Written in Stone: Evolution, the Fossil Record, and Our Place in Nature

by Brian Switek

  Just as with Owen’s attempt to separate humans from apes on the basis of the hippocampus minor, however, meat eating, hunting, and tool use would not last long as hallmarks of humanity. What made the difference came not from fossils, but a shift in our understanding of living apes. Prior to the 1960s chimpanzees were seen as peaceful, fruiteating apes that offered an alternative to our violent ways. The work of Jane Goodall, who was selected by Louis Leakey to study the chimpanzees at Gombe Stream Preserve in Tanzania starting in 1960, changed all that.83

  What Goodall observed drastically altered our understanding of chimpanzees. Not only did they hunt and eat meat, but they also made simple tools. Goodall observed chimpanzees carefully selecting twigs and stripping them of leaves in order to “fish” for termites, identifying the apes as not only tool users but tool manufacturers. (Subsequent studies have enlarged the chimpanzee toolkit and even shown cultural differences in the ways tools are used in different populations.) When Goodall telegraphed Louis Leakey with this news, he replied, “Now we must redefine tool, redefine Man, or accept chimpanzees as humans.”

  While chimpanzees were not regularly having bushpig for dinner or making machines, Goodall’s observations threatened the apehuman boundary. Horrific observations made at Gombe in the 1970s would bring them even closer. As in the Planet of the Apes mythos, it was widely thought that “ape did not kill ape,” at least not unless there was something pathologically wrong with an individual. Conflict sometimes occurred between one group of chimpanzees living in a forest and another, but it was relatively peaceful and did not result in death.

  Then, in the early 1970s, a small group of chimpanzees split from a larger group and took up residence in the jungle nearby. Conflicts between the groups were common, and in 1974 researchers were horrified to see a group of males from one group chase down and savagely beat a lone male from the other. The male died from his injuries, and this was just the first of the fatalities. By 1977, all the males from the smaller group had been killed. Murder, and perhaps even warfare, was not restricted to our species.

  While studies of gorillas and orangutans helped to clear misunderstandings about those apes, it was chimpanzees that, in a multitude of ways, threatened notions of human uniqueness. Even so, humans remained taxonomically divided from living apes. Most scientists placed humans within the Hominidae and all apes into the Pongidae, an arrangement similar to Darwin’s notebook diagram of 1868. This was reinforced by the notion among paleoanthropologists that the fossil ape Ramapithecus was the earliest-known hominid, placing the date of the human-ape split about fifteen to twenty-five million years ago. Comparisons of the differences in the blood-protein chemistry of humans and apes made during the early years of Goodall’s studies, however, suggested a much closer family relationship.

  In 1967, biochemists Vincent Sarich and Allan Wilson set out to create a “molecular clock” for human evolution. The concept behind it was relatively simple. If there had been steady, regular rates of mutation during the evolution of humans and apes, then it should be possible to look at differences between certain proteins and determine how long it took for those differences to be created. The accumulation of mutations in a lineage of organisms over time would be like the regular ticking of a clock that would allow Sarich and Wilson to count backward to the last ancestor common to apes and humans.

  When Sarich and Wilson carried out the experiment they came to a hypothetical divergence date of about five million years ago, and there was no sign of any slowdown in mutations that would have skewed their results. If the biochemists were correct, then the australopithecines sat near the base of our family tree and Ramapithecus would be booted out of it entirely. This did not sit well with many paleoanthropologists. For them, the fossil evidence was clear that humans and apes did not diverge earlier than the fourteen-million-year-old hominid Ramapithecus. The paleontologists resented that a few lab monkeys were making pronouncements about their field, while many biochemists regarded the paleoanthropologists as fossils who refused to get with the times.

  Biochemistry alone could not resolve the issue. Future fossil discoveries were required to confirm or refute the dueling hypotheses. One of the most promising areas for this research was the Rift Valley in northeastern Africa. Pockmarked by active and dormant volcanoes, it was the right place, with accessible deposits of the right age, to potentially yield more fossil hominids. It was in the northern range of this region that the French anthropologist Maurice Taieb discovered a two- to three-million-year-old site at Hadar in the Afar region of Ethiopia in 1970. By 1973, he had organized the International Afar Research Expedition. The American paleoanthropologist Donald Johanson was among the early members of the group.

  Early work was difficult, both in terms of fieldwork and camp politics, but toward the end of the first field season Johanson had discovered the upper part of a tibia (or shinbone) and the lower part of a femur. Together they reconstructed a knee of a bipedal creature, and since only humans habitually walked on two legs Johanson was sure he had found a hominid.

  When we stand up straight, our femurs angle inward toward the midline of the body. Our hip sockets are set farther apart than where our knees nearly meet beneath our hips. This arrangement is important to balance and allows us to walk upright, and it was these characteristics that had allowed W. E. Le Gros Clark to identify the australopithecines of South Africa as being more humanlike in posture years before. Johanson’s find, too, had more in common with us than living apes.

  If Johanson was right, his discovery was a major find, but he needed something for comparison and he could not wait to get back to the anatomy lab to check. With Tom Gray, Johanson raided a burial chamber set up by the local Afar people and stole a human femur. This sort of looting ran sharply against the basic ethical standards of science, but despite his theft, Johanson was relieved when the recent bone showed the same orientation as its fossil counterpart.

  The discovery of a three-million-year-old bipedal hominid was exciting by itself, but a more momentous find would be made the next year. To the joy of Johanson and his fellow team members, parts of the skull, jaw, arms, legs, fingers, ribs, vertebrae, and hips from a single individual hominid of the same type were found at Hadar. As the scientists reveled that night, the Beatles song “Lucy in the Sky with Diamonds” played endlessly—so it seemed only fitting to name the individual “Lucy.”

  The skeleton had an informal name, but Johanson was still unsure of just what kind of hominid it was. With the permission of the Ethiopian government he brought the remains back to the United States, where he teamed up with another young paleoanthropologist, Tim White, to pick away at the affinities of the bones. The quandary was not easily solved. It took years to sort out, during which time more fragmentary remains of this kind of hominid came to light. A whole group of the same type of creature—the “First Family”—was found at Hadar. Similar fossils were also found at Laetoli in Tanzania, where White had worked with Mary Leakey.

  The large collection of bones showed a mosaic of traits, similar to both later humans and to apes, and the specimens did not fit neatly into the known varieties of Homo or Australopithecus. This caused nearly ceaseless arguments between Johanson and White, and among both of them and Mary Leakey, but by the fall of 1977 the duo knew they had found a new species. They decided to call it Australopithecus afarensis. It was a hominid with legs and hips suited to walking bipedally but an upper body retaining adaptations to moving through the trees. It had long arms, curved fingers, and a funnel-shaped ribcage like living apes, but it also had hips and legs that indicated an upright posture. This was reinforced by the presence of the track way of a bipedal hominid at Laetoli, where they found type material for A. afarensis, which seemed to be a likely candidate for the track maker. Clearly, this was a creature set to shake up the evolutionary tree.

  FIGURE 91 - The skull and neck of a juvenile A. afarensis discovered in Ethiopia and reported in 2007.

  Ever since the scientific
recognition of the Neanderthals, experts had debated whether one type of hominid or another was a true ancestor of our species or represented a side branch that left no living descendants. Personal philosophies influenced these arrangements as much as the bones did, from the early rejection of australopithecines as hominids to Louis Leakey’s dogged search for the earliest members of the genus Homo. Australopithecus afarensis, though, appeared to occupy a critical junction in human evolution. It was older than the other australopithecines (A. africanus and Paranthropus) and Homo habilis, yet it showed resemblances to both lines. A. afarensis seemed to occupy a place near the split of later australopithecines and the earliest members of our own genus. Given that A. afarensis was only about three million years old and exhibited so many apelike traits, though, it hinted that our forebears diverged from the ancestors of living apes more recently than many paleoanthropologists were prepared to accept.

  The transitional status of Lucy was becoming established just as Ramapithecus was being torn down from its vaunted position. When the first jaw fragments of the supposed hominid were found they were thought to take a distinctively human shape. In apes, the molars and premolars form a straight line with the incisors and canines slightly curving to give the jaw a rigid U shape. In humans, however, the rows and molars and premolars are more curved and have a parabolic shape. The fragments of Ramapithecus’s jaw seemed to fit the human pattern, but when more complete fossils of the ape were found in the 1970s it turned out that the fossils called “Ramapithecus” belonged to another creature.

  In many primate species males and females are highly sexually dimorphic, meaning that they differ significantly in traits like size, skeletal structure, canine tooth length, musculature, and other features. Orangutans and gorillas are good examples of this; if you looked at the skulls of a male and female gorilla side by side and did not know any better you could be excused for thinking they belonged to two distinct species. Rather than being products of natural selection, these differences are the result of sexual selection—such as female choice and the competition between males for mates—and this would have held true for ancient primates, too. When more complete skulls of Ramapithecus came to light it became apparent that they represented the more gracile (and probably female) form of Sivapithecus, a fossil ape that closely resembles living orangutans. This revision made it no longer reasonable to push the divergence of humans and apes back past fourteen million years.

  The fall of Ramapithecus only represented half of the major shift occurring in anthropology, however. At about the same time, molecular biologists Charles Sibley and Jon Ahlquist were pioneering a new technique called DNA hybridization to determine the relatedness of species. In this technique, segments of DNA from two species are hybridized with each other, and the more similar the DNA strands of the two species are the more closely they will match (indicated by how much energy it takes to split them apart again). The technique had worked well enough on birds that Sibley and Ahlquist decided to try it on primates. Not only did they pin the human-ape divergence as occurring seven to nine million years ago, but they confirmed that chimpanzees were more closely related to humans than they were to other apes.

  Further studies showed that the difference between the genetic codes of humans and chimpanzees was very small, only a few percent, but scientists were even more surprised when they found our “missing” chromosome. Chimpanzees, gorillas, and orangutans have twenty-four pairs of chromosomes and we have twenty-three. Since we shared a common ancestor with these apes it is likely that our ancestors also had twenty-four pairs. At some point, the chromosome number in the human lineage was reduced to twenty-three, but rather than disappearing entirely the “missing” chromosome still remains in our cells. It is part of our chromosome 2. At some time during our evolution the “missing” chromosome fused to the end of chromosome 2, and so the end of the original chromosome 2 became the middle of the new, fused one. Not only does our present chromosome 2 show that such an event occurred, but the entire chromosome is strikingly similar to the chimpanzee chromosomes 2p and 2q combined.

  In light of all this new evidence the classic division between the Pongidae and Hominidae could no longer be upheld. Our species was situated within the ape group, not outside of it, and our place in nature had to be reconciled with the new evidence.

  Our species, and all of our extinct relatives more closely related to us than to chimpanzees, belong to the hominin lineage. Since chimpanzees, belonging to the panin line, are our closest living relatives, the two lineages together constitute the group Hominini. This group is itself nested in a larger group that includes the gorilla and orangutan lineages called the Hominidae, and the Hominidae is part of an even more inclusive group, of which gibbons are a part, called the Hominoidea. These terms are confusingly similar, but each rank represents differing degrees of specificity that relate all apes to each other. We are hominins, but, as far as our family tree is concerned, we are also hominids and hominoids.

  By the 1990s much had changed in paleoanthropology. Our species was identified as being a part of the ape family tree, not outside it, and the preoccupation with “Man the Hunter” had faded. Even Dart’s horrifying view of the past had been permanently laid to rest. In his investigation of the Sterkfontein and Mapakansgat caves, the paleoanthropologist C. K. Brain discovered that the bone, tooth, and horn “tools” Dart found had actually been made naturally. Most of the remains in the caves had been washed in or deposited by carnivores, causing wear that Dart mistook as signs of tool use. The weight of the evidence revealed that the australopithecines had probably been prey more often than predators, particularly the skullcap of a young Paranthropus that had two puncture marks in it indicating that it had been bitten, and probably killed, by a leopard. It was only in the younger cave deposits, from when the caves were inhabited by Homo erectus, that clear signs of human hunting (like herbivore leg bones bearing tool cut marks) were found.84

  Homo erectus, like its australopithecine relatives, also received a makeover in the last quarter of the twentieth century. When the first remains of Homo erectus were found by Dubois (“Pithecanthropus”) they were thought to have belonged to a creature intermediate between human and ape. As the discoveries from Dragon Bone Hill (“Sinanthropus”) and sites from Africa were compared, though, it became clear that Homo erectus was more similar to us than our common ancestor with chimpanzees. Key to this revision was a specimen found by Kamoya Kimeu near Lake Turkana, Kenya, in 1984.

  Kimeu was part of a team working with Richard Leakey, son of Louis and Mary, at a 1.5-million-year-old site called Nariokatome when he discovered the nearly complete skeleton of a male juvenile Homo erectus. Nicknamed “Turkana Boy,” this nine to ten year old was already five feet, six inches tall, putting him head and shoulders above his australopithecine ancestors. He also had a relatively narrow pelvis and lanky proportions, quite different than the stooped images of “Java Man” from early twentieth-century books and magazines.

  Even though the Nariokatome skeleton was of a juvenile male, its hips provided paleoanthropologists with a starting point for determining the pelvis shape of adult female Homo erectus. This was important, as in order to understand how our species was born we had to know how individuals were born at different times in our ancient history. As our lineage evolved larger brains, the increasing size of the infant head had to be accommodated by the hips, and in our species birth is extremely painful and stressful due to a convoluted birth canal and the large size of the infant skull. Even though the infant skull is not fully fused and can distort somewhat, it is still a tight squeeze.

  Apes, however, do not go through the same discomfort human mothers do. They have a straight birth canal and give birth with relative ease. The differences continue after birth, as well. Since we have such large heads we have to be born “prematurely” to make sure the skull will fit through the birth canal; we are helpless for a much longer time than infant apes are. If we developed any longer in the womb, bi
rth might not be possible, though, and so a long period of helplessness after birth is a trade-off for larger adult brains. The narrow-hipped Homo erectus appeared to have a pattern of birth and growth similar to our own, with a longer period of infant helplessness outside the womb.

  What was needed to test this idea, however, was the pelvis from an adult female Homo erectus. It remained elusive for years, but in the fall of 2008 Scott Simpson and colleagues announced that they had found the coveted prize from 1.2-million-year-old sediment of Ethiopia. Given that Homo erectus was closely related to our species, it was possible to use the same anatomical landmarks in the hips that identify human females, so the researchers could be sure they had sexed the bones properly.

  The newly discovered Homo erectus hips were much wider than those of the Nariokatome skeleton, and even of modern Homo sapiens females. These hips housed a more capacious birth canal and would have allowed for an infant head 30 percent larger than previously calculated to pass through, making it probable that Homo erectus children were born relatively more developed than Homo sapiens infants and spent a shorter amount of time being helpless. The pelvis also showed that there were probably some stark differences between Homo erectus males and females. The males may have been tall and lanky like Turkana Boy, while the females might have been shorter and wider. (More specimens will be needed to test this and make sure it is not simply a case of regional variation.) This sexual dimorphism and a growth rate intermediate between that of our species and living apes emphasizes our gradual, mosaic evolution from ancient apes.

  Linnaeus was right. There is no character or trait that can be zealously wielded to obliterate our blood relationship with other primates. We are apes, just of a different sort, and many of our traits can be traced back millions of years into the past. Opposable thumbs, fingernails, binocular vision, and many other traits that we think of as distinctively human evolved among primates during the sixty-five million years after the extinction of the dinosaurs, and the first members of our own hominin lineage between five and seven million years ago were little different from the apes from which they evolved.