Thumbs, Toes, and Tears Page 2
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Because of these shifts, we are descended from a branch of the primate family that became geographically orphaned. Though anthropologists verbally brawl over the details,3 the creatures from which we evolved seem to have been a breed of ape that was trapped on the drier, eastern side of the Rift Valley when it began to form five million years ago or so.
While our progenitors found themselves forced to cluster around what shrinking forests remained in the east, some primates followed the familiar safety of the jungles retreating to southern and central Africa beyond the western rim of the Rift Mountains. They evolved into today’s three subspecies of gorillas and two species of chimpanzees—Pan troglodytes, the chimps you see at the circus, and bonobos (Pan paniscus), very likely our closest relatives. They still make their homes in these jungles, though they may not survive much longer as we deforest their habitats and hunt them down for food or sale.
But given the circumstances five million years ago, it would have been perfectly reasonable to expect that the other apes, those caught on the eastern side of the rift, were the ones marked for extinction. Yet they managed to survive, and in time they even split into several species of their own. Anthropologists didn’t understand this, even a few decades ago. They had assumed that our kind had marched cleanly from a single line of protohumans into the present, like new models of cars or computers, each subsequent version an improvement over the previous one. We know better now, though the picture is far from perfectly clear.
Some species, for example, may have evolved in Chad,4 much farther west than anthropologists once thought. A handful may even have met and interbred as they wandered the drying, broken landscape. Others may simply have gone the way of the dinosaur, leaving behind no descendants, not a single gene, just a few petrified scraps of bone—muddled messages from another epic that we haven’t yet deciphered.5 Our emergence was a far messier evolutionary business than many scientists suspected at first. Nevertheless, piece by piece, we are getting a handle on how we came to be the upright walking, naked apes we are.
The Human Family Tree
The fossil record is jumbled and controversial, and won’t be resolved in this sidebar or even in this book. But it is helpful to have an overview of our family tree based on the fossil finds paleoanthropologists have made so far. The problem with the fossil record, of course, is that it is an incomplete puzzle. Though our picture of human evolution grows, in some ways, clearer with each new find, each find also raises questions. There have been many lines of primates that evolved at different times in different parts of Africa, each with their own distinct capabilities, intelligence, and anatomy. Some of these lines may or may not have interacted.
The general picture is this: In various parts of Africa, apes that had previously spent most of their time in the jungle began to stand upright. They probably did so haltingly and possibly for different reasons. Based on the most recent fossil evidence, the earliest primate to evolve into the line that ultimately led to us is Sahelanthropus tchadensis, a creature that lived in north-central Africa as many as seven million years ago. Some scientists disagree and argue that tchadensis may have been an early form of modern gorillas. The question has yet to be answered.
About six million years ago another primate, known as Orrorin tugenensis, arose in what is now western Kenya. Tugenensis may have walked on all fours as well as two feet, and he may be related to us. The paleoanthropological community is still working that out.
Between 4 million and 5.8 million years ago two other controversial primates evolved in East Africa: Ardipithecus ramidus and Australopithecus anamensis. It is not clear how fully bipedal ramidus was, but there is little doubt that anamensis spent at least much of his time on two feet.
Beginning about 3.5 million years ago and continuing until 1 million years, there seems to have been a miniboom of new savanna apes, each species struggling to eke out a living on the increasingly naked plains of Africa. This might simply be an anomaly in the fossil record. There may have been more species earlier, but fewer of their bones may have survived. Or there may have been others who came afterward that simply haven’t been found. Whatever the case, two species stand out: Australopithecus afarensis and Australopithecus africanus. They would have been a little taller than a chimpanzee, with both legs and arms proportionately longer than ours. If you were to come upon a group of either africanus or afarensis making their way through the southern African Transvaal or over a ridge in the African Rift Valley, you might mistake them for a troop of chimpanzees except for their upright posture. They were small, weighing in at 65 to 150 pounds and no taller than five feet. Unlike the gorillas evolving in Africa’s central jungles, their limbs were thin and gracile. Their fossils tell us that they would have been smarter than the very first big-toed apes that emerged from the receding jungles like Ardipithecus, with brains that weighed roughly 450 cubic centimeters (cc), about the same size as today’s bonobo chimps. There were also other gracile apes in the mix, though there is debate about which of them actually represent separate species and which are simply other forms of A. africanus or A. afarensis. These include Kenyanthropus platyops and Australopithecus garhi. (See chart on pages 6–7.)
While these more gracile apes roamed the African landscape, another group of bipedal apes were also evolving. They were taller, thicker, and stronger than A. africanus and A. afarensis; paleoanthropologists like to call them “robust.” Their brows were sloped, their chests bigger, and their faces and jaws large, the better to house the square rows of chunky teeth inside, which were adapted for eating nuts, roots, and leaves harvested as they roamed Africa’s woodlands and savannas. To support the enormous muscles needed to grind the food they ate, they evolved large sagittal crests, a thick row of bone that ran from the front to the back of their skulls and to which long jaw muscles were anchored. These creatures included Australopithecus robustus, Australopithecus aethiopicus, and Australopithecus boisei (also known as Zinjanthropus boisei). The brain sizes of these creatures vary widely, with most weighing about 400 cc.
Some scientists argue that the eating habits of these two groups of apes were a crucial turning point in their evolution. The more gracile apes may have been more prone to eat meat, mostly scavenged. Even partial meat eaters would have required a smaller digestive tract and less energy to digest their food. That may have resulted in two fundamental evolutionary shifts. First, the energy needed to operate larger digestive tracts may have been used instead to build bigger brains; and second, the more concentrated forms of protein in the meat gracile apes scavenged would have supplied the building blocks that accelerated their cerebral growth.
Even if A. africanus and A. afarensis were meat eaters, they were not feared predators; they didn’t have the natural equipment for it. For the most part they were probably gentle, highly social, and above all, dependent on the other members of their troop for survival. And survival would have been a full-time job because life for these creatures could not have been easy. They had no real tools, no fire, no language, no claws or weapons. Their numbers would have been measured in the thousands, if that, certainly not the millions. Infant mortality would have been high and life spans short.
There is a good chance that at least some of these species interacted with one another. They may have interbred or they may have helped wipe one another out or they may have peacefully coexisted, like wildebeests and elephants, we don’t know.
Whatever the case, the bigger, robust lines all died out, but the gracile apes evolved through one line or another into Homo habilis, the first of our genus and the first toolmaker (see chapter 3). Truthfully, the fossil specimens of Homo habilis could arguably be divided into more than one species, but for now they all tend to fall under the habilis nomenclature.
Most agree that Homo erectus is a descendant of Homo habilis, but their brains and body sizes are so different (some erectus specimens’ brains are 50 percent larger and their bodies nearly a foot taller than Homo habilis) tha
t it’s likely other, still undiscovered species came in between. One candidate was found in 2002 in Dmanisi, Georgia, known fittingly as Homo georgicus. Georgicus was about the same height as habilis, five feet or so, but its brain was larger than most habilis specimens, weighing approximately 650 cc. This creature lived north of the Middle East about 1.8 million years ago, before Homo erectus reemerged from Africa and began fanning out all around the planet.
Following Homo erectus came a series of creatures that have shed light on our lineage but hardly paint a perfectly clear picture. Homo ergaster and Homo antecessor preceded Homo sapiens neanderthalensis (Neanderthal Man) and Homo floresiensis, a primate discovered on the Indonesian island of Flores in 2003. Floresiensis was clearly very advanced. It probably made tools and possibly spoke or used some sort of language, but stood no higher than three feet full grown, with a brain roughly the size of a chimpanzee’s. (This seems to prove that it is the structure of the brain, not its size, that matters.) Current theory holds that floresiensis is a form of Homo erectus that evolved to dwarflike dimensions like other mammals on the island as an adaptation to the scarcity of food and the small size of the island’s ecosystem.
Neanderthals are a different matter entirely. They were intelligent early Homo sapiens, but not, by common consensus, directly related to us. (Anthropologists debate whether our ancestors and Neanderthals might have mated.) In fact, our direct ancestor, Cro-Magnon Man, may well have wiped them out, probably because he had developed more advanced tools.
In between modern Homo sapiens sapiens and Homo erectus came another archaic form of Homo sapiens, known in most paleoanthropological circles as Homo heidelbergensis, a creature who had some of the facial features of erectus—a sloping brow, for example—but also some of our modern features, such as smaller teeth and a more rounded skull. They flourished between 500,000 and 200,000 years ago, when the first modern humans emerged.
Though fossils indicate that these earliest modern humans were physically identical to us, the wiring of their brains may not have been because it isn’t until around 160,000 years later that we see the first flowering of complex artwork, sculpture, and other forms of truly modern culture and communication. Did disparate parts of a complex brain connect in ways that led to consciousness and insight that made us far more fully self-aware? That mystery still remains unresolved.
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Under a searing noonday sun on November 30, 1974, Donald Johanson and his colleague Tom Gray were heading back to base camp for a break. Noon was a good time for a break because it was a poor time for fossil hunting in Hadar, Ethiopia. Not only was it hot, but the midday sun also threw no shadows, and the bone and rock all tended to blend into one indistinguishable morass of tan dust.
That morning neither Johanson nor Gray had turned up anything better than a few old pig and monkey bones. But then as they walked through a small gully just beyond the place they had been excavating, Johanson noticed what appeared to be an elbow joint lying at the bottom of a slope. Both men knelt down to look more closely when they suddenly realized that they were surrounded by hominid bones—the elbow, a femur, a chunk of pelvis, various vertebrae, and several ribs. As he picked through the rock, Johanson recalled fervently hoping that all of these fossilized messages belonged to a single human precursor. But he was afraid if he said so out loud, it might somehow jinx the possibility.6 He needn’t have worried.
Johanson called the creature they had found that day Lucy, for the popular Beatles song “Lucy in the Sky with Diamonds,” but he named the species she represented Australopithecus afarensis, the southern ape from Afar. (The area where Lucy was found sits squarely in Africa’s Afar Triangle, a chunk of land where three tectonic plates are still pulling that part of the world in separate directions.)
Lucy revamped the scientific view of human evolution and remains one of the most important paleoanthropological finds of the twentieth century because the bones Gray and Johanson uncovered that day told the world that Lucy walked upright at a time earlier than any scientist thought possible. Her pelvis, femur, and tibia all sent a clear message that despite being chimpanzeelike in size and stature (she was about three feet, six inches tall and weighed about sixty-two pounds), Lucy’s view of the world was nothing at all like a chimpanzee’s.7
Lucy’s skeleton (left) compared with the skeleton of a modern woman. Lucy’s pelvic girdle was not like a modern chimpanzee’s, nor was it fully human, but it was getting there. (Reprinted from How Humans Evolved (second edition) by Robert Boyd and Joan B. Silk, used by permission of W. W. Norton & Company.)
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At about the time Donald Johanson was assembling Lucy’s bones, fabled anthropologist Mary Leakey (wife of anthropologist Louis Leakey) had returned to an area called Laetoli in Tanzania, a spit of dusty flatland about thirty miles south of one of her and her husband’s favorite fossil haunts, Olduvai Gorge. Neither had been back to Laetoli for decades, and with good reason. Even in their earlier visits Laetoli had failed to yield any significant fossil finds. This particular expedition didn’t turn up any bones of importance either. But it did produce the record of one of the most famous walks in history: Eighty feet of three sets of footprints perfectly preserved in a combination of mud and volcanic ash.
Initially, Leakey didn’t think much about the discovery. The prints, found beneath a relatively thin layer of dirt, were interesting at first glance but not apparently very important. They looked as if they had been made a few thousand years earlier by three people strolling through the gorge. Leakey figured that Mount Sadiman, once an active volcano that loomed nearby, had probably spewed the ash that had created a kind of cement that preserved the walkers’ journey.8
But then in 1976 Leakey finally got around to having the rock containing the prints dated, and she was astonished to find that the walk had been made not a few thousand years earlier, but 3.5 million years ago, long before any modern humans roamed Africa. Or put another way, the footprints revealed that a species, probably like Johanson’s Lucy, walked upright with a fully human gait. That, Leakey admitted years later, was when “we got excited!”9
“Make no mistake about it,” Timothy White, an anthropologist who worked with both Johanson and Leakey later said, “[the Laetoli footprints] are like modern human footprints. If one was left in the sand of a California beach today, and a four-year-old was asked what it was, he would instantly say that someone [human] had walked there. He wouldn’t be able to tell it from a hundred other prints on the beach, nor would you.”10
Leakey’s discovery pushed the date of bipedal hominids back even further than Lucy had—four hundred thousand years further. Before these discoveries no one had dreamed that upright-walking apes could have lived this long ago. But all of the evidence was there in Lucy’s bones and in the footprints Leakey had uncovered: the slender foot with its knobby big toe supporting the weight of the smallish, long-armed bodies, pushing them away from the volcano and toward their destination. The heel was already elongated, the toes ran in parallel, and the arch was engineered and in place, absorbing the weight and transferring it along the outside, then across the balls of their feet, just as our feet do today.
Laetoli footprints preserved in Tanzania (left); close-up of a footprint (right). These footprints are at least 3.6 million years old, yet they are nearly identical to our own. (Used by permission of Science Source Photo Research, Inc., in New York, New York.)
Imagine the haunting image of these three creatures—two adults and a child, by the looks of the prints, all unlike any primate that lives on Earth today—passing together through a flat spit of land. Behind them loomed Mount Sadiman, rumbling, belching hot ash, rattling the ground beneath their feet. Yet they weren’t running scared. Perhaps they were accustomed to irascible volcanoes. The footprints are measured; there is no sign of flight. In fact, the impressions of their feet show that one of the creatures stopped briefly, turned to look eastward at the angry volcano, and then continued onward.11
; No one can say precisely what these three creatures looked like. Apparently they survived their journey and went on to live out their lives among the shattered and restless landscape of the Rift Valley, so we don’t have their remains. But if they had the same anatomy as Lucy, and most scientists believe they did, from a distance their walking, if not their bodies, would very likely have resembled the gaits of a toddler and two adolescents making their way through a park.12 Their hips would have twisted like ours, and their arms, though slightly longer than a modern human’s, would have swung in a very human way. Despite their small stature, their legs would have angled inward, unbowed, and anchored to a slender pelvis. From the hips down they would have looked remarkably similar to us.
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Johanson’s and Leakey’s discoveries turned theories about human evolution on their heads. Until Lucy, scientists were mostly certain that if anything distinguished our ancestors from their simian cousins it was their brain, not their feet. Brains, they theorized, led to bipedalism rather than the other way around. But apparently they had it wrong. Lucy was not big-brained, at least by our standards. The skull fragments that Johanson and his team found indicated that her cerebral capacity weighed in at about 450 cc, roughly the same as a modern chimp. Yet Lucy’s locking knee joint and short, narrow pelvis indisputably indicated that she stood upright.
The footprints at Laetoli sent the same clear message. Our ancestors had begun walking upright sooner than we thought, perhaps a million years after our line split off from the common ancestor we shared with chimpanzees. In an evolutionary blink, they had gone from knuckle-walking, tree-climbing jungle apes to striding, sure-footed, savanna apes that walked pretty much the way the rest of us do today.