WE OWE it all to climate change. Scientists have suggested that the intelligence and adaptability of modern humans only arose because our ancestors faced repeated challenges from massive shifts in prehistoric climate that forced them to evolve.  The researchers have looked back over 2m years of human history and found that humanity has evolved and spread fastest during times of climatic instability - when the Earth has undergone rapid warming or cooling.  It means, they suggest, that Homo sapiens may be intrinsically adapted to cope with climatic complexity and rapid change rather than stability.  "Climate change has been a major player in our evolution," said Chris Stringer, of London's Natural History Museum and author of The Origin of Our Species.  "It created the conditions that encouraged our early ancestors to come down from the trees and later to spread out of africa and across the globe. It made us what we are today."  Details of the research will emerge this week at a conference on human evolution organised by the Royal Society.  The scientists are not suggesting that modern global warming is beneficial. "What intrigues them is the growing evidence that human evolution and climate change have been inextricably linked for hundreds of thousands of years," said co-organiser Rhiannon Stevens, of Cambridge University. Stringer has identified five crucial eras when big changes in climate seem to have accelerated humanity's genetic and social evolution. The earliest was about 2m years ago when the forests covering east africa, where the first humans evolved, dried out and turned to savannah. This led to the emergence of Homo erectus, an early human evolved for running and hunting in the open grasslands rather than for life in and among trees.  "