The first modern humans to have existed in Africa lived alongside a hominin species that also appears to have buried its dead, behaviour it shared with Homo sapiens, South Africa's University of the Witwatersrand announced Tuesday.

The species known as Homo naledi, the discovery of which was initially announced in September 2015, had been thought to have lived up to 3 million years ago.

It has now been discovered that it lived only between 335,000 and 236,000 years ago, overlapping with modern humans.

During that period, "it was previously thought that only Homo sapiens existed in Africa," the university said.

"More critically, it is at precisely this time that we see the rise of what has been called 'modern human behaviour' in southern Africa - behaviour attributed, until now, to the rise of modern humans ... such as burial of the dead, self-adornment and complex tools," the statement added.

The Homo naledi discoveries are based on remains found in two caves at the archaeological World Heritage Site known as the Cradle of Humankind near Johannesburg.

The second cave, the discovery of which was announced Tuesday, contains remains from three people, including a complete skull.

The fact that the caves could only be accessed through narrow tunnels makes scientists believe Homo naledi used them as burial chambers.

"If there is one other species out there that shared the world with modern humans in Africa, it is very likely there are others," said professor Lee Berger from Witwatersrand. "We just need to find them."

'All sorts of possibilities'

"We're looking at a diversity of species in Africa in the latest stages of our evolutionary history that no one had suspected would be there," said Hawks, an academic at the University of Wisconsin in the United States.

"That implies that as our species arose, it arose with others, that there were a diversity of hominid species in Africa occupying these environments during what we had considered to be the critical time period of modern human origins."

Work has begun to also date the latest fossils to be found.

Paul Dirks, a professor at Australia's James Cook University which is involved in the project, said that Homo naledi's hand structure and the more recent era in which it is thought to have lived means it could have been a toolmaker.

"We have many different branches on the family tree and it is only fairly recently that there is only one survivor on the landscape. The new dating of the fossils opens up all sorts of possibilities for an interchange of... behaviours between Homo naledi and Homo sapiens," Dirks added.

"This will have profound effect on archeology... (it is a) critical missing part of what happened in human evolution," said Berger, who described the findings as "a Rosetta Stone for us".

Homo naledi had a tiny brain, about the size of an orange and stood about 1.5 metres (5 feet) tall, weighing around 45 kilogrammes (100 pounds), experts say.

Homo naledi's teeth and skull are similar to those of early humans while their shoulders are more similar to those of apes.

They had a brain about a third of the size of a modern human brain and curved fingers that are seemingly well suited for climbing.