New York University neuroscientist Nikolai Kukushkin traces the evolution of human consciousness in his book One Hand Clapping: Unraveling the Mystery of the Human Mind (Prometheus/Swift Press, 2025). He begins his story with the first appearance of DNA on Earth and then focuses on the key evolutionary landmarks that paved the way for us, modern humans. In the excerpt below, Kukushkin describes the “social brain hypothesis,” which posits that human intelligence arose in part to help us keep track of increasingly complex social groups.
what made us human
Until now, many explanations of human uniqueness have focused not on why we want to be so intelligent, but on what gives us the ability to be as intelligent as we are. We take for granted that intelligence is something that all animals clearly desire, and we have simply found a better evolutionary path to achieving it. One classical explanation for this, for example, is that the transition from trees to grasslands led to bipedal walking, which eliminated the need for hands to climb and allowed for more complex tasks. Another explanation focuses on increased meat-based diets and increased brain size. These factors certainly play an important role in making us who we are. But they alone don’t necessarily explain what’s so good about being intelligent in the first place. We just assume it’s obvious.
I think that’s a bit of a selfish assumption, like a jellyfish wondering why no one ever managed to evolve stinging cells. We all want to believe that we have somehow won evolution. This is a concept we discussed in Chapter 3 when we talked about complexity and perfection. We have the image of a monkey standing up, picking up a stick, and taking credit for its accomplishment with a giant brain.
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But the truth is, intelligence comes at a price, and for many species the benefits are not worth it. Brains like ours take enormous amounts of energy from bodies that are already burning fuel. One gram of brain tissue uses 10 times more nutrients than the average gram of human body. Additionally, the larger the brain, the heavier it becomes and the more easily it is damaged. Brain expansion therefore comes at a significant evolutionary cost. In any given species, these costs ultimately outweigh the diminishing benefits of brain enlargement. All brains have evolutionary stages in which they become large enough. If twice the size of the brain gave the rhinoceros a survival advantage, the rhino’s brain would certainly have become twice as large over millions of years. Even with little awareness of our evolutionary history, we can believe that humans alone have cracked a code that has eluded everyone for many years. In the case of the rhino, there was no special advantage to having a larger brain, so the rhino’s brain ended up being what it was. The question is not, as we tend to think, why humans succeeded while others failed, but why did we need a supercomputer when everyone else was fine with a calculator?
There’s an interesting pattern that might explain it. If you measure the size of the cerebral cortex (the brain’s “understanding machine”) of different primate species relative to the rest of the brain and plot it against the typical number of group members for each species, the two numbers fall on a straight line. The more members there are, the larger the cerebral cortex. Humans are number one in both respects. Our cerebral cortex is the largest compared to other parts of the brain, and our typical population size is estimated to be around 150 people. This is the size of a typical hunter-gatherer society, and the typical upper limit for the number of active social acquaintances that we modern humans can maintain. For example, business organizations are often naturally segmented into units of about 150 people.
Why is that? This is far from a settled question, but proponents of the so-called social brain hypothesis argue that social behavior is a uniquely demanding task that places an unprecedented strain on our brain’s capabilities. All mammals, to some extent, use their brains as mirrors and understand the behavior of others by modeling it in their own minds. But primates whose defensive groups swell to dozens or even hundreds had to contend with dozens or even hundreds of complex, interconnected models of other group members: their personalities, emotions, interactions, and vast amounts of complex data, including which members did what to whom and when. For us humans, it’s as natural as eating dinner, but it’s a repository of complex data that would baffle even the smartest non-primate. In other words, the social brain hypothesis states that social life is what makes us intelligent.
What makes this explanation different from other explanations is that it provides an incentive, not just a means to achieve it. Yes, free hands, meat eating and many other factors made our brain possible, but the reason we needed it in the first place was to remember all our friends who helped us fight monsters.
I know it sounds cheesy, but I think about it all the time. Various fables have been told about the birth of humanity. It was work that made humans human (this is the story of the communists, the monkey picking up tools), or it was violence (this is the story of 2001: A Space Odyssey, the monkey picking up weapons). They were not just scientific theories, but origin stories as important to the modern mind to understand itself as myths were to the ancient mind. An origin story is told to explain what you really are, and in doing so provides a template for the present, rather than simply describing the past. When it comes to work, work should naturally become the mainstay of your life. If it’s about violence, there’s no point in trying to avoid it. But the more we learn about ourselves, the clearer it becomes what we really think about others. Our true nature is to have dozens or even hundreds of companions in our brains, to navigate the ups and downs of their emotions and relationships, and to derive meaning and joy from living life with them. For example, it has long been recognized that happiness depends more on the richness of social contacts than on individual happiness. Social life has a deep impact on us not only mentally but also physically. For example, the Harvard Adult Development Study, which began in 1938 and followed hundreds of people for decades, famously showed that close relationships were a better predictor of a long and happy life than social class, IQ, or even genes. Too often in modern life we forget the solid fact that life is worth living for your friends. The social brain hypothesis puts the origin story behind this simple truth.
It also places the birth of our species in a broader context. Our brains began increasing in size long before the first Homo sapiens appeared. All primates share group size and cortical relationships, which means that a large brain is always required to handle large numbers of mates.
And that means that sooner or later, a human-like being was inevitable.
When eukaryotes first began extracting energy from other organisms, this set the trajectory for humanity. Eventually, someone will emerge who can control fire and even nuclear fission. At the deepest level, something similar to what the social brain hypothesis points out exists. As primates feel the urge to expand their groups and their brains, eventually someone will emerge with a large enough group and a sufficiently advanced brain to start talking to each other, inventing symbols and abstract categories, and eventually some form of culture, art, and civilization will emerge from this.
This last essence, the abstract, symbolic language passed down from person to person through cultural transmission, completes the human design that we have seen gradually crystallize over billions of years. But to understand why language is so important to our species, we must take a detour here. Most books on human evolution begin here, and over the past several million years, the apes have gradually evolved into several species of hominin, of which only the “smart” genus Homo, or Sapiens, survives. But our quest instead takes us inside the human brain, into the ocean of electrical signals that pulsate inside this amazing machine that powers consciousness.
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