Lone Survivors (8 page)

As with radiocarbon dating, procedures have continuously been refined, so now even single grains of sand can be dated by luminescence. Equally, in the case of ESR, where previously a large chunk of a tooth had to be sacrificed, we have moved to a situation in which, using the microscopic technique of
laser ablation
, it is now possible to directly date a tiny area of fossil human tooth enamel. Another potential complication of ESR dating is the fact that fossils take up uranium when they are buried; hence they contribute to their own accumulated radiation dose. Estimating the rate of uranium uptake is critical (did most of it get in soon after burial or did it come in gradually?), but this unknown can now be addressed by combining, or “coupling,” an ESR determination with a U-S date on the same piece of enamel, and looking for the age estimate that is most compatible when comparing the two.

An excellent example of the tremendous impacts that luminescence and ESR dating have made on human evolution came out of the Middle East, from the famous Israeli caves of Tabun and Skhul (Mount Carmel) discussed in chapter 1. I was fortunate enough to be involved in some of the pioneering work on dating these sites in the late 1980s and early 1990s, since the Natural History Museum has a share of the human fossils, artifacts, and sediments from them.

Ranges of the main dating methods for recent human evolution.

These fossils had a key role in developing ideas of Neanderthal–modern human relationships: essentially, did they represent one single rather variable population, perhaps 40,000 years old, or did the more modern-looking Skhul people succeed Neanderthals like Tabun, and perhaps evolve from them? Even when more Neanderthal fossils were added to the mix from Israeli sites like Amud and Kebara (both with quite complete skeletons from apparent burials), and more modern-looking skeletons were added from the site of Qafzeh (near Nazareth), the picture did not get any clearer. Relative dating using similarities in stone tools suggested that they were all rather close in age, while a radiocarbon date on some charcoal from Tabun suggested the Neanderthal from there was not much more than 40,000 years old. In turn, based on the known succession of Neanderthals and modern humans in Europe about 35,000 years ago, it was assumed that a similar sequence would be found in the Middle East, though perhaps it would be a little older. Thus in the early 1980s it seemed reasonable to assume that the Qafzeh and Skhul “moderns” were about 40,000 years old, and two different evolutionary scenarios were proposed in the region. Erik Trinkaus favored the view that the Neanderthals had evolved fairly rapidly into moderns there, while I took the view that there had been a replacement of the Tabun people by the Skhul and Qafzeh moderns—but we were both wrong! And there was already a clue as to why we were wrong in some relative dating work on animal remains from the sites.

Qafzeh Cave, like many of the sites, contained fossilized rodent remains as well as the human burials, and these can provide useful information not only about the local environment but also about the dating of sites. Pioneering studies of these small mammal remains by Israeli researchers suggested that Qafzeh could in fact be older than the Neanderthal sites rather than younger. This led the archaeologist Ofer Bar-Yosef to propose that the Qafzeh early moderns could date to as much as 70,000 years. Yet such an age was clearly beyond the reach of radiocarbon, so how could this view be tested? At last, with refinements in ESR and luminescence dating that came during the 1980s, this became possible.

The first significant application of these emerging techniques (thermoluminescence applied to flints that had been heated in a hearth) came from French–Israeli collaborations and initially seemed to reinforce the expected pattern in the Middle East, dating the recently discovered Neanderthal burial at Kebara to about 60,000 years. However, shortly afterward in 1988, the first application was made to the site of the Qafzeh early modern material, giving an astonishing age estimate of about 90,000 years, more than twice the generally expected figure, supporting or even exceeding the relative dating suggested by the rodents! Next up were the Skhul and Tabun sites, and for these I started working with dating specialists like Rainer Grün and Henry Schwarcz. Henry, a Canadian, is the doyen of dating in this time range, and Rainer, a German now working in Canberra, had studied and worked with him. Analyzing samples of animal teeth from both the sites for ESR dating gave equally revealing results. Within three years we had shown that the Skhul early moderns were at least as old as the Qafzeh ones, while the deep Tabun Cave sequence covered hundreds of thousands rather than tens of thousands of years. We also suggested that the Neanderthal burial from Tabun was much older than the 40,000-year radiocarbon date: it was perhaps as old as the moderns from Skhul and Qafzeh.

There was obviously a much more complex sequence than any of us had envisaged, and in some ways the expected chain of events was turned on its head: the modern-looking people from Skhul and Qafzeh were older than the Kebara Neanderthal. Further work showed that they were also older than the Amud Neanderthal. Thus they could not have evolved from these late Neanderthals, and, puzzlingly, those late Neanderthals were in the Middle East after the early modern humans, and not before them. Continuing dating work using all the available techniques now suggests that the Skhul and Qafzeh people actually range from about 90,000 to 120,000 years old, while the Tabun Neanderthal is most likely about 120,000 years old. So the emerging scenario is one where populations apparently ebbed and flowed in the region, which makes sense given its geographic position between the evolving worlds of the Neanderthals to the north and early moderns to the south.

Bar-Yosef suggested that the moderns came up into the region during a particularly warm and wet period, about 120,000 years ago, but as the succeeding Ice Age cooled and dried the north, the Neanderthals were pushed down there and took over the region—an intriguing reversal of the usual Replacement model! In fact, I think such changes could have been long-standing and even more complex, going back deep into the evolutionary history of the two species. At times when conditions were favorable, one or the other group, or perhaps both, would have moved into the region, while at times of severe aridity it might even have been completely abandoned. Whether populations were generally pushed there by unfavorable conditions in their home territories, or were pulled there by climatic ameliorations leading to population expansions, we do not yet know, but new climatic data are emerging.

The potential of ESR to match the ability of AMS radiocarbon in directly dating human fossils is at last being realized. In 1996 the first of an increasing number of applications of this technique to significant human fossils was made when Rainer Grün and I collaborated with colleagues including James Brink from South Africa to date the Florisbad human skull. This fossil, which had been found in 1932, is actually rather incomplete but seems to combine a large and fairly modern-looking face with a strong brow ridge and somewhat receding forehead. For many years it was assumed to date from about 40,000 years ago, based on a radiocarbon date from peat deposits at the site, and on that basis it seemed to be a relic hanging on in the margins of southern Africa, while moderns were evolving and spreading in western Asia and Europe. As such, it supposedly demonstrated the backward role of Africa in modern human evolution—the primitive Florisbad humans merely marking time until moderns arrived from farther north and replaced them. However, the fossil preserved one upper molar tooth, and a tiny fragment of its enamel was taken to Rainer's lab in Australia for ESR dating—with sensational results. The fossil was not 40,000 but about 260,000 years old! Thus its potential role in human evolution was revolutionized at a stroke: rather than representing a southern African equivalent of the Neanderthals, on the brink of extinction, it could instead have been an ancestor to us all.

There are some situations where even the best physical dating techniques need a helping hand, and combinations of physical and relative methods are required. The Neanderthals apparently disappeared about 30,000 years ago, but the factors leading to this and the time scale for their demise are still fiercely debated. While accelerator radiocarbon dating gives excellent precision in the measurement of an age, it does have problems of accuracy compared with calendar years during this critical period of time, both because the rate of formation of radiocarbon in the atmosphere was unusually variable then, and because even a tiny amount of contamination from young or old carbon will make a significant difference to the age obtained. As I explained earlier, this latter problem is being addressed through techniques that very effectively remove contaminants before dating is attempted. But to address the former issue, there were fortunately other significant events in Europe during that period to provide new and potentially very accurate ways of relative dating. As I discuss further in chapter 4, a massive volcanic eruption took place in the Campania region of central Italy about 39,300 years ago (which we know from argon dating). As well as enormous quantities of local deposits such as lava, pumice, and ash, the eruption also produced much finer volcanic dust, known as
crypto
- or
microtephra
because it cannot be seen with the naked eye. This microtephra may be ejected into the upper atmosphere and travel for many thousands of miles, and the Campanian Ignimbrite—from the Latin words
igni
(fire) and
imbri
(rain)—settled eastward as far as Russia and North Africa.

The CI, as it is known, has now been found in dozens of archaeological sites, including the famous Russian localities at Kostenki, in levels which we already knew from radiocarbon dating were at least 35,000 years old. Each volcanic eruption took place as a result of unique combinations of factors like chemical composition, temperature, and pressure, and thus can be “fingerprinted” and recognized. So wherever the special CI chemical signature is found in an archaeological site, we can be pretty confident that the level concerned, with its associated fossils and artifacts, was laid down just over 39,000 years ago. In turn, all such sites can be correlated to this age by a lattice of synchronous volcanic deposits.

This approach gave rise to a large collaborative project called RESET (Response of Humans to Abrupt Environmental Transitions), in which I am involved. Over a five-year research period, RESET is correlating tephras from their volcanic sources to where they fell in deep ocean and lake sediments, and even farther into important archaeological sites in Europe, western Asia, and North Africa. The aim of RESET is to investigate the effects of climate and environmental changes on the human populations of the region, including the last Neanderthals and the first moderns. The tephras themselves are markers of volcanic eruptions, of course, most of which were only local and short-lived in their effects, but a few did have major—even global—impacts, as we will see later in this chapter.

Using volcanic deposits to date human fossils has a long history, as I explained in relation to Olduvai Gorge earlier, and the mapping of outputs from successive volcanic eruptions has played an important role in refining the age of many important fossil sites in East Africa, including Omo Kibish in Ethiopia. The two most complete human fossils from there, the Omo 1 skeleton and the Omo 2 braincase, were found in 1967 by a team led by Richard Leakey and were important in early proposals for a recent African origin. But although there were initial estimates that the material was over 100,000 years old, some of these were based on the application of uranium-series dating to shells in the deposits—not the most reliable material for such determinations—and so doubts remained. Over thirty years after the original discoveries, an international team led by the anthropologist John Fleagle returned to the Kibish region, relocated the 1967 find-spots, and found further fossils and stone tools. Both Omo 1 and Omo 2 were originally recovered from the lowermost portion of the massive Kibish Formation, a series of annual but episodic sediments laid down by the ancient Omo River when it periodically flooded, before it entered Lake Turkana. These deposits lie about one hundred kilometers farther north than its present delta, close to the Ethiopian border with Kenya. Occasionally, volcanic eruptions deposited volcanic ash and pumice over the river and lake sediments, and these can be dated through their contained argon. A layer of ash about three meters below the location of Omo 1 was placed at about 196,000 years old, while a second ash about fifty meters above the location was dated to about 104,000 years. Because there were also clear signs of geologic erosion (the removal of sediments when the river and lake level fell) between the level of Omo 1 and the higher ash, it seemed likely that the age of Omo 1 was much closer to the age of the 196,000-year ash than to the 104,000-year one.

Additional indirect support for this came from much farther afield, on the seabed of the Mediterranean. During ancient monsoon periods, rain and snowmelt in the Ethiopian highlands sent annual floods pouring into the sources of the River Nile, causing
sapropels
(dark layers of sediment) to be deposited when these waters eventually flowed out into the Mediterranean. A particularly strongly marked sapropel can be dated from its position in Mediterranean seabed cores to about 195,000 years, suggesting that it correlates perfectly with the major monsoon event that sent floods in the opposite direction down the Omo River, producing the vast deposits of the lower part of the Kibish Formation, in which the Omo 1 skeleton and the underlying volcanic ash were found. The Omo 2 braincase was a surface find rather than a fossil excavated from sediments (which was the case for the Omo 1 partial skeleton), but the surrounding location consisted of the lowermost part of the Kibish Formation. Thus the team that revisited the region and published the new dating work remains confident that Omo 1 and Omo 2 are very close in age, at about 195,000 years, despite some strong contrasts in their level of modernity—something to which I will return in chapter 9.