Scientists have discovered a ‘hidden chapter’ in human evolution – and it suggests our history is much more complex than we thought.

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Now, a team from the University of Cambridge has found that modern humans descended from not one, but at least two ancestral populations.

These two ancestral populations – referred to as Group A and Group B – split around 1.5 million years ago. 

This was possibly due to a migration event where one group trekked thousands of miles to new terrain. 

But around 300,000 years ago, the two groups came back together, before breeding and eventually spawning homo sapiens.

Group A contributed 80 per cent of the genetic makeup of modern humans, while Group B contributed 20 per cent. 

MailOnline’s graphic below shows this ‘hidden chapter’ in human evolution spanning from 1.5 million years ago (top) until the modern-day (bottom).  

Scientists have discovered a ‘hidden chapter’ in human evolution – and it suggests our history is much more complex than we thought 

For the study, the team used data from the 1000 Genomes Project, a global initiative that sequenced DNA from populations across Africa, Asia, Europe and the Americas.

The method relied on analysing modern human DNA, rather than extracting genetic material from ancient bones – letting the scientists infer the presence of ancestral populations that may have otherwise left no physical trace. 

For decades, it’s been thought Homo sapiens first appeared in Africa around 200,000 to 300,000 years ago having descended from a single lineage. 

Although the new study does not contest the time of Homo sapiens’ emergence, it does show that there were two lineages, not one. 

Around 1.5 million years ago, a small population (A) diverged from the main group (B) and slowly grew in size over a period of 1 million years. 

‘A divergence event is when a population splits into two or more genetically distinct populations, [but] it is not necessarily a migration event,’ lead author Dr Trevor Cousins told MailOnline. 

Interestingly, Group A seems to have been the ancestral population from which Neanderthals and Denisovans emerged around 400,000 years ago. 

Around 300,000 years ago, Group A and Group B came back together – although exactly how this happened is unclear. 

Group A seems to have been the ancestral population from which Neanderthals and Denisovans emerged around 400,000 years ago. Pictured, a recreated head and pieced-together skull skull of Shanidar Z, a 75,000-year-old Neanderthal skeleton

From then on, the two reunited groups evolved and eventually spawned modern humans – non-Africans, west Africans and other indigenous African groups, such as the Khoisans. 

Where exactly this all happened, however, is a matter of speculation.  

Dr Cousins said it’s ‘likely’ that groups A and B both originated and stayed in Africa, but there are other possibilities regarding location. 

For example, group A may have stayed in Africa while group B migrated to Eurasia, or B stayed in Africa while A migrated to Eurasia. 

‘The genetic model can not inform us about this, we can only speculate [but] in my view there are valid arguments for each scenario,’ he told MailOnline. 

‘Due to the diversity of fossils found in Africa, perhaps scenario one – A and B both originated and stayed in Africa – is the most likely.’ 

The study authors do not know the identity of the ancient species that make up the A and B groups, although fossil evidence suggests that species such as Homo erectus and Homo heidelbergensis lived both in Africa and other regions during this period.

This makes them potential candidates for these ancestral populations, although more evidence will be needed to confirm this. 

Fossil evidence suggests that species such as Homo erectus and Homo heidelbergensis lived both in Africa and other regions during the period of Group A and Group B. Pictured, the most complete skull of an Homo heidelbergensis ever found

Homo erectus (depicted here) was the first hominin to evolve a truly human-like body shape 

‘It is not even clear that they would correspond to any species currently identified through fossils,’ Dr Cousins told MailOnline. 

‘We speculated at the end of the paper what species that may belong to – but it is just that – speculation.’ 

The new results, published in the journal Nature Genetics, reveal an intriguing hidden chapter in human evolution. 

Beyond human ancestry, the researchers say their method could help to transform how scientists study the evolution of other species, like bats, dolphins, chimps and gorillas. 

‘Interbreeding and genetic exchange have likely played a major role in the emergence of new species repeatedly across the animal kingdom,’ added Dr Cousins. 

WHAT DO WE KNOW ABOUT HOMO HEIDELBERGENSIS?

Homo heidelbergensis lived in Europe, between 650,000 and 300,000 years ago, just before Neanderthal man. 

Homo heidelbergensis, shares features with both modern humans and our homo erectus ancestors.  

The early human species had a very large browridge, and a larger braincase and flatter face than older early human species. 

Homo heidelbergensis lived in Europe, between 650,000 and 300,000 years ago, just before Neanderthal man

It was the first early human species to live in colder climates, and had a ­­­short, wide body adapted to conserve heat. 

 It lived at the time of the oldest definite control of fire and use of wooden spears, and it was the first early human species to routinely hunt large animals.

This early human also broke new ground; it was the first species to build shelters, creating simple dwellings out of wood and rock.

Males were on average 5 ft 9 in (175 cm) and weighed 136lb (62kg) while females averaged 5 ft 2 in (157 cm) and weighed in at 112 lbs (51 kg).

Source: Smithsonian