Did Humans Originate From A Single Place In Africa? Study Rejects Old Theory, Presents New Timeline

Scientists have long argued that humans originated from a single place in Africa, during a single period of time, but a new study rejects this theory. 

According to the study, the earliest population divergence among contemporary populations occurred 1,20,000 to 1,35,000 years ago. This is the earliest population split detectable among early humans.

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Human beings are said to have originated in Africa, but there are several theories surrounding where, when and how they emerged. Models on divergence and migration of humans across Africa have a lot of uncertainties. Scientists have long argued that humans originated from a single place in Africa, during a single period of time, but a new study rejects this theory. 

The study, published May 17 in the journal Nature, suggests that humans lived in different regions of Africa, migrated from one place to another, and mixed with one another over a period of hundreds of thousands of years. Thus, the study, led by researchers at McGill University and University of California, Davis, negates some of the dominant theories about human origins in Africa. 

The reasons the uncertainties surrounding human origins in Africa exist are a shortage of fossil and genomic data, and variability in previous estimates of human divergence times. 

What do old theories on human origins suggest?

One of the theories surrounding human origins says that there was a single ancestral population in Africa about 1,50,000 years ago, and it is from this population that other populations diverged. Another theory is that the mixing of modern humans with Neanderthal-like hominins resulted in the central ancestral population. This, in turn, resulted in a leap forward in human evolution, the theory says. 

According to a statement released by McGill University, Brenna Henn, co-lead author on the paper, said that at different times, people who embraced the classic model of a single origin for Homo sapiens suggested that humans first emerged in either East or Southern Africa. She also explained that it has been difficult to reconcile these theories with the limited fossil and archaeological records of human occupation from sites such as Morocco, Ethiopia, and South Africa, which show that Homo sapiens lived across Africa at least 300,000 years ago. 

Researchers analysed genomes of 290 living individuals

The study marks the first systematic test of competing anthropological models against genetic data. As part of the study, the researchers analysed the genomic material of 290 living individuals from four geographically and genetically diverse African groups to find similarities and differences between the populations over the past million years. The aim of the researchers was to obtain information about genetic interconnections and human evolution across the continent. 

Which African groups were analysed?

The African groups analysed as part of the study were the Nama (Khoe-San from South Africa); the Gumuz, who are recent descendents of a hunter-gatherer group from Africa; the Mende, who are from Sierra Leone; and the Amhara and Oromo, who are agriculturalists from eastern Africa. Newly sequenced genomes from 44 modern Nama individuals were included. In order to take into consideration colonial infiltration and mixing in Africa, the researchers also included some Eurasian genetic material. 

In order to remove the uncertainties, the researchers made detailed demographic models for populations across Africa, including eastern and western representatives. 

The researchers stated that they inferred a "reticulated" African population history in which present-day population structure dates back to Marine Isotope Stage 5. Marine Isotope stages refer to alternating periods of warm and cold episodes in Earth's paleoclimate, and are deduced from the oxygen isotope data derived from deep sea samples. Marine Isotope Stage 5 is a complex period of warm and cool episodes that occurred 1,28,000 to 73,000 years ago. This means that the present-day population structure emerged during Marine Isotope Stage 5.

When did the earliest population split among early humans occur?

According to the study, the earliest population divergence among contemporary populations occurred 1,20,000 to 1,35,000 years ago. This is the earliest population split detectable among early humans.  Before the population split, there were two or more weakly differentiated ancestral Homo populations that were connected by gene flow over hundreds of thousands of years.

These ancestral populations are called stem populations. 

Migration continued between stem populations 

People continued to migrate between the stem populations even after the population split. This created a weakly structured stem. However, the mixing of populations even after the split explains the genetic variation seen among individual humans and human groups, a statement released by the University of California, Davis, said.

What lineages emerged from stem 1?

By analysing their models, the researchers found that one of the stems, called stem 1, diverted into lineages that led to contemporary populations in western, southern and eastern Africa, the study said. 

Stem 1 gave rise to Neanderthals, after a small branch split from the stem about 600,000 years ago, a New York Times (NYT) article said. 

What was the contribution of stem 2 to evolution?

The other stem, called stem 2, contributed to variable ancestry to those populations.

Migration from stem 2 was the highest with the Mende, and this process occurred until five thousand years ago.

According to the Natural History Museum, Neanderthal and modern human lineages separated between 6,50,000 to 500,000 years ago. This proves that before the split of Neanderthal and modern human lineages, there was a common ancestor. 

After the split of Neanderthals from stem 1, both stem 1 and stem 2 thrived in Africa for hundreds of thousands of years. 

The NYT article also said that the merging of Stem 1 and Stem 2 in southern Africa gave rise to a new lineage that eventually led to the Nama and other living humans in the region. 

According to the article, a separate fusion of stem 1 and stem 2 occurred elsewhere in Africa, which produced a lineage that eventually gave rise to living people in West Africa and East Africa, and also to the people who expanded out of Africa. 

Variation in stem populations accounts for some genetic differences in populations

Based on their model, the authors predicted that one to four per cent genetic differentiation among contemporary human populations can be attributed to variation in the stem populations. 

The authors also said that since migration occurred between the branches, the multiple lineages were probably morphologically similar. 

The authors concluded that if, as their model predicts, the genetic differences between the stems were similar to those among contemporary human populations, the most morphologically divergent fossils are unlikely to represent the branches that contributed to the evolution of humans.