Early human migration out of Africa was driven by climate change induced by variations in the Earth's rotation, scientists have said. They have identified different waves of migrations that took place over the past 125,000 years.

When and why Homo Sapiens left Africa is one of the most important question that scientists studying the origins of modern humans have tried to answer for years.

A lot of studies have frequently put forward the role of climate as a driver of human migration. Their basic argument is that climate has an impact on resources and thus limits the number of people that can survive in a given region. This in turn leads to human dispersal.

Some have hypothesised that sea level fluctuations and variations in the orientation of the Earth's axis of rotation resulted in significant climate shifts that were primarily responsible for early Homo Sapiens migrating out of Africa – but with limited evidence to corroborate this.

Indeed, the way scientists can investigate the role of climate on early human dispersion, is by coming up with spatio-temporal models of ancient climates that can be compared with available fossils as well as archaeological and genetic evidence. The problem is that this data is often sparse in key regions of the world, and the date of some fossils is uncertain.

The recent research, published in the journal Nature, is the most comprehensive climate, vegetation and human-dispersal modelling study conducted to date – it tries to avoid the shortcomings of previous models.

"Our study provides one of the first computer simulation of human migration out of Africa. It shows that the 'single exit scenario' – the hypothesis of a single major human dispersal – is inconsistent, suggesting it was not a single event but occurred in waves over the last 125,000 years", lead author Axel Timmermann, from Hawaii University at Manoa told IBTimes UK.

Most comprehensive model to date

The Hawaii University-led team constructed a numerical model which included data regarding Earth's orbit, solar-radiation levels, carbon dioxide levels, glacial ice and sea levels from the past 125,000 years. The model quantifies the effects of past climates and sea level changes on human dispersal. With it, the scientists show that the main driver of human dispersal out of Africa was a climate shift induced by changes to the Earth's rotational axis at regular points in time.

"The waves of migration were quite periodic because they are closely linked to changes to the orientation of the Earth's axis of rotation, every 21,000 years, which led to wetter climates in Sahara and Arabian peninsulas and the emergence of savanna-like corridors where fauna thrived. This was favourable to human movement, as early humans were hunters and followed the animals", Timmermann said.

Today, the Sahara and Arabian peninsulas are covered by deserts and thus form an effective barrier to dispersal of fauna out of Africa. However, the model indicates that changes to the Earth's orbit may have resulted in monsoons in these regions and established wetter conditions in the past, creating migration paths out of Africa along vegetated, resource-rich corridors. The scientists found out that waves of migrations associated with these environmental changes occurred around 106–94, 89–73, 59–47 and 45–29 thousand years ago. These findings are in close agreement with archaeological and fossil data.

The findings are also significant, because they confirm that climate has a crucial role to shape life on the planet, including its peopling.