Modern humans left Africa twice as early as previously thought, spreading in a number of climate-driven waves, new research suggests.

The paper, published in Quaternary International, pours fresh doubt on the previously-held consensus that humans spread from Africa in a single cohort.

'The consensus view has been that modern humans left Africa around 60,000 years ago by a coastal route, skirting around some very arid places, and spread to Australia very quickly,' explains Professor Michael Petraglia of the University of Oxford, one of the study's authors.

'We think that's wrong. We think people left Africa multiple times, probably a long time before, and we think it was terrestrial rather than coastal.'

Previous attempts to put a date on the exit of modern humans from Africa have relied heavily on evidence from genetics and archaeology.

Petraglia and his team believe that, by adding evidence on climate and environment into the mix, they will be able to unlock new clues as to both how and why humans spread from the continent.

'We know that the climate has shifted a lot of times. We think that has acted like a pump out of Africa, pushing waves of people into South Asia.'

'When the climate was humid, there would have been rivers and lakes across the Asian continent. We think modern humans would have used those as routes, but what we don't know is what happened to those populations when it became arid again.'

The idea goes against a well-established and widely-held consensus. But Petraglia sees signs that academics across the spectrum are beginning to change their minds.

'There are lots of people buying into this idea in many different fields; in genetics, in archaeology, in environmental fields. We're seeing major cracks in the consensus view,' he says.

The team will now zoom in to examine some important sites in more detail, as they attempt to add flesh to their theory.

Petraglia believes that the research has important implications for understanding our present, as well as our past.

'It's in the public imagination. People are fascinated by our own species and how we populated the Earth,' he says.

'But we're also trying to understand this climate pump - how the climate affects the movement of populations and the speed at which that happens - and that could clearly have important implications for today.'

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More information: Bolvin, N., Fuller, D., Dennell, R., Allaby, R., and Petraglia, M. Human Dispersal Across Diverse Environments of Asia during the Upper Pleistocene, Quaternary International, 2013, Journal information: Quaternary International Bolvin, N., Fuller, D., Dennell, R., Allaby, R., and Petraglia, M. Human Dispersal Across Diverse Environments of Asia during the Upper Pleistocene,, 2013, doi: 10.1016/j.quaint.2013.01.008

This story is republished courtesy of Planet Earth online, a free, companion website to the award-winning magazine Planet Earth published and funded by the Natural Environment Research Council (NERC).