London, Feb 21 (IANS): Did human migration out of Africa happen all at once or in phases? A new study provides fresh insights into the long-standing debate.

The new study goes by the model that suggests that dispersal into the Arabian interior began in multiple phases approximately 75,000 to 130,000 years ago when increased rainfall provided sufficient freshwater to support expanding populations.

"The dispersal of early human populations out of Africa is dynamically 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," wrote Ash Parton of the University of Oxford in Britain.

"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," he added.

Another dominant hypothesis states that human populations expanded rapidly from Africa to southern Asia via the coast lines of Arabia approximately 50,000 to 60,000 years ago.

Parton and colleagues, however, present a unique alluvial fan aggradation (the deposition of material by a river, stream, or current) record from south-east Arabia spanning the past approximate 160,000 years.

Situated along the proposed southern dispersal route, the Al Sibetah alluvial fan sequence provides a unique and sensitive record of landscape change in south-east Arabia.

This record is to date the most comprehensive terrestrial archive from the Arabian Peninsula, and provides evidence for multiple humid episodes during both glacial and interglacial periods.

Evidence from the Al Sibetah alluvial fan sequence indicates that during insolation maxima (period of greatest solar activity), increased monsoon rainfall led to the widespread activation of drainage systems and grassland development throughout regions that were important for the dispersal of early human populations.

Previously, the timing of episodes of increased humidity was largely linked to global interglacials, with the climate of Arabia during the intervening glacial periods believed to be too arid to support human populations.

Parton and colleagues suggest that periods of increased rainfall were not driven by deglaciations every 100,000 years approximately, but by periods of maximum incoming solar radiation every 23,000 years approximately.

The findings were reported in the journal Geology.