The Origin of Humankind
Page 3
With this realization in place, the task of the anthropologist in accounting for human origins refocused on the origin of bipedalism. Even stripped down to this single event, the evolutionary transformation was not trivial, as Owen Lovejoy, an anatomist at Kent State University, has noted. “The move to bipedalism is one of the most striking shifts in anatomy you can see in evolutionary biology,” he wrote in a popular article in 1988. “There are important changes in the bones, the arrangement of the muscles that power them, and the movement of the limbs.” A glance at the pelvises of humans and chimpanzees is sufficient to confirm this observation: In humans, the pelvis is squat and boxlike, while in chimps it is elongated; and there are major differences in the limbs and trunk, too (see figure 1.2).
The advent of bipedalism is not just a major biological transformation but a major adaptive one as well. As I argued in the preface, the origin of bipedal locomotion is so significant an adaptation that we are justified in calling all species of bipedal ape “human.” This is not to say that the first bipedal ape species possessed a degree of technology, increased intellect, or any of the cultural attributes of humanity. It didn’t. My point is that the adoption of bipedalism was so loaded with evolutionary potential—allowing the upper limbs to be free to become manipulative implements one day—that its importance should be recognized in our nomenclature. These humans were not like us, but without the bipedal adaptation they couldn’t have become like us.
What were the evolutionary factors that promoted the adoption of this novel form of locomotion in an African ape? The popular image of human origins often includes the notion of an apelike creature leaving the forests and striding onto the open savanna. A dramatic image no doubt, but entirely inaccurate, as has recently been demonstrated by researchers at Harvard and Yale Universities who have analyzed soil chemistry in many parts of East Africa. The African savannas, with their great migrating herds, are relatively recent in the environment, developing less than 3 million years ago, long after the first human species evolved.
FIGURE 1.2
Different modes of locomotion. The shift from quadrupedal to bipedal locomotion demanded substantial changes in the body’s anatomical structure. For instance, humans have longer hind limbs, shorter fore-limbs, a squatter pelvis, shorter and noncurved digits, and a reduced lumbar region, compared with chimpanzees and gorillas. Australopithecus afarensiSy the earliest-known hominid, undoubtedly was a biped, but retained some anatomical features of tree dwellers. (Courtesy of John Fleagle/Academic Press.)
If we take our minds back to an Africa of 15 million years ago, we find a carpet of forest from west to east, home to a great diversity of primates, including many species of monkeys and apes. In contrast with the situation today, ape species greatly outnumbered monkey species. Geological forces were stirring, however, which would dramatically alter the terrain and its occupants during the next few million years.
The earth’s crust was tearing itself apart beneath the eastern part of the continent, in a line from the Red Sea through present-day Ethiopia, Kenya, Tanzania, and into Mozambique. As a result, the land rose blisterlike in Ethiopia and Kenya, forming great highlands more than 9000 feet in altitude. These great domes transformed not only the continent’s topography but its climate. Disrupting the previously uniform west-to-east airflow, the domes threw the lands to the east into rain shadow, depriving the forests of their sustenance. The continuous tree cover began to fragment, leaving a mosaic environment of forest patches, woodland, and shrubland. Open grassland, however, was still rare.
About 12 million years ago, a continuation of tectonic forces further changed the environment, with the formation of a long, sinuous valley, running from north to south, known as the Great Rift Valley. The existence of the Great Rift Valley has had two biological effects: it poses a formidable east-west barrier to animal populations; and it further promotes the development of a rich mosaic of ecological conditions.
The French anthropologist Yves Coppens believes the east-west barrier was crucial to the separate evolution of humans and apes. “By force of circumstance, the population of the common ancestor of [humans] and [apes] . . . found itself divided,” he wrote recently. “The western descendants of these common ancestors pursued their adaptation to life in a humid, arboreal milieu; these are the [apes]. The eastern descendants of these same common ancestors, in contrast, invented a completely new repertoire in order to adapt to their new life in an open environment: these are the [humans].” Coppens dubs this scenario the “East side story.”
The valley has dramatic highlands with cool, forested plateaus, and precipitous slopes that plunge 3000 feet to hot, arid lowlands. Biologists have come to realize that mosaic environments of this kind, which offer many different kinds of habitat, drive evolutionary innovation. Populations of a species that once were widespread and continuous may become isolated and exposed to new forces of natural selection. Such is the recipe for evolutionary change. Sometimes that change is toward oblivion, if favorable environments disappear. This, clearly, was the fate of most of the African apes: just three species exist today—the gorilla, the common chimpanzee, and the pygmy chimpanzee. But while most ape species suffered because of the environmental shift, one of them was blessed with a new adaptation that allowed it to survive and prosper. This was the first bipedal ape. Being bipedal clearly bestowed important survival advantages in the changing conditions. The job of anthropologists is to discover what those advantages were.
Anthropologists tend to view the importance of bipedality in human evolution in two ways: one school emphasizes the freeing up of the forelimbs for carrying things; the other emphasizes the fact that bipedalism is a more energy efficient mode of locomotion, and sees the ability to carry things merely as a fortuitous by-product of the upright stance.
The first of these two hypotheses was proposed by Owen Lovejoy and published in a major paper in Science in 1981. Bipedalism, he argued, is an inefficient mode of locomotion, so it must have evolved for carrying things. How could the ability to carry things give bipedal apes a competitive edge over other apes?
Ultimately, evolutionary success depends on producing surviving offspring, and the answer, suggested Lovejoy, lay in the opportunity that this new ability gave male apes to boost the reproductive rate of the female, by gathering food for her. Apes, he pointed out, reproduce slowly, having one infant every four years. If human females had access to more energy—that is, food—they might successfully produce more offspring. If a male helped provide a female with more energy by collecting food for her and for her offspring, she would be able to boost her reproductive output.
There would be a further biological consequence of the male’s activity, this time in the social realm. Since it would not benefit the male in a Darwinian sense to provision a female unless he were sure she was producing his offspring, Lovejoy suggested that the first human species was monogamous, with the nuclear family emerging as a way of increasing reproductive success, and thus outcompeting the other apes. He supported his argument by further biological analogy. In most primate species, for example, males compete with each other for sexual control of as many females as possible. They often fight with one another during this process, and are endowed with large canine teeth, which they use as weapons. Gibbons are rare in that they form male-female pairs, and—presumably because they do not have reason to fight with one another—the males have small canine teeth. The small canines in the earliest humans may be an indication that, like gibbons, they formed male-female pairs, Lovejoy argued. The social and economic bonds of the provisioning arrangement would in turn have driven an increase in the size of the brain.
Lovejoy’s hypothesis, which enjoyed considerable attention and support, is powerful because it appeals to fundamental biological issues, not cultural ones. It has weak points, however; for one thing, monogamy is not a common social arrangement among technologically primitive people. (Only 20 percent of such societies are monogamous.) The hypothesis was the
refore criticized for seeming to draw on a trait of Western society, not one of hunter-gatherers. A second criticism, perhaps more serious, is that the males of the known early human species were about twice the size of females. In all species of primate that have been studied, this great difference in body size, known as dimorphism, correlates with polygyny, or competition among the males for access to females; dimorphism is not seen in monogamous species. For me, this fact alone is sufficient to sink a promising theoretical approach, and an explanation other than monogamy must be sought for the small canines. One possibility is that the mechanism of masticating food required a grinding rather than a slicing motion; large canines would impair such a motion. Lovejoy’s hypothesis enjoys less support now than it did a decade ago.
The second major bipedalism theory is much more persuasive, partly for its simplicity. Proposed by the anthropologists Peter Rodman and Henry McHenry, of the University of California, Davis, the hypothesis states that bipedalism was advantageous in the changing environmental conditions because it offered a more efficient means of locomotion. As the forests dwindled, food resources in woodland habitats, such as fruit trees, would have become too dispersed to be efficiently exploitable by conventional apes. According to this hypothesis, the first bipedal apes were human only in their mode of locomotion. Their hands, jaws, and teeth would have remained apelike, because their diet had not changed, only their manner of procuring it.
To many biologists, this proposal initially seemed unlikely; researchers at Harvard University had shown some years earlier that walking on two legs is less efficient than walking on four. (This shouldn’t be a surprise to anyone with a dog or a cat; both run, embarrassingly, much faster than their owners.) The Harvard researchers had, however, compared the energy efficiency of bipedalism in humans with that of quadrupedalism in horses and dogs. Rodman and McHenry pointed out that the proper comparison should have been between humans and chimpanzees. When these comparisons are done, it turns out that bipedalism in humans is more efficient than quadrupedalism in chimpanzees. An energy-efficiency argument as a force of natural selection in favor of bipedalism, they concluded, is therefore plausible.
There have been many other suggestions for the factors that drove the evolution of bipedalism, such as the need to look over tall grass while monitoring predators and the need to adopt a more efficient posture for cooling during daytime foraging. Of them all, I find Rodman and McHenry’s the most cogent, because it is firmly biologically based and fits the ecological changes that were occurring when the first human species evolved. If the hypothesis is correct, it will mean that when we find fossils of the first human species, we may fail to recognize them as such, depending on which bones we have. If the bones are those of the pelvis or lower limbs, then the bipedal mode of locomotion will be evident, and we will be able to say “human.” But if we were to find certain parts of the skull, jaw, or some teeth, they might look just like those of an ape. How would we know whether they were those of a bipedal ape or a conventional ape? It’s an exciting challenge.
If we could visit the Africa of 7 million years ago to observe the behavior of the first humans, we would see a pattern more familiar to primatologists, who study the behavior of monkeys and apes, than to anthropologists, who study the behavior of humans. Rather than living as aggregations of families in nomadic bands, as modern hunter-gatherers do, the first humans probably lived like savanna baboons. Troops of thirty or so individuals would forage in a coordinated way over a large territory, returning to favored sleeping places at night, such as cliffs or clumps of trees. Mature females and their offspring would make up most of the troop’s numbers, with just a few mature males present. The males would be continually looking for mating opportunities, with the dominant individuals achieving the most success. Immature and low-ranking males would be very much on the periphery of the troop, often foraging by themselves. The individuals in the troop would have the human aspect of walking bipedally but would be behaving like savanna primates. Ahead of them lay 7 million years of evolution—a pattern of evolution that was complex, as we shall see, and by no means certain. For natural selection operates according to immediate circumstances and not toward a long-term goal. Homo sapiens did eventually evolve as a descendant of the first humans, but there was nothing inevitable about it.
CHAPTER 2
A CROWDED FAMILY
By my count, fossil specimens of varying degrees of incompleteness, representing at least a thousand individuals of various human species, have been recovered from South and East Africa from the earliest part of the record—that is, from about 4 million years ago up until almost a million years ago (many more have been found in the later record). The oldest human fossils found in Eurasia may be close to 2 million years old. (The New World and Australia were populated much more recently, some 20,000 and 55,000 years ago, respectively.) It is fair to say, therefore, that most of the action of human prehistory took place in Africa. The questions anthropologists must answer about this action are twofold: First, what species populated the human family tree between 7 million years ago and 2 million years ago, and how did they live? Second, how were the species related to each other evolutionarily? In other words, what was the shape of the family tree?
My anthropological colleagues face two practical challenges in addressing these problems. The first is what Darwin called “the extreme imperfection of the geological record.” In his Origin of Species, Darwin devoted an entire chapter to the frustrating gaps in the record, which result from the capricious forces of fossilization and later exposure of bones. The conditions that favor the rapid burial and possible fossilization of bones are rare. And ancient sediments may become uncovered through erosion—when a stream cuts through them, for instance—but which pages of prehistory are reopened in this way is purely a matter of chance, and many of the pages remain hidden from view. For instance, in East Africa, the most promising repository for early human fossils, there are very few fossil-bearing sediments from the period between 4 million and 8 million years ago. This is a crucial period in human prehistory, because it includes the origin of the human family. Even for the time period after 4 million years we have far fewer fossils than we would like.
The second challenge stems from the fact that the majority of fossil specimens discovered are small fragments—a piece of cranium, a cheekbone, part of an arm bone, and many teeth. The identification of species from meager evidence of this nature is no easy task and is sometimes impossible. The resulting uncertainty allows for many differences of scientific opinion, both in identifying species and in discerning the interrelatedness of species. This area of anthropology, known as taxonomy and systematics, is one of the most contentious. I will avoid the details of the many debates and concentrate instead on describing the overall shape of the tree.
Knowledge of the human fossil record in Africa developed slowly, beginning in 1924 when Raymond Dart announced the discovery of the famous Taung child. Comprising the incomplete skull of a child—part of the cranium, face, lower jaw, and brain case—the specimen was so named because it was recovered from the Taung limestone quarry, in South Africa. Although no precise dating of the quarry sediments was possible, scientific estimates suggest that the child lived about 2 million years ago.
While the Taung child’s head had many apelike features, such as a small brain and a protruding jaw, Dart recognized human aspects too: the jaw protruded less than it does in apes, the cheek teeth were flat, and the canine teeth were small. A key piece of evidence was the position of the foramen magnum—the opening at the base of the skull through which the spinal cord passes into the spinal column. In apes, the opening is relatively far back in the base of the cranium, while in humans it is much closer to the center; the difference reflects the bipedal posture of humans, in which the head is balanced atop the spine, in contrast to ape posture, in which the head leans forward. The Taung child’s foramen magnum was in the center, indicating that the child was a bipedal ape.
Although Dart was convinced of the hominid status of the Taung child, almost a quarter of a century was to pass before professional anthropologists accepted the fossil individual as a human ancestor and not just an ancient ape. The prejudice against Africa as the site of human evolution and a general revulsion at the idea that anything so apelike might be a part of human ancestry combined to consign Dart and his discovery to anthropological oblivion for a long time. By the time anthropologists recognized their error—in the late 1940s—Dart had been joined by the Scotsman Robert Broom, and the two men had found scores of early human fossils from four cave sites in South Africa: Sterkfontein, Swartkrans, Kromdraai, and Maka-pansgat. Following the anthropological custom of the time, Dart and Broom applied a new species name to virtually every fossil they discovered, so that very soon it appeared that there had been a veritable zoo of human species living in South Africa between 3 million and 1 million years ago.
By the 1950s, anthropologists decided to rationalize the plethora of proposed hominid species and recognized just two. Both were bipedal apes, of course, and both were apelike in the way that the Taung child was. The principal difference between the two species was in their jaws and teeth: in both, these were large, but one of the creatures was a more massive version of the other. The more gracile species was given the name Australopithecus africanus, which was the appellation Dart had given to the Taung child in 1924; the term means “southern ape from Africa.” The more robust species was called, appropriately, Australopithecus robustus (see figure 2.1).
From the structure of their teeth, it was obvious that both africanus and robustus had lived mostly on plant foods. Their cheek teeth were not those of apes—which have pointed cusps, suited to a diet of relatively soft fruit and other vegetation—but were flattened into grinding surfaces. If, as I suspect, the first human species had lived on apelike diets, they would have had apelike teeth. Clearly, by 2 million to 3 million years ago the human diet had changed to one of tougher foods, such as hard fruits and nuts. Almost certainly this indicated that the australopithecines lived in a drier environment than that of apes. The huge size of the robust species’ molars suggests that the food it ate was especially tough and needed extensive grinding; not for nothing are they referred to as “millstone molars.”