How An Ape Got Up And Walked

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    One question that biologists hear often regarding evolution reflects the fact that many, if not most, lay people do not properly understand the theory of evolution. In the popular imagination the idea of evolution seems to represent a kind of linear growth like that depicted in popular presentations of the evolution of animals, such as horses; in those presentations we see a kind of straight line, a timeline of simple progress, extending from eohippus to the modern equus equus, on which diagram one species follows the preceding one. So the question often arises: If humans evolved from chimpanzees, why do we still have chimpanzees?

    To devise and understand an answer to that question we need to look at the most basic feature that separates hominids from the other Great Apes. If you watch humans and chimpanzees for any length of time, you will notice, aside from the obvious differences in form, that chimpanzees typically walk hunched over on all fours while humans walk upright on what, in any other animal, would be their hind legs. Although chimpanzees can walk upright much as we do, and they do so when they carry things, the chimpanzee's body plan promotes a four-legged stride. Thus the modern theory of evolution contains a proposition stating that humans began to evolve as creatures separate from the chimpanzees when a population of proto-chimpanzees evolved a body shape that made upright walking their primary means of locomotion. Other differences, such as the humans' proportionately longer legs and relative hairlessness, accompany bipedalism as secondary effects. So now we need to know how bipedalism evolved.

    Bipedalism didn't come out of nowhere, it didn't just happen, so what, then, made us bipedal? How and when did waddling chimpanzees become striding hominids? Scientists have devised over a dozen hypotheses in their efforts to answer those questions, but I want to consider only those that connect strongly to natural selection. We know that bipedalism evolved long before the large human brain did and long before the invention of stone tools, so we only want to consider hypotheses that involve selective influences that act at the purely animal level. We must also note that the different hypotheses do not necessarily exclude each other: a number of mutations and selective forces may have acted together to evolve human bipedalism.

    We begin with a creature that has the possibility of evolving upright walking. According to paleontologists' interpretation of the fossils Tribe Hominini began to split into the genera Homo (humans) and Pan (chimpanzees) about 6 million years ago. Although they spend a substantial fraction of their time on the ground, apes also climb trees and spend a lot of time in them, so they evolved the ability to stand upright and to use their hands to grasp things. (Note that cats never evolved that ability, even though they climb trees. Cats evolved retractable claws that enable them to grip the bark of trees.) So now we want to figure out how a semi-arboreal creature left the trees and evolved the ability to walk upright and why some did not. Actually, we find that our remote ancestors didn't leave the trees; the trees left them.

    When we contemplate the grand waltz of mutation and natural selection we often forget that it occurs on a vastly wide and ever-changing dance floor. Even if we restrict our attention to Africa, we find room enough and time enough for an ape to become human. We find time for great environmental changes to come over the land and change the creatures that live there. At one time forests, with rivers and lakes, covered the Sahara, now one of the driest, most barren deserts on Earth. At one time thick forest covered all of Africa; now it doesn't. How did the changes implicit in that statement lead to the evolution of a bipedal ape?

    To conform to the standard theory of evolution, our explanation of how tree-dwelling apes became bipedal hominids must assert a genetic basis for the change. More specifically, we need to know what factors promoted the mutations that changed the semi-arboreal ape into the peripatetic hominid. Ideally we want to find a feature of the environment that makes a life-or-death difference between the mutated and unmutated apes. A lack of trees constitutes that feature and that feature came to East Africa as a consequence of a significant change in climate - the Ice Ages.

    To see how climate change came into play consider the Pliocene epoch - a time 5.33 to 2.6 million years ago. In that epoch the Isthmus of Panama formed, blocking currents flowing between the Atlantic and Pacific Oceans, and the climate consequently cooled and dried out. The movement of Earth's tectonic plates completed the formation of the isthmus about three million years ago and triggered ice ages, leading to drought in East Africa. But the process had been well begun by 15 million years ago, as the Caribbean Plate slid northeastward between North and South America, followed by island volcanoes and the shallowing of the seafloor by tectonic uplift beginning the process of interfering with the ocean currents between the Pacific and the Atlantic. We see the effect that such interference with inter-ocean heat flow had in the facts that the ice sheet on Antarctica had already begun growing 20 million years ago and we entered the deeper part of the current series of Ice Ages 2.58 million years ago.

    Further, over the last ten million years the Himalaya Mountains, raised by India's collision with South Asia, have caught so much rain that they have drawn down the carbon dioxide in Earth's atmosphere by converting it into carbonates as the rain eroded rocks, drawing down enough to cause a significant climatic change. The collision began about 55 million years ago, closing the Tethys Sea and then raising the mountains, which created the famous monsoon.

    Rearrangement of heat flow in the oceans and the cooling of Earth led to changes in rainfall patterns. Areas that once received enough rain to support a forest no longer did and the forest slowly passed away. But that process did not necessarily result in a desert; rather, it led to an environment to which apes could adapt. About seven million years ago something new spread across the deforested area of East Africa - wide grasslands. As forests died back away from some areas, those areas filled in with grass. Over wide enough areas tall grass became the primary plant cover of prairies, steppes, and savannahs. And on the savannahs of East Africa apes became human.

    Consider the interaction between a population of proto-chimpanzees and their environment millions of years ago. In particular consider what happened to the proto-chimpanzees when the onset of the Ice Ages caused the environment to change. In East Africa the onset of drier conditions severely reduced the amount of wooded habitats available to the apes.

    During this period, when the forests thinned, the proto-chimpanzees had to adapt to an environment which had become more like the liminal forest-savannah mosaic zones of equatorial Africa.

    Of course, those proto-chimpanzees who found themselves in a thinning forest would have tried to migrate into still-thick forest. They would not have succeeded. Like us, chimpanzees are viciously territorial and for good reason: territory correlates with food. To protect their food supply the forest proto-chimpanzees would certainly have fought to repel the invaders coming from the growing savannah. Driven back into the savannah, those invaders would have had to adapt to the new environment. Among the first bipedal apes to come out of such adaptation we find those in Genus Ardipithecus.

    Ardipithecus existed 5.8 to 4.4 million years ago, showing us the first split of a hominid from the common ancestor of Homo and Pan. The toe structure found in the fossils provides evidence that Ardipithecus ramidus walked upright much as we do. Paleontologists have found fossil evidence, in the Middle Awash area of Ethiopia, indicating that Ardipithecus ramidus and Ardipithecus kadabba lived in a mosaic of forest and savannah that had an array of lakes, swamps, and springs to provide water. Although ardipithecines likely preferred the forest, they had to cross wide spans of grassland to get from one patch of forest to the next. In that mosaic we envision the environment that led knuckle-walking apes to evolve into purely bipedal ape-folk.

    That mixture of savannah and scattered forests forced proto-chimpanzees to travel between clusters of trees, so in order to remain effective in gathering food the apes had to travel relatively long distances over land covered with tall grass. In that circumstance we see a factor that had an intimate relation with the difference between life and death, just what natural selection needs in order to promote one trait over another.

    Tall grass created a terrible risk for proto-chimpanzees. Knuckle walking through the grass could result in the apes blundering into predators. Encounters with leopards generally don't end well for chimpanzees. But walking upright enables the apes to see farther, to look over the top of the grass and thus see danger before wandering into it. Over tens of thousands of generations, over hundreds of thousands of years, those facts would promote any mutations that enabled the apes to walk upright as a normal means of locomotion. Those apes who walked upright most of the time would more likely survive long enough to reproduce themselves than would those apes who spent less time walking upright. Those mutations, the changes in the relations between the thighbone and the hipbone, the longer legs enabling a longer stride, etc., lead to the more energy efficient locomotion permitted by bipedalism, which would have given the creature an additional advantage. Thus proto-chimpanzees on the savannah became Aridipithecus.

    Australopithecus (the australopiths), the next stage in hominid evolution, existed 3.9 to 3.0 million years ago. Footprints found in a deposit of volcanic ash at Laetoli in Tanzania, footprints that two individuals left behind as they walked across that ash 3.7 million years ago, offer proof that australopithecines also walked upright just as we do, though not quite as well. Fossil anatomy shows that Australopithecus afarensis had features of the hand and shoulder similar to those of the chimpanzee, which indicates hanging arms. Also, the Australopithecus hip and hind limb clearly indicate bipedalism, but these fossils also indicate very inefficient locomotive movement when compared to humans. Then, about three million years ago, Australopithecus began to evolve into the first species of Genus Homo when another factor came into play.

    That factor also had the intimate relationship with life-or-death situations that evolution requires. Some scholars distinguish between adaptations for bipedal walking and adaptations for bipedal running, which they assume came long after our ancestors became bipedal. Actually our version of bipedalism has its greatest advantage in our style of running, which no chimpanzee can match. They may run faster, when they get down on all fours, but they don't have the endurance. If a human has enough of a head start, no predator can catch him: the predator "runs out of gas" long before the human does. Note that lions do not run marathons. So how does that factor work?

    An old joke has two hikers on a mountain trail seeing an enraged bear charging down the trail at them. One hiker quickly sheds his backpack and his heavy hiking boots.

    "Surely you don't think you can outrun the bear!" his astonished companion says.

    "I don't have to outrun the bear," the first hiker says. Then he takes off running down the trail, calling over his shoulder, "I only have to outrun you."

    That joke tells us how early hominids evolved the ability to outrun predators. Certainly the proto-humans who lived on the savannah had to have the necessary mutations to evolve into runners. Those mutations gave some individuals slightly longer legs, reshaped hips, and arches in their feet, among other mutations. Another mutation that promoted running consists of a shortening of the toes: If our toes had remained as long as our fingers, then running would sprain or break them. In any event in which a predator went after a troop of proto-humans those individuals who lacked those mutations had the greater probability of falling victim to the predator. Over time that reduction in the proportion of people who lacked the necessary mutations led to the mutations spreading throughout the population, thereby producing a permanent change in the proto-humans.

    Do we have any evidence to support that hypothesis? Certainly we have fossils, which show how the bones of hominids changed over time. And since we are the last species of hominid left on Earth, we can assume reasonably that some of those fossils reflect our own evolution. But can we come up with something a little more direct? Consider how we cope with excess heat.

    DNA analysis tells us that three million years ago gorilla lice became human pubic lice, distinct from head lice. That fact implies that our ancestors had lost their body hair at that time and began sweating to cool ourselves. That fact, in turn, implies that hominids of Genus Australopithecus had begun to do something that generates large amounts of heat, amounts so large as to justify the sacrifice of some liquid.

    That inference implies our unique ability to run for long stretches of time and to do so in the heat of mid-day. Other animals can run ten minutes or less, so we became hunters who simply chased prey until they overheated and could no longer run from us. Of course, no hominid set out to become a running hunter: instead, the ability to run likely evolved as the ability to outrun predators.

    But some people hypothesize that our relative hairlessness caused our bipedalism. By standing up to walk, they argue, our hominid ancestors gained access to breezes and cooler air that would take excess heat away from them. Those individuals who had the least hair and the most sweat glands would lose heat fastest and, thus, could act more vigorously in the heat of the savannah. But our ancestors didn't lose their hair so that they could stand up and walk. The DNA analysis of lice implies that hominids evolved bipedalism before they became hairless. Those hominids could walk without generating so much heat that they could not dissipate the excess by panting, as chimpanzees do and as, to a lesser extent, we do. Only when our ancestors became runners did hairlessness offer an advantage significant enough to engage natural selection. Over the millenia, as chimpanzees watched from their refuges in the forests, our ancestors suffered the process that one writer calls Darwinnowing, changing them drastically and turning them into a species greatly different from the relatively unchanged chimpanzees.

    Then, about 1.8 million years ago, one species those runners evolved into the species Homo erectus. And from that time the evolution of humans involved less an improvement in locomotion and involved more the enlargement and elaboration of the human brain.

    Thus we see why our species evolved from a creature similar to a chimpanzee and yet we still find chimpanzees in the world. The theory of evolution tells us that when an environment suffers little or no change the creatures that live in it also do not change in any significant way. That fact tells us that the proto-chimpanzees who originally lived in the forests of Africa have changed little in the past six million years, having lived in the forests continuously and become modern chimpanzees. But when an environment undergoes a significant change, those creatures that survive the change will adapt to it. So when Africa began to dry out and the forests shrank, certain groups of proto-chimpanzees attempted to move into the forests, only to be repelled by the troops of proto-chimpanzees that already lived there. The losers in those contests had to live on the savannah, where they eventually evolved into humans, while the winners, remaining in the forests, evolved into the modern chimpanzees.

    In that picture we may see a faint reflection of one of the subtexts that we find in H.G. Wells' novel "The Time Machine". Wells' Time Traveler goes nearly a million years into the future and finds that Homo sapiens has evolved into two separate species: today's winners, the rich, have evolved into the indolent Eloi, living in lavish palaces and parklands, while today's losers, the working class and the poor, have evolved into the hypercompetent Morlocks, who creep out of their subterranean factory-homes at night to kidnap Eloi and eat them.

Appendix:

Why Humans Have Protruding Noses

    What makes evolution complicated lies in the fact that natural selection does not change just one feature of any given life-form. As an example consider the fact that evolving the ability to walk upright also gave our ancestors noses that protrude from the face. Why didn't we keep the noses that lie flush against the plane of the face that our Great Ape ancestors had and that the other Great Apes still have?

    First, notice the tacit assumption that we just made. We have implicitly assumed that the protruding human nose evolved along with the human ability to walk upright and, thus, had the same cause. But the truth of the matter gives us an excellent example of the subtleties that can arise in the theory of evolution.

    We need to ask whether some aspect of upright walking promoted mutations that made the nose grow outward and downward to form the little tent-like protrusion that covers the nostrils or whether the environment in which our hominid ancestors evolved had a more direct effect on the evolution of the nose. To answer that question we need to consider how the environment in which hominids evolved differed from the environments occupied by the other Great Apes. We notice that, unlike the other Great Apes, which continued to live in tropical forests, hominids evolved in the dry, dusty savannahs that spread into formerly forested areas as Africa dried out during the Ice Ages. We can see that in that savannah environment the protruding nose might confer one or more of several possible advantages:

1. Smell Detector

    The elongated nose gave hominids an improved sense of smell. We can dismiss this possibility on the basis that other apes smell as well as we do.

2. Sweat Shed

    Shedding perspiration so it won't get inhaled. We perspire most copiously when we run, but we breath through our mouths when we run, so this would not offer an advantage. Further, if some perspiration gets into our nose, a quick snort will expel it.

3. Air conditioning

    In the dry savannahs that our ancestors occupied the extra length of moist tissue helps in pre-moistening the person's inhaled air before it hits the lungs. However, we cannot see clearly the advantage gained by adding less than half an inch to a roughly eight-inch passage.

4. Parasol

    It shades the interior of the nose from sunburn. A sunburn on one of the most sensitive tissues of the body would promote infection, so we might expect anything that reduces the incidence of such sunburns would get promoted by natural selection.

5. Dust filter

    Moist nose hairs and membranes act as air filters. In the protruding nose we find hairs that carry a sticky moist substance that traps dust and viruses, becoming thereby the unlovely substance vulgarly called snot. Snot production in dusty environments is more copious than it is in clean environments. Dust includes bacteria and viruses, so this mutation helps to maintain the individual's health. In a dusty environment that feature gives an ape a selective advantage over the flat-nosed apes.

    Note that we don't have to pick only one factor that promotes the growth of the nose. Benefits that individually would do effectively nothing, can act together to harness selection.

    What does the elongated nose do for us? We notice that, unlike the other great apes, hominids evolved in the dry, dusty savannah, where they endured greater exposure to direct sunlight. In the damp forests apes don't need air filters or nasal parasols, which partly explains why apes lost the lemurs' snout. On the African savannah that latter factor would have played on the natural differences among people. Those individuals who had a slight overhang on their noses would have gotten less sunburn on their delicate nasal tissues, which would have filtered out more dust, viruses, and bacteria, and, thus, would have suffered less exposure to infection. Natural selection would have worked on that difference and over time created humans who each came equipped with their own fleshy parasol and snot factory.

    My own experience of the world gives me probable cause to accept that hypothesis as true to Reality. For a long time I lived in a dusty environment that obliged me to used a facial tissue, sometimes several times a day, to clean out my nose. When I moved to a less dusty environment I noticed a substantial reduction in snot production and in my use of the tissues.

    So how can we validate this hypothesis? The nose itself doesn't fossilize, but the nasal bones, though fragile, can. We simply need enough hominid skulls from the fossil record to see when the nose evolved. As they evolved, the nasal bone and the nasal process (the projection from the skull on which the nasal bone rests) would have grown in such a way that they elongated and swung upward and outward. Our remote ancestors began with the flat-against-the-face nose of the chimpanzee. At about the time some of them began to develop improved ability to walk upright or shortly thereafter their descendants would have begun showing noses that protruded a little bit and then more and more as the savannah lifestyle reshaped the hominid body. A look at the fossils would validate or refute that hypothesis.

    If we examine hominid skulls, we see that in the older examples, as with chimpanzees, we can run a finger upward over the nasal opening and encounter no resistance. The edges of the nasal opening lie flat relative to the surrounding bone. But beginning with Homo erectus, which originated in Africa about two million years ago, we find the same maneuver stymied by the bone at the top of the nasal opening flexing outward.

    Australopithecus, which existed four million to two million years ago, does not show the indication of a protruding nose. These creatures looked very much like modern bonobos and likely lived in a similar environment. We can infer, tentatively, that they had to begin running in earnest when the savannahs became even wider, and therefore even drier and dustier, than they had been when Australopithecus first evolved. Thus bipedal running and protruding noses evolved more or less together, but not as one consequent to the other. Both represent, rather, separate adaptations of an upright walking ape to life on a wide, open savannah.

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