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Homo naledi, the newly discovered species of ancient human, explained

It lived millions of years ago — and may have buried its dead.

Over the past two years, an international team of scientists has discovered more than 1,500 mysterious fragments of bone in a tiny cave in South Africa. Thursday morning, the team announced that the fossils are from a new species of ancient human that is believed to have lived 2 to 3 million years ago: Homo naledi.

They appear to resemble another ancient hominin species, Homo erectus, but they have much smaller brains, more along the lines of a gorilla's. There's even tantalizing evidence that they might have buried their dead — something scientists previously thought only modern humans did.

When it comes to the study of human evolution, all this is a really big deal. To date, we know of only a handful of other species similar enough to us to fit in to our genus: Homo. Scientists will debate this designation for H. naledi — as they do for all newly discovered species — but the bottom line is that these fossils, detailed in a pair of new papers in the journal eLIFE, give us a fascinating glimpse into a new part of our ancient history.

How are we related to the new species?

That's hard to answer. They could, theoretically, be our direct ancestors. But it's much more likely they were our ancestors' cousins, who diverged, evolved their own unique traits, and at some point died off.

Why don't we know? A good analogy for our current situation is that the many semi-human-like species that predated us made up a complicated, densely branching family tree. But today, every single branch of that tree has disappeared, minus the last twig: us.

Using fossils, we can get a rough idea of what some of the branches looked like, but it's really difficult to put them all together accurately.

As such, we know of a dozen or so species that evolved sometime in the past 10 million years — when our ancestors first split off from the ancestors of chimps — but we don't know exactly how all these species fit together. In some cases, we're not even sure if certain fossils are from different species or the same one. The best we can do, at the moment, is roughly put the species into groups, based on shared traits:

The scientists say H. naledi belongs to the Homo genus.

(S.V. Medaris/University of Wisconsin–Madison)

The Homo group (technically called a genus) includes modern humans, as well as a handful of relatively recent species that are most closely related to us: H. erectus, for instance, walked upright. By contrast, Australopithecus — the older group that includes the famous "Lucy" fossil — had much smaller brains.

The scientists behind this new discovery, led by Lee R. Berger of the University of the Witwatersrand in Johannesburg, say the H. naledi species belongs in the Homo group. Other scientists, though, might dispute that — and even if it does fit there, it doesn't mean H. naledi were actually our ancestors.

What did this new species look like?

(John Hawks/University of Wisconsin–Madison)

If you came across a live H. naledi, you might have trouble deciding if it was a human or an ape. The average individual, scientists believe, was around 5 feet tall and 100 pounds. Many of the beings' features — especially their feet and relatively long legs, seemingly suited to long-distance walking — look a lot like ours. And the proportions of their skulls (with small teeth and a narrowing just behind the eyes, at the temple) also roughly resemble ours and other species in the Homo genus.

At the same time, these skulls are way, way smaller than ours: They held brains about the size of an orange, more along the lines of the older, more ape-like Australopithecus species. The fossils also have chests and hips that look more like Australopithecus than Homo, as well as arms, shoulders, and curved fingers that seem to be adapted for climbing.

How did this new species live?

H. naledi's hand.

(John Hawks/University of Wisconsin–Madison)

We don't know much about their lifestyle yet. H. naledi's legs and arms make it seem likely that they walked upright, but also spent time climbing trees. The scientists speculate that their hands might have been capable of making tools, but we don't know for sure, and no stone tools were found in the cave

The one fascinating glimpse we have into their lifestyle, though, is where the fossils were found: jammed into a tiny crevice in a cave that's more than 200 feet from the surface. What's more, the 15 different bodies — which include infants, juveniles, and one elderly individual — appear to have been brought in whole, over time.

All this makes scientists suspect that H. naledi may have intentionally buried their dead or dropped them down a chute that led to the cave. (It's not certain, but there's no evidence of bite marks or anything else to suggest predators brought the bodies there.) If it's true, it would be the oldest instance of intentional burial by far. Currently the oldest burial site, in modern-day Israel, is 100,000 years old and is filled with anatomically modern humans.

Why are scientists so excited about this?

H. naledi's foot looks surprisingly like ours.

(John Hawks/University of Wisconsin–Madison)

One reason is the absolute bonanza of fossils that prompted this discovery. Some ancient hominin species are known by just a few shards of bone. Here, scientists found more than 1,500 fossil pieces from at least 15 different individuals, of varying ages, which lets us see the variation within the species. The find includes nearly complete hands and feet, something essentially unheard of in the fossil record. All this gives us a much clearer picture of H. naledi's anatomy than we have for species we've known of for decades.

But H. naledi's particular characteristics are also surprising, and in some ways, unique. Some scientists have thought that ancient hominins went from climbing trees to walking on the ground, but here's a species that seemingly did both at once. Others thought that many distinctively human-like behaviors (such as burial of the dead and toolmaking) depended upon the large brains we've evolved more recently, but H. naledi challenges that idea too.

Figuring out the answers will take a lot of time. One problem is that scientists have been unable to figure out the age of the H. naledi fossils because they were found in soft sediments, rather than hard, easily dateable rock.

But the scientists are now beginning the process of radiocarbon dating (which was avoided so far because it requires destroying small pieces of the material). Between that and study of the many more pieces of H. naledi fossil that remain in the cave, we'll hopefully learn a lot more.