Homo naledi: our “decidedly strange” cousins

Ah, families are so complicated. You think you’re clear about who is related to whom, and then a long-lost relative surfaces and it all gets hazy again.

That’s the current situation with the evolutionary family tree of every one of us. The discovery four years ago of a new member of the Homo genus—that means humans, of which Homo sapiens is the only surviving member—created a fresh puzzle in the story of human evolution. And now a new family member, called Homo naledi, is proving even more disruptive. According to the latest results on H. naledi skeletons and bones, remarkably well preserved in a cave in South Africa, the species may be far younger than was suspected. These slight beings could still have been living around 250,000 BC. The earliest known anatomically modern humans in Africa date back to around 200,000 years ago, and so something like big-brained Homo sapiens would have coexisted with these smaller, more “primitive” creatures.

Primitive, perhaps—but their discoverers think they might have elements of a Stone Age culture, including burial and tool use. Regardless of whether that theory pans out, the new fossils seem likely to supply one of the biggest shake-ups in human evolution for several years, comparable to the discovery of the diminutive Homo floresiensis (the so-called “hobbit”) on the island of Flores in Indonesia in 2003.

These and other discoveries in the human fossil record show that we can’t any longer think of our having evolved in linear, sequential fashion from steadily less ape-like ancestors. Instead, there seem to have been several very different-looking groups of humans living alongside one another—and perhaps even interbreeding. We already knew that was so throughout most of the last ice age (since about 100,000 years ago), when humans coexisted with Neanderthals and another subspecies of Homo called Denisovans identified from fossils in Siberia. But now it appears that various types of Homo lived side by side for far longer.

The new results are published in three papers* in the journal eLife by palaeoanthropologist Lee Berger of the University of Witwatersrand in Johannesburg and his coworkers. They describe 130 H. naledi fossils, including an almost complete (probably male) skeleton dubbed Neo, found in a second underground chamber within the Rising Star cave system in which the species was first identified by this team in 2013. (Those findings were reported in 2015**.) The researchers were previously unable to date the bones reliably, but now they’ve done so using several different dating techniques. The findings pinpoint these H. naledi remains to between 335,000 and 236,000 years ago.

The fossils look decidedly strange. Some of the features are, for want of a better term, rather ape-like: a small brain cavity, curved fingers, simian hip joints. But other features, such as the hands, legs and feet and jawbone, look more like those of Neanderthals and modern humans.

In short H. naledi is something of a chimera—or more properly, a “relic species,” probably originating close to the origin of the Homo genus about 2.8m years ago but surviving for a long time with some preserved primitive features. In that respect H. naledi may be similar to H. floresiensis (thought to have survived until about 50,000 years ago, except that it’s far easier to see how the “hobbits” could survive in their isolated island setting. To judge from the new African fossils, H. naledi wasn’t clinging on to existence either: the bones look like those from healthy, well-nourished individuals. An adult like Neo was about 1.4m tall and weighed around 40 kg.

The big question, according to palaeontologist Chris Stringer of the Natural History Museum in London, is then how these creatures managed to persist alongside more “advanced” humans in Africa. If they were hunter-gatherers like other humans around that time—not just the precursors to Homo sapiens but also Homo rhodesiensis, identified in Zambia (then Northern Rhodesia) in 1921—then how could they have competed in the same evolutionary niche? Or perhaps they had a different, less competitive lifestyle, such as sticking to an ape-like practice of living partly in trees?

It’s still a mystery too how these remains ended up within this deep, totally dark cave system. Berger and colleagues think that the only way the bodies could have got down there was by intentional disposal in underground tombs, suggesting that H. naledi had some kind of burial culture. If so, the burial process would seem to require the use of fire to see the way. And the researchers suggest that these creatures may have made tools. None has been found in the cave, but Berger says the “modern” hands of H. naledi would have been up to the task of making and manipulating them, and stone tools of about this age have been found elsewhere in southern Africa.

Stringer, though, thinks that’s unlikely for creatures with such small brains. He wonders whether further exploration will reveal other cave entrances down which the bodies might have been dropped—or just fallen by accident.

At any rate, Stringer says that the discoveries remind us how much we still don’t know about our family tree and early history. “About 95 per cent of the area of Africa is still essentially unexplored for its fossil human record,” he says. “At around 300,000 years ago there were probably at least three kinds of humans across the African continent [ancestors of H. sapiens, H. rhodesiensis and H. naledi]. Who knows what else might be out there?”

* The three papers can be found here, here and here

* Links to these findings can be found here and here