In my last two blog posts, I’ve detailed my connection with the team that discovered our new human relative Homo naledi, deep in the Rising Star Cave in South Africa. This discovery was announced to the world formally in September 2015 and was featured as the cover story of the October 2015 issue of National Geographic magazine.

Ever since the announcement, I’ve been teaching about Homo naledi to interested science students and enthusiasts of all ages—from kindergarteners to senior citizens.

Now, I want to take this space to share the science behind this amazing discovery with you here at National Geographic Education. Today, I am covering the science side of the Homo naledi discovery, and in next week’s post, I will address why I think the discovery of Homo naledi is relevant beyond “bones & stones” science.

The initial discovery of Homo naledi is credited to two amateur South African cavers, Rick Hunter and Stephen Tucker. They were exploring the Rising Star Cave outside Johannesburg when they explored beyond the existing map and descended down a 40-foot-long, 7-inch-wide chute. The chute opened into a fossil chamber where they saw human-like bones.

Hunter and Tucker brought their discovery to the attention of paleoanthropologist and National Geographic Explorer-in-Residence Dr. Lee Berger, who immediately recognized these as a potentially world-class hominin discovery.

By early November 2013, Berger put out a call for qualified excavators who also were skinny enough to fit into the chute that Hunter and Tucker first navigated. Within three weeks, he had assembled a highly qualified team of six women who would serve as excavators. You can get to know the team better through my series of Rising Star interviews.

Given the dangerous and challenging nature of getting to the fossils in what became known as the Dinaledi (“Many Stars”) Chamber, the support team laid several miles of AV cabling to allow senior scientists to communicate with the “Underground Astronauts” in the fossil chamber.

Over the course of several weeks, the “partial skeleton” the team hoped to recover turned out to be the largest-ever human fossil excavation on the African continent. The Rising Star Expedition of November 2013 excavated a pit that was 80 square centimeters in area and 20 centimeters deep—the size of a card table and as deep as your hand. They recovered more than 1,500 fossils that represented 15 individuals ranging in age from infants to the elderly and including both sexes. You can relive the daily details of the expedition via my “Twitter Play by Play“.

On the final day of the 2013 expedition, Tucker and Hunter alerted team members to another potential fossil site in a different area of the Rising Star system.Subsequent exploration and excavation of this new chamber—now known as the Lesedi (“Light”) Chamber—yielded three more partial skeletons, including a very complete adult skeleton named “Neo” (“Gift”) and fragments from an infant.

Subsequent exploration and excavation of this new chamber—now known as the Lesedi (“Light”) Chamber—yielded three more partial skeletons, including a very complete adult skeleton named “Neo” (“Gift”) and fragments from an infant.

Such a stunning treasure trove of hominin fossils presented a challenge about how to study them.Traditionally, a small group of senior scientists would spend years analyzing the fossils for subsequent publication. This collection would overwhelm such a system, so Berger chose to expand the team by inviting dozens of early-career scientists to Johannesburg in May 2014 to get the best young minds working on describing and identifying these bones.

The Rising Star Workshop divided these scientists into the body regions of interest (skull group, spine group, hand group, etc). The teams effectively divided the labor needed to describe these fossils and also brought a diversity of ideas to the table when it came to analyzing what species was represented by the Rising Star collection.

What immediately came of the Rising Star Workshop was a sense that this was a unique species. While parts of the collection had obvious connections with existing species, there was no other hominin species that matched the unusual combination of primitive and modern traits that were present in these bones. For example, had the team recovered just the foot bones, they could have called it Homo sapiens due to the similarity to modern feet. (Learn more about the fossils’ Nike-ready feet here.) If they had just the vertebrae, they could have linked it to Homo neanderthalensis. The shoulders looked like those of an ape. The skull had similarities to both Homo sapiens and Homo erectus but was far smaller than either of those species. In short, this species was a fascinating mosaic of traits that could only be a new species. Due to this, the team named it Homo naledi (“Star human”) due to its discovery in the Rising Star cave.

As I detailed in my last post, the announcement of this new species unleashed a media typhoon. What was of greatest interest to both scientists and the public was not the science behind the naming of a new species, but the circumstances of how Homo naledi wound up more than 100 feet underground in a hard-to-access chamber that requires modern cavers more than half-an-hour to reach.

When I pose this dilemma to groups I speak to, they invariably offer the following logical reasons for the accumulation of bones in the Dinaledi (and now the Lesedi) chamber.

1. The bones were gathered by a predators or scavengers such as hyenas.
2. The group was in the cave and died in a catastrophic cave collapse of some sort.
3. The group got lost in the cave and starved.
4. The bones were washed into the cave by a flash flood.

Each of these scenarios, while sensible, are not supported by the evidence from Rising Star.

In scenario #1, there are absolutely no predator or scavenger markings on the bones. This suggests no carnivore involvement in the accumulation of these bones. In one of my favorite statements of the whole expedition, Berger claimed, “These are the healthiest dead things you’ll ever see!”

Scenario #2 is not supported as the accumulation and layout of bones suggests that the bodies arrived in the Dinaledi chamber over an extended period, not at once. The geology of the chamber suggests that it has been stable prior to and since the arrival of Homo naledi. The fact there were fully intact (articulated) hands and feet suggests no significant geologic activity in the chamber.

Scenario #3 is not supported due to the lack of other sediments or organic debris of any kind. This, as well as the lack of any other species of fauna, shows that the chamber has not been accessible from the outside in any other significant way.

Finally, scenario #4 is not supported by the sediments of the cave, which do not include any dirt or sediments from outside the chamber. The chemical nature of the sediments also shows no action of moving water at any time in the past. Those articulated hands and feet also show no water-based movement of the fossils.

Based on ruling out so many other logical possibilities, the team settled on the controversial hypothesis that Homo naledi brought their dead to the chamber by way of the narrow chute. They called this “Ritualized body disposal” and deliberately did not try to address the question of intent or “why” they did this.

Last year while Skyping with a library group in New Jersey, staff arranged for a local forensic detective to speak with students on a different matter. The detective was unaware of the Homo naledi story, so we asked him his thoughts about how the bones got into the chamber before mentioning the body disposal hypothesis. He was quick in his assessment. The only place you would find a mixed age and mixed sex group of otherwise healthy dead bodies in a small area with no other species represented would be a modern graveyard or a mass grave of some sort.

Given my interest in cliffhangers, I’ll save two lingering questions for my next post: How old is Homo naledi and how can we investigate the “Ritualized Body Disposal” hypothesis? These will help explain why I think the Homo naledi adventure is something all students should be learning about!