Out of Africa: did humans migrate quickly and all at once, or in phases based on weather?

Dr Ash Parton and Prof Adrian Parker (lower figure) examining alluvial fan deposits.

Understanding the past climate of the United Arab Emirates (UAE) is important for studying human evolution, as the region lies at one of the major bottlenecks for human migration out of Africa. Two models for the movement of modern humans have been put forward. The coastal migration hypothesis suggests human populations expanded rapidly from Africa to southern Asia via the coastlines of Arabia approximately 50 000 to 60 000 years ago, the interior being too arid to support human populations. An alternative model suggests that humans migrated into the Arabian interior several times, beginning much earlier (about 75 000 to 130 000 years ago) when increased rainfall provided sufficient freshwater to support expanding populations.

Research by BGS in collaboration with colleagues at Oxford Brookes University suggests that periodic changes in rainfall linked to changes in global climate, in particular shifts in the Indian Ocean monsoon, created several opportunities for humans to migrate across the interior of Arabia. By analysing river sediments near Al Ain, UAE, we unearthed a unique and sensitive record of landscape change in southeast Arabia spanning the past 160 000 years, which provides evidence for several wet climatic interludes during both glacial and interglacial periods.

Previously, phases of increased humidity were thought to be linked to global interglacial periods, with the climate of Arabia during the intervening glacial periods believed to be too arid to support human populations. Now it is likely migration pathways through the interior of the Arabian Peninsula may have been viable approximately every 23 000 years since at least 191 000 years ago.

Ashely Parton, lead author on the paper, says, "The dispersal of early human populations out of Africa is linked with the changing climate and environmental conditions of Arabia. Although now arid, at times the vast Arabian deserts were transformed into landscapes littered with freshwater lakes and active river systems. Such episodes of dramatically increased rainfall were the result of the intensification and northward displacement of the Indian Ocean monsoon, which caused rainfall to reach across much of the Arabian Peninsula."

This research is an excellent example of how detailed geological mapping can underpin not only the economic well-being of a country, but can lead to important scientific discoveries.

Reference

Parton, A, Farrant, A R, Leng, M J, Telfer, M W, Groucutt, H S, Petraglia, M D, and Parker, A G.  2015.  Alluvial fan records from southeast Arabia reveal multiple windows for human dispersal. Geology; DOI: 10.1130/G36401.1