Masters of the Planet (3 page)

It may seem amazing that the same genes or gene families can influence structure across a spectrum of organisms that look as vastly different as, say, a human being and a fruit fly. But it makes sense when you consider not only that all organisms share an ultimate common ancestry, but also that the form of any creature is not solely a reflection of the structure of its individual genes. Instead, adult anatomy is the endpoint
of
a developmental process that is heavily influenced not just by the underlying genes themselves, but also by the sequence in which the genes are switched on and off; by exactly when this switching happens; and by how strongly the genes themselves are expressed while they are active. This multilayered process (genes, timing, activity) explains the apparent paradox of extreme genomic conservatism together with huge anatomical variety among organisms. And, at the same time, it limits future possibilities. For while changes in the genetic code occur at an astonishingly high rate as a result of simple copying errors (mutations) when cells multiply, few such changes survive in the gene pool. Some mutated genes may linger simply because they don't get in the way (and they may, indeed, turn out to be useful in the distant future, though that won't count for much at the time); but not many will produce a viable result, let alone an adaptively advantageous one. For all these reasons, radical makeovers of the basic structures of heredity are simply not in the cards.

THE ROLE OF CHANCE

Another
big reason for not expecting that evolution should be a process of fine tuning is that not all evolutionary changes are the work of natural selection. Chance—technically known as “genetic drift”—is also a huge factor. As a result of those constant mutations, isolated or semi-isolated local populations of creatures belonging to the same species will always tend to diverge from each other purely as a result of what is known as “sampling error”—even in the absence of significant selective forces for change. This is especially true if those populations are small, because the smaller your sample size, the greater your chances of such error. Just think of flipping coins instead of mutations. If you flip a coin only twice, there is a good chance it will come up heads both times; if you flip it ten or a hundred or a thousand times, it is progressively less likely it will always show heads. Tiny populations are equivalent to just a few flips.

Of course, it's also true that not all mutations are equal. Some will have little or no effect on the adult organism; but a few may have a radical influence on developmental processes, and thus upon the creature's final structure. Also important are differences in the degree to which a
gene's
effects are expressed, or how active its products are in determining the final physical outcome. For all these reasons we should not expect significant evolutionary change in physical form to happen always, or even usually, in tiny and incremental steps. As we will see, sometimes a very small change in the genome itself can have extensive and ramifying developmental results, producing an anatomical or behavioral gap between highly distinct alternative adult states.

None of this is an optimally efficient way to produce adaptation. But, as the luxuriant branching of the Great Tree of Life amply demonstrates, given enough time it
works.
And it works not only as a general explanation of how life diversified over billions of years, but also as an aid to understanding how the deep cognitive gulf separating humans and all other living organisms was so improbably bridged.

This brings us back to the central subject of this book: the story of how human beings came to be the extraordinary creatures they are—as physical entities, of course, but also as an unprecedented cognitive phenomenon. It was a long and eventful (albeit rapid by evolutionary standards) journey from humanity's humble beginnings as a vulnerable prey species, out in the expanding woodlands of ancient Africa, to the position we now occupy of top predator on Earth. But the major outlines of this dramatic story are now becoming clear. And they fit comfortingly well with our emerging views of the multilevel mechanisms underlying evolutionary change. For it's worth repeating that, remarkable as we may think we are, we are actually the product of a routine biological process.

MAJOR
EVENTS IN HUMAN EVOLUTION

ONE

ANCIENT ORIGINS

Among the most important influences not only on how ancient creatures evolved, but on their preservation as fossils, has been the geography and topography of the Earth itself. This has been as true for our group as for any other, so it's worth giving a bit of background here. During the Age of Mammals that followed the demise of the dinosaurs some 65 million years ago, much of the African continent was a flattish highland plateau. This slab of the Earth's crust lay over the roiling molten rocks of the Earth's interior like a great thick blanket, trapping the heat below. Heat must rise, and eventually ascending hot rock began to swell the rigid surface above.

Thus began the formation of the great African Rift, the “spine of Africa,” that formed as a series of more or less independent but ultimately conjoined areas of uplift known as “domes.” These blistered and split apart the continent's surface along a line that started in Syria, proceeded down the Red Sea, then south from Ethiopia through East Africa to Mozambique. The Rift's major feature, the Great East African Rift Valley, formed as a complex chain of sheer-sided depressions when the swelling below cracked the inflexible rock at the surface. As the continent continued to rise with the injection of more hot rock from below, erosion by water and wind began to deposit sediments in the valley floors— sediments that contain an amazingly rich assortment of fossils. As a category, fossils technically include any direct evidence of past life, but the
overwhelming
majority of them consist of the bones and teeth of dead animals that were luckily—for paleontologists—covered and protected by marine or lake or river sediments before they could be obliterated by scavengers and the elements. And, as fate would have it, the sedimentary rocks of the Rift Valley include the most remarkable fossil record we have, from anywhere in the world, of the long history of mankind and its early relatives.

In eastern Africa, Rift sediments began to be deposited in the Ethiopian Dome about 29 million years ago, and similar deposits mark the initiation of the Kenya Dome only a few million years later, at about 22 million years. This occurred during the period known to geologists as the Miocene epoch, and it happens to have been an exceptionally interesting time in primate evolution, as the fossil record shows. It was what you might call “the golden age of the apes,” and it set the stage for the evolution of the human family, which appeared toward its end.

Today's Great Apes—the chimpanzees, bonobos, gorillas, and orangutans—constitute a mere handful of forest species now restricted to tiny areas of Africa and a couple of southeast Asian islands. But the Miocene was the apes' heyday, and over its 18-million-year extent, scientists have named more than 20 genera of extinct apes from sites scattered all around the Old World, though mostly in East Africa. The earliest of these ancient apes are known as “proconsuloids.” They scampered along the tops of large branches in the humid forests of the eastern African early Miocene in search of fruit, some 23 to 16 million years ago. Like today's apes, they already lacked tails; but in many ways they were more monkeylike, with less flexible forelimbs than those their descendants eventually acquired.

Around 16 million years ago, African climates seem to have become drier and more seasonal, changing the character of the forests. True monkeys began to flourish in the new habitat, and the proconsuloids themselves yielded to “hominoid” apes that more closely resembled their modern successors. Most notably, the apes of the later Miocene developed mobile arms that they could freely rotate at the shoulder joint, allowing efficient suspension of the body beneath tree branches and imparting all-around greater agility. These early hominoids also typically had molar teeth with thick enamel that were set in robust jaws, allowing
them
to tackle a broad range of seasonally available forest foods as they began spreading beyond the Afro-Arabian region into Eurasia.

In both Eurasia and Africa, paleontologists have found the remains of several different hominoid genera that date back between about 13 and 9 million years ago. These probably represent the group that gave rise to the first members of our own “hominid” family (or “hominin” subfamily; for most purposes the distinction is merely notional). Most of the genera concerned are known principally from teeth and bits of jaw and cranium; but one of them, the 13-million-year-old
Pierolapithecus,
is well known from a fairly complete skeleton discovered not long ago in Spain.
Pierolapithecus
was clearly a tree climber, but it also showed a host of bony characteristics that suggest it habitually held its body upright. Such a posture—in the trees, at least—may actually have been typical for many hominoids of the time (as it is for orangutans today). However, the skull and teeth of
Pierolapithecus
are different from those of any of the putative early hominids that we'll read about in a moment.

WILL THE EARLIEST HOMINID PLEASE STAND UP?

The earliest representatives of our own group lived at the end of the Miocene and at the beginning of the following Pliocene epoch, between about six and 4.5 million years ago. And they appear just as the arrival of many new open-country mammal genera in the fossil record signals another major climatic change. Oceanic cooling affected rainfall and temperatures on continents worldwide, giving rise in tropical regions to an exaggerated form of seasonality often known as the “monsoon cycle.” In Europe this cooling led to the widespread development of temperate grasslands, while in Africa it inaugurated a trend toward the breakup of forest masses and the formation of woodlands into which grasslands intruded locally. This episode of climatic deterioration furnished the larger ecological stage on which the earliest known hominids made their debut.

Before we look at the varied cast of contenders for the title of “most ancient hominid,” perhaps we should pause for a moment to consider just what an early hominid
should
look like. How would we recognize the first
hominid,
the earliest member of the group to which we belong to the exclusion of the apes, if we had it? The question seems straightforward, but the issue has proven to be contentious, especially since members of related lineages—such as our own and that of the chimpanzees—should logically become more similar to each other, and thus harder to distinguish, as they converge back in time toward their common ancestor. But while the characteristics that define modern groups should even in principle lose definition back in the mists of the past, attempts to recognize very early hominids have paradoxically been dominated by the search for the early occurrence of those features that mark out their descendants today.