH supporters, since if the affinity for water is shared by many higher primates, it was probably present in early hominins and could have been a driving force in human evolution.
There certainly are a lot of hominin fossil sites found near water. However, fossils are most often preserved near water because they are rapidly buried in sediment and in most cases water is the transporter of that sediment. Fossil sites without water burial or at least some water interference are rare unless they are found in extreme desert or arctic conditions or in cases of instantaneous burial by volcanic lava or ash. Paleontologists are well aware of the preservation bias and expect fossils to be found near ancient rivers, streams, deltas, and lakes because of the preservation benefits they provide.
Although the AAH is not a strong or mainstream scientific hypothesis, we should not overlook the possibility that ancient hominins dipped, waded, and wallowed to stay cool like we and many other mammals do. --
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