It was probably a warm interglacial interlude within the Ice Age, between about 130,000 and 90,000 years ago, that initially triggered large-scale Homo sapiens migrations across Africa.
Then, from about 70,000 years ago, the climate cooled, causing glaciers to form on the tops of mountain ranges so that parts of north-west and north-east Africa were cut off from each other, as well as from the south. As Charles Darwin discovered, whenever a species is physically separated, small variations begin to creep into its respective gene pools, creating diversity. So it was with modern man, giving us our four main ethnic groups: Khoisan (African), Caucasian (European), Mongolian (Chinese and American Indian) and Aboriginal (Australian).
From about 60,000 years ago, these four groups of humans emigrated from Africa separately and in their own time across the world, taking their small genetic differences with them. Some Homo sapiens swept across Asia, displacing the last of the Neanderthals either by depriving them of food, or by hunting them, or maybe occasionally by absorbing them into their own species through limited interbreeding. Some turned south and reached India and China. They learnt to build rafts. From about 40,000 years ago, Australia, for many millions of years the preserve of marsupial mammals, became another human hunting-ground, as the first people paddled ashore.
The first Homo sapiens to arrive in Europe walked eastwards out of Africa about 50,000 years ago, and then came north via the Middle East. They brought with them enormous changes in lifestyles, technology and culture, including the world's first spears that were specially designed for flight, rather than for close-range use as with Neanderthal-style clubs.
The time from about 50,000 years ago marks the start of the final second before midnight on the 24-hour clock of Earth history. This period has been described as "the Great Leap Forward", because the complexity of human tools increased dramatically. Bones, tusks and antlers were used for the first time to carve out ornaments as well as to craft useful household items such as needles for sewing, and spoon-like oil lamps that burned animal fats. Jewellery, in the form of necklaces and pendants, has been found buried inside graves of these people. The first ceramic pots date from this period, as do the world's first known sculptures, such as the Venus of Willendorf, a female fertility figure found in Austria in 1908 which is thought to date from about 24,000 years ago. Some of the first known cave paintings date back to the same era. They can be seen to this day in the caves of Lascaux, in the Dordogne region of France.
The Earth was far cooler back then. The last of the great ice sheets swept down from the North Pole about 22,000 years ago, to disappear rather quickly 12,000 years later. During this time some people adapted to the changes in climate by developing paler skin that helped produce sufficient quantities of vitamin D for bone formation despite the weaker sunlight of the Ice Age.
Homo sapiens arrived in Britain about 20,000 years ago. They walked across the Channel from France, since it didn't flood until the end of the last big Ice Age melt, about 10,000 years ago. But they weren't the first to arrive. Up to seven previous attempts were made by earlier people to populate the British Isles, starting with Homo erectus some 700,000 years ago. Each time, the populations of humans died out, probably because of the horrendously icy conditions that periodically swept over the islands as far south as present-day London. Even in the far south, the cold would sometimes have been too much for any type of human to bear.
About 15,000 years ago, giant glaciers still locked up much of the Earth's waters, sinking sea levels so that a massive land bridge the size of Poland, called Beringia, connected the eastern tip of Russia to Alaska across what is now a 95km-wide stretch of sea called the Bering Strait. In those days people could cross by foot from Asia to North America, a land that had until then probably been free from human habitation (although some scientists think people may have rafted there a few thousand years before from south Asia, via the Pacific islands). North and South America were the last of the great habitable continents to be populated by man, and are still appropriately called "the New World" even today. It was an opportunistic walk all the way across Asia, following big animals, hunting on the move, making the most of nature's twisting and turning climate changes.
With another land bridge via Panama linking the two great Americas, it wasn't long before the first people from North America wandered down to the southern American continent where the climate was warmer and the land rich in vegetation and game.
The arrival of Stone Age humans in this part of the world – as in Australia – came with dramatic consequences for much of the world's wildlife. Although a few of nature's ecosystems lingered on without any human representation – New Zealand and Iceland were untouched by humans until about AD800 or later – many of the world's living creatures were by now beginning to succumb to mankind's growing influence as he spread out to envelop the whole of planet Earth.