Human beings share 96% of their genome with chimpanzees,1 which is why modern science has accepted the concept that humans and apes share a recent common ancestor. However, our understanding of the transition from these ancient primates to the bipedal, tool-wielding species that conquered the globe is less clear than many realize. One crucial missing chapter in the evolutionary story is the origin of our very own genus, Homo. Scientists believe that somewhere between two and three million years ago, the hominid species Australopithecus afarensis evolved into the first recognizably human species, Homo erectus. However, the details of this genealogical shift have remained a mystery. In 2013, a discovery made in the Rising Star cave by two recreational cavers may have provided revolutionary insight into this intractable problem.
The Rising Star cave lies 30 miles outside the city of Johannesburg in northern South Africa. A popular destination for spelunkers for the past 50 years, this cave is well-known and has been extensively mapped.2 Two years ago, Steven Tucker and Rick Hunter dropped into the Rising Star cave in an effort to discover new extensions to the cave, with the hope of finding something more.2 They found a tight crevice that was previously unexplored, which led to a challenging forty-foot drop through a chute. At the bottom, Hunter and Tucker came across scattered bones and fossils in what would later be named the Dinaledi chamber.2 Hunter and Tucker consulted with Dr. Lee Berger, a paleoanthropologist at the University of Witwatersrand. It was clear to Dr. Berger that these fossils were not of modern humans -- an ancient hominid species had been discovered.2
Within weeks of this discovery, Dr. Berger assembled a qualified team and set up camp at the mouth of the Rising Star cave. In the largest hominid artifact discovery in Africa, over one thousand bones from multiple bodies were extracted and analyzed.2
As the fossils were being transferred out of the cave, paleoanthropologists at the surface worked to piece together a skeleton. Some aspects of this species’ bone structure were distinctly human, like the long thumbs, long legs, and arched feet.2 Other features, including curved fingers and a flared pelvis, were indicative of a more primitive animal.2 A large skull fragment from above the left eye of one of the skeletons allowed scientists to definitively determine this hominid’s genus.
The Australopithecus skull is characterized by a large orbital ridge above the eye, with a deep concavity behind it, leading to a flatter face with pronounced brows.3 The skull fragment collected by the team, however, had a shorter ridge and less of an indentation above the frontal lobe.3 This finding led the team to conclude that they had discovered a new member of the Homo genus, which Dr. Berger named Homo naledi. ‘Naledi’ in the Sotho language means ‘star,’ a reference to the vivid stalactites emanating from the ceiling of the Dinaledi chamber.3
Dr. Berger’s discovery in the Rising Star cave was an incredible breakthrough, but finding fossils is only half the battle. The next step is to find a place for this species in the million-year narrative of human evolution we have created.
In accomplishing this feat, a logical place to start is considering how the fossils of Homo naledi ended up in their final resting place. There were no signs of predation, as no other animal fossils were found at this location. In addition, these fossils accumulated gradually, meaning that the bodies did not all die from a single event. Dr. Berger postulated that these bodies were placed there with purpose, but intentional body disposal is an advanced social behavior which, up to this point, has only been exhibited by more evolved Homo species. The brain size of the discovered hominids is estimated to be between 450 and 550 cubic centimeters, about one third the size of Homo sapiens brain and only marginally larger than that of a chimpanzee.3 The possibility of such a small-brained animal engaging in intentional body disposal challenges ideas about the cognitive abilities necessary for such advanced social behavior. Dr. William Jungers, chair of anatomical sciences at Stony Brook University, argues that advanced social intelligence was not likely at play in this instance. He claims that “intentional corpse disposal is a nice sound bite, but more spin than substance […] dumping conspecifics down a hole may be better than letting them decay around you.”4
The idea of intentional body disposal is not the only one of Dr. Berger’s conclusions that has attracted criticism. Some in the scientific community argue that Homo naledi is a distant cousin, not a direct ancestor, of modern humans. Others, like UC Berkeley’s Dr. Tim White, argue that "new species should not be created willy-nilly,” and believe that these discoveries may just be fossils of Homo erectus.5 Biologist Dr. David Menton takes the small brain size of these hominids as well as well as their “sloped face” and “robust mandible” as indication that Homo naledi does not even belong in the Homo genus.6
It is clear that while the Homo naledi fossils are extremely significant in the scientific community, their placement within the story of human evolution is contentious. Our inability to definitively date the fossils makes the task even more challenging. However, Homo naledi’s unique mosaic of human and ape-like features provides support for a new model of human evolution that has recently gained traction in the scientific community. While scientists would prefer to draw a family tree of human ancestors with modern humans at the top, our evolution is not so simple. Dr. Berger likens the reality of evolution to a braided stream.2 Like a collection of tributaries all contributing to a river basin, humans may have been the product of a collection of human ancestors, each contributing to our existence differently. We may never fully understand where we came from, but discoveries like Homo naledi bring us a little bit closer to completing the evolutionary puzzle.
- Spencer, G. New Genome Comparison Finds Chimps, Humans Very Similar at the DNA Level. National Human Genome Research Institute [Online], August 31, 2005. https://www.genome.gov/15515096 (accessed March 1st, 2016)
- Shreeve, J. This Face Changes the Human Story. But How? National Geographic [Online], September 10, 2015. http://news.nationalgeographic.com/2015/09/150910-human-evolution-change/ (accessed January 17, 2016)
- Berger, L. R. et al. ELife [Online] 2015, 4. http://elifesciences.org/content/4/e09560 (accessed January 16, 2016)
- Bascomb, B. Archaeology's Disputed Genius. PBS NOVA NEXT [Online], September 10, 2015. http://www.pbs.org/wgbh/nova/next/evolution/lee-berger/ (accessed January 19, 2016)
- Stoddard, E. Critics question fossil find, but South Africa basks in scientific glory. UK Reuters [Online], September 16, 2015. http://uk.reuters.com/article/us-safrica-fossil-idUKKCN0RG0Z120150916 (accessed January 19, 2016)
- Dr. Mitchell, E. Is Homo naledi a New Species of Human Ancestor? Answers in Genesis [Online], September 12, 2015. https://answersingenesis.org/human-evolution/homo-naledi-new-species-human-ancestor/ (accessed January 17, 2016)