Box 1   Multiple events: key points in recent human evolution. (1) The (approximate) time scale for considering the fossil evidence for the origins of H. sapiens is around half a million years. By this stage, hominid populations were well established in Africa, Europe, and Asia, and there is evidence for diversity among them. The recent estimate of the coalescence point for modern human and Neanderthal mtDNA is a little longer ago than this date, whereas the diversification of the African ancestral population leading to modern humans and that leading to European Neanderthals is likely to be more recent than this date (Krings et al. 1997). Based on fossil evidence, the likely ancestral population leading to all these half-million-year-old hominids was in Africa between 1.5 and 2.0 million years (Myr) ago. (2) The environmental context for considering later human evolution is that of repeated glacial cycles; this is shown best by the marine isotope record, which estimates the temperature of the ocean by measuring the ratio of two stable oxygen isotopes (delta 18 values on the y-axis) (Shackleton 1996). There have been several major glaciations during this period, each of which would have produced profound climatic and biogeographical changes. The last interglacial and glacial period is but one of many that have occurred, and each one is itself highly variable. During very cold periods of the glacial (glacial maximum), much of Europe would have been under glaciation, the rest open tundra and steppe. Africa would have been cold and arid, and the rain forests reduced and fragmented. During interglacials, Africa and Eurasia would have been far more forested; during periods of decreased aridity, migration of African faunas across the Sahara would have been more possible. (3) A snapshot of the hominid populations between half a million years and 300,000 years ago would have suggested the following: (1) persistence of H. erectus with a simple chopper and flake industy in Southeast Asia and possibly in parts of Europe and eastern Asia as well and (2) an Afro-European distribution of a more derived and larger brained hominidHomo heidelbergensiswith Acheulean handaxes and associated tools; this population may also have spread into parts of eastern Asia. (4) Around 300,000 years ago a new type of stone tool technology appeared in Africa, and this was probably associated with the evolution and dispersal of a new and more adaptable hominid species, the ancestor of both humans and Neanderthals (Homo helmei). This Mode 3 technology [a Middle Stone Age/Middle Paleolithic technology based on prepared core technology, where the core or pebble that is used is prepared in advance before a flake is struck off (see Fig. 2)] persisted in the early modern humans and is probably evidence of a very much more advanced behavioral capacity. The Mode 3 or Middle Stone Age dispersals were among the first of the ones that established the modern human world (Foley and Lahr 1997). (5) The Isotope Stage 6 glacial maximum was a period of sustained and extreme cold. In Africa there would have been isolation and fragmentation of human populations, a process possibly associated with the ancestral modern human bottleneck. In Europe there may have been a virtual loss of hominid populations. It was during this period that modern human anatomy may have evolved among isolated African populations, and the first modern human fossils (the Omo Kibbish sample from Ethiopia) probably date from this period. (6) The last interglacial was a period of ~10,000 years of warming, much of it equivalent to today's climate. It was during this period that the fossil and archeological evidence suggest that modern human populations expanded to the south (the Klasies River Mouth sample in South Africa) and to the north, out of Africa for the first time (the Skhul and Qafzeh samples in Israel). These early modern populations may well have become locally extinct at the end of the last glaciation. One such early modern population may have dispersed through a southern route, around the Indian Ocean Rim, and into Australia and New Guinea (Lahr and Foley 1994). (7) Although not as cold as the Last Glacial Maximum, the early part of the last glaciation (~70,000 years ago) saw the contraction of African environments and populations, their disappearance from the Middle East, and an apparent expansion of Neanderthals from Europe into areas previously occupied by modern humans in the Levant. (8) From ~45,000 years ago, there were a number of dispersals, often associated with a new technology (Klein 1992; Foley and Lahr 1997). These were not global but a series of regional ones: the Upper Paleolithic in Europe, the Later Stone Age in sub-Saharan Africa, and one with populations using microliths in southern and eastern Asia. The subsequent 15,000 years appear to have seen recurrent expansions and contractions, and changes in population distribution. It was during this period that the last of the nonmodern populations became extinct: the Neanderthals in Europe, H. erectus in Southeast Asia, and possibly other populations in eastern Asia. (9) From 21,000 years ago to 18,000 years ago, world climates reached their coldest and driest point of the last 130,000 years, the last glacial maximum (LGM). During this period, human populations would have been under major selective pressures again. These would not have been uniform, resulting in some populations concentrating demographically in favorable enclaves, others diluting throughout vast arid landscapes, whereas still others suffered significant demographic loss. (10) The global warming out of the LGM was extremely rapid (although with a sharp reversal for a short period of time), and the succeeding demographic and geographical expansions placed human populations under new pressures for survival, out of which agriculture emerges in various parts of the world. This in turn led to increasing human population density and new dispersals, resulting in both asimilation and extinction of populations marginal to these developments.                  

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