Homo naledi represents a recent discovery in the study of human evolution. This ancient hominin species has sparked discussion and intrigue, particularly due to its unexpected combination of physical characteristics. The species’ brain size, in particular, has become a key interest, prompting re-evaluation of assumptions about the relationship between brain volume and complex behaviors in early human relatives. This aspect of Homo naledi challenges traditional narratives of our evolutionary past.
Discovering Homo Naledi
The initial discovery of Homo naledi occurred in 2013 within the depths of the Rising Star Cave system in South Africa, a region part of the Cradle of Humankind World Heritage Site. Two cavers, Rick Hunter and Steven Tucker, found fossil bones strewn across the floor of a difficult-to-access chamber called Dinaledi. This discovery prompted paleoanthropologist Lee Berger to organize an expedition to excavate the site.
Subsequent expeditions in 2013 and 2014 yielded over 1,550 fossil specimens, representing at least 15 individual Homo naledi. This makes it the largest assemblage of a single hominin species ever found in Africa. Homo naledi displays a mix of primitive, ape-like features, such as in its pelvis and shoulders, alongside more modern human-like traits, particularly in its hands and feet. The species has been dated to between 335,000 and 236,000 years ago, placing it in a period when other Homo species with much larger brains were present.
The Enigmatic Brain Size
The brain size of Homo naledi is a central aspect of its intrigue, ranging from 465 to 610 cubic centimeters (cc). Two male skulls from the Dinaledi chamber showed cranial volumes around 560 cc, while two female skulls measured about 465 cc. This volume is notably smaller compared to modern humans, whose brains average between 1,270 and 1,400 cc.
The brain size of Homo naledi is more akin to that of Australopithecus, an earlier hominin genus, and falls at the lower end of the range seen in early Homo species like Homo habilis. For context, Homo erectus had an average brain size of around 900 cc, making Homo naledi’s brain significantly smaller than many of its Homo relatives. Brain size is estimated from fossil skulls using endocasts, internal molds of the braincase that allow volume calculation and shape examination. The small brain of Homo naledi, despite its human-like body proportions and relatively recent existence, presents a puzzle that challenges established ideas about hominin evolution.
Rethinking Intelligence and Behavior
The small brain size of Homo naledi prompts a re-evaluation of how intelligence and complex behaviors developed in early hominins. Traditionally, a larger brain was considered a prerequisite for advanced cognitive abilities. However, evidence associated with Homo naledi challenges this view. The deep location of the Dinaledi Chamber, accessible only through narrow passages, suggests that Homo naledi individuals may have intentionally navigated these dark spaces.
Claims, though debated, suggest Homo naledi may have engaged in intentional burial practices within the cave system. If true, this would represent a complex behavior previously attributed primarily to larger-brained hominins like Homo sapiens and Neanderthals. The presence of possible markings on cave walls and discussions about the controlled use of fire in the deep cave add to the complexity, although definitive evidence for these behaviors directly linked to Homo naledi remains debated. These discussions highlight that brain volume alone may not dictate cognitive capacity, suggesting that brain organization and connectivity could play a greater role in enabling complex actions.
Broader Evolutionary Implications
The combination of traits observed in Homo naledi, particularly its small brain size alongside human-like skeletal features and potential complex behaviors, challenges the previously held linear model of human evolution. Rather than a straightforward progression, Homo naledi suggests a more intricate, branching pattern of development. This species demonstrates that different human-like characteristics evolved independently and at various times across different lineages.
The existence of Homo naledi during a period when larger-brained Homo species were also present indicates that multiple hominin forms coexisted, each with a distinct set of adaptations. This discovery prompts reconsideration of what defines the genus Homo and the diverse pathways that led to modern humans. Homo naledi underscores the complexity and mysteries in understanding our evolutionary history, emphasizing that the story of human origins is far from complete.