The earliest modern human colonization of Europe.

T he expansion of modern humans over the planet is one of the most spectacular events in the course of human evolution. During millions of years, distinctive forms of hominins evolved in parallel and sometimes coexisted in the same regions. Between 60,000 and 40,000 y ago, one species expanded out of its African birthplace and replaced all others. The Neanderthals are the best-known archaic humans to go extinct at this time. In PNAS, Lowe et al. (1) resolve pending issues surrounding the mechanism of this evolutionary drama. Their study also fuels an increasing number of questions on the antiquity of the modern human colonization of Eurasia. Scenarios accounting for the demise of the Neanderthals are much debated. For some, their replacement resulted from intrinsic biological and behavioral differences with our species (2). For others, external causes precipitated their decline at the time of modern human expansion. Of these, climatic disasters are most often envisioned (3) but a mega-volcanic eruption (4), and even an inversion of the magnetic field resulting in a brutal increase of deleterious radiation (5), have also been proposed. Epidemics devastating Neanderthal populations represent an intermediate category of explanations (6). Crucial to demonstrating any of these scenarios is the establishment of accurate chronologies for each region and their synchronization on a continental scale. For this time period, archeologists mostly use the decay of the radioactive C isotopes in organic matter to assess the ages of sites and human remains. Other methods are less precise or are not easily applied in archeological contexts. However, C isotopes virtually disappear from bones and charcoal more than 50,000 y old, which makes the application of this technique increasingly delicate beyond 40,000 y. Only recently have some improvements in the pretreatment of the dating samples (7), and in the conversion to calendar dates (8), started to produce reliable C dates for this crucial time period. A dramatic volcanic eruption surpassing anything known in the historical record took place in central Italy 39,300 y ago. It is known as the Campanian Ignimbrite (CI) eruption. Atmospheric circulation carried clouds of ash all over the eastern Mediterranean, and the products of the eruption reached regions as distant as southwestern Russia. The mineralogical nature of these ash deposits allows this event to be very accurately dated. It also has a distinctive chemical signature that can be recognized, even when the ashes themselves are invisible to the naked eye in sediments. This has allowed Lowe et al. (1) to accurately synchronize the chronologies of deposits between very distant areas and to compare archeological sequences in caves with more detailed geological sections in open air sites, lake, or even sea deposits. This method provides a precise chronological marker to relate cultural changes and human occupations to well-recorded environmental changes. This large-scale study demonstrates that lithic assemblages of the Upper Paleolithic associated with the spread of modern humans predate the CI eruption in several European sites. This applies in particular to some forms of the Aurignacian, a lithic industry with sophisticated art objects and musical instruments (9) and associated with modern human remains (10). This observation falsifies the hypothesis proposing that the CI eruption itself, and/or the short but intense cooling episode known as Heinrich event 4, which started 40,000 y ago, could have triggered the cultural changes or population replacements witnessed around this time. Rather, it suggests that the main threat to indigenous populations were the modern human invaders themselves and not environmental hazards. Several recent studies based on C dating also support this conclusion. They assign diagnostically modern human remains (11), as well as early Aurignacian assemblages (9), to a period predating the CI and Heinrich event 4. This raises the question of when exactly the first modern humans reached Europe. A major difficulty in answering this question comes from the extreme scarcity of human remains from this time period. The makers of most lithic assemblages dated to between 50,000 and 40,000 y ago remain unknown. Long have archeologists recognized the Aurignacian as a proxy for the first expansion of modern humans into Europe. Thus, discussions of a possible chronological overlap between these modern invaders and late Neanderthal populations have Fig. 1. The Bohunician and other initial Upper Paleolithic assemblages of Eurasia display the retention of Levallois blank production (Middle) and a large use of various types of points (Top). This pattern is reminiscent of North African assemblages predating the out-of-Africa exodus of modern humans. Upper Paleolithic-style tools such as end-scrapers and burins (Bottom) complete this toolkit. Unfortunately, at most Bohunician sites, sediments do not allow the preservation of human remains, bone tools or body ornaments. (Scale bar: 5 cm.) [Reprinted with permission from ref. 20 (Copyright 2008, Elsevier).]