Ice ages and us
What have ice ages got to do with people? Quite a lot, according to current thinking among scientists. Peter Forster, an archaeologist at Cambridge University in England, points out that the successive waves in which humans have spread across the world have mainly been determined by ice ages, both in their timing and in the routes they took.
The route which early humans took from Africa into Asia about 60,000 years ago was probably aided by low sea levels (usual in an ice age because so much water has turned to ice) helping them cross into Yemen. The same happened 20,000 years ago when people crossed what is now the Bering Strait between Asia and Alaska to reach America. The end of the last ice age allowed areas of northern Europe from which people had been driven by the cold to be reinhabited.
However, some experts make even bigger claims about the connection between humans and climate. The first accepted stone tools are about 2-5 million years old and come from Africa. Their appearance coincided with climate change in East Africa associated with the spread of plains vegetation and a reduction in the amount of tree cover.
Some scientists claim that the establishment of the present weather patterns in the Indian Ocean, complete with the monsoon that dumps huge amounts of rain in and around India, was accompanied by drier weather elsewhere in the region. Jungles need more water than savannah, so the vegetation changed and walking, tool-using social species of hominids came down from the trees and adapted to the new conditions.
On this theory, the cooling may have been caused by the slow but sure rise of the Himalayas. As they rose, more carbon-rich matter would be eroded off them, swept down to the sea and buried, reducing the amount of carbon dioxide in the atmosphere and setting up a reverse greenhouse effect that would cause global cooling. This is a neat line of speculation and has some evidence to back it up. But its advocates are probably overplaying their hand by presenting it as 'the'' story of the origin of Homo sapiens.