Why Evolution Is True (35 page)

Genetic and fossil evidence supports the “out of Africa” theory, but the debate continues. Why? Probably because it boils down to the significance of race. The longer human populations have been separated, the more genetic differences they will have accumulated. The multiregional hypothesis, with its splitting of populations over a million years ago, would predict fifteen times more genetic difference between races than if our human ancestors left Africa only 60,000 years ago. But more about race later.

One population of earlier hominins may have survived the worldwide extinction of
H.
erectus, and it is perhaps the most bizarre twig on the human family tree. Discovered in 2003 on the island of Flores in Indonesia, individuals of
Homo floresiensis
were promptly dubbed “hobbits,” for their adult height was a scant one meter (thirty-nine inches), and they weighed only fifty pounds—roughly the size of a five-year-old child. Their brains were also proportionately small—about australopithecine size—but their teeth and skeletons were indisputably those of Homo. They used stone tools and may have preyed on the Komodo dragons and dwarf elephants that populated the island. Amazingly,
floresiensis
fossils date to a mere 18,000 years ago, well after Neanderthals disappeared and twenty-five centuries after modern H.
sapiens
had already reached Australia. The best guess is that
floresiensis
represents an isolated population of
H
.
erectus
that colonized Flores and was somehow bypassed by the spread of modern H. sapiens. Although
floresien
sis was probably an evolutionary dead end, it is hard not to be charmed by the idea of a recent population of tiny humans who hunted dwarf elephants with miniature spears; and the hobbits have drawn wide public interest.

But the nature of the
floresiensis
fossils is disputed. Some contend that the tiny size of the one well-preserved skull may simply represent a diseased individual of modern
Homo sapiens—
perhaps one suffering from hypothyroid cretinism, a condition producing abnormally small skulls and brains. Recent analysis of fossil wrist bones, however, do support
H. floresiensis
as a genuine species of hominin, but questions remain.

Looking at the whole array of bones, then, what do we have? Clearly, indisputable evidence for human evolution from apelike ancestors. Granted, we can’t yet trace out a continuous lineage from an apelike early hominin to modern
Homo sapiens.
The fossils are scattered in time and space, a series of dots yet to be genealogically connected. And we may never have enough fossils to join them. But if you put those dots in chronological order, as in figure 24, you see exactly what Darwin predicted: fossils that start off apelike and become more and more like modern humans as time passes. It’s a fact that our divergence from the ancestor of chimps occurred in East or Central Africa about seven million years ago, and that bipedal walking evolved well before the evolution of large brains. We know that during much of hominin evolution, several species existed at the same time, sometimes at the same place. Given the small population size of humans and the improbability of their fossilization (remember, this usually requires that a body find its way into water and be quickly covered with sediment), it’s amazing that we have as good a record as we do. It seems impossible to survey the fossils we have, or look at figure 25, and deny that humans have evolved.

Yet some still do. When dealing with the human fossil record, creationists go through extreme, indeed almost humorous, contortions to avoid admitting the obvious. In fact, they’d prefer to steer clear of the issue. But when forced to confront it, they simply sort hominin fossils into what they see as two discrete groups—humans and apes—and assert that these groups are separated by a large and unbridgeable gap. This reflects their religiously based view that although some species may have evolved from others, humans did not, but were the object of a special act of creation. But the whole folly is exposed by the fact that creationists can’t agree on exactly which fossils are “human” and which are “ape.” Specimens of
H. habilis
and
H. erectus,
for example, are classified as “apes” by some creationists and “humans” by others. One author has even described a
H. erectus
specimen as an ape in one of his books and a human in another!
⁴⁸
Nothing shows the intermediacy of these fossils better than the inability of creationists to classify them consistently.

What, then, propelled the evolution of humans? It’s always easier to document evolutionary change than to understand the forces behind it. What we see in the human fossil record is the appearance of complex adaptations such as erect posture and remodeled skulls, both of which involve many coordinated changes in anatomy, so there’s no doubt that natural selection was involved. But what sort of selection? What were the precise reproductive advantages of larger brains, erect posture, and smaller teeth? We’ll probably never know for sure, and can only make more or less plausible guesses. We can, however, inform these guesses by learning something about the environment in which humans evolved. Between ten million and three mil- lion years ago, the most profound environmental change in East and Central Africa was drought. During this critical period of hominin evolution, the climate gradually became dryer, and was later followed by alternating and erratic periods of drought and rainfall. (This information comes from pollen and African dust blown into the ocean and preserved in sediments.) During the dry periods, the rainforests gave way to more open habitat, including savanna, grassland, open forest, and even desert scrub. This is the stage on which the first act of human evolution played out.

Many biologists feel that these changes in climate and environment had something to do with the first significant hominin trait to evolve: bipedality. The classic explanation is that walking on two legs allowed humans to travel more efficiently from one patch of forest to another across newly open habitat. But this seems unlikely, because studies of knuckle-walking and bipedality show that these forms of locomotion don’t use significantly different amounts of energy. Still, there are a host of other reasons why walking erect may have had a selective advantage. It could, for instance, have freed the hands to gather and carry newly available types of food, including meat and tubers (this could also explain our smaller teeth and increased manual dexterity). Walking erect could also have helped us deal with high temperature by raising our body off the ground, reducing the surface area exposed to the sun. We have far more sweat glands than any other ape, and since hair interferes with the cooling evaporation of sweat, this may explain our unique status as “naked apes.” There is even an improbable “aquatic ape” theory, arguing that early hominins spent much of their time foraging for food in the water, with erect posture evolving to keep our heads above the surface. Jonathan Kingdon’s book on bipedality, Lowly Origin, describes still more theories. And of course these evolutionary forces are not mutually exclusive: several might have been operating together. Unfortunately, we can’t yet distinguish among them.

The same goes for the evolution of increased brain size. The classic adaptive story is that once our hands were freed by the evolution of two-legged walking, hominins were able to fashion tools, leading to selection for bigger brains that allowed us to envision and fashion more complex tools. This theory has the advantage that the first tool appeared around the time that brains started getting larger. But it ignores other selective pressures for bigger and more complex brains, including the development of language, negotiating the psychological intricacies of primitive society, planning for the future, and so on.

These mysteries about
how
we evolved should not distract us from the indisputable fact that we
did
evolve. Even without fossils, we have evidence of human evolution from comparative anatomy, embryology, our vestigial traits, and even biogeography. We’ve learned of our fishlike embryos, our dead genes, our transitory fetal coat of hair, and our poor design, all testifying to our origins. The fossil record is really the icing on the cake.

IF WE DON’T YET UNDERSTAND why selection made us different from other apes, can we at least find out how many and what sort of
genes
differentiate us? “Humanness” genes have become almost a Holy Grail of evolutionary biology, with many laboratories engaged in the search. The first attempt to find them was made in 1975 by Allan Wilson and Mary-Claire King at the University of California. Their results were surprising. Looking at protein sequences taken from humans and chimps, they found that they differed on average by only about 1 percent. (More recent work hasn’t changed this figure much: the difference has risen to about 1.5 percent.) King and Wilson concluded that there was a remarkable genetic similarity between us and our closest relatives. They speculated that perhaps changes in just a very few genes produced the striking evolutionary differences between humans and chimps. This result garnered tremendous publicity in both the popular and scientific press, for it seemed to imply that “humanness” rested on just a handful of key mutations.

But recent work shows that our genetic resemblance to our evolutionary cousins is not quite as close as we thought. Consider this. A 1.5 percent difference in protein sequence means that when we line up the same protein (say, hemoglobin) of humans and chimps, on average we’ll see a difference at just one out of every hundred amino acids. But proteins are typically composed of
several hundred
amino acids. So a 1.5 percent difference in a protein three hundred amino acids long translates into about four differences in the total protein sequence. (To use an analogy, if you change only 1 percent of the letters on this page, you will alter far more than 1 percent of the sentences.) That oft-quoted 1.5 percent difference between ourselves and chimps, then, is really larger than it looks: a lot more than 1.5 percent of our proteins will differ by
at least one amino acid
from the sequence in chimps. And since proteins are essential for building and maintaining our bodies, a single difference can have substantial effects.

Now thatwe’ve finally sequenced the genomes of both chimp and human, we can see directly that more than 80 percent of all the proteins shared by the two species differ in at least one amino acid. Since our genomes have about 25,000 protein-making genes, that translates to a difference in the sequence of more than 20,000 of them. That’s not a trivial divergence. Obviously, more than a few genes distinguish us. And molecular evolutionists have recently found that humans and chimps differ not only in the
sequence
of genes, but also in the
presence
of genes. More than 6 percent of genes found in humans simply aren’t found in
any form
in chimpanzees. There are over fourteen hundred novel genes expressed in humans but not in chimps. We also differ from chimps in the number
of copies
of many genes that we do share. The salivary enzyme amylase, for example, acts in the mouth to break down starch into digestible sugar. Chimps have but a single copy of the gene, while individual humans have between two and sixteen, with an average of six copies. This difference probably resulted from natural selection to help us digest our food, as the ancestral human diet was probably much richer in starch than that of fruit-eating apes.

Putting this together, we see that the genetic divergence between ourselves and chimpanzees comes in several forms—changes not only in the proteins produced by genes, but also in the presence or absence of genes, the number of gene copies, and when and where genes are expressed during development. We can no longer claim that “humanness” rests on only one type of mutation, or changes in only a few key genes. But this is not really surprising if you think about the many traits that distinguish us from our closest relatives. There are differences not only in anatomy, but also in physiology (we are the sweatiest of apes, and the only ape whose females have concealed ovulation),
⁴⁹
behavior (humans pair-bond and other apes do not), language, and brain size and configuration (surely there must be many differences in how the neurons in our brains are hooked up). Despite our general resemblance to our primate cousins, then, evolving a human from an apelike ancestor probably required substantial genetic change.

Can we say anything about the specific genes that did make us human? Right now, not very much. Using genomic “scans” that compare the entire DNA sequence of chimps and humans, we can pick out
classes
of genes that have evolved rapidly on the human branch of our divergence. These happen to include genes involved in the immune system, gamete formation, cell death, and, most intriguingly, sensory perception and nerve formation. But it’s a different matter entirely to zero in on a single gene and demonstrate that mutations in that gene actually
produced
human/chimp differences. There are “candidate” genes of this sort, including one (FOXP2) that might have been involved in the appearance of human speech,
⁵⁰
but the evidence is inconclusive. And it might always remain so. Conclusive proof that a given gene causes human/chimp differences requires moving the gene from one species to another and seeing what difference it makes, and that’s not the kind of experiment anyone would want to try.
⁵¹

TRAVELING AROUND THE GLOBE, you quickly see that humans from different places look different. Nobody, for example, would mistake a Japanese for a Finn. The existence of visibly different human types is obvious, but there’s no bigger minefield in human biology than the question of race. Most biologists stay as far away from it as they can. A look at the history of science tells us why. From the beginning of modern biology, racial classification has gone hand in hand with racial prejudice. In his eighteenth-century classification of animals, Carl Linnaeus noted that Europeans are “governed by laws,” Asians “governed by opinions,” and Africans “governed by caprice.” In his superb book
The Mismeasure
of
Man,
Stephen Jay Gould documents the unholy connection between biologists and race in the last century.