The modern human brain is one of the defining features that separates us from our great ape relatives. When it emerged in our distant ancestors, it allowed us to gain self-awareness, grapple with abstract concepts, use knowledge to manipulate our environment, and use information-rich language to communicate. 

However, it appears that our unique brain structure may have arrived on the scene much later than previously believed, most likely as late as 1.5 million years ago. This suggests that many Homo species in our family tree, including the first wave that left Africa, were walking around with a surprisingly ape-like brain. 

As reported in the journal Science, anthropologists led by the University of Zurich (UZH) in Switzerland carried out computed tomography to examine the skulls of Homo fossils that lived in Africa, Georgia, and Southeast Asia some 1 to 2 million years ago. 

"The problem is that the brains of our ancestors were not preserved as fossils. Their brain structures can only be deduced from impressions left by the folds and furrows on the inner surfaces of fossil skulls," Christoph Zollikofer, lead study author from the Department of Anthropology at UZH, said in a statement. 

Their findings indicate that modern human brain structures only emerged 1.5 to 1.7 million years ago in African Homo populations, which is surprisingly recent. The first populations of the genus Homo emerged in Africa about 2.5 million years ago and settled around Dmanisi in present-day Georgia. Despite their puny ape-like brains, these hominids were able to adapt to the new environmental conditions of Eurasia and display some fairly impressive skills, such as tool making and developing complex social groups.

Meanwhile back in Africa, Homo cultures were becoming increasingly advanced and early traces of human language were starting to emerge. By around 1.7 million ago, the evidence suggests a second wave of hominins had left Africa with notably larger and more “human” brains, eventually settling around the southeast Asian island of Java.

Many of the key differences between the human brain and ape-like brain can be found in regions of the frontal lobe. Since it's widely known that the frontal lobe is responsible for higher cognitive functions, including things such as memory, emotions, problem-solving, and social interaction, it makes sense that these changes were associated with the rise of more complex behavior and culture. 

"The features typical to humans are primarily those regions in the frontal lobe that are responsible for planning and executing complex patterns of thought and action, and ultimately also for language," explains first author Marcia Ponce de León.

Even though the modern brain structure only arrived some 1.5 million ago, it would be a long time until hominins reached the brainpower of 21st-century humans. Although they had much of the hardware ready to go, these early humans would have not behaved particularly "human." While anatomically modern Homo Sapiens evolved around 200,000 years ago, it wasn't until some time in the past 70,000 years that they started developing a sophisticated culture, displayed signs of abstract thought, and indulging in creating artworks. Only then could species of the Homo genius perhaps be truly recognized as "truly human.," whatever that means.