Human Descent
To many persons the conclusion that man is the naturally modified descendant of apelike ancestors appears to be destructive of the belief in an immortal soul, and in the teachings of Christianity; and accordingly, they either reject Darwinism altogether, or claim for man a special exemption from the mode of origin admitted for other animals. ~ Ray Lankester in 1880
Though the advent of modern humans happened only a few hundreds of thousands of years ago, their descent has a very long history. Homo sapiens is but one leaf of many branches on the tree of primates; a tree with roots dating at least 65 million years, and with the leaves of over 1,000 species from then to now.
Untangling some confusing terminology is merited before proceeding. The 18th-century Linnaeus system of primate taxonomy held to the 1980s. This earlier classification had hominoid subfamilies of hominids (eventuating into humans) and anthropoids (monkeys and apes).
Recent genetic studies put humans, gorillas, and chimps closer to one another than orangutans. So, hominoids were bifurcated into subfamilies: Ponginae (orangutans) and Homininae (humans, gorillas, and chimps (including bonobos)). The Homininae subfamily is further broken down to Hominini (the evolutionary line which led to humans), Panini (chimps), and Gorillini (gorillas).
Hominin refers to the proximate clade descending to humans. A hominid is an ape descendant, some of which became hominin: contributors to the human genome.
Anthropology is the study of humans. Paleoanthropology is the study of ancient humans by the fossil record and other artifacts.
Over 3 dozen hominid species have lived, depending upon judgmental accounting for speciation. Many overlapped in time. New species keep being discovered.
Now only 1 human species is named: Homo sapiens. This supposed singularity comes despite numerous races and sociological barriers to interbreeding; a fact which, by any decent biologist’s definition, comprises a world populated today by numerous Homo subspecies.
Why, then, has evolutionary fate treated ape and man so differently? The one has been left in the obscurity of its native jungle, while the other has been given a glorious exodus leading to the domination of earth, sea, and sky. ~ American paleontologist Henry Osborn
Traditionally, the story of human evolution has cited 4 milestones: terrestriality (from being in the trees to living on the ground); bipedality (walking upright); encephalization (brain expansion relative to body size); and culture (civilization). For a long time, debate about human descent centered on the primacy of milestone: which came first, leading to the other facets. This shallow discourse ignored that none of the traits are unique to humans. Above all, the one thing taken for granted as unquestionable truth was the superiority of man.
Man differs conspicuously from all the other primates in being almost naked. But a few short straggling hairs are found over the greater part of the body in the man, and fine down on that of the woman. ~ Charles Darwin in The Descent of Man
In The Descent of Man (1871) (women merely being along for the ride with their “fine down”) Darwin identified intelligence, manual dexterity, bipedality, and technology as making humans special; further arguing that any ape with even modest endowment of these traits would surely yield a competitive edge over other apes.
According to Darwin, once a hominin emerged with these qualities, competitive natural selection rendered Homo sapiens inevitable. This seductive but sophistic formula held sway among most paleontologists until quite recently and remains the preponderant account among the populace who ponder such. What happened is more mosaic. The path to modern humans owes to several circumstances and coincidences, and remains uncertain in many regards.
That humans descended from apes is likewise not long held. Even in the 1960s the predominant theme in the story of human evolution was that humans diverged from early ape stock, not directly from more-evolved primates. Modern apes and humans were considered an example of parallel evolution. Today’s great apes were conjectured closely related by lineage, whereas humanity was a long-standing clade, with the progenitor originating in the far-distant past.
Species names should be banned as end points in cladograms. Cladograms do not document evolutionary changes in whole animals or species. Instead, they document only changes in the character (usually few) under consideration. ~ American anthropologist Glenn Conroy
From the 1940s into the 1970s the accumulating pile of hominid fossils and stone tools shifted the story. Culture, centered on the butchery of animals with stone tools, became the dominant theme that begat the path of hunter-gatherer to concrete-and-glass entombed office worker.
To tidy the tale, in the 1960s a couple of American paleoanthropologists promulgated what became popular in passing – the single-species hypothesis: that only 1 species of hominin existed at any one time. (Loring Brace & Milford Wolpoff were proponents for human single-species descent.) In step with orthogenesis, human history was supposed a linear lineage of steady steps up the evolutionary ladder.
The rationale behind single-species descent was a rigged rule of ecology: competitive exclusion, which posits that 2 species with selfsame adaptations cannot coexist. Darwin surely would have approved.
Under the single-species silliness culture was a powerful evolutionary push. The notion that multiple cultural species could thrive side by side was inconceivable. Hence, because all hominins are by definition cultural, only 1 hominin species could exist at a time. Once that loop is closed, circular logic looks like a speedway of rationalized rectitude. Using assumption as a crutch, such is inference at its worst.
The single-species myth was shuttered in the mid-1970s. Fossil discoveries in Kenya undisputedly demonstrated coexistence of 2 dramatically different hominids: Homo erectus, an ancestor to Homo sapiens, and Australopithecus boisei, a hominid destined for the extinction dustbin.
This disruption in presumption knocked culture off its pedestal of predominance. In its place arose the concept that some aberrant African ape adopted a decidedly un-apelike lifestyle, evolving from there.
Once one gets over the sophistry that humans are inherently superior, the heritage of the hominid lineage falls into the realm of behavioral ecology, with issues by no means unique from the rest of animate Nature. In this proper view, the origins of human descent are entirely within the context of primate biology.
Human beings are special in many ways – of course we are – but so is every other species. ~ English paleontologist and evolutionary biologist Henry Gee
None of the physiological traits that supposedly typify the human body are novel. Let’s look at a few prominent features.
Neoteny
The genetic basis for humans has not strayed from its ape ancestry. While becoming sexually mature, humans retain many features of juvenile chimps. Neoteny in humans includes a larger ratio of brain to body size, lighter bones, a flattened and broader face, larger eyes, smaller nose, smaller teeth and jaw, and glabrousness (hairlessness). Human limbs and body posture are also indicative: limbs proportionately short compared to torso length, longer leg than arm length, foot structure, and upright stance.
Neoteny in the genitals is seen by lack of a penis bone (baculum) in males, and in females a forward-facing vagina and the presence of a hymen.
Female faces that are more neotenized are most alluring to men, while less neoteny is the least attractive, regardless of age. As women choose a mate largely on concerns other than physicality, neoteny is not as looming an issue in male attractiveness.
Neoteny has been seen in several animal species. It typically results in larger individuals with longer lifespans, as it has with humans. There is a notable exception: birds. Avian evolution included downsizing, partly attained by sticking with the skeletal structure of the young dinosaurs birds descended from.
Neoteny in animals is often indicative of a change in the vector of evolutionary descent. This was the case with salamanders, birds, and humans. Chordates arose over a half-million years ago via neoteny.
Another association with neoteny is a transformation of sociality. Animal domestication has repeatedly resulted in neoteny, as dogs illustrate. Hominin neoteny likely reflected a shift toward comity.
Brain Size
Brain size and cerebral organization evolve independently. ~ American evolutionary anthropologist Lauren Gonzales et al
Even a partial skull fragment can indicate cranial capacity. The question then becomes what to make of relative brain size. It was long thought that brain size was adaptive to cognitive faculty; specifically, sophistication in sociality: more complex social relations spelled a larger brain. Nonsense. With their tiny bird brains, corvids have comparable social complexities, better problem-solving faculties, and superior memory capability than humans.
Woodpeckers live in stable social groups and deal with the politics of social living: competition, manipulation, and deception. The relative brain size of these woodpeckers is significantly smaller than other, less-sociable birds.
The simple fact is that evolutionary biologists do not know why brain size varies in primates. Body and brain size are only roughly related. One study found a much tighter correlation between diet and brain size than any measure of sociality, and that fruit eaters had the biggest brains.
Correlating overall brain size and cognition is pointless, since an increase in brain size will not lead to higher intelligence. ~ German paleontologist Marc Srour
Speculation based upon statistics may be intriguing, but there is no evidence that brain size has anything to do with cognitive faculty. Statistical analyses of modern humans show no correlation between brain size and mental abilities. Little women may be brainier than big brawny men.
1/3rd of the human brain is dedicated to vision. Yet flying insects, with minuscule brains and meager neural connections between the eyes and brain, have much better sight.
One thing the brain is certainly good for is eating energy. The human brain accounts for only 2–3% of body weight, but guzzles 25% of the body’s energy when at rest. By comparison, other apes require only 8% of resting energy.
What is known is that the brain is a physiological showpiece, with activity that is coincidental with mentation, not causal nor even correlative. Organisms without brains are known to have impressive cognitive abilities. The mind-body complex manifests from an energetic gyre: an entangled intricacy barely understood and utterly ignored by the matterists who foolishly believe that the brain conjures the mind.
The brain is not an organ of thinking but an organ of survival, like claws and fangs. ~ Hungarian physiologist Albert Szent-Györgyi
Vocalization
Numerous species are capable of complex sounds. Baboons utter tones resembling the vowels in human speech. Chimps flexibly employ facial expressions, gestures, and vocalizations to express themselves. Still, the human voice box is something special, though not the epitome of sophistication in sound production. Birdsong makes human vocals seem like baby talk.
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We want to find a simple logical progression for primate and human evolution, but evolution was a very messy process. ~ American anthropologist Brenda Benefit
Proconsul
Proconsul was a hominoid that lived 23–5 MYA. There were at least 2 Proconsul species, possibly 4. The species had distinct body sizes. One was as small as a gibbon. Another had the girth of a female gorilla.
The frugivorous Proconsul was built for an arboreal existence. The forest was so dense that the canopy was closed. A Proconsul could travel through the trees without ever touching the ground.
Most of Proconsul‘s features were monkey-like, including its brain and gait. But it had much of a human hand: similar thumb joint and overall proportions, thus able to manipulate small objects precisely – the same as humans. When a chimp tries to pick up a small object, it has to hold the item between its thumb and the side of its hand, rather than between fingertips, as Proconsul did, and as humans do now.
Proconsul was a hominoid offshoot. It is therefore only illustrative of hominid radiation and the numerous adaptations that would result in hominins.
Independent evolution of similar features is a common phenomenon, but it seems to be particularly rampant in primates. ~ Spanish paleontologist David Alba
Pierolapithecus
Weighing around 33 kg, Pierolapithecus lived 13 MYA. Its limbs were different from modern apes. Its hands lacked the long, curved fingers that allowed easy suspension in trees. Its pelvis gave greater stability for moving using its forelimbs. Pierolapithecus illustrates a transition in lifestyle between living in trees and on the ground – the 2 different directions arboreal apes and hominids took.
Pliobates
A gibbon-sized (4–5 kg) hominoid lived 11.6 MYA that had a mosaic of ape and hominid features. Pilobates was a cautious climber, fond of fruit.
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14–8 MYA in Africa apes and hominids evolutionarily parted ways. (The date range owes to a discrepancy between genetic mutation estimates (earlier) and the fossil record (later). Further, that there was a common ancestor is likely a simplification. DNA data indicates that hominids diverged from apes 14 MYA. But genetic molecular clocks are somewhat unreliable in assumptions about mutation rate. Statistical modeling based upon both fossils and genetics points to divergence 8 MYA.) Orangutans were the first to split from the clade that descended to humans, followed by Dryopithecus, an extinct ape; then gorillas, and finally chimpanzees and bonobos.
Dryopithecus lived in Africa and Eurasia ~12.5 MYA, during the Late Miocene. It had a large brain and delayed development. Dryopithecus spent most of its time in the trees, where it ate fruit and soft leaves.
The supposed last common ancestor of apes and hominids was small, weighing some 5.5 kg. Small size continued until the arrival of Homo erectus, long after hominids had taken to living on the ground rather than in the trees.
The snapshots of evolutionary events do not make an altogether coherent movie, but the picture of hominid emergence is fairly clear: progressively erect posture, changes in the brain, loss of body hair, and refinements in manual dexterity.
Hominoids underwent several adaptive radiations, producing a great abundance and variety of species. Climate change greatly reduced hominoid habitat through the Late Miocene and was probably responsible for the drop in the diversity of hominoids. ~ Roger Lewin & Robert Foley
Climatic pulses spurred adaptation as well as foreclosing some species to extinction. As habitats and food resources became more variable, more generalist species emerged, better able to cope with changing conditions. Hence erratic climate was a primary driver of hominid descent.
Climate
Global temperatures over the past 6 million years are known by measuring chemical signatures in the shells of fossilized benthic microorganisms. These changes are due in part to Milankovitch cycles: fluctuations in Earth’s orbit, axis of rotation, and proximity to the Sun. Otherwise climate is a gyre with multiple feedback loops that unwind through time.
It was long thought that both evolution and climate change had glacial time frames, on the order of 10,000 years – wrong on both counts. At times, major shifts from one climatic mode to another have occurred in 10 years.
The descent of hominins transpired over 4 geological epochs: the Miocene (23 to 5.3 MYA) and the Pliocene (5.3 to 2.6 MYA) in the Neogene period, along with the Pleistocene (2.6 to 0.0117 MYA) and the Holocene (11.7 TYA to the present) in the Quaternary period.
The prelude to human emergence was the Oligocene (34–23 MYA), a relatively warm epoch to start but cooling in the close. During the Oligocene came further evolution of flowering plants, including the culmination of coevolution between plants and pollinating animals. The epoch also saw the rapid evolution of and diversification of animals, especially mammals.
The Miocene and epochs to follow witnessed dramatically changing climates, punctuated by ice ages. The Himalayas started their ascent at the end of the Miocene (7–6 MYA), as did a sudden expansion of grasslands globally. Forests fell victim to a generally cooler and drier climate. This continued in the Pliocene (5.33–2.588 MYA), as well as more seasonal weather, similar to the modern climate (before manmade pollution jumped into the climate driver’s seat).
The continents continued to drift. Most tellingly, South America became linked to North America during the Pliocene. This resulted in a cutoff of the warm equatorial ocean currents, thus beginning an Atlantic cooling cycle that rendered the Arctic and Antarctic waters frigid. Water locked up in ice caps created a short-lived land bridge between Asia and Alaska: Beringia. This passage led to animal migrations to the Americas from Asia. The first humans had already come before the land corridor formed, making their way on the water, their sea craft hugging close to the coast.
Pleistocene
Though continental configuration looms large in climatic inclinations, as does biotic influence, Earth has a tendency toward icehouse, as it orbits on the cool side of the planetary habitable zone. (This zone is more formally known as the circumstellar habitable zone, and less formally as the Goldilocks zone. Astronomers sometimes call this ambient range amenable to life the comfort zone, as if it were a cozy stellar sofa for lounging life.)
By the Pleistocene (2.588–0.0117 MYA) the plates upon which the modern continents sit were fairly settled into place. But the climate was not.
Continuing a trend set in the late Pliocene, the Pleistocene was a time of repeated glacial cycles with global impact. Advances of ice depopulated large areas of continents as plants and animals migrated. Climate swings went from a largely ice-free world to almost 1/3rd of the planet’s surface sheeted in ice.
Starting 3 MYA the character of Earth’s climate changed in 2 ways. 1st, the long-term trend of global cooling steepened. 2nd, the amplitude of temperature oscillation increased. Glaciations became more bitter, while interglacial intervals heated up.
During the early Pleistocene, ~2 MYA, the glacial-interglacial cycle was ~40,000 years. 1 MYA this duration lengthened to ~100,000 years. Though Milankovitch cycling was still in play, the reasons for the climate-cycle stretch are not well understood.
Despite rapid extreme climate pulses, the average rates of extinction until approaching the close of Pleistocene were less than 2% – one of the lowest for any epoch. Life had largely acclimatized to fickle climate.
The Last Glacial Maximum
During the Last Glacial Maximum (LGM) (26.5–19.5 TYA) vast ice sheets blanketed much of North America, Europe, and Asia. The greatest extent of glaciation was 22 TYA, though there were regional differences in glacier advance and retreat. At maximum glacial extent, some 30% of Earth’s surface was iced over. Glaciers formed on African mountains. Continental glaciers pushed to the 40th parallel in some places. The north 40th parallel runs near Denver, Philadelphia, Madrid, Ankara, and Beijing.
Because of reduced ocean evaporation, the climate was not only cold but also drier. Forests were fewer, steppe and deserts more extensive.
Massive ice sheets locked water away. Sea levels dropped 130 meters. Continental shelves were exposed, joining land masses together and creating extensive coastal plains.
The world was inhospitable, with frequent storms and an atmosphere laden with dust: 20–25 times dustier than today. This owed to reduced vegetation, stronger global winds, and aridity: less precipitation washing dust out of the sky.
Conversely, interglacials are conducive to life on land, which expanded accordingly. Many remote Pacific islands, including Australia, were settled by humans during the warmer, wetter period preceding the LGM.
Rainfall in Australia declined 90% during the LGM. The Amazon rainforest was bifurcated, with a swath of savanna between them. The tropical rainforests of southeast Asia were similarly affected.
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The planet was jolted from the grip of glaciation by a sudden wobble in Earth’s orbit. This changed Earth’s orientation to the Sun, increasing the sunlight reaching higher latitudes, particularly the polar regions.
Though still in a glacial period, the LGM was followed by wild oscillations in global climate. This partly reflects oceanic releases of carbon dioxide into the atmosphere, the first of which helped trigger the end of the Last Glacial Maximum. The deep Southern Ocean ventilated ancient CO2-rich water. The concentration of atmospheric CO2 rose from 180 parts per million by volume (ppmv) to 280 ppmv from the end of the LGM into the onset of the Holocene 11.7 TYA. Other large carbon releases occurred contemporaneously, perhaps outside the ocean-atmosphere gyre, from sources as yet unidentified.
In the context of European climate, gradual warming from 22 TYA was interrupted by 3 stadials before the Holocene began. A stadial is an extended cold spell of insufficient duration or intensity to be considered a glacial period.
The Oldest Dryas stadial was 18–15 TYA. The term Dryas derives from Dryas octopetala, an artic-alpine angiosperm. The Dryas stadials are so named because the flower’s pollen, found in core samples of glacial ice and peat bogs, are used for dating these periods.
The interstadial after the Oldest Dryas was the Bølling oscillation, termed after a peat sequence discovered at Bølling lake in central Jutland, the peninsula of Denmark. Sea level rose 100 meters from glacial melt during the Bølling oscillation. Temperate forests expanded.
The next stadial was the 100–150-year Older Dryas, centered at its coldest time 14.1 TYA. The Older Dryas had variable frigidity.
Then came the Allerød oscillation, which foreshadowed the climate of the 20th century. Forests prevailed in Eurasia, with more evergreens in the north, and more deciduous trees toward the south. The Allerød oscillation is termed after the town in Denmark where the soil samples used to first identify the climate period were obtained. Comparing the Bølling and the Allerød oscillations: the Bølling was warmer and came on more suddenly.
A cosmic impact ~12.8 TYA in the northern hemisphere provoked the last Dryas stadial. Debris thrown into the atmosphere cooled the climate quickly. Megafauna especially suffered from the rapid climate change.
The Younger Dryas stadial began ~12.8 TYA. Lasting 1,100 years before suddenly shifting into the interglacial Holocene, the Younger Dryas was a period of considerable climactic variability.
The Holocene Extinction
The mass extinction that concluded the Pleistocene killed numerous mammal megafauna in Eurasia, including giant ground sloths, saber-toothed cats, mastodons, and enormous armadillos (glyptodons). Most were gone 130 TYA.
Hominin hunting, helped by increasingly arid and erratic climate in the 450,000-year period prior to the Holocene, did the big beasts in. The ones that survived climate change were hunted to extinction. (This was not the first time that large animals were done in by predators. Giant insects were eaten up by nascent birds during the early Cretaceous. A further decrease in maximum insect size occurred during the Cenozoic when bats arose.) More modestly sized animals took their place. Several smaller terrestrial vertebrates also suffered extinctions, though the percentages are not impressive.
Termed the Holocene extinction, this event is misnamed. Though the slaughter of the last megafauna was only 1,000 years ago, it largely happened long before the cusp of the Holocene epoch.
Dodos probably had a similar intelligence level to pigeons. ~ American anatomist Maria Gold
Killing off animals is a continuing human endeavor. The Dutch took possession of the Indian Ocean island Mauritius in 1598. By 1662 they had wiped out the local large flightless bird that was fearless, and so easy game; whence the phrase: “dead as a dodo.”
This was not the first flightless bird that lost its lease on life to rampaging primates. Moa in New Zealand suffered the same fate in the late 13th century after humans arrived. 1,500 settlers managed to kill off the moa in short order; but nobody says: “dead as a moa.”
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Modern civilization enjoyed a brief interglacial stage; an exceptional time. 90% of the past 500,000 years has been colder. Thanks to the industry of men, the pleasantness won’t last. The world is already rocketing into hothouse.
Bipedalism
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.
Savanna Hypothesis
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.
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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.
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Quick climate fluctuations invoke tolerance adapta-tion: being able to survive in a wider variety of habitats and climates. For hominids, behavioral facets of adaptive tolerance included flexibility in diet, tool use, and clothing.
As hominids lived in groups under taxing environmen-tal conditions, the demands of sociality also drove mental development. As with phenotype, a combination of prob-lem-solving and social pressures fomented mentotypic evolution.
