This is an article from the spring 2016 issue of LSA Magazine. Read more stories from the magazine.
 

Paleoanthropologists researching human evolution haven’t had a lot of material to work with. The early human fossil record’s thousands of entries have been inscribed in fragments: Isolated teeth and solitary bones are the scant dots that connect us to our earliest ancestors across millions of years. Those occasions when paleoanthropologists have found semi-intact figures—the Taung child or the skeleton we know as Lucy—are so extraordinary they became international headlines.

So when a peculiar Facebook ad appeared in 2013, the anthropology world took notice. Paleoanthropologist Lee Berger, working at a UNESCO World Heritage Site known as the Cradle of Humankind, was looking for individuals with excellent archaeological excavation skills who were skinny, small, fit, and not claustrophobic.

In early October 2013, two recreational cavers had brought Berger some photos they’d taken in a cave called Rising Star. Located 30 miles northwest of Johannesburg, South Africa, Rising Star is part of a cave system that has more than a kilometer of twisty underground passages, making it a popular spot for weekend spelunkers to explore, often searching for new passages to put on the map. That September, the cavers had found just such a passage and were able to squeeze into it. And squeeze is the word: In places, the passage was just eight inches wide. The passage led them to an incredible chamber they photographed and showed to Berger.

The Facebook ad yielded six slender excavators—who all happened to be women—whom Berger called “the underground astronauts.” The sextet spent five weeks working underground and recovered a whopping 1,550 pieces of bone fossils.

Homo naledi fossils are confusing in many ways. The hands have human-like wrists and fingertips, but very curved fingers. Photo by John Hawks

Back on the surface, an international team of senior paleoanthropologists cleaned and prepared the fossils, and were joined by an even larger team of paleoanthropologists who reconstructed and identified them. Two years later, the team announced that the bones belonged to a previously unknown branch of the human family tree, a new species they called Homo naledi.

New Branches on the Family Tree

The earliest humans belonged to the genus Australopithecus, which included species that shared both human and ape-like traits. Like humans, they walked on two legs and had small canine teeth. Like apes, they had small brains and long, curved fingers. Australopithecus includes several species who all lived between 4.4 and 1.3 million years ago.

The Homo genus began about 2.5 million years ago. “In anthropology, we tend to classify things in Homo if they have large brains and hands that look like they’re capable of making tools,” explains John Hawks (M.A. ’96, Ph.D. ’99), a leader of the Rising Star Project. “The idea is these different traits belong together, that the Homo genus happened because some sort of hominin—the name of the tribe to which we, extinct humans, and our immediate ancestors all belong—became smarter, made tools, and got larger brains and smaller teeth. For a long time, that’s the way we thought these things went together.”

But anatomy specialists working to reconstruct the Homo naledi skeletons discovered that its bones didn’t fit that way. Part Australopithecus and part Homo, the Rising Star bones were all over the map. Within the profession, anthropologists call specimens whose traits demonstrate different stages of evolution mosaics. And Homo naledi is an example of a mosaic so extreme that Berger described it as “a Mr. Potato Head disaster.”

From Hand Land to Hip Heaven

Before publishing, anthropologists usually keep their discovery within a small circle of collaborators as they study the nut they’re trying to crack. With a discovery as big as Rising Star’s, following a model like that would take decades. Plus, they had a hunch that the Rising Star fossils were going to be pretty big. In other words, they needed a lot of pairs of eyes to get the job done.

A typical endeavor of this sort includes about a dozen senior scientists, but the Rising Star team asked about 20 senior scientists to join the project. Even more surprising, they also invited more than 30 early-career scientists. The anthropologists divided themselves by specialty. Hand people worked with fossils related to the hand in what became known as Hand Land. The Tooth Booth was concerned with fossilized teeth. The pelvis fossils were located in Hip Heaven.

With fossils from 15 individuals, there were multiple samples of each bone for the scientists to review and compare. Working all together in a windowless vault, the scientists analyzed the fossils within their areas of expertise and engaged other stations in constructive cross talk that allowed researchers to discuss the ways in which Homo naledi was similar and the ways in which it was different from previous finds.

“The Homo naledi have the most modern-looking feet I’ve ever seen in a fossil human,” says Jeremy DeSilva (Ph.D. ’08), a project leader and paleoanthropologist whose expertise is in foot and ankle anatomy. “They have a human-like foot and leg that’s well-adapted for a long-distance striding gait.”

Their feet might be familiar, but their pelvis is not, says biological anthropologist Caroline VanSickle (M.A. ’10, Ph.D. ’14). “The upper part of the pelvis is similar to Lucy’s,” VanSickle explains, referring to the famous Australopithecus afarensis skeleton discovered in 1974, “but the lower part shares traits with members of our genus. It seems like the pelvis is in transition between an older-looking torso and human-like lower limbs. It is not a pelvis we would have expected anything named Homo to have.”

The Homo naledi's teeth are nearly the same size as human teeth, but the jaw and the skull are smaller. Photo by Peter Schmid

In the Tooth Booth, paleoanthropologist Juliet Brophy (B.S. ’02) compared the recovered teeth to other pre-human species. Small brains, like Homo naledi’s, should come with large teeth, but Homo naledi’s teeth are small with subtle but significant differences in their features. “The shape of some of the teeth are complete outliers,” says Brophy. “There is no overlap with any other human ancestor we’ve recovered.”

Understanding Homo naledi is complicated: Is it a convergence for separate species, or a point of origin? Is it a case of something more human-like reverting to pre-human characteristics? Is this an ancestor, the first of our kind, or the result of species interbreeding?

“Working out the tree is difficult,” says Hawks. “In the olden days, when Homo habilis was discovered, we said, ‘Well, it has a large brain and tool-making hands. Clearly, that’s Homo, that’s us.’ But maybe Homo naledi isn’t Homo, or maybe Homo isn’t what we thought.”

Aping Behavior

In addition to defying anatomical expectations, Homo naledi also seems to elude expectations about its behavior. The recovered bones appear to have been deliberately deposited into the remote, pitch-black chamber where they were found. In other words, says Hawks, the evidence suggests Homo naledi deliberately disposed of their dead.

The bones have no tooth marks or cracks, Hawks explains, so they weren’t dropped into the cavern by predators. The chamber has no opening, and tests indicate its sediments never had contact with the surface, so the chamber wasn’t an open pit Homo naledi could have fallen into. Articulated appendages, such as their hands, were found together, which means that the bones weren’t carried there by water. Other than birds and rodents, there were no bones from other species, meaning the chamber was probably inaccessible. And the bodies are layered, suggesting they were added over time, rather than in a single, unlucky event where individuals climbed into a chamber from which they couldn’t escape.

The one theory left standing is that Homo naledi ritually deposited their dead. “We’re looking for evidence that rejects that scenario,” says Hawks, “but so far it’s the one that fits. If it proves correct, it means that a fossil species whose brain was the size of a chimpanzee’s was engaging in behavior we’ve long held as a hallmark of being human.”

Open Source Paleoanthropology

From its onset, the Rising Star team flouted convention. They found scientists through social media, welcomed researchers still early in their careers, and built a team of 60 scientists who came and worked together on site all at once rather than working in the more traditional way, with a small team of senior scientists doing their research at different times and then collaborating over email. “We wanted to do things much differently,” says Hawks.

“The field of paleoanthropology can be needlessly competitive, and some of our colleagues have, at times, restricted access to fossils,” explains DeSilva. “With Rising Star, we had a large team of people who shared data to interpret fossils that are available to anyone interested in studying them.”

“Lee Berger, John Hawks, and others decided to create teams that included some wonderfully talented young scholars who have large and innovative data sets, and to anchor the teams with more senior specialists, like myself,” DeSilva continues. “A find of this magnitude really benefits from the field’s collective knowledge, and including younger scholars allowed us to say a lot about these fossils in what is a comparatively short amount of time. We didn’t rush the analysis; we just had more people working on them than is typical.”

Openness has remained one of the Rising Star team’s guiding principles. The team’s paper was published online and is available for anyone to download. It has been read more than 250,000 times around the world. Fossil scans are also available on an open-source site for anyone to download and print as everyone from senior scientists to K–12 teachers have done. “There are huge scientific advantages to such openness,” says Hawks. “It makes everyone’s work better, and this is the first time it’s been tried in anthropology.” And it is a practice that recalls his experience as a U-M student, where he, along with many key members of the Rising Star team, were mentored by Milford Wolpoff, a professor of anthropology and a committed supporter of fledgling paleoanthropologists.

“Milford was such a leader in anthropology that whenever there was a new discovery, we knew about it in advance,” recalls Hawks. “People shared fossil casts with him, and he shared them with us so we could assess them. This project and all of the Michigan people that were involved in it are part of his open legacy.”

“These fossils are extremely precious,” concludes DeSilva, bringing us back to the bones. “Each one has a story to tell. And now, after all of these years, we have this extraordinary opportunity to tell them.”