For decades, scientists believed modern humans came from one main ancestral group in Africa, an idea known as the 'Out of Africa' model.

But new DNA research suggests the story may be far more complicated. 

Instead of coming from a single isolated group, early humans likely developed from several groups spread across Africa that stayed in contact and mixed over hundreds of thousands of years.

Scientists led by researchers at the University of California–Davis reached this conclusion by studying DNA from modern African populations. 

A key part of the research involved 44 newly sequenced genomes from the Nama people of southern Africa, whose unusually rich genetic diversity offered important clues about humanity's distant past.

They then used computer models to test different theories of human origins, comparing whether modern DNA was better explained by one ancestral group or several connected populations. 

The results showed the evidence fit much better with the idea of multiple early human groups that continued mixing over long periods of time.

According to the study, the earliest detectable split among these ancient populations happened roughly 120,000 to 135,000 years ago, but even after that, the groups continued exchanging genes for thousands of generations.

Scientists broadly agree that Homo sapiens originated in Africa. 

The harder question is how early human groups separated, moved, reconnected and shaped one another across the continent.

Brenna Henn, professor of anthropology and the Genome Center at UC Davis and co-author of the study, said the uncertainty comes from gaps in both fossils and ancient DNA.

'This uncertainty is due to limited fossil and ancient genomic data, and to the fact that the fossil record does not always align with expectations from models built using modern DNA,' she said in a statement. 'This new research changes the origin of species.'

A major part of the study stemmed from 44 newly sequenced genomes from modern Nama individuals in southern Africa. 

The Nama are an Indigenous population known for carrying unusually high levels of genetic diversity compared with many other living groups.

This group of people is unique for their ancient origins dating back 100,000 to 140,000 years. 

Researchers collected saliva samples from people in their villages between 2012 and 2015 while participants were going about their daily lives. 

Those samples helped the team examine whether human origins fit a single source model or something broader and more interconnected.

The best-fitting model suggested that the earliest population split among early humans, still detectable in living people, happened roughly 120,000 to 135,000 years ago. 

Before that split, two or more weakly differentiated Homo populations had been exchanging genes for hundreds of thousands of years. Even after the split, movement and mating continued between these early groups.

The researchers described this as a weakly structured stem, meaning the roots of modern humans were not one isolated population, but a loose set of connected populations with ongoing gene flow.

That network-like model may explain human genetic diversity better than older models, according to the authors.

Instead of needing to assume major contributions from an unknown archaic hominin population in Africa, the model shows how patterns in modern DNA could have emerged from structure within ancestral human populations themselves.

'We are presenting something that people had never even tested before,' Henn said of the research. 'This moves anthropological science significantly forward.'

Co-author Tim Weaver, a UC Davis professor of anthropology who studies early human fossils, said the results shift how scientists should think about older explanations.

'Previous, more complicated models proposed contributions from archaic hominins, but this model indicates otherwise,' he said.

Weaver contributed comparative fossil expertise to the study, helping connect genetic models with what early human remains looked like.

The model also has consequences for how scientists interpret the fossil record. According to the authors, only 1 to 4% of genetic differentiation among living human populations can be traced to variation between these ancestral stem populations.

Because the early branches continued mixing, they were probably similar in appearance. 

That means fossils with very different physical traits, such as Homo Naledi, are unlikely to represent lineages that directly contributed to the evolution of Homo sapiens, the authors said.