Why Evolution Is True (33 page)

Dart’s 1924 discovery of the Taungs child set off a hunt for human ancestors in Africa, eventually leading to the famous excavations of the Leakeys at Olduvai Gorge beginning in the 1930s, the discovery of “Lucy” by Donald Johanson in 1974, and a host of other finds. We now have a reasonable fossil record of our evolution, although one that’s far from complete. There are, as we’ll see, many mysteries, and more than a few surprises.

But even without fossils we’d still know something about our place on the tree of evolution. As Linnaeus proposed, our anatomy places us in the order Primates along with monkeys, apes, and lemurs, all sharing traits such as forward-facing eyes, fingernails, color vision, and opposable thumbs. Other features put us in the smaller superfamily Hominoidea along with the “lesser apes” (gibbons) and “great apes” (chimpanzees, gorillas, orangutans, and ourselves). And within the Hominoidea we are grouped with the great apes in the family Hominidae, sharing unique features like flattened fingernails, thirty-two teeth, enlarged ovaries, and prolonged parental care. These shared characters show that our common ancestor with the great apes lived more recently than our common ancestor with any other mammal.

Molecular data derived from DNA and protein sequences confirms these relationships, and also tells us roughly when we diverged from our relatives. We are most closely related to the chimpanzees—equally to the common chimp and the bonobo—and we diverged from our joint common ancestor about seven million years ago. The gorilla is a slightly more distant relative, and orangutans more distant yet (12 million years since the common ancestor).

Yet to many, fossil evidence is psychologically more convincing than molecular data. It’s one thing to learn that we share 98.5 percent of our DNA sequence with chimps, but another entirely to see the skeleton of an australopithecine, with its small, apelike skull perched atop a skeleton nearly identical to that of modern humans. But before we look at the fossils, we can make some predictions about what we’d expect to find if humans evolved from apes.

What should our “missing link” with apes look like? Remember that the “missing link” is the
single ancestral species
that gave rise to modern humans on the one hand and chimpanzees on the other. It’s not reasonable to expect the discovery of that critical single species, for its identification would require a complete series of ancestor-descendant fossils on both the chimp and human lineages, series that we could trace back until they intersect at the ancestor. Except for a few marine microorganisms, such complete fossil sequences don’t exist. And our early human ancestors were large, relatively few in number compared to grazers like antelopes, and inhabited a small part of Africa under dry conditions not conducive to fossilization. Their fossils, like those of all apes and monkeys, are scarce. This resembles our problem with the evolution of birds, for whom transitional fossils are also rare. We can certainly trace the evolution of birds from feathered reptiles, but we’re not sure exactly which fossil species were the direct ancestors of modern birds.

Given all this, we can’t expect to find the single particular species that represents the “missing link” between humans and other apes. We can hope only to find its evolutionary cousins. Remember also that this common ancestor was not a chimpanzee, and probably didn’t look like either modern chimps or humans. Nevertheless, it’s likely that the “missing link” was closer in appearance to modern chimps than to modern humans. We are the odd man out in the evolution of modern apes, who all resemble one another far more than they resemble us. Gorillas are our distant cousins, and yet they share with chimps features like relatively small brains, hairiness, knuckle-walking, and large, pointed canine teeth. Gorillas and chimps also have a “rectangular dental arcade”: when viewed from above, the bottom row of their teeth looks like three sides of a rectangle (see figure 27). Humans are the one species that has diverged from the ape ground plan: we have uniquely flexible thumbs, very little hair, smaller and blunter canine teeth, and we walk erect. Our tooth row is not rectangular, but parabolic, as you can see by inspecting your lower teeth in the mirror. Most striking, we have a much larger brain than any ape: the adult chimp’s brain has a volume of about 450 cubic centimeters, that of a modern human about 1,450 cubic centimeters. When we compare the similarities of chimps, gorillas, and orangutans to the divergent features of humans, we can conclude that, relative to our common ancestor, we have changed more than have modern apes.

Around five to seven million years ago, then, we expect to find fossil ancestors having traits shared by chimpanzees, orangutans, and gorillas (these traits are shared because they were present in the common ancestor), but with some human features too. As the fossils become more and more recent, we should see brains getting relatively larger, canine teeth becoming smaller, the tooth row becoming less rectangular and more curved, and the posture becoming more erect. And this is exactly what we see. Although far from complete, the record of human evolution is one of the best confirmations we have of an evolutionary prediction, and is especially gratifying because the prediction was Darwin’s.

But first a few caveats. We don’t (and can’t expect to) have a continuous fossil record of human ancestry. Instead, we see a tangled bush of many different species. Most of them went extinct without leaving descendants, and only one genetic lineage threaded its way through time to become modern humans. We’re not yet sure which fossil species lie along that particular thread, and which were evolutionary dead ends. The most surprising thing we’ve learned about our history is that we’ve had many close evolutionary cousins who died out without leaving descendants. It’s even possible that as many as four humanlike species lived in Africa at the same time, and maybe in the same place. Imagine the encounters that might have taken place! Did they kill one another, or try to interbreed?

And the names of ancestral human fossils can’t be taken too seriously. Like theology, paleoanthropology is a field in which the students far outnumber the objects of study. There are lively—and sometimes acrimonious—debates about whether a given fossil is really something new, or merely a variant of an already named species. These arguments about scientific names often mean very little. Whether a humanlike fossil is named as one species or another can turn on matters as small as half a millimeter in the diameter of a tooth, or slight differences in the shape of the thighbone. The problem is that there are simply too few specimens, spread out over too large a geographic area, to make these decisions with any confidence. New finds and revisions of old conclusions occur constantly. What we must keep in sight is the general trend of the fossils over time, which clearly shows a change from apelike to humanlike features.

FIGURE 24
. Fifteen hominin species, the periods over which they occur as fossils, and the nature of their brain, teeth, and locomotion. Fossils designated by open boxes are too fragmentary to draw conclusions about locomotion and brain size.

 

On to the bones. Anthropologists apply the term hominin to all the species on the “human” side of our family tree after it split from the branch that became modern chimps.
⁴³
Twenty types of hominins have been named as separate species; fifteen of these are shown in rough order of appearance in figure 24. I show the skulls of a few representative hominins in figure 25, along with those of a modern chimp and human for comparison.

Our main question is, of course, how to determine the
pattern
of human evolution. When do we see the earliest fossils that might represent our ancestors who had already diverged from other apes? Which of our hominin relatives went extinct, and which were our direct ancestors? How did the features of the ancestral ape become those of modern humans? Did our big brain evolve first, or our upright posture? We know that humans
began
evolving in Africa, but what part of our evolution happened elsewhere?

FIGURE 25
. Skulls of modern humans
(Homo sapiens),
earlier hominins, and a chimpanzee
(Pan troglodytes).

 

Except for some bone fragments whose classification is unclear, until recently the hominin fossil record didn’t go back beyond four million years. But in 2002, Michel Brunet and his colleagues announced the astounding discovery of an older possible hominin,
Sahelanthropus tchadensis,
from the Central African deserts of Chad, the region known as the Sahel. The most surprising thing about this find is its date: between six and seven million years ago, right when molecular evidence tells us that our lineage diverged from that of chimps.
Sahelanthropus
might well represent the earliest human ancestor—or it could be a side branch that went extinct. But its mix of traits certainly seems to place it on the human side of the human/chimp divide. What we have here is a nearly complete skull (albeit a bit squashed during fossilization), but one that is a
mosaic,
showing a curious mixture of homininlike and apelike traits. Like apes, it had a long cranium with a small, chimp-sized brain, but like later hominins, it had a flat face, small teeth, and brow ridges (figure 25).

Lacking the rest of the skeleton, we can’t tell if
Sahelanthropus
had the critical ability to walk upright, but there is a tantalizing hint that it could. In knuckle walkers like gorillas and chimps, the animal’s usual posture is horizontal, so its spinal cord enters the skull from the rear. In erect humans, however, the skull sits directly atop the spinal cord. You can see this difference in the position of the opening in the skull through which the spinal cord passes (the
foramen magnum,
Latin for “big hole”): this hole is set farther forward in humans. In
Sahelanthropus,
the hole is farther forward than in knuckle-walking apes. This is exciting, for if this species really was on the hominin side of the divide, it suggests that bipedal walking was one of the first evolutionary innovations to distinguish us from other apes.
⁴⁴

After Sahelanthropus, we have a few six-million-year-old fragments from another species,
Orrorin tugenensis,
including a single leg bone that has been interpreted as evidence of bipedality. But then there is a two-million-year gap with no substantive hominin fossils. This is where, one day, we’ll find crucial information about when we began to walk upright. But, beginning about four million years ago, the fossils reappear, and from them we see branches beginning to sprout from the hominin tree. In fact, several species might have lived at the same time. Among these are the “gracile” (slender and graceful) australopithecines, which again show mixtures of apelike and humanlike traits. On the ape side, their brains are roughly chimp-sized, and their skulls are more apelike than humanlike. But the teeth are relatively small, and set in rows midway between the rectangular shape of apes and the parabolic palate of humans. And they were definitely bipedal.

An early set of fossils from Kenya, grouped together as
Australopithecus anamensis,
shows tantalizing hints of bipedality from a single fossilized leg bone. But the decisive find was made by Donald Johanson, an American paleoanthropologist prospecting for fossils in the Afar region of Ethiopia. On the morning of November 30, 1974, Johanson awoke feeling lucky, and made a note to that effect in his field diary. But he had no idea how lucky he’d be. After searching vainly all morning in a dry gulley, Johanson and Tom Gray, a graduate student, were about to give up and head back to camp. Suddenly Johanson spotted a hominid bone on the ground, and then another, and another. Remarkably, they had stumbled on the bones of a single individual, later formally designated AL 288-1, but more famously known as “Lucy,” after the Beatles’ song “Lucy in the Sky with Diamonds,” played repeatedly in camp to celebrate the find.

When Lucy’s hundreds of fragments were assembled, she turned out to be a female of a new species,
Australopithecus afarensis,
dating back 3.2 million years. She was between twenty and thirty years old, three and a half feet tall, weighing a scant sixty pounds, and possibly afflicted with arthritis. But most important, she walked on two legs.