Humanity’s strange new cousin is shockingly young - and shaking up our family tree

PUBLISHED : Wednesday, 10 May, 2017, 10:28am
UPDATED : Wednesday, 10 May, 2017, 8:54pm

Homo naledi, a strange new species of human cousin found in South Africa two years ago, was unlike anything scientists had ever seen.

Discovered deep in the heart of a treacherous cave system - as if they’d been placed there deliberately - were 15 ancient skeletons that showed a confusing patchwork of features. Some aspects seemed modern, almost human. But their brains were as small as a gorilla’s, suggesting Homo naledi was incredibly primitive. The species was an enigma.

Now, the scientists who uncovered Homo naledi have announced two new findings: they have determined a shockingly young age for the original remains, and they found a second cavern full of skeletons. The bones are as recent as 236,000 years, meaning Homo naledi roamed Africa at about the time our own species was evolving.

And the discovery of a second cave adds to the evidence that primitive Naledi may have performed a surprisingly modern behaviour: burying their dead.

“This is a humbling discovery for science,” said Lee Berger, a paleoanthropologist at the University of the Witwatersrand in Johannesburg. “It’s reminding us that the fossil record can hide things . . . we can never assume that what we have tells the whole story.”

Berger and his colleagues report Naledi’s age and the new chamber in two papers published Tuesday in the open-access journal eLife. In a third paper, they argue that Naledi must be a long-lasting lineage that arose 2 million years ago during the early days of the genus Homo and somehow survived long enough to coexist with modern humans, who emerged about 200,000 years ago.

Several scientists not involved the Naledi research urged caution about some of Berger’s bolder claims, including the suggestion that Naledi was burying its dead and crafting the sophisticated stone tools that characterise southern Africa’s “Middle Stone Age.”

But they agreed with Berger on this point: Naledi reminds us that human history is even richer than we realised.

“The past was a lot more complicated than we gave it credit for and our ancestors were a lot more resilient and lot more varied than we give them credit for,” said Susan Anton, a paleoanthropologist at New York University who was not involved in the research. “We’re not the pinnacle of everything that happened in the past. We just happen to be the thing that survived.”

The original Homo naledi skeletons were discovered in 2013 in the Rising Star cave system, one of the twisted and branching limestone caverns that make up a World Heritage Site known as “the Cradle of Humankind.”

The Dinaledi (“star” in the Sesotho language) chamber, which contained the Naledi skeletons, was so narrow and difficult to access that Berger had to seek out an all-women team of petite, extremely agile spelunkers to excavate it. What they found astonished the paleoanthropology community - not only had a new species been discovered but, with 15 skeletons, it was suddenly the best-documented species in the history of hominins.

But the Rising Star system wasn’t done giving up its secrets. As soon as the Dinaledi excavation was complete, the team went back to a second chamber, dubbed Lesedi (“light”).

Somehow, more than 130 hominin bones wound up in this dark and humid cavern hundreds of thousands of years ago. The excavators uncovered remains from at least three Homo naledi individuals.

Berger and his colleagues don’t yet have an age for the Lesedi individuals, and without DNA evidence from both caverns, it will be impossible to tell whether they are related to those from Dinaledi. But he and his colleagues argue that the presence of a second cavern full of bones bolsters the theory that Homo naledi was deliberately leaving its dead in these chambers.

“One, perhaps, was a singular event,” Berger said. “Two is not a coincidence.”

In dating the original Dinaledi skeletons, the research team employed six different dating techniques at 10 labs around the world. Based on analysis of teeth and several measures of radioactivity in the cave, the team concluded that the fossils date back to between 236,000 and 335,000 years ago - just before the arrival of modern humans.

“Our ancestors did not live in a single species world the way we do,” Brooks said. “The real take-home message of this paper is that we were not alone until very recently.”

Several other hominin species roamed the globe during this period, known as the Middle Stone Age: Homo erectus in Asia; tall, large-brained Homo heidelbergensis in Africa and Europe; eventually Neanderthals and the mysterious Denisovans (who are known only from DNA and a few fossil teeth). But these species were a lot like us: They walked primarily on two feet, used tools and probably mastered fire. Even the smallest-brained species had a brain that was three-quarters the size of ours.

Homo naledi complicates that narrative. Its limbs and teeth suggest that it had a human’s walking habits and diet, and perhaps roamed the same lands and ate the same foods as our recent ancestors. But its brain was only 30 per cent the size of a human’s, and no bigger than that of a gorilla today.

“We keep finding stuff that we didn’t think existed,” said John Hawks, a paleoanthropologist at the University of Wisconsin at Madison who helped lead the Rising Star expedition. “This is not the first, and it’s not going to be the last.”