Genetics of Original Sin (15 page)

We must remember that the prehistory of our species is known to us only through an extremely small number of fossil fragments—often no more than a few teeth, a jawbone, or a femur—tiny bits of evidence left over huge expanses of time. It is like having a kneecap or wisdom tooth from Moses as sole trace of the whole of recorded human history. Anthropologists have proved remarkably perspicacious detectives, capable of squeezing information out of even the slightest of clues, such as a scratch on a tooth or the tip of a toe bone. Nevertheless, linking these sparse fragments of evidence into a coherent historical narrative is still a highly risky exercise. Considering the frequency of evolutionary branching, on one hand, and of
evolutionary convergence, on the other, to mention only the most obvious causes of uncertainty, there is little assurance that the clues refer to a single story. It is certainly impressive, in this respect, to find that all the most ancient pre-human vestiges discovered so far, with the exception of Toumaï, have been found in East and South Africa. Until proven otherwise, this part of the world appears as the birthplace of our species.

Note that the African origin of humankind is hardly in doubt. Not a single sign of hominid presence earlier than two million years ago has been detected anywhere else in the world. The question is whether the adventure unfolded only in the eastern part of the African continent. Focus on this area could well be due to chance factors, such as fossil preservation and accessibility, the influence of earlier findings on subsequent research, the whim of investigators, their origin and nationality, the political constraints on their work, not to mention an accidental observation by a completely naïve person, as has happened several times (in Lascaux, for example). Other regions, such as Chad, could well hold some surprises in store for future investigators.

The second stage in this extraordinary saga, still restricted exclusively to Africa, took place around two and a half million years ago, with the appearance of more humanlike forms called
Paranthropus
(“next to human”) or, anticipating the third stage,
Homo,
which is Latin for human. Characteristic of this stage is a sizable expansion of the brain volume, accompanied, presumably, by enhanced manual skill and the related development of stone tools.

Tools, contrary to what one might suspect, are not a
human invention. Many animals use stones or sticks to break, dig, or even kill. The most intelligent primates, such as chimpanzees and bonobos, are even known to fashion a tool with what seems like a purpose in mind. A typical example, first reported by Jane Goodall, the pioneer of primate research in the wild, is that of chimpanzees denuding a branch, sticking it into a termite nest, waiting a few moments, and pulling it out again to eat the insects that cling to it. It is difficult not to see some intentionality behind such behavior. The devising of tools by Stone Age hominids is no more than a continuation and elaboration of this primate faculty, but manifesting considerably greater foresight and skill.

These tools can be seen in any museum of natural history or anthropology. You will find them ranged in orderly fashion, from roughly hewn pebbles, with only a few flakes chopped off, to exquisitely honed axes and blades of various shapes, obviously adapted to specific uses. What you may not realize is that the evolution of tool-making skills covers more than two million years, more than two thousand millennia. In comparison, it took us only a small fraction of a millennium to move, for example, from the abacus to the slide rule, and even less from the slide rule to the computer. The difference is clear. The jump from abacus to computer was accomplished by the same kind of humans with the same kind of brains, who shared and transmitted their experience by communication. Computers are products of
cultural
evolution. Prehistoric stone tools are the products of
biological
evolution. The first tools were made by prehumans, appropriately called
Homo habilis,
handy man, with brains only half the size of our brains today. The improvement of the tools reflects the increase in the size and complexity of the prehuman brain and the gain in manual dexterity that went together with this increase by way of back-and-forth interactions between brain and hands. Cultural transmission,
of course, was also involved. Even chimpanzees have a culture. The point is that mental and manual ability were pace-limiting factors of technological progress. Hence the extreme slowness of tool development.

The next stage on the road to humankind was inaugurated some two million years ago by two new groups,
Homo ergaster
(from the Greek
ergon,
work) and, especially, the anachronistically named
Homo erectus
(his ancestors were upright for more than two million years), who persisted for a particularly long time. These newcomers could almost be called human. They were tall (about six feet), had brains up to two-thirds the size of a modern human brain, probably had lost most of their body hair, walked and even ran, if necessary, on two legs, and manufactured sophisticated tools. They also formed more closely knit and socially organized bands than did their predecessors, built shelters, centralized tool-making and butchering in special areas, and, at some stage, learned to use the fire ignited by lightning, husband that fire, and, eventually learn to light it on their own. They no longer lived only on gathered fruits, roots and tubers, and on carrion stolen from predators; they started hunting on their own. Most impressive, they began to move out of Africa.

Those migrations are amazing, considering the physical fragility of the migrants. Brain power, no doubt, was a major asset in their success. Bands of
Homo,
presumably driven by climatic changes and by the movements of the herds from which they drew their subsistence, spread out east of North Africa, to invade much of Asia, up to China and Indonesia,
where they left some of the first hominid fossils to be discovered outside Europe, including the famous
Java Pithecanthropus
(literally ape man), so named by the Dutchman Eugene Dubois, who discovered it in 1891, as well as
Sinanthropus,
or Peking man, which contributed to the celebrity of the French Jesuit Pierre Teilhard de Chardin.

Other bands moved north and northwest to create settlements in many parts of Europe. A late wave is represented by
Homo heidelbergensis
—first discovered in Germany, as its name indicates, but present over much of Europe, Africa, and even Asia—which was still around less than half a million years ago.

Surprisingly, these first invading groups, which at some time occupied much of the Eurasian continent, all died out, leaving Africa as the sole point of departure of the final stage that gave rise to modern humans. This question has long been disputed, pitting the partisans of polygenism, who believe that several distinct lines have independently produced representatives of
Homo sapiens
in different parts of the world, against those who defend monogenism. As long as only fossils were available to settle the issue, the matter remained debatable, though with majority opinion slowly veering toward monogenism, much to the satisfaction of those who put their faith in the Bible. The clinching argument has come from molecular phylogenies.

We have seen (
chapter 3
) that mitochondria, which are the major centers of oxidative energy production in the vast majority of eukaryotic cells (including human cells), are derived from bacteria that were adopted as endosymbionts. The most convincing proof of this origin is provided by the pres
ence in mitochondria of small amounts of DNA, inherited from the ancestral bacterium. We have also seen (
chapter 5
) that, in sexually reproducing organisms, the male sperm cells lose their mitochondria in the course of maturation, so that these organelles are transmitted from generation to generation exclusively through the female line, by way of oocytes. Comparative sequencing of samples of human mitochondrial DNA collected in various parts of the world has allowed a reconstruction of this history, which leads back to a single ancestral female—the so-called mitochondrial Eve—who lived somewhere in Africa about two hundred thousand years ago. Similar studies on the male-specific Y chromosome have likewise led to a “Y Adam,” who dwelled in Africa at about the same time.

It should be noted, for those who would like to see a biblical analogy in these findings, that they do not refer to a specific couple, but to two unspecified individuals who are estimated, on the basis of theoretical calculations, to have been part of an initial population of about five thousand members of each sex sharing the same genetic endowment. As time went on, all but one of the same-sex lines initiated by members of this population were cut short by the failure of females to beget daughters or of males to beget sons, leaving only two uninterrupted lines, one female, the other male, going back to two individuals that most likely never have had anything to do with each other. Y Adam almost certainly never copulated with mitochondrial Eve.

According to these findings, all extant human beings are descendants from this single African branch. Contrary to the polygenism hypothesis, the bands of
Homo ergaster
and
Homo erectus
that spread over much of Eurasia one and a half million years ago all became extinct long before our times without leaving any extant descendants. Hopes that some may have
survived much later were recently lit by the discovery, in 2004, on the Indonesian island of Flores, of the remains of some pygmy-like individuals, which were assumed to go back to a group of
Homo erectus
that had become stranded on the island and had undergone dwarfism, as is known to happen to isolated species under such circumstances. Doubts have since been expressed as to the reality of so-called
Homo floresiensis,
which could be no more than an abnormally developed modern human. The matter is still being debated as I write this book.

The fact that, after all these wanderings, the final step to humanity should have started from a single point in Africa, its early cradle, is remarkable. It could be mere coincidence. Or, perhaps, the core process of hominization never left the African continent, and only lateral branches have spread out over other territories.

Among the acquisitions that marked this final stage, a special place must be given to the capacity for speech. All mammals communicate by sounds. Some even have a rudiment of a language, with different sounds having different meanings. Vervet monkeys, for example, use different alarm calls to signal the presence of a leopard, an eagle, or a snake. However, even the most meaningful sounds emitted by mammals, including the higher apes, are no more than varieties of grunts or shrieks, nothing like the articulated sounds used for communication by human populations all over the world. The difference is not just mental.

In all nonhuman mammals, the larynx, or voice box, is situated close to the pharynx, the funnel-shaped conduit that leads from mouth to esophagus. This proximity severely limits
the kinds of sounds that can be produced. Humans stand out from all other mammals by having their larynx situated distinctly lower, which endows them with their unique ability to emit the wide variety of sounds that allows articulated speech. In typical evo-devo manner, this evolutionary history is “re-capitulated” in early child development. Newborns have the typical larynx-to-pharynx closeness of the other mammals and, as a result, can emit only unformed cries. In the course of the first two years of development, the baby's larynx slowly descends, and the ability to speak appears. Interestingly, in acquiring language, we lost the ability to drink and breathe at the same time. Apes and babies can do this; human adults can't.

According to cranial castings, the larynx probably started descending only in the last stage of hominization, sometime between a hundred thousand and fifty thousand years ago. This crucial event may well have signaled what is sometimes called the “great breakthrough” or the “great leap forward,” the dramatic expansion of human achievements that occurred in the last fifty thousand years. In the span of a few millennia, tools of much greater sophistication, including needles and awls, harpoons, spear points, and bows and arrows, began to be manufactured, not only out of stones but also out of bones, horns, antlers, wood, and other biological products. Animal skins were prepared and converted to protective clothing. Vegetal fibers were worked into ropes and nets. Settlements became better organized on a communal basis. Shelters were constructed, and fire became routinely used for heating and cooking. Big game hunting was developed to become an efficient, cooperative activity, employing specially designed weapons. Ships were built and used, not only to follow rivers or coastlines, but even to cross wide expanses of water. Especially, artworks and jewels started to be made, and the dead began to
be buried according to special rites. Awe and fear became expressed in what may have been the first religious ceremonies.

These skills were not all developed in Africa, but the aptitude to exercise them must have done so, to be spread over all parts of the world in a new wave of migrations, even more extraordinary than the preceding one. Not only did the humans of those days retrace the earlier wanderings of their ancient
ergaster
and
erectus
predecessors into Europe and Asia. They went on to the extreme Far East, even crossing over to Australia in what must have been a most hazardous passage. At the other end, they reached north Russia and walked across the land junction that spanned the Bering Strait at that time, invading the American continent long before it was discovered by Christopher Columbus. Whether they crossed the Pacific Ocean from Polynesia to South America on balsam rafts, as was done successfully on his
Kon Tiki
by the Norwegian explorer Thor Heyerdahl to substantiate this theory, is more problematic.