Much of human prehistory is about movement. Sometimes these move­ments involved the colonisation of a new niche brought about by environ­mental change, or when our ancestors crossed a behavioural threshold that enabled them to inhabit new regions that were previously beyond their capabilities.

Often, these movements were ones of contraction and expansion because of climatic changes that made a region either habitable or uninhabit­able. At times, these movements brought groups into closer contact with each other; at other times, these movements resulted in their isolation.

Biogeographers define many different types of movement by animal and plant species.^[522] Dispersals, or population expansions, refer to the extension of a species' range into new areas. Colonisation occurs if this leads to the establishment of a permanent population. Most dispersals involve no behav­ioural change, and result from either an expansion of a species' preferred habitat - as when, for example, a fall in sea level results in the expansion of the type of coastal plain already familiar to it - or (especially with birds), the ability to cross barriers such as mountains or the sea to reach favourable new habitats. Dispersals should be differentiated from migrations, which usually refer to seasonal movements within a defined annual range: many modern pastoral societies are examples of this type of seasonal movement between winter and summer pastures, as are modern hunting communities with distinct winter and summer territories, and many species of birds. This type of migration is distinct from emigration, which is the planned, and usually one-way movement of people from one region to another. Rapid dispersal events (over a matter of decades or a few centuries) that are at the expense of indigenous communities (whether red squirrels or Australian aborigines) are sometimes termed invasions or irruptions; although there are numerous examples from historic contexts, these are unlikely during the Palaeolithic because population densities were so low.

Dispersals by humans and their ancestors are particularly interesting because they were often facilitated by anatomical, technological, or social changes. Examples might be an increase in brain size, the invention of stone­tipped projectiles, or the development of exchange networks that allowed groups to obtain resources such as high-quality stone that were not available locally. Dispersals can also occur if the colonising species is a predator that the indigenous fauna had never previously encountered. For example, the rapid dispersal of our species into and across Australia 40,000-50,000 years ago, and the Americas after 12,000-15,000 years ago may have been possible because the local fauna was ‘naive' with respect to the predatory nature of humans: the same process may also have occurred when Homo erectus first entered Asia over 1.8 million years ago.^[523] In Palaeolithic Asia, human evolu­tion is often discussed as the result of two, and possibly three major, continental-level dispersals. The first, known as Out of Africa 1, comprises the earliest expansion and subsequent colonisation of our own genus Homo from Africa into the Eurasian landmass.^[524] The second, or Out of Africa 2, summarises a similar expansion of our species, Homo sapiens, from Africa across Eurasia and ultimately to Australia and the Americas.^[525] Some research­ers also recognise a third, which was the expansion of an African type of Acheulean, bifacial technology into Southwest Asia and perhaps India and

Europe c. 600,000-800,000 years ago.^[526] As outlined below, some recent work in India and the Caucasus challenges this scenario.