The discovery of a trove of bones in South Africa is raising fascinating questions about whether some of humans’ earliest relations behaved in a more complex way than previously thought.
Researchers suggest that Homo naledi, which emerged an estimated 2.5 million to 2.8 million years ago, deliberately and persistently used a cave to dispose of their dead. Burial or entombment practices are generally thought to have emerged some 350,000 years ago, once humans developed the complex cognitive abilities needed to perform them.
It is but one remarkable aspect of what is a remarkable find on several levels.
The bones were exceptionally well preserved. There were lots of them – so far, more than 1,550 skeletal features representing more than 15 individuals ranging in age from infants to seniors. And the shapes of the bones are remarkably uniform from one individual to another, pointing to all as members of the same species.
Altogether, H. naledi exhibited a bizarre mix of physical characteristics, such as a Homo-like skull, but a softball-size brain more like Homo's parent genus, Australopithecus.
But what the site implies about the behavior of H. naledi is perhaps most significant. If the research team’s interpretation of its site is correct, it suggests that more-complex behaviors might not have been limited to H. naledi.
"It does mean that we ... have to explore the possibility that the behavior of other species was equally as complex," says Lee Berger, a paleoanthropologist at the University of Witwatersrand in Johannesburg and the leader of the international team that announced the find Thursday.
At the least, the find puts a spotlight on the evolutionary experiments the genus Homo underwent en route to Homo sapiens, the researchers say.
"Homo naledi is truly unlike any other species that we've discovered within our genus," says John Hawks, a biological anthropologist at the University of Wisconsin at Madison and a member of the research team.
The skull, about one-third the volume of a human skull, "is giving us some sort of signal about where this might fit in the family tree," Dr. Hawks says.
Hips, shoulders, and fingers were more like those of Australopithecus. But the team found that the feet of H. naledi were the most human-like in the fossil record after those of humans and Neanderthals. The species sported legs made for walking long distances – long and slender. Wrists and teeth, and aspects of the hands, were human-like.
After toting up the traits, the team concluded that its discovery was a better fit as an early example of the genus Homo than it was to Australopithecus. The species also adds another piece of evidence supporting the idea that growth in brain size doesn't necessarily precede growth in body size, researchers say.
The hominin fossils – and there are more to be retrieved – were discovered in a lightless chamber at the farthest reaches of a cave dangerous even to modern spelunkers.
No evidence exists for ceremonial rituals of any kind, Dr. Berger says. But the team infers the habitual use of the site for disposing of the dead from several lines of evidence.
- The bodies apparently were brought intact; while fossilized bones were broken, the evidence so far suggests that the breaks occurred at various times after the bodies were deposited.
- The bones show no evidence of cannibalism or of animal toothmarks.
- No animal or plant remains appear in the chamber.
- And the chamber has only one entrance, which does not reach the surface, ruling out a random series of accidental plunges.
Although the team is confident in its classification of the new species, others are less sure, suggesting that H. naledi raises more questions that it answers.
During the past five years, in particular, new finds have highlighted how diverse – and old – the genus Homo was before humans emerged from the pack, notes Susan Antón, an anthropologist at New York University. That diversity is greater than previously believed. And because specimens come from so few locations in Africa, the diversity may be greater still.
But where H. naledi fits "is the $64,000 question," says Dr. Anton, who was not part of the team making the discovery.
"The assemblage is fabulous but weird," she says. "The mosaic of anatomy is weird, with some very derived characters and some primitive ones. The pattern doesn't fit well with that of early Homo in East Africa – and some derived characters are similar to later Homo."
Given the embarassment of riches at this site, "it will take a while for us to sort out all its implications," Anton says.
For most Hominid species, skulls and teeth are often the only bones available for sorting one species from another.
"We were able, with our incredible sample, to go beyond the skull to look at the entire body," says Hawks.
One crucial piece of information still missing is the age of the specimens. Based on anatomy and Homo's evolutionary timeline, the team estimates that the lineage itself is more than 2 million years old. But the cave contained little in the way of material the team could analyze to date the fossils themselves.
The team plans to try taking DNA samples to see if they can read the bones' genetic clock. But that process requires destroying bone samples, something they did not want to do before they had a chance to provide a detailed description of the discovery. The description appears in two papers published by the online journal eLife.
If the fossils are more than 2 million years old, they would represent the earliest evidence for Homo based on more than a few bone fragments. If the fossils are less than 1 million years old, they would provide additional evidence that several evolutionary experiments en route to humans overlapped in the same general region – southern Africa – at the same time.
It wouldn't be the first example of a small-brained, primitive species of Homo sharing turf with more-recent species, the team notes. It would join Homo floresiensis, a.k.a. the hobbit, discovered in Indonesia in 2003.