In a series of papers published in the journal eLife
on Friday, a global group of scientists say Homo naledi roamed the grassland of Southern Africa near modern-day Johannesburg between 335,000 and 236,000 years ago.
It means the mysterious small-brained hominim could have lived at around the same time as the first modern humans in Africa, according to the research team, led by Professor Lee Berger of the University of Witwatersrand.
"If there was one other species out there that shared the world with 'modern humans' in Africa, it is very likely there are others. We just need to find them," says Berger.
Berger and other members of his team originally believed that the fossils could be much, much older.
The remains were first discovered at the Rising Star Cave complex in 2013, with the results first published in 2015.
The cave system -- the richest fossil site of its kind in Africa -- revealed skeletal structures with a strange mix of archaic and modern features.
Naledi's brain is much smaller than ours and its curved fingers are ideal for climbing trees -- more like an ape's -- but its legs, feet and wrists resembled modern humans.
On Friday, in a separate paper in eLife, the team announced the discovery of a second chamber within the cave system that contains even more remains of Homo Naledi.
In the chamber the scientists say they found more fossils and a beautifully complete skull.
"I think some scientists assumed they knew how human evolution happened, but these new fossil discoveries, plus what we know from genetics, tell us that the southern half of Africa was home to a diversity that we've never seen anywhere else," says John Hawks of the University of Wisconsin-Madison and Wits University, an author on all three papers.
Perhaps the most controversial hypothesis by Berger and his team is that Naledi buried its dead -- a behavior that most scientists believe is limited to modern humans.
Found deep in the caverns of the limestone Rising Star Cave complex, Berger believes that Naledi could even have used fire to light the way.
The chambers were so difficult to reach, that the team used underground astronauts to slither through the dark and dangerous tunnels to collect the enormous fossil haul representing adults, elderly and children. Most of the fossils were in remarkably good condition.
Hawks says the new haul could help prove their earlier theories.
"This likely adds weight to the hypothesis that Homo naledi was using dark, remote places to cache its dead," says Hawks. "What are the odds of a second, almost identical occurrence happening by chance?"