Lone Survivors (33 page)

Another possibility is that there was interaction between human populations that had been distinct for much longer than 200,000 years, which implies contact between modern and archaic humans such as Neanderthals or, as we have seen, the Denisovans. The kinds of interaction cannot be determined, but it could range from interbreeding to contact with bedding, through to aggressive confrontations or even cannibalism, where lice could have jumped from victims to the perpetrators. As an example of the latter, historical studies showed that the Torres Straits islanders, living between New Guinea and Australia, used to keep the heads of both their deceased relatives and their enemies. In the latter case, this sometimes involved eating parts of the face and eyes of the trophy head; such behavior in the past could have allowed the spread of parasites between distinct human populations and even species. If they were able to jump species before their host populations became extinct, these louse lineages literally gave themselves a new lease on life. And of course the transfer of pathogens could have gone in the reverse direction too, and it remains possible that infections of one kind or another brought out of Africa by modern humans contributed to the demise of archaic humans elsewhere.

In this chapter we have discussed the data within our genomes and those of the Neanderthals and Denisovans: genomes that document the evolution of, and at least occasional contact between, these closely related lineages of humans. The evidence confirms that we have a predominantly recent African origin, but worldwide our species is not purely and entirely Out of Africa. Within that continent, our ultimate ancestors were few in number and probably lived in small pockets. Our earlier discussion of the development of behavioral modernity showed its patchy genesis across Africa—a genesis that I compared to brief episodes, like a candle flickering and then being extinguished. So what finally changed to keep that flame burning and then intensified, in order for our species to begin its seemingly inexorable rise to world domination? There are many ideas and theories, and I will start to explore them in the next chapter.

8

Making a Modern Human

As I explained in the first chapter, when I began my doctoral research in 1970, the origin of modern humans was hardly recognized as a specific topic worthy of scientific study. The standard classification of humans had living people, the Neanderthals, and diverse remains from sites like Broken Hill in Africa and Solo in Java all classified as members of our species. With such different-looking fossil members within
Homo sapiens
, the origins of features like a chin, a small brow ridge, and a globular skull were, not surprisingly, lost in a morass of diversity. Moreover, with the predominance of Multiregional or Neanderthal Phase models, the origins of those features were apparently scattered among many different ancestors living right across the Old World, so modern human evolution was not so much an event as a tendency; we were merely the end result of continuing long-term trends in human evolution in features like increasing brain size and decreasing tooth and face size. For human behavior too, there was an emphasis on gradual evolutionary trends; for example, in France the “transition” from the Middle Paleolithic Mousterian to the Upper Paleolithic Gravettian via the Châtelperronian industry was seen as supporting a parallel local evolution from Neanderthals to Cro-Magnons.

It looks very different forty years on. For most scientists, Africa has been established as the center for both our physical and our cultural origins. The evolution of “modern”
Homo sapiens
can be viewed as an important physical and biological event, backed up by both fossil and genetic evidence. Many researchers would also trace back to Africa the origins of the complex behavior apparent in the Upper Paleolithic figurines and painted caves of Europe. And yet, much as I am delighted with the way the subject of modern human origins has taken off to become one of the most dynamic areas of research in paleoanthropology, I am still puzzled by many aspects of the African origin of our species. When I look critically at what we do know and, more important, what we still don't, I feel we are not yet close to a full understanding of those origins, as I hope to explore in these final chapters.

In the 1980s the issue for people like me, Günter Bräuer, and Desmond Clark was to get people to take the idea of a recent African origin for modern humanity seriously at all, let alone discuss how that origin might have come about. In what was a real struggle against some very influential and at times vitriolic opposition, I'm sure at times we oversimplified both our views and those of the multiregionists, and played down complexity in the data, in what became an increasingly polarized and sometimes bitter debate. At times also, as my views on a recent African origin developed, I favored the idea that our species evolved very rapidly in one small area—a sort of African “Garden of Eden.” But the general view has been that there was probably a relatively gradual evolutionary sequence in Africa from archaic humans (
Homo heidelbergensis
, sometimes also called
Homo rhodesiensis
) to our species,
H. sapiens
.
Heidelbergensis
fossils in both Africa and Europe are dated to about 500,000 years old, while, as we have seen, fossils representing our species have been found in Ethiopia at Omo Kibish and Herto, dating to between 160,000 and 195,000 years, with more fragmentary remains from Guomde in Kenya perhaps 250,000 years old. The assumption has been that a gradual accumulation of modern characteristics in Africa would have paralleled a comparable buildup of Neanderthal traits in Europe, from a similar
heidelbergensis
ancestor.

What triggered the evolution of modern humans in Africa, and why this happened at all, is still uncertain. Did social or technological advances promote evolutionary change, or was geographic isolation following severe climatic change responsible? It is not yet even clear where the first “modern” population or populations lived, but the areas of eastern and southern Africa have vied for the title “Cradle of Modern Humanity.” The fossil record has highlighted Ethiopia and Kenya in East Africa as the most probable location for our origins, but this is also the region with the best fossil record for the period. In contrast, South Africa has a poorer fossil record but a much richer behavioral one for the Middle Stone Age, which is why some workers claim that region as the real focal point for modern human origins. Recent discoveries also shifted the focus farther north to Morocco, where reevaluation of previous finds and the discovery of new ones suggest that even northwest Africa cannot be excluded as a center for modern human origins. We must also remember that at least 50 percent of African regions that have stone tools from this period have not yielded a single fossil human relic to show us who was making the tools in question. So bearing these points in mind, I would like to take a fresh look at several aspects of our evolution that we already discussed, ranging from biology to behavior, to the role of climate in our evolution, in the hope of throwing further light on our mysterious African origins.

First we'll look at the brain, because one leading theory about our African origins—that of the archaeologist Richard Klein—argues that the development of modern human behavior came about suddenly around 50,000 years ago, as a result of genetic mutations that enhanced the workings of our brain, essentially making us “modern” at a stroke. A similar view is espoused by the neuroscientist Fred Previc, who highlights the importance of the neurotransmitter dopamine to human creative thought and hypothesizes that it reached critical levels about 80,000 years ago, driving behavioral evolution to modernity. Unfortunately it is very difficult to test such ideas properly from the surviving evidence, since while we can make a real or a virtual model of the inside of a fossil skull, that will only reflect the external shape and proportions of the ancient brain that was once inside that skull. Such a model can tell us nothing about the internal workings and wirings of the once-living brain, which would have contained billions of interconnected nerve cells. However, from such data we do know that during human evolution our brains have certainly increased in overall volume relative to body mass (this ratio is known as the
encephalization quotient
, or
EQ
). Early humans had EQs of only about 3.4 to 3.8, and this even included
H. heidelbergensis
individuals, who had human-sized brains but much larger bodies than the average today. More evolved humans such as our African ancestors prior to 200,000 years ago and Neanderthals had EQs between about 4.3 and 4.8, and when we arrive at early moderns such as those from Skhul and Qafzeh and the Cro-Magnons, EQ reaches its highest values at around 5.3 to 5.4.

Since then,
H. sapiens
seems to have leveled off at those values or has even suffered a slight decline in EQ. But in brains, as in many other things, size isn't everything, and we can infer that there must also have been significant reorganizations in the human brain for activities like toolmaking and speech. In order to maximize the surface area of the outermost cortical layer of our brain (the “gray matter,” which includes nerve cells and their interconnections), it is complexly folded into convolutions, thus allowing our cortical surface area to be about four times that of a chimpanzee's, matching the increase in overall brain volume. While there have been many careful studies of the impressions of these convolutions of
sulci
(furrows) and
gyri
(ridges) on the inner surface of fossil braincases, such markings are often faint and difficult to interpret. Work done in the last century on the fake Piltdown skull's convolutions found many supposedly apelike features, and yet we now know that the skull in question was actually that of a recent human, so much of that work consisted of wishful thinking or even fantasy—I even compared some of the old work to the pseudoscience of phrenology. Yet another approach to the analysis of ancient brains has focused on changes in the relative proportions of the various components rather than their convolutions, as these can be determined quite well from the preserved inner surface of the braincase or from CT data.

The cortex or cerebrum is by far the largest part of the brain in humans and is divided centrally into two cerebral hemispheres—the left and right—which have different specializations, but which are interconnected by bundles of nerve fibers. The cerebral hemispheres are made up of four lobes, corresponding in position to the cranial bones of the same name: the frontal, parietal, temporal, and occipital lobes. We know quite a lot about the general roles that these lobes play in our brain: the frontal is involved with thinking and planning; the parietal in movement and the senses; the temporal with memory, hearing, and speech; and the occipital with vision. Tucked underneath and behind the cerebral hemispheres is the smaller cerebellum, which is predominantly concerned with regulation and control of the body.

However, recent studies showed that the cerebellum also plays a role in many so-called higher functions and is extensively interconnected with the cerebrum. As well as regulating body functions, it seems the cerebellum is also concerned with the processes of learning. The increase in both gross brain size and EQ really took off about 2 million years ago, soon after the first clear archaeological evidence of both meat eating and toolmaking appeared in Africa. All areas of the brain enlarged, but proportionately the cerebral hemispheres increased more than the cerebellum. The pace of cerebral enlargement accelerated in
heidelbergensis
and peaked in Neanderthals and early moderns, seemingly correlated with an increase in behavioral complexity. But interestingly, in recent humans this long-term pattern has reversed, since the cerebellum today is proportionately larger. At the moment it is not clear what, if anything, this change signifies. On average, human brains have shrunk some 10 percent in size over the last 20,000 years, so is the cerebellum having to maintain its size more than the cerebrum, or, as some have claimed, does a relatively larger cerebellum perhaps provide greater computational efficiency? We simply do not know the answer yet.

It is certain, however, that the overall shape of the brain and the braincase that envelops it changed from archaic to modern humans, becoming shorter and higher, narrower lower down and broader higher up, with a particular expansion in the upper parietal area. Brain shape is inevitably closely matched to skull shape, since the two must develop and grow in harmony—but which is the driver and primary determinant of their closely matched shapes? This is not a simple question to address, since even the braincase and the brain do not grow and exist in isolation. For example, the base of the skull anchors the upper parts of the vocal, digestive, and respiratory tracts and articulates the head and the spine, while the front of the skull contains the teeth and jaws and the muscles that work them. Those factors seem to have constrained brain shape from changing much in those regions.

But the upper areas of the skull and brain are not so constrained, and my Ph.D. results back in 1974 highlighted changes in the frontal, parietal, and occipital bones of modern humans. Each contributed to the increased globularity of the braincase in modern
sapiens,
and our domed forehead is particularly noticeable. There are data from my collaborative research with Tim Weaver and Charles Roseman that suggest that many of these cranial changes might not be significant in evolutionary terms and could simply be the result of chance changes (genetic drift) as modern humans followed their own separate path in isolation in Africa. I will return to this issue in chapter 9, but nevertheless the cranial shape of modern humans is so idiosyncratic, compared with the patterns found in all the other known human species, that I do think it is worth considering whether brain evolution might lie behind our globular vault shape. This is particularly so as research by Philipp Gunz and his colleagues suggests that this shape change began to separate archaic and modern skulls soon after they were born.