Every other organism is similarly nested into the living world; and graphically, this pattern of resemblance is best expressed in the form of a repeatedly branching tree. Ultimately, every one of all the many millions of living organisms can be embraced within one single gigantic Tree of Life. In this greatest of all trees, biologists group the tiniest branch tips (species, e.g., Homo sapiens) into genera (e.g., the genus Homo), which are in turn grouped into families (Hominidae), which are grouped into orders (Primates), and so on. As you move up the tree, each successive level departs farther in its configuration from the common ancestral form at the bottom, and from equivalent neighboring branches. And although it is possible to study this self-evident Tree of Life in purely structural terms, the most interesting thing is to know what caused it.
The only testable (and thoroughly tested) scientific explanation of this pattern of resemblance is common ancestry. The similarities that clue us in to the shape of the tree are inherited from a series of shared ancestral forms, whose descendants have diverged from them in various respects. Similar forms share a recent common ancestor, while more disparate ones shared an ancestor much more remotely in time—allowing differences to accumulate over a longer period. No matter how dissimilar they may now appear to the eye, all life forms are ultimately linked at the genomic level to a single common ancestor that lived more than 3.5 billion years ago.
The nineteenth-century naturalists Charles Darwin and Alfred Russel Wallace were the first to come up with a convincing mechanism by which divergence from a common ancestor could occur. Darwin called this instrument of change “natural selection.” Once pointed out, this natural process seemed so self-evident that Darwin’s famous contemporary Thomas Henry Huxley publicly berated himself for his own failure to think of it. In a nutshell, natural selection is simply the preferential survival and reproduction of individuals that are better “adapted” to their environments than their fellows, in features inherited from their parents. And it is pretty much a mathematical consequence of the fact that, in all species, each generation produces more offspring than survive to reproduce. The idea here is that, over enough time, those with more advantageous inherited characteristics will have greater reproductive success, and therefore will nudge the population in the direction of better adaptation. In this way, members of the lineage will change in average appearance and ultimately evolve into a new species.
That was the theory, anyway, though it has subsequently been noticed that natural selection may well act mostly to trim off both extremes of the available variation, keeping the population more or less stable. Another complication is that, when we think of adaptation, we usually have in mind one single anatomical feature, or behavioral property, of the animal in question: the structure of its foot or pelvis, say, or its “intelligence.” Thinking of just one feature in isolation, it is easy to envision how that structure might have been improved over time by natural selection. Yet we now know that all organisms are astonishingly complex genetic entities, in which a remarkably small number of structural genes (exactly how many we humans have isn’t known for sure, though most current bets are in the 23,000 range) govern the development of an enormous number of bodily tissues and processes. In the end, natural selection can only vote up or down on the entire individual, which is a real mash-up of genes and of the characteristics they promote. It cannot single out specific features to favor or disfavor.
This, though, blurs the “fitness” picture. It is, for example, of little value to be the smartest member of your species if, in an environment crawling with predators, you are also the slowest—or even just the most unfortunate. What’s more, in an indifferent world your reproductive success may not in the end have much to do with how magnificently you are adapted to any one thing. Whether or not that predator gets you, or whether or not you get the girl, may simply be a function of blind luck and circumstance. The upshot of complications such as these is that evolutionary histories, certainly as we see them reflected in the fossil record, are not produced by the reproductive fates of individuals alone. Indeed, in a world of constantly changing environments, and of ceaseless competition among different kinds of organisms for ecological space, it is more often the fates of entire populations and species that determine the larger evolutionary patterns we observe when we look back at the fossil record.
And there are yet other reasons for not expecting that evolution should produce tidy perfection. As I’ve already suggested, change can only build on what is there already, because there is no way that evolution can conjure up de novo solutions to whatever environmental or social problems may present themselves. As a result, we are all built on modified versions of a template ultimately furnished us by an ancient ancestor. History severely limits what you can potentially become not simply because you must necessarily be a version of what went before, but because genomes, dedicated as they are to the propagation of mind-bogglingly complex systems, turn out to be hugely resistant to change. In fact, they provide the ultimate example of “if it ain’t broke, don’t fix it.” After all, fiddling around with anything as intricate as a genome is asking for trouble: most random changes to a functioning system this complicated simply won’t succeed. The fact that changes in the genetic code carry huge risks explains the inherent conservatism of genomes. It also explains why some organisms that look hugely different to the eye have amazingly similar genes: I’ve heard it said that we share over 40 percent of our genes with a banana, while a gene that is highly active in determining human skin color is also responsible for regulating the dark stripes on the side of a zebrafish.
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
Event
Thousand Years Ago
Origin of Life
3,500,000
Origin of Primates
60,000
Group containing humans and apes begins to diversify
23,000
Earliest hominids (bipeds) appear in Africa
7–6,000
First Australopithecus
4,200
Earliest possible use of sharp stone for cutting
3,400
Beginning of glacial cycle
2,600
Distinct expansion of grassland fauna in Africa
2,600
Earliest documented manufacture of stone tools
2,600–2,500
Claimed “early Homo” fossils appear
2,500–2,000
First Homo of modern body proportions in Africa
1,900–1,600
Hominids first leave Africa (Dmanisi)
1,800
First stone tools made to deliberate shape
1,760
Homo erectus appears in Asia
1,700–1,600
First Homo fossils in Europe
1,400–1,200
Earliest evidence of domesticated fire in hearths
790
Homo antecessor appears in Europe
780
First Old World–wide hominid, Homo heidelbergensis
600
First evidence of Neanderthal lineage in Europe
> 530
Earliest blade tools in Africa
500
Earliest wooden spears, hafted tools
400
First evidence of constructed shelters
400–350
Earliest prepared-core tools
300–200
Origin of anatomically recognizable Homo sapiens in Africa
~200
First possible beadwork
~100
Earliest engravings, heat-treatment of silcrete
~75
Exodus of cognitively symbolic Homo sapiens from Africa
70–60
First modern humans in Australia
60
First modern humans in Europe, flowering of art and symbols
40–30
Extinction of Neanderthals, Homo erectus
~30
Homo floresiensis extinct
14
Last Ice Age ends
12
Plant cultivation and animal domestication begin
11
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 th
e 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.
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