To fully grasp how we evolved to become modern humans, scientists argue that we need to better understand our ancient ancestors’ brains. Adult human brains today are large and globular, but whether ancient human brains looked like that when our species first emerged has been subject to much speculation.

In a study published Wednesday in Science Advances, researchers from the Max Planck Institute for Evolutionary Anthropology announced that the earliest Homo sapiens did not have globular brains like we have today. Instead, their brains had a shape intermediate between that of Homo erectus and that of the Neanderthals, both of which were somewhat more elongated horizontally. The brain, the authors write, gradually became globular over evolutionary time, and those changes in turn, induced neurological shifts that coincide with archaeological evidence of modern behavior.

“The brain is an organ that is really important for what makes us human,” co-author Simon Neubauer, Ph.D., tells Inverse. “Our findings add to the accumulating archeological and paleoanthropological evidence demonstrating that Homo sapiens is an evolving species with deep African roots and long-lasting gradual changes in behavioral modernity, brain organization, and potentially brain function.”

On the left, the globular endocranial shape of a modern human brian, and on the right the Neanderthal brain shape. Simon Neubauer/Philipp Gunz

Modern behavior, in this context, refers to actions like creating tools and art that represent the possession of abstract thought. Scientists call the period of their emergence the “human revolution,” dating it to the Middle to Upper Paleolithic eras, approximately 50,000 years ago. This new paper argues that this time wasn’t a reaction to some rapid evolutionary event, as scientists have thought before, but rather was “merely a point in time where gradual changes supported the full suite of modern behaviors.”

According to the new paper, the size of the early Homo sapiens brain entered the range of modern human brain size as early as 300,000 years ago, but its globular, round features emerged only 40,000 years ago. This unexpected revelation means that the brain reached its current shape much later than anticipated during evolution.

To come to this conclusion, the team used tomographic scans and 3-D analysis to create virtual endocranial casts of 20 different Homo sapiens fossils. These fossils were divided into three groups: the oldest came from North and East Africa and represented the earliest known representatives of humans after the population split with Neanderthals, others lived in East Africa and the Eastern Mediterranean regions between 130,000 to 100,000 groups, and the final group lived between 35,000 to 10,000 years ago.

They also created virtual endocasts — internal casts of the brain that approximate its size and morphology — of modern human samples to compare. Because brains are not preserved in the fossil record, the only direct evidence researchers have are from endocasts.

Brain shape reconstruction of early Homo sapiens that lived 300,000 years ago. Simon Neubauer, Philipp Gunz

Analyses and comparison of these endocranial shape variations revealed that the present-day human shapes overlapped with the specimens from the Upper Paleolithic era, which means that brains became rounded between 100,000 to 35,000 years ago. This is later than when Homo sapiens evolved to have the larger brain size we still have today.

Globularity itself likely didn’t give us advantages, says Neubauer, but the features that contributed to the rounding probably did: the bulging of parietal areas and the bulging of the cerebellum. The parietal lobe is an important hub in connecting brain regions and is involved in functions like orientation, attention, and the sensorimotor transformations that underlie planning and visuospatial integration. Meanwhile, the cerebellum relates to motor-related functions, like balance, as well as integral functions like working memory, language, affective processing, and social cognition. It’s likely that the emergence of these skills prompted the “human revolution.”

“It’s also interesting to point out that, in present-day humans, brain globularity emerges developmentally during a few months around the time of birth,” says Neubauer.

“Our new data therefore suggests evolutionary changes to early brain development in a critical and vulnerable period for neural wiring and cognitive development.”