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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