New Delhi: It was believed by scientists, until recently, that the major exodus of modern humans (Homo sapiens) out of Africa took place only 65,000 years ago. This belief was based on the dating of the fossils of modern humans found outside Africa.
However, a recent study on the migration of Homo sapiens out of Africa, by a group of researchers from various universities, explains that there were many favourable climatic windows available for migration in the last 300,000 years.
According to the study titled ‘Climatic windows for human migration out of Africa in the past 300,000 years’, published in the Nature Communications journal, the timings and routes of the migrations are not known because fossils and ancient DNA, from which suitable data could be obtained, are unavailable. The scientists behind this research used paleoclimatic models, which can understand past climatic changes and make predictions of future climate change by deciphering computer models consisting of information about physical processes of the climate system, to estimate conditions like rainfall availability — one of the most important factors to determine successful migration.
By reconstructing the climate of northeastern Africa over the last 300,000 years, the researchers determined that sufficient rainfall was a crucial parameter for modern humans to remain alive during their voyage from Africa to the Arabian Peninsula.
On the basis of anthropological (related to the study of humankind) and ecological (related to the study of human-human and human-environment interactions) estimates, the scientists deciphered the suitable timings and geographical paths for migration, which were dependent on appropriate climatic conditions and minimum amount of rainfall that would ensure the survivability of the hunter-gatherers.
The study also mentions that archaeological and genetic evidence of human expansions out of Africa, and the timings and routes estimated by scientists to have been adopted by modern humans to migrate from Africa to Eurasia, are compatible with each other. The scientists explain in the paper that modern human dispersals were largely dependent on paleoclimatic variability, which refers to the change in climatic conditions according to different geological eras.
The study was conducted by researchers from the University of Cambridge, UK; Potsdam Institute for Climate Impact Research, Germany; GNS Science, Lower Hutt, New Zealand; University of Tartu, Estonia; and King’s College London, UK.
What Do Fossils Found In China, Saudi Arabia, Israel, Greece Tell?
Archaeological evidence from different regions of the world shows that migration of modern humans could have taken place many years prior to the major exodus. Fossils of modern humans as old as 80,000, 85,000, 100,000 and 210,000 years have been found in China, Saudi Arabia, Israel and Greece, respectively, suggesting excursions from Africa much before the major exodus.
The study says that though there were many windows when Homo sapiens could have left Africa, the timings, routes and outcomes of these migrations were unconfirmed. Empirical (based on experience rather than logic or theory) models have not been able to give proper information about migration routes into Eurasia.
Demographic rules, synchronous with archaeological and genetic evidence, have been included in the models and were used to obtain information quantitatively. However, these models are not necessarily biologically accurate, because they make assumptions such as the human thermal niche (ability to raise body temperature above environmental temperature) changed by 50°C, and that there was a 6,000% increase in migration speed, over a period of 125,000 years, the researchers note.
The scientists have identified the favourable windows for migration of Homo sapiens in two steps. First, they estimated the tolerance of modern humans to adverse climatic conditions like low rainfall and aridity (desert like conditions), required for the successful exodus from Africa, using high-resolution paleoclimatic simulations of the past 300,000 years. In the second step, the researchers deduced the timings of climatic windows out of Africa by estimating the actual climatic tolerances of hunter-gatherers, from anthropological and ecological data, the study states. Then, they compared their findings with the already existing empirical data about these migrations.
There Was A Suitable Migration Window Even 275,000 Years Ago
The analysis helped the scientists infer that previously obtained data about timings and routes from anthropological and genetic data were associated with the presence of wet corridors into Eurasia. This was indicative of the fact that paleoclimatic conditions, especially rain, were important determining factors for the out-of-Africa expansion.
The scientists also deduced the hunter-gatherer precipitation tolerance, which is the lowest value of rainfall required for survival during migration from Africa to Eurasia. They found the precipitation threshold (value of rainfall below which survival is not possible) to be around 90 mm of rainfall per year, which resulted in arid conditions, and which supported a certain kind of vegetation including scattered seeds, grasses, herbs and shrubs.
After making this estimate of the threshold, the scientists found a number of climatic windows of opportunity for northern or southern expansions out of Africa. At time intervals between 246,000 and 200,000 years ago, it was possible to cross the Nile-Sinai-Land Bridge.
The scientists also inferred that similar migrations would have been feasible 96,000 years ago or around 78,000 and 67,000 years ago.
From 275,000 to 242,000 years ago, from 230,000 to 221,000 years ago, and from 182,000 to 145,000 years ago, there were periods of sufficient rainfall and relatively low sea levels, making them suitable windows for migration. From 135,000 to 115,000 years ago, an early northern exit could have occurred, and the modern humans were likely to have come across southern migrants on the Arabian Peninsula.
The study found that the migrants faced difficulty crossing the Red Sea. Along with climatic constraints like low precipitation and aridity, another challenge was the need to cross the strait of the Bab-e-Mandeb, a strait located between Yemen on the Arabian Peninsula, and Djibouti and Eritrea in the Horn of Africa, which connects the Red Sea to the Gulf of Aden.
Earlier Migrations Mostly Failed
It is already known that the large scale colonisation of the world began from 65,000 years ago, and though migrations took place before that too, the adverse environmental conditions in southwest Asia, and likely competition for survival among different hominins (group of modern humans, both extinct and alive) could have resulted in the deaths of early colonists.
However, it is still unclear why those migrations had failed.
Whether Homo sapiens could have crossed the Taurus-Zagros Mountain Range, and survived after competing with Neanderthals in the north, would determine successful migration beyond the Arabian Peninsula. There also existed other competing hominins like the Denisovans, who resided in East Asia. The mountain constraints, and competition from other hominins led to limited human expansions.
Periods of rainfall in which humans can't thrive also interrupted their voyage, the paleoclimatic reconstructions suggest. Climatic fluctuations could have led to local extinctions of those who migrated to the Arabian Peninsula. All this led to the isolation of the early colonists.
Why Was The Exodus That Occured 65,000 Years Ago Successful?
Between 65,000 and 30,000 years ago, hugely favourable climatic conditions along the southern route would have outweighed the effect of climatic fluctuations, which led to the successful mass expansion of Homo sapiens, from Africa to the Arabian Peninsula, where the populations stabilised, and subsequently into Eurasia, the study notes.
The decline of the Neanderthals and the economic, social, technological and cognitive changes in human societies, together led to the success of the exodus from Africa 65,000 years ago, and subsequent colonisation of Eurasia.