"Mystery ancestors" gave humans 20% of our current DNA, but who were they?

For decades, scientists believed modern humans descended from a single ancestral lineage. However, new research has not only challenged this view, it has simultaneously spawned a new mystery surrounding human evolution.

Using full genome sequencing, researchers discovered that modern humans originated from two distinct populations that diverged around 1.5 million years ago and later merged.

This genetic reunion, occurring roughly 300,000 years ago, resulted in one group contributing 80% of modern human DNA, while the other provided the remaining 20%.

These findings suggest human evolution was far more intricate than previously assumed.

Genetic mixing and complex evolution

“The question of where we come from is one that has fascinated humans for centuries,” said Dr. Trevor Cousins from the University of Cambridge Department of Genetics.

He emphasized that while a single continuous lineage was the dominant theory, genetic evidence now reveals a more complicated history.

Professor Richard Durbin, co-author of the study, noted that these ancient groups remained separate for over a million years before recombining to form the genetic foundation of modern humans.

This suggests our species’ origins were shaped by deep evolutionary interactions rather than a straightforward lineage.

Humans and Neanderthals – the basics

Humans and Neanderthals didn’t just coexist – they mingled, competed, and even had children together.

Neanderthals lived in Europe and Asia for hundreds of thousands of years before modern humans migrated out of Africa around 60,000–70,000 years ago.

For at least 5,000–10,000 years, the two species overlapped, sharing landscapes, caves, and possibly even knowledge.

Archaeological evidence suggests Neanderthals created tools, used fire, and may have practiced symbolic behaviors like burying their dead.

Some of their cultural traits look strikingly similar to those of early humans, which hints at possible interactions – maybe even trade or shared ideas.

However, this new research highlights an earlier, more significant genetic mixing event around 300,000 years ago.

Unlike Neanderthal DNA, which comprises roughly 2% of non-African modern human genomes, this earlier event contributed up to 20% of the genetic makeup found in all humans today.

Decoding the past with DNA

The research team used modern human DNA rather than ancient fossils to reconstruct this evolutionary history.

They analyzed data from the 1000 Genomes Project, which includes genetic samples from populations worldwide.

Their approach enabled them to infer the presence of ancestral populations that left no direct fossil evidence.

By developing an advanced computational tool called cobraa, the researchers modeled how human populations split and later merged.

They first tested the algorithm on simulated data before applying it to real human genetic sequences.

This method provided fresh insights into the genetic diversity of early humans.

One human population nearly disappeared

“Immediately after the two ancestral populations split, we see a severe bottleneck in one of them – suggesting it shrank to a very small size before slowly growing over a period of one million years,” said Professor Aylwyn Scally from the Department of Genetics.

This smaller group later contributed most of the genetic material found in modern humans and was also the ancestral population from which Neanderthals and Denisovans evolved.

Meanwhile, the second population, which contributed a minority of modern human DNA, carried genes related to brain function and neural processing.

“Some of the genes from the population which contributed a minority of our genetic material may have played a crucial role in human evolution,” said Cousins.

Natural selection and genetic compatibility

The experts found that genes inherited from the second population were often located in regions of the genome unrelated to primary gene functions. This suggests natural selection filtered out less compatible genetic traits over time.

The process, known as purifying selection, gradually removed harmful mutations, ensuring only beneficial genetic changes persisted.

Expanding the study beyond humans

Beyond human evolution, the researchers applied their computational model to genetic data from species such as bats, dolphins, chimpanzees, and gorillas.

They found evidence of similar ancestral population structures in some species, reinforcing the idea that evolutionary history is more intertwined than previously thought.

“What’s becoming clear is that the idea of species evolving in clean, distinct lineages is too simplistic,” Cousins explained.

He suggested that interbreeding and genetic exchange likely influenced species development across the animal kingdom.

Identifying our ancestral candidates

The identities of these ancestral human populations remain uncertain.

Fossil evidence indicates that species such as Homo erectus and Homo heidelbergensis lived in Africa and other regions during this period.

These species could be potential candidates for the populations identified in the genetic study, but more research is needed.

Future studies aim to refine the genetic model to account for gradual genetic exchanges rather than abrupt splits and reunions.

Scientists also plan to integrate their findings with fossil records to build a clearer picture of early human diversity.

Our understanding of human evolution

“The fact that we can reconstruct events from hundreds of thousands or millions of years ago just by looking at DNA today is astonishing,” said Scally.

He emphasized that our evolutionary history is far richer and more complex than previously imagined.

This remarkable research reshapes our understanding of human origins, proving that our past was shaped by intricate genetic interconnections rather than a single ancestral line.

The findings open new avenues for studying evolutionary history across multiple species, offering a deeper appreciation of the forces that shaped modern life.

The study is published in the journal Nature Genetics.

—–

Like what you read? Subscribe to our newsletter for engaging articles, exclusive content, and the latest updates. 

Check us out on EarthSnap, a free app brought to you by Eric Ralls and Earth.com.

—–