Why Did Early Humans Begin to Develop Larger Brains Nearly 2.5 Million Years Ago? — Here’s What Scientists Suggest

Before, the brain size of our ancestral lineage was no bigger than that of modern-day Chimpanzees, but around 2.5 million years ago, it began to develop. During this period, our brains saw a significant increase in size and gradually grew beyond the normal hominid or ape-like proportions.

Along with this change was the appearance of the genus Homo, which pertains to a group that includes the modern human species, Homo sapiens, along with our extinct close relatives like Homo erectus, Homo heidelbergensis, and Neanderthals. But why exactly did these brains get larger around this time? And how did it develop even further to resemble the brains that we use today? Let’s dive deep into our ancestral past and explore their world.

The Appearance of the Homo Genus and Bigger Brains

Around 4.2 million years ago, there was a group of species that survived in Africa for nearly 2 million years, namely the Australopithecus. They are a genus of hominins that were believed to be part of the early ancestors of humans. However, they had a small brain that is roughly the same size or slightly larger than that of a chimpanzee. They also had a diverse diet, but it was mainly plant-based, such as fruits, leaves, underground roots, and seeds.

Then, from this group of hominins, different traits and characteristics developed, and a new genus arose, which was the Homo genus. One of the key developments they had was their larger brain size relative to their bodies. With this change comes various abilities and cognition that they previously did not have or rarely had, such as their more complex behavior, allowing the development and use of stone tools, and a new diet that includes more meat.

One of the earlier species from this genus was Homo habilis. They are known to have bigger brains than the ancient Australopithecus. The arrival of species like them, and the overall Homo genus, is an important period in human evolutionary history because it marks the time when brain growth and cognition start to accelerate gradually.

Although the most rapid expansion of our brains and intelligence was believed to be about 800,000 to 200,000 years ago, the fact that Homo emerged from this period around 2.5 million years ago is one of the major turning points in brain evolution in early humans. But what exactly could be the reasons for this development? Let’s explore some possible factors.

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Environmental Change and Brain Evolution

The Evolution of the Human Brain

One of the strongest hypotheses for why this period marks the arrival of the Homo genus, and the start of the gradual development of bigger brains, was environmental pressure. Around 2.5 million years ago, it is believed that the natural habitats of early human ancestors were shifting.

Climate change may have caused the previously forested areas and woodlands to become open spaces like savannas. In other words, some forests may have receded, and deserts or grasslands expanded. There may also have been fluctuations in rainfall and glacial periods. All these different changes may have brought new ecological challenges that our ancestors had to adapt to in order to survive.

When an environment changes, species that have adapted are the ones that are most likely to reproduce and pass on their genes. So, having even slightly bigger brains that allow for better problem solving during ecological uncertainty and challenges is evolutionary advantageous. Those individuals have a higher chance of survival. Over time, this selection pressure may have progressively led to the development of larger brains in early humans, ultimately contributing to the arrival of the genus Homo.

Diet, Tools, and the Energy Cost of Big Brains

The brain is one of the most metabolically expensive organs to grow and maintain. In order for a species like the early hominins to develop a bigger brain, the diet and nutrition they consume have to be able to allow for this change. For example, the genus that precedes Homo, Australopithecus, mostly has a plant-based diet. It would be hard for them to have enough calories and nutrients to support a bigger brain.

This is most likely why the development of bigger brains coincides with the change in diet of the human lineage. In other words, our ancestors began to eat more meat and bone marrow. This new diet would have allowed them to develop a larger brain than the genus that came before them. However, without stone tools, they would have just relied on their ability to chew on the meat or their strength to crack the bones to access the nutrient-rich marrow in the carcasses that they scavenged, which would prove difficult.

This could also be the reason why the Oldowan industry stone tool appeared around this time, too, along with the arrival of the genus Homo. Around 2.6 million years ago, these stone tools were basic, sharp flakes made by striking one stone against another.

But this simple change provided them access to new dietary food sources that gave them more energy, calories, and nutrients, supporting the development of bigger brains. These new diets and the invention of basic stone tools contributed to the eventual arrival of the different Homo genus species like Homo habilis and Homo erectus.

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Social Life, Cooperation, and the Selection for Brains

Why We Got Smart - The Story of the Hominid Brain ~ with ZACHARY COFRAN

Another key factor for the development of a bigger brain is the increasing social complexity of early humans. We are hypersocial creatures that need each other in order to survive. The individuals who are more capable of working together in a group, sharing food, scavenging, and living together, were evolutionary advantaged. They are the ones most likely to survive, reproduce, and pass on their genes to the next generations.

Over a long period, this selection pressure for social cooperation could have contributed to the development of bigger brains. It is because higher cognition from a larger brain provides more capability for humans to manage relationships, remember allies or foes, and cooperate on social tasks like scavenging, foraging, and gathering resources. The earlier genus, Australopithecus, was already believed to be social, but the environmental pressures may have driven them toward increased cooperation and large social groups.

To meet these demands, their brains developed to be bigger, and then the bigger brains allowed for even more social complexity, fueling a cycle where living in groups pushes brain and cognitive development, also known as the social brain hypothesis. However, it is important to remember that the brain growth was most likely caused by the culmination of all the factors discussed, each one playing a crucial role.

Author's Final Thoughts

The shifting environment and development of stone tools that allowed a transition to a more nutrient-rich diet that includes meat and bone marrow instead of just mostly plants may have all played a crucial role in the development of the human brain. Then this rise of larger brains in the human lineage around 2.5 million years ago may have led to the arrival of a new genus Homo, which includes earlier human ancestors like Homo habilis and Homo erectus.

Moreover, the ecological pressure could have pushed the species that preceded the genus Homo to be more social and live in bigger groups to survive. It could have driven them to develop more complex relationships that demand higher cognition from a bigger brain.

Ultimately, the interplay of these different factors culminated in the eventual arrival of our species’ direct ancestral lineages. Without these developments that led to slightly bigger brains over time, we may never have been here today.

Read next: Did Homo sapiens get into Conflicts and Fights With the Neanderthals around 50,000 Years Ago? — Here’s What New Discoveries Suggest

References & Further Reading

Aiello, L. C., & Wheeler, P. (1995). The expensive-tissue hypothesis: The brain and the digestive system in human and primate evolution. Current Anthropology. https://doi.org/10.1086/204350

Maslin, M. A., Brierley, C. M., Milner, A. M., Shultz, S., Trauth, M. H., & Wilson, K. E. (2014). East African climate pulses and early human evolution. Quaternary Science Reviews. https://www.sciencedirect.com/science/article/pii/S0277379114002418

Wrangham, R. (2017). Control of fire in the Paleolithic: Evaluating the cooking hypothesis. Current Anthropology. https://doi.org/10.1086/692113

Rightmire, G. P. (2004). Brain size and encephalization in early to Mid-Pleistocene Homo. American Journal of Physical Anthropology. https://onlinelibrary.wiley.com/doi/10.1002/ajpa.10346

Dunbar, R. I. M. (1998). The social brain hypothesis. Evolutionary Anthropology. https://doi.org/10.1002/(SICI)1520-6505(1998)6:5%3C178::AID-EVAN5%3E3.0.CO;2-8

McPherron, S. P., et al. (2010). Evidence for stone-tool-assisted consumption of animal tissues before 3.39 million years ago at Dikika, Ethiopia. Nature. https://www.nature.com/articles/nature09248

Antón, S. C., Potts, R., & Aiello, L. C. (2014). Evolution of early Homo: An integrated biological perspective. Science. https://www.science.org/doi/10.1126/science.1236828