When it comes to brain volume, previous research established that Homo naledi‘s was 465-560 milliliters. (In today’s study, the authors acknowledge those earlier numbers, based on virtual reconstructions, but also perform their own physical reconstructions and measure the volume with a water displacement method, arriving at a similar range of 460-555 mL.)This is very small. As the authors note, contemporary hominins possessed brains well over 1000 cc at this point. It was the complexity of H. naledi's brain that surprised people:
Homo naledi‘s inferior frontal and lateral orbital gyri were organized notably more like that of other members of the genus Homo than that of australopiths. These are parts of the brain associated with complex behaviors, such as communication, planning and tool-making, that are particularly important in our lineage.As we discover that early Homo was behaviorally more complex than we thought, it also forces us to rethink what went on later in hominin evolution and re-evaluate evidence of complexity in H. erectus and archaic H. sapiens.
Finding Homo naledi‘s brain was structurally similar to that of larger-brained members of the genus tells us two important things. First, it means Homo naledi itself was likely capable of more complex behavior than australopiths with a similar brain volume but different brain structure. Second, it torpedoes the old notion that Homo brains grew steadily in size and complexity until reaching the evolutionary pinnacle that is Homo sapiens (/sarcasm, just a touch).