London, Oct 31 : A study that has accurately dated sophisticated stone tools made by our ancestors has suggested that technological innovation is more likely to have spurred the first modern humans to migrate out of Africa than climate change.
Scientists have long argued about the forces that drove the transition to modern human behaviour after our species evolved in Africa up to 280,000 years ago.
Most scholars agree that Homo sapiens passed a threshold between 60,000 and 80,000 years ago, with evidence for more complex technology, ornaments and symbolic art turning up in the archaeological record.
Human genetics research also suggests that the population expanded markedly during this time.
Some say that language was behind the transition. Others put it down to environmental factors that forced humans to migrate across the globe between 60,000 and 70,000 years ago. Both have been linked to technological developments around the time, such as tool-making.
According to a report in Nature News, Zenobia Jacobs, of the University of Wollongong, Australia, and her colleagues, have now reanalyzed tools from southern Africa that were long seen as markers for the transition, but had not been conclusively dated.
Jacobs's team looked at tools from nine sites in South Africa, Namibia and Lesotho, using a method called optically stimulated luminescence.
This relies on the tendency for natural background radiation to knock electrons out of place in minerals, with more and more of these displaced electrons accumulating over time.
The technique then prompts these electrons to snap back into place, releasing a measurable amount of light as they do so, which can be used to date the rock.
The tools studied are grouped into two types, named after the original sites where they were discovered: Still Bay and Howieson's Poort.
The Still Bay tools, fashioned from a hard, fine-grained rock called silcrete, were in vogue between 71,000 and 71,900 years ago, according to Jacob's analysis.
"People went to a lot of effort to make them look pretty," Jacobs said of the precision tools, used as spearheads. "And when they made a mistake, they discarded the tool and started again," she added.
The Howieson's Poort implements, also made of silcrete, were composite tools, with components hafted to wood to form weapons. They flourished between 59,500 and 64,800 years ago, the team found.
Comparing these dates with palaeoenvironmental records failed to turn up a close correlation between the industries and dramatic climatic changes.
"We see no consistent pattern between the timing of these industries and major climatic changes, although local conditions probably influenced where people preferred to live," she said.
The team concludes that these innovations, and the migration of humans associated with them, cannot necessarily be explained by changes in the environment.