H. heidelbergensis, present in Africa and Europe more than 500,000 years ago, shows combinations of features found in the more primitive erectus fossils and those found in the later Neanderthal and modern sapiens fossils, as befits a possible intermediate species: a brow ridge like erectus, but often filled with extensive sinuses (voids); an occipital bone like that of erectus; a wide interorbital breadth like erectus; a superior pubic ramus like erectus; an iliac pillar like erectus; rounder femora like erectus; brain volumes that overlap the smaller values of erectus and the larger ones of H. sapiens and H. neanderthalensis; a braincase higher than erectus, and parallel-sided in rear view; a face intermediate between erectus and later humans in its overall projection from the braincase; a temporal bone more like those of sapiens and neanderthalensis; a tympanic like Neanderthals and moderns; increased projection of the middle of the face and nose (as in Neanderthals and moderns); and, in some cases, inflated cheekbones that retreat at the sides, like those of Neanderthals.
The Neanderthals are advanced humans and thus share features with both heidelbergensis and with us. Yet there are also some retained primitive traits and those that betoken a separate evolutionary pathway. They have an elongated superior pubic ramus like erectus and heidelbergensis; rounder femora like erectus and heidelbergensis; a large brain volume like ours; a high and arched temporal bone like ours; reduced interorbital breadth; reduced total facial projection; a lightly built tympanic; in many Neanderthals, simplification and shrinkage of tooth crowns as in sapiens; weak or absent iliac pillar.
Then there are the features that seem to distinguish the Neanderthals as an evolutionary lineage. Some of these are concerned with a distinctive body shape, rib cage, and limb proportions, but the clearest ones are on the skull: a double-arched brow ridge with central sinuses; a double-arched but small occipital torus with a central pit (the suprainiac fossa); a spherical vault shape in rear view; distinctive shape of the semicircular canals of the inner ear (see chapter 3); strong midfacial projection and cheekbones that are inflated and retreat at the sides; a high, wide, and projecting nose; large and nearly circular orbits; a high but relatively narrow face; and enlarged front teeth (incisors), which are hollowed (shoveled) on the inside surfaces of the upper centrals.
(Clockwise from top left) Skulls of erectus (Sangiran, Java), heidelbergensis (Broken Hill, Zambia), sapiens (Indonesia), and neanderthalensis (La Ferrassie, France).
The feature that stands out (literally) in these comparisons of modern and archaic species is the strong brow ridge of the latter, and its absence in the former. The anatomist Hermann Schaaffhausen, one of the first describers of the original Neanderthal skull, called its strong brows “a most remarkable peculiarity,” and although there have been many scientific hypotheses to explain their presence or absence, none really convince me. The fact that many of the huge brow ridges in fossils are hollowed inside, with large sinuses (air spaces), suggests that they are not there to bear or transmit physical forces from blows to the head or heavy chewing. The eccentric anthropologist Grover Krantz even strapped on a replica brow ridge from a Homo erectus skull for six months to investigate its possible benefits, finding that it shaded his eyes from the sun, kept his long hair from his eyes when he was running, and also scared people out of their wits on dark nights. For me, that last clue might be significant, and like the paleontologist Björn Kurtén I think it may even have had a signaling effect in ancient humans, accentuating aggressive stares, especially in men. Thus its large size could have been sexually selected through the generations, a bit like antlers in deer. But if that is so, why don’t we have large brows like our predecessors? Well, I think the rest of this book will show that modern humans have developed so many other ways to impress each other, from weapons to bling, that perhaps the selective benefits of large brows wore off in the last 200,000 years.
(Clockwise from top left) Side view of skulls of erectus (Sangiran, Java), heidelbergensis (Broken Hill, Zambia), sapiens (Indonesia), and neanderthalensis (La Ferrassie, France).
If there were, in fact, different human species in the past, could they have interbred? In my view, RAO has never precluded interbreeding between modern and archaic people during the dispersal phase of modern humans from Africa. This is undoubtedly one of the main areas of confusion in studies of modern human origins: how to recognize species in the fossil record, and what this signifies. Some researchers argue that many distinct morphological groups in the fossil record warrant specific recognition, with the existence of at least ten such species of the genus Homo during the last 2 million years (that is, Homo ergaster, erectus, georgicus, antecessor, heidelbergensis, rhodesiensis, helmei, floresiensis, neanderthalensis, sapiens).
At the other extreme, some multiregionalists argue that only one species warrants recognition over that period: Homo sapiens. An additional complication is that different species concepts may become confused; for example, some multiregionalists have applied what is called the Biological Species Concept (BSC) to the fossil record to justify their belief that H. neanderthalensis and H. sapiens must have belonged to the same species and would have been fully interfertile. This concept, developed from the study of living organisms, argues that a species consists of the largest community of a group of plants or animals that breeds among itself, but not with any other community. Thus it is “reproductively isolated” with reference to other species, but its own varieties can interbreed with each other. Living Homo sapiens would be a good example of this, since people from all over the world are potentially able to mate and have fertile children, but we are apparently reproductively quite distinct from our ape cousins. I say “apparently” because there are persistent rumors that in the 1940s and 1950s scientists in the United States and/or the Soviet Union conducted unethical experiments impregnating female chimpanzees with human sperm—the results of which, so the rumors go, have been suppressed.
And what if we could still meet a Neanderthal—could modern humans interbreed with one? First this brings up the potential conflict between the BSC (which relates to living species) and the completely different concepts that I just used to recognize species in the fossil record, such as the degree of variation in the skeleton. Using the latter measure (a morphological species concept based on what is preserved in the fossil record), I and many other anthropologists recognize the Neanderthals as specifically distinct from Homo sapiens. But there is a conflict at the heart of the BSC: the fact that many closely related mammal species can hybridize and may even produce fertile offspring. Examples are wolves and coyotes, bison and cows, chimps and bonobos, and many species of monkey. So we have to recognize that species concepts are humanly produced categories which may or may not always work when compared with the reality of nature. So in my view, even if there was Neanderthal-modern hybridization (and I will discuss that thorny question in chapter 7), it would not necessarily mean that Neanderthals belonged to the same species as us—it would depend on the scale and impact of the interbreeding.
Fossils—the relics of ancient species—sparked my interest in the distant past when I began collecting them as a boy, and they still fascinate me. But on their own they are just mineralized and inert bones and teeth. In the next two chapters I will show how a range of exciting new techniques are helping us return these inanimate fossils to their ancient environments and bring them back to life.
2
Unlocking the Past
Just down the corridor from my room at the museum, locked in their own special cabinet, are some of the most notorious relics in the story of human evolution, mentioned already in chapter 1—Piltdown Man. They were found and announced, to an unsuspecting world, about a hundred years ago, and they provide a sobering reminder to all scientists to beware of something that seems too good to be true—because it may well not be true! British paleoanthropologists of the time had seen German, Dutch, and French scientists discover fossils of possible ancient ancestors, but Britain had nothing to compare with these. Moreover some
of these British experts were, as we have seen, supporters of the view that our species had a deep and separate evolution from people like Java Man and Neanderthal Man. Imagine their delight, then, when a “missing link” was apparently discovered in their own backyard, in the county of Sussex. It seemed to have an apelike jawbone and a very human braincase, and these were combined to make the ape-man called “Eoanthropus dawsoni.” Of course we now know that its “ape” jaw and “human” skull were exactly that—two completely different and relatively recent specimens maliciously combined to create a misleading transitional fossil. But the hoaxer or hoaxers were knowledgeable enough to not just rely on anatomy to fool the experts—they knew enough about how fossils were dated in 1912 to also misuse that knowledge, to make a case that the Piltdown assemblage of bones and stone tools were as ancient as the remains of Java Man. They were able to get away with it because none of the physical dating techniques that I will discuss in this chapter (such as radiocarbon dating) were known a hundred years ago, and, instead, human fossils could really only be relatively dated—that is, dated in relation to the material found alongside them. The hoaxer(s) planted genuine fossils of primitive mammals from other sites alongside the remains of Piltdown Man, so they would look suitably ancient. In 1953 the whole sorry story began to unfold, and when radiocarbon dating was finally applied to show the ape and human remains were both less than a thousand years old, it was the ultimate nail in Piltdown Man’s coffin!
So, in this chapter, I will show how new dating techniques have revolutionized our view of human evolution in each of the main regions and time periods in which they have been applied, and I will use a variety of examples to look at the way the records of past climates and environments are influencing the story of our evolutionary origins. We now think that Neanderthals and modern humans evolved along parallel paths, the former lineage north of the Mediterranean and the latter south of it, in Africa. After several false starts, modern humans finally emerged from Africa and spread along Asian coastlines toward China and Australia. But Europe, perhaps the last bastion of the Neanderthals, seems to have remained beyond modern reach until about 45,000 years ago. We have only recently dated some of the most important human fossils, work that has revolutionized the time scale of our evolution. Fascinating new environmental and archaeological evidence also shows the complexity of the process of our evolution, and of the extinction of our close relatives the Neanderthals.
There are two main categories of dating: relative and physical (that is, based on the laws of physics, sometimes also called radiometric or absolute) dating. The first relates an object or layer to another object or layer in time; one may be younger than the other, or (within the limits of the method) they may be about the same age. The geologic law of superposition supposes that, unless there has been major disturbance, a layer in a geologic sequence is always younger than the layer below it; this is the main principle at work in relative dating. More rarely, a geologic event such as a tsunami or a volcanic eruption can be traced across a region, and fossils or artifacts associated with that event can be assumed to be contemporary with it, and thus with each other. But such relative dating cannot tell us how old the materials in question actually are; it can only place them in relation to each other—that is, show they are relatively older, younger, or correlated (similar) in age. Thus if I dug in my garden and found Roman pottery that looked similar to pottery found at, say, Fishbourne Roman Palace in Sussex, I could assume that my finds were about the same age as those at Fishbourne; but without independent evidence of how old Fishbourne Palace was, or the pottery was, that would be as far as I could go. I could get more detailed relative dating by, say, researching the age of Roman coins found at Fishbourne, or I could attempt a physical determination by asking a specialist in luminescence dating (see discussion later in this chapter) to use physical signals within the clay of my pottery to tell me how long ago it had been fired.
So to go farther than a relative date, we need physical clocks that will tell us how far back some rocks were laid down, how long it is since an animal or plant died, or when an event happened, such as the heating of clay or flint. Many of these clocks measure time using the natural radioactive decay of isotopes. Isotopes are distinct atoms of substances, such as argon or carbon, that have different atomic weights (because they contain different numbers of particles called neutrons). An example of such a technique is potassium-argon dating, which can be used on volcanic rocks. Potassium partly consists of an unstable isotope called potassium-40, and this isotope gradually changes over many millions of years into the gas argon. When there is a volcanic eruption, the liquid lava or hot ash contains a small proportion of potassium-40, and when the lava or ash cools and solidifies, this unstable isotope of potassium begins to change into argon, such that half of it decays into argon about every 1.25 billion years (this is its half-life). Provided the volcanic eruption was sufficiently energetic to drive out any previous argon gas (usually a reasonable assumption), and provided any newly formed argon gas remained trapped in the volcanic layer once it set hard, the amount produced can be used as a natural measure of time since the volcanic rock was deposited. In one of the first and most famous applications of this technique to archaeology, lava at the base of the site of Olduvai Gorge in Tanzania was shown to be about 1.8 million years old. This caused a sensation in 1960 because it indicated for the first time just how ancient the artifacts and humanlike fossils in Bed I at Olduvai might really be, doubling their expected age at a stroke. A more recent development from potassium-argon dating is to use the decay of argon-40 to argon-39 instead, since this can be used to date single crystals of volcanic rock with much greater accuracy over the time span of human evolution.
The most famous physical dating method is radiocarbon dating, based on an unstable form of carbon. The method relies on the fact that radiocarbon (an isotope of carbon called carbon-14) is constantly produced in the upper layers of the Earth’s atmosphere by cosmic radiation acting on the element nitrogen. This unstable form of carbon gets taken up into the bodies of living things, along with the much more common, and stable, carbon-12. However, when the plant or animal dies, no more carbon-14 is taken in, and the amount left begins to break down by radioactive decay, such that the amount present halves about every 5,700 years—a very much shorter time span than that of potassium-argon dating. So measuring the amount of carbon-14 left in, say, a piece of charcoal or a fossil bone allows us to estimate how long it is since the plant or animal concerned was alive.
In 1949 the American chemist Willard Libby and his colleagues first applied it to a sample of acacia wood from the tomb of the pharaoh Zoser (who lived nearly 5,000 years ago). Libby reasoned that since the half-life of radiocarbon was close to 5,000 years, they should obtain a carbon-14 concentration of about 50 percent of that found in living wood, and this was confirmed. That work, and much that followed, earned Libby a Nobel Prize in 1960. The method cannot be used on very ancient materials because the amount of carbon-14 left behind is too small to measure accurately, and hence radiocarbon dating becomes increasingly unreliable beyond about 30,000 years ago. Moreover, the assumption of constant production and uptake of carbon-14 is now known to be only an approximation, due to past fluctuations in cosmic rays and changes in the Earth’s atmospheric circulation—thus scientists talk of dates in radiocarbon years rather than real (calendar) years.
This means that other methods are needed to cross-check (calibrate) the accuracy of radiocarbon dates. Several methods have been particularly useful for dates in the last 10,000 years or so, and all of them require the counting and dating of annual layers. The first uses tree rings (dendrochronology) and builds up overlaps in patterns of growth rings from timbers preserved in buildings, boats, or natural deposits, in order to establish a long sequence where the age assessed from the wood is compared with a radiocarbon date obtained on rings within the wood. A comparable method uses varves (annually deposited layers in the bottom of deep lakes), wher
e spans of time can be measured through counting varves, and also by radiocarbon dating of plant or animal residues within the varves. Yet another method uses radiocarbon dates obtained within annual layers of ice, and this can be taken even farther since trapped bubbles of gas in the ice preserve a snapshot of the composition of the atmosphere when a particular layer was deposited. Beyond these methods, very ancient trees preserved in bogs in New Zealand hold the promise of accurately calibrating radiocarbon to beyond 40,000 years, while ancient coral terraces can be dated both by radiocarbon and by uranium-series dating (discussed later in this chapter), giving a cross-check between independent physical dating methods, each with different assumptions.
Comparisons so far suggest that radiocarbon dating, while not exact over the last 40,000 years, is quite reliable, although sometimes off by as much as 10 percent. Unfortunately, one of its least accurate phases covers the demise of the Neanderthals and much of the spread of modern humans around the world—hence the need to further refine radiocarbon dating or supplement it with other methods wherever possible, as I will explain later in this chapter.
Many technical improvements have been made in radiocarbon dating procedures since Libby’s initial work. For example, he analyzed solid carbon, while later techniques convert the carbon to gas or dissolve it in solvents. The early methods also required large sample sizes to detect radiocarbon decay, so that important artifacts or bones had to have large chunks sawn off them to attempt a date—permission for which was understandably often refused by concerned museum curators. Luckily, from about 1977, the method of accelerator mass spectrometry (AMS) has increasingly taken over, and this counts individual atoms of carbon-14 directly, rather than measuring their radioactivity. So now only milligram-sized samples are needed, allowing the dating of relics as precious as the Shroud of Turin, the Dead Sea Scrolls, the Alpine iceman “Ötzi,” and the Ice Age art of the Lascaux and Chauvet caves.
Lone Survivors Page 4