Bipedality
The main goals in any evolutionary game are to eat, stay alive, and reproduce. The edge was not speed because most four-legged animals move faster than humans. It was not efficiency because moving on two legs uses no more nor less energy than moving on four legs unless one is traveling a long distance. If this is the case, the number of calories that are expended for travel are lower in a biped than in a chimp walking on four legs or an antelope on four legs.
Consider the following insights about the relationships between male and female chimpanzees:
Two chimps, one male and one female, were fed food from a chute to see who ate first. Over a 32 day test, the male was there first for 18 days while the female was there for 14. The results were that a 44% to 56% differential existed. Why was this so? One would expect that the male, who is larger in size, would be there 100% of the time. Let us consider when the female ate first. When the female was at maximum genital swelling, when she was sexually receptive, she ate first on a regular basis. There are implications for this. If the period of sexual interest is, by implication, an extra-natural phase for women (for it makes them act dominant when they are really naturally subordinate or a male gives way because of the signals given off by the female), is there a lesson here? It does looks like female chimps spend about 14 days out of every 32 in toils to the male anyway where they got to the food first is almost a secondary part of the relationship during these 14 days. During estrus, the female advertises - call me sexually interested - for that period. Is there small consolation to getting food first during those 14 days? Unfortunately, we can not ask the chimps. There is a second question to ask ourselves. Is the female simply hungrier during her period of sexual receptiveness and, therefore, wants the food more than the male? The female could gesture a sexual invitation and put the male in a place where he has to choose between food or sex. In this sense the male is exchanging sexual access for food, a form of prostitution. As we can see from other studies of chimpanzees may simply be enjoying life in the process of this exchange. This is particularly the lesson of studies of Bonobo chimps who appear to trade sexual access for food. The Bonobos actually go beyond this to participate in sexual promiscuity as a means to develop bonds between members of a troop. They almost do away with the notion of dominance by doing this.
Also consider the following features of anatomy that are crucial to understanding some of the physical changes that were necessary to enable ancestral forms walk upright on two legs. These go beyond the changes in the pelvis (hip) that are quite obvious.
(1) The femur of a chimpanzee is built for climbing and that of a human for walking. The femur is our long leg bone that articulates between the hip and the knee. The femur of a chimp cannot withstand stress. There is a dense ring of bone around the outside of the femur that is suited to absorb the force from compression but not tension. The human femur has a substantially longer femur neck. The outer layer of dense bone of a chimp is unevenly distributed in the human femur and thins at the top. Longer femur necks are crucial to walking upright as well. Humans have longer femur necks. This enables muscle attachments to stabilize the hip while one leg swings forward. As soon as one leg lifts off the ground, the muscles fire and press the femur neck into the hip socket. This action relieves the tension caused by weight bearing down on the top of the neck and focuses stress along the bottom of the neck where bone density is maximized. The spongy bone inside the neck absorbs shock that can reach the force equal to three times the body weight on the leg as it supports the body. The neck bone of a Homo Erectus femur is actually longer that ours today.
(2) A human heel is four times the size of a gorilla heel and twice that of a chimpanzee. This is because it contains a lot of spongy bone that absorbs the shock of walking. Besides being smaller than a human heel, a chimp heel has a thicker coating of bone around the outside. The Australopithecus Afarensis heel has only a thin outer layer like that a modern human heel and is partly inflated with spongy bone.
An Overview to Bipedality Origins by Linda Trujillo
Bipedality, the ability to walk upright using only the lower limbs, is the most ancient of all characteristics that distinguish humans from apes. Other primates are basically quadrupedal, but hominids walk on two limbs instead of all four. Bipedality deprives us of speed and agility and all but eliminates our ability to easily climb trees. We must propel our body with two limbs instead of four, and we are generally unable to use the hip and back as parts of our propulsive muscle system.
A terrestrial animal must propel itself by applying a force against the ground in a direction opposite to the direction of travel. In the quadrupedal posture found in most primates, the center of mass lies well in front of the lower limbs, thereby increasing the efficiency of forward movement. However, the center of mass for bipeds is positioned almost directly over foot and so a repositioning of the center of mass is necessary to initiate forward movement.
The pelvic features of a biped reflect the very different mechanics of two-and four-legged locomotion. Muscle groups that function to maneuver and propel quadrupeds, function to stabilize and balance the bipeds' upright torso. An example of how certain muscle groups function differently is demonstrated by the gluteus maximus, a relatively minor muscle in the chimpanzee, but the largest muscle in the human body where it serves as the major propulsive muscle in upright walking.
In addition, the knee is adapted for withstanding greater stress during complete extension than the knee of other primates which is unable to fully extend or lock. The knee is designed in a way that brings the femur and the tibia together so that the foot can be easily planted directly under the body's center of mass and thereby support the entire body weight as it shifts from leg to leg. The ankle is also modified for supporting the entire body weight, and a shock-absorbing arch helps the foot to cope with this added load. In addition, the big toe is no longer opposable, as it is in quadrupeds who use it for grasping and climbing. The big toe in bipeds is in-line with the other toes and serves as a propulsive lever for upright walking.
A long period of skeletal readjustment of bones and muscles must have taken place over many generations to yield a descendant that walked with an efficient bipedal gait. Unfortunately, there are currently no fossils to document how or why this trait was acquired. All we know is that upright walking was skeletally well established by the time of Australopithecus afarensis ("Lucy"), approximately 3.1 million years ago.
During its earliest evolution, bipedality is thought to have been a very costly form of locomotion. However, a reproductive advantage must have fallen to those in each generation that walked more frequently in a bipedal posture. Many theories have been developed to try and explain the evolutionary success of bipedality. These theories include:
Owen Lovejoy's theory, which suggests that bipedality freed the hands to carry food and allowed males to provision female(s) and offspring. According to this theory, reproductive success would be increased because the female(s) and offspring could be secluded in greater safety. In addition, the female(s) would be better able to manage multiple or overlapping offspring.
Another biologically based theory suggests that bipedality provided the possibility of improved efficiency of travel by modification of hind/lower limbs only. This left the more ape-like arms and hands free for arboreal feeding. Because the food was found in widely scattered pockets of open woodland rather than densely packed forest, bipedality would allow more efficient movement between these food bearing sources. In addition, the bipedal stance would provide a wider view of the surrounding countryside and enable earlier detection of potential predators.
Pat Shipman's theory suggests that man became a savannah scavenger, locating animals already killed by a predator, chasing off the predator, and carrying away the usable parts of the kill. Such activity would require endurance for long periods of time necessary to search for food. In addition, the bipedal stance would again elevate the reposition of the head atop the spinal column and above the shoulders, which would make seeing potential food, or danger, from a distance not only easier, but safer.
A Good Brain Is Hard to Cool by Dean Falk
(Natural History, 8/93, page 65)
Even a rise of several degrees in body temperature, from 98.6° to 107° F, poses a special threat to the human brain, leading rapidly to convulsions, hallucinations, permanent neural damage, and sometimes death. Because of its large size, the brain itself generates lots of heat, which can accumulate to dangerous levels if not dissipated. Fortunately, like the engine of a car, the brain has a radiator to protect it from overheating - a network of tiny veins that originate in the scalp and face. Although blood passing through these veins can drain directly through vessels outside the skull, it can also travel into the braincase (through "emissary veins") and join the blood draining from the brain. This process helps cool the brain.
When a person exercises, for example, his or her face will flush as arteries dilate to bring more blood near the skin surface, which is cooled by the evaporation of sweat. Much of this extra blood, now cooled, enters the emissary veins and is delivered into the meningeal veins and sinuses of the dura mater, which covers the brain. From there, some of the blood flows to veins within the surface of the brain itself. As it circulates, the blood removes heat from all these regions. By the time it joins other venous blood leaving the skull, it is wanner than the oxygenated, arterial blood that supplies the brain. This venous network does not have valves, allowing the blood to move freely as required.
Unlike modern humans, apes lack a venous radiator. The evolution of bipedalism necessitated changes in blood circulation and may have laid the groundwork for its emergence. An examination of fossil skulls, however, reveals that a venous radiator did not arise immediately. It is absent in the earliest-known hominid (Australopithecus afarensis) and in some of its successors, the "robust" australopithecines. Brain size remained conservative in these species. But a prototype radiator is present in the "gracile" australopithecines. This is most evident in the greater number of holes in the skull, called emissary foramina, to accommodate emissary veins. I therefore conclude that the gracile australopithecines gave rise to our own genus.
Beginning about two million years ago, the number of emissary foramina in the genus Homo began to increase dramatically.
Brain size also began to increase rapidly in this group, and these trends continued in tandem until the time of the Neanderthals (about 100,000 years ago). The evolving venous radiator apparently removed a major constraint on the increase of brain size.
Humans are not the only large-brained animals that evolved cranial radiators. Some fifty million years ago, the terrestrial ancestors of whales (including dolphins) began a major change in posture and locomotion that culminated in their becoming exclusively adapted to an aquatic habitat. Like humans, whales have evolved large, highly convoluted brains that generate potentially damaging heat. But unlike humans (and other terrestrial mammals), whales do not receive blood for their brains from the internal carotid and vertebral arteries. Instead, their entire blood supply to the brain comes through an elaborate body of convoluted vessels, called a rete, that originates in the vessels of the thorax and spine. Recently, Tom J. Ford, of Boston College, and Scott D. Kraus, of the New England Aquarium, discovered that in at least three species of whales (right, bowhead, and pygmy right) another rete exists in the mouth. The rete in the mouth is thought to lose heat to ocean water. They speculate that the two retia are connected in some way, and serve to cool blood that supplies the brain. This mechanism is reminiscent of, but distinct from, the arrangement in many mammals adapted to hot terrestrial habitats. Thus radiators may have evolved independently at least four times - in terrestrial mammals with cooling muzzles, in whales, humans, and automobiles.
Blood from the face and scalp may return to the heart directly through the external jugular vein or by a more circuitous path, entering the braincase and ultimately exiting through the internal jugular vein. The second route enables blood cooled at the skin surface to help cool the brain.
Human Ancestors Walked Tall, Stayed Cool by Pete Wheeler
(Natural History, 8/93 pages 65 - 66)
Hominids, or humanlike primates, first appeared in Africa five to seven million years ago, when that continent's climate was becoming increasingly arid and large tracts of woodland and savanna were replacing the unbroken canopy of the equatorial forest. While the ancestors of chimpanzees and gorillas remained in the moist forests, the hominids started to exploit the more open, drier habitats. The exact nature of the transition remains hidden, because the oldest-known hominid fossils (Australopithecus afarensis) are only four million years old. But the intense sunshine in the new environment, combined with a scarcity of drinking water, must have severely challenged the ability of early hominids to regulate their body temperature.
Many savanna mammals do not even attempt to dissipate all the additional heat they absorb during the day, allowing it instead to accumulate within their bodies until nightfall, when they can cool off without expending precious water. But this strategy works only if delicate tissues, such as the central nervous system, are protected from surges in body temperature. Most savanna mammals possess special physiological mechanisms to cool the brain - notably the carotid rete, a network of fine arteries near the base of the brain, coupled with venous circulation through the muzzle.
Humans, apes, and monkeys, however, lack these features. Although humans appear to have eventually evolved an alternative mechanism to help cool the surface of their enlarged brains (see "A Good Brain Is Hard to Cool," page 65), the first hominids could have prevented damaging elevations of brain temperature only by keeping their entire body cool. Any adaptations that either reduced the amount of heat absorbed from the environment or facilitated its rapid dissipation would have proved highly advantageous.
Walking on two feet - the unique mode of terrestrial locomotion that is widely recognized as the first key development in hominid evolution - conferred precisely these benefits. Bipedalism dramatically reduces exposure to direct solar radiation during the middle of the equatorial day. I have placed scale models of early australopithecines in quadrupedal and bipedal postures to measure how the sun would hit them. These experiments show that when the sun is high, bombarding the earth's surface with intense radiation (because the rays pass through less atmosphere), far less body surface is exposed on a biped than on a quadruped. When the sun is directly overhead, the heat load on an upright hominid is only about 40 percent of that received by a quadruped of similar size.
Bipedalism also raises most of the body well above the ground, so that the skin contacts cooler and faster-moving air currents. This favors heat dissipation through convection. Allowing for variation in environmental conditions and vegetation, I calculate that hominids would have lost about one-third more heat through convection by adopting a bipedal posture.
Finally, human bipedalism at low speeds uses less energy than does either true quadrupedalism or the knuckle walking used by African apes. This reduces both dietary requirements (and the time and effort spent foraging) and the rate at which heat is generated internally as a byproduct of muscular activity.
Taking these factors into account in calculating the overall energy and water budgets of the early hominids, I conclude that bipedalism significantly decreased early hominids' dependence on shade, allowing them to forage in the open for longer periods and at higher temperatures. Bipedalism also greatly reduced the amount of drinking water they needed for evaporative cooling through sweating. I estimate that a knuckle-walking ape, active throughout the day on the savanna, would typically need to drink about five pints of water. Just by assuming bipedal posture and locomotion, a hominid of similar size would get by with three pints daily.
Bipedalism appears to be an ideal mode of terrestrial locomotion for a mammal foraging in the equatorial savanna, where food and water resources are dispersed and far from abundant. But if so, why do we find bipedalism only in humans? Probably because all other mammals of the African savanna, including monkeys such as baboons, are descended from ancestors that were already true quadrupeds.
In contrast, humans, along with chimpanzees and gorillas, probably descended from tree-dwelling primates that brachiated, or swung from branches using their arms. These ancestors were not strongly committed to one particular mode of terrestrial locomotion and may have been predisposed to walking upright. As they moved into more open habitats, the overheating problems they encountered may have tipped the balance in favor of bipedalism. (An alternative possibility is that bipedalism was first perfected in the forest habitat for some entirely different reason, and that our ancestors just happened to be preadapted for the problems they would encounter on the expanding savanna.)
Following the acquisition of an upright posture, humans evolved in other ways that enabled them to keep cool. Average body weight rose, slowing dehydration under savanna conditions. Larger hominids would have been able to forage for longer periods, and across greater distances, before needing to drink.
Later hominids - members of our own genus, Homo are also taller for their body weight than their stockier and rather more apelike ancestors, the australopithecines. By at least 1.6 million years ago, H. erectus had acquired the tall, linear physique, with relatively narrow shoulders and hips, characteristic of many human populations inhabiting hot, arid regions of the tropics today. A tall, thin body maximizes the skin area available for heat dissipation, while minimizing the exposure of these surfaces to the overhead sun. Longer legs help by raising the body still farther above the hot ground.
On all fours (top row), a hypothetical human progenitor exposes considerable body surface to direct solar radiation, whether the sun is low on the horizon, intermediate, or directly overhead. In contrast, the same creature in a bipedal posture is far less vulnerable to the intense rays of the midday sun (bottom row, right).
Scientists have long reasoned that one of the most obvious and unusual human features, the loss of insulating body hair, is an adaptation to the hot savanna. Although follicles are still densely distributed over most of the human body, the hairs they produce are so short and fine that the underlying skin is exposed directly to the flow of air, promoting the shedding of excess heat by convection and, when necessary, enhancing the effectiveness of sweating.
The problem with this hypothesis has always been explaining why humans differ from other savanna mammals, which have retained dense coats of hair. In environments where mammals are exposed to strong solar radiation, the coat acts as a shield, reflecting and reradiating heat before it reaches the skin. For most mammals, the loss of this insulation would create more problems that it would solve: I calculate that on the savanna, naked quadrupeds would actually need to drink additional water to cope with the extra heat load. For a biped, in contrast, a naked skin saves water because so little skin surface is exposed to the sun. Mainly the head and upper shoulders are exposed, and these can be protected by the retention of a relatively small amount of hair cover. Bipedalism and the strategy of cooling the whole body (rather than just the brain) probably explain why humans evolved a naked skin, while other savanna mammals of comparable size did not.
The stability in body temperature provided by bipedalism and a naked skin may have been an essential step in allowing our large, heat-sensitive brains to evolve further. A parallel to this can be seen in the development of modem computers. Information-processing systems - semiconductor as well as biological - generate substantial heat and are vulnerable to damage from overheating. This presents a major obstacle to electronics engineers attempting to build ever more capable machines. The circuits of the Cray 2 supercomputer, for example, are so densely packed that they must be immersed in a tank of fluorocarbon liquid maintained at about 65° F. As in the case of the evolution of the human brain, the development of such an elaborate cooling system does not inevitably lead to higher-performance machines, but it does make them possible.
As humans spread outward from Africa, they encountered different levels of heat, exposure, and moisture. Many studies suggest that these factors determined, at least in part, the variation we now observe in features as diverse as nose shape, limb proportions, hair structure, skin pigmentation, and eye color. Modern humans inhabiting savanna and desert environments near the equator, such as the Nilotic peoples of Africa and the Australian aborigines, commonly have tall, thin physiques resembling that of early Homo erectus. Their skin, especially among groups that have traditionally worn little clothing, is generally very dark, owing to the high concentration of melanin pigment that protects underlying tissues from sunburn and the carcinogenic effects of ultraviolet radiation.
As humans migrated north into colder regions, where retaining heat became more vital, they evolved proportionately shorter limbs, a trend seen in many other groups of mammals. In these populations the skin has lost most of its pigmentation, apparently because of the milder impact of ultraviolet radiation at high latitudes. The reduction in pigment may simply reflect the relaxation of the need for it, or it may have been demanded to allow sufficient penetration of ultraviolet radiation (which humans need to synthesize essential vitamin D).
As they colonized - or recolonized - tropical rain forests, humans faced another obstacle. Although the canopy affords shade, the humidity inhibits the evaporation of sweat. The resultant dependence on convective heat loss favors a body form with a large surface area relative to volume. Unfortunately, the tall, linear physique that works so well in open equatorial habitats is not practical when negotiating dense vegetation. A better solution may be a small body, exemplified by the Mbuti Pygmies of the Congo Basin, who benefit from a high surface-to-volume ratio and can move with agility across the forest floor.
