Walking upright on 2 legs is the trait that defines the hominid lineage. ~ American anthropologist Erin Wayman
Anthropologists make a big deal about humans being bipedal, but it really is not hard at all for apes. ~ Dutch primatologist Frans de Waal
If you were living in Africa 3 million years ago without fire, without structures, and without any means of defense, you’d better be able get up in a tree when the Sun goes down. ~ American anthropologist Jeremy DeSilva
Paleoanthropologists have long argued – often contentiously – about the climbing abilities of early hominins, and whether a foot adapted to terrestrial bipedalism constrained regular access to trees. However, some modern humans climb tall trees routinely in pursuit of honey, fruit, and game, often without the aid of tools or support systems. A more excursive calf muscle facilitates climbing with a bipedally-adapted ankle and foot by positioning the climber closer to the tree, and it might be among the mechanisms that allow hunter-gatherers to access the canopy safely. As there is no skeletal correlate for this observed behavior, this implies that derived aspects of the hominin ankle associated with bipedalism remain compatible with vertical climbing and arboreal resource acquisition. This challenges the persistent arboreal-terrestrial dichotomy that has informed behavioral reconstructions of fossil hominins. ~ American anthropologist Nathaniel Dominy, American evolutionary anthropologist Vivek Venkataraman, & American ethnologist Thomas Kraft
Motility is a sometime driver of evolution. Being able to migrate considerable distances was a factor in the survival of nascent hominid species, owing to the climatic turbulences of the past few million years. For one, food resources became more dispersed.
The modern human brain has had incremental adaptations from even its most recent ancestor, Cro-Magnon; noticeable changes in the past few tens of thousands of years. Bipedalism, by contrast, was old news 4 million years ago. Dinosaurs and birds were on 2 legs long before hominids decided to get off all fours.
Bipedalism and the brain are physiologically related. They both adaptively adjusted to walking upright. Hands, arms, and shoulders adapted to better manipularity as an outcome of standing upright. For instance, the ability to throw objects improved 2 MYA.
A long-held savanna hypothesis suggested that change in vegetation in sub-Saharan east Africa – the hominin cradle – evoked bipedal adaptation. This supposedly transpired during a time when forests dwindled and savanna grasslands flourished, owing to shifts in the African plate rift.
Hominid fossils of the same species have been found in grasslands and woodlands, wet and dry lands. Most damning, the biome of sub-Saharan east Africa was always one of woodlands separated by savanna, though the grasses which grew changed over millennia. Though efficiently covering ground was an impetus for bipedality, the savanna hypothesis is bunk.
To understand the origins of human bipedalism, scientists should stop assuming a ‘chimpanzee starting point’. ~ Spanish paleoanthropologist Sergio Almécija
Beyond new vegetation patterns from climate change, terrain shifts were a likely ecological push to bipedalism. The rugged landscapes of east and south Africa were shaped during the Pliocene epoch by geophysical events: tectonic pulses and volcanism. The rocky outcroppings and gorges that emerged opened new foraging opportunities and shelter. Faster movement over ground would have been adaptively advantageous, especially considering the changing climate of the times.
Terrain and climate may not have been the only drivers to getting up on 2 feet. The mating systems of apes involve intense male competition involving force or its threat. Male apes often fight from a bipedal posture, using their forelimbs to strike with more power than otherwise possible. For hominid males at least, standing tall might have been sexually advantageous.
While striding on 2 legs was significant phenotypically, an equally important development was a contemporaneous improvement in manual dexterity: as much an adaptation of mind-body coordination as bodily transformation, driven by augmented tool use. The changes in the hominid hand were physically trivial compared to the alterations required for bipedalism. In evolutionary time both developments were rapidly achieved.
Ambulation is multifaceted. Variations are possible using the same basic technique. Modern chimps and gorillas knuckle-walk differently.
Hominids did not go through a knuckle-walking stage. Instead, they went from a largely arboreal to a ground-based lifestyle as bipedals.
Adaptation to walking upright resulted in considerable energy efficiency. Chimpanzees consume 75% more energy walking, whether on 2 legs or 4. This owes mainly to hominids walking on relatively straight legs.
Trees were somewhat reluctantly abandoned. Ardipithecus hominids were still regularly climbing trees 3.4 MYA.
Adapting to walking upright was not a singular progression. In the evolution of bipedalism, different hominids had distinct gaits.
Hominids were unusual in lacking fur, though other heat-adapted mammals, including elephants, lack much body hair. This improves thermal conductance.
There is also a thermoregulatory advantage to bipedalism. The human brain is increasingly impaired by prolonged exposure to heat above 37 °C. Temperatures as low as 40.5 °C can sometimes prove fatal.
Many arid-adapted African mammals have an ingenious cooling system via venous blood evaporative cooling within the nasal cavity. This is enhanced by a mesh of blood vessels (carotid rete) near the base of the cranium, which creates a countercurrent thermal exchange mechanism for optimizing brain temperature.
Early hominids had small nasal passages, limiting the potential for nasal cooling. Further, the carotid rete is absent in higher primates. Localized endothermic regulation potential would have been much less than in other mammals. Hominids were more dependent upon whole-body cooling to avoid damaging elevations of brain temperature.
Bipedality offers 2 endothermic advantages: 1) reducing the surface area of the body exposed to solar radiation and 2) placing more of the body’s surface higher, where airflow is more favorable for convective heat loss. This reduces reliance on evaporative cooling (such as sweating), which requires greater water intake.
The height advantage of standing tall is considerable. It improves both distance vision and the ability to communicate by signaling over greater distances. This may have hastened social evolution in species with males prone to provocation.
While the advantages of bipedality were manifold, walking upright was an evolutionary innovation with a price. Bipedality meant adaptations in the toes, feet, knees, hips, pelvis, spine, shoulder, and bone structure. The burdens of bipedality plague us to this day. Back pain is common, as are aching feet, and not just from wearing shoes.
Bipedalism affects the entire skeletal structure. Rotator cuff injuries in the shoulder are more easily had from walking upright.
Added load on 2 feet required knee and hip joints to expand: more surface area to absorb the footfall forces of walking and running. These joints, and vertebrate, evolved to be bigger by enlarging the spongy inner bone and hardening the outer bone. The result: less dense bones than other primates. Apes lose bone mass as they age, but elderly apes do not suffer fractures like humans because their bones were much denser to begin with. Lifestyle can compensate. Vigorous exercise during youth creates more apelike bones.
Walking upright was instrumental in shaping hominid cognition. Sustained ambulation – walking and/or running – is one of the best mental exercises.
Bipedality required a substantial number of sensory and motor-control modifications, such as changes in visual perception, as well as freeing the forelimbs for different functionality.
Walking on 2 legs also altered brain development. Pelvic changes needed for bipedality meant a narrower birth canal, just when babies’ heads were getting bigger. This delayed much brain growth until after birth, increasing altriciality and prolonging childhood. In contrast, chimpanzees’ brains largely develop in the womb.
Narrowing the hips to facilitate bipedality made giving birth difficult, partly from the need to rotate the baby in the birth canal. It seems that prenatal growth reached its size limit. To develop as far as chimps do in the uterus, human pregnancy would last 21 months.
Instead, human infants are precariously helpless at birth compared with baby apes. Their survival depends utterly upon continuous maternal care, which if not immediately forthcoming yields outrageously loud curdling cries. Such behavior in the wild could attract predators. In similar need, young monkeys remain quiet. One can only suppose that by the time this habit evolved, desertion was a greater threat to a neonate than predation.
In their inconsolable screaming, starvation is not as much the issue as bonding, which breast-feeding stimulates through hormone production. Human infants have layers of white fat in far greater quantities than any other primate; blubber which may serve as an energy reserve as well as giving an adorable roundedness to the baby body. Baby fat also provides bodily protection from bumps and falls.