Large brains are humans’ most distinctive anatomical feature. Our brains are four times bigger than chimpanzees’ and gorillas’ brains.
Brains use twenty times the calories of muscles at rest. Brains require maintaining a constant temperature. Large brains are easily injured, and make childbirth difﬁcult. Intelligence has many costs, yet doesn’t directly help an animal survive, e.g., a big brain doesn’t make you run faster or survive cold winters.
Our ancestors’ brains began to enlarge about two million years ago. In evolutionary time, two million years is short. Why our ancestors rapidly evolved large brains — speciﬁcally, a large, uniquely human cerebral cortex — is unknown, according to Sapiens, by Yuval Noah Harari. For example, the “man the toolmaker” hypothesis is unsupported by archeological evidence.
The cerebral cortex learns new things. Animals without a cerebral cortex act only as their genes programmed them to act. Animals with a cerebral cortex can ﬁnd new foods, survive in new environments, or change their mating tactics to improve reproductive success.
In The Origin of Species (1859), Charles Darwin wrote that species evolve via random mutations. Environmental changes — e.g., changing food sources, predation, climate — favor one mutation over another. He called this process natural selection.
In The Descent of Man (1871), Darwin proposed an second theory of evolution. Species evolve when males and females select each other for certain qualities. He called this sexual selection. Biologists ignored this idea for over a century.
Females choose males with features that make the males less able to survive. E.g., a peacock’s bright colors make him visible to predators, and his huge tail slows his escapes. His beautiful tail communicates to peahens that he’s an especially ﬁt individual, i.e., he’s so fast that he can escape predators despite his heavy tail. Sexual selection is, in general, the opposite of natural selection.
Natural selection advances via slow environmental change. Natural selection advances evolution in harsh environments (e.g., predation, climate change). Natural selection produces animals better able to survive — usually smaller, more efﬁcient, and less conspicuous.
In contrast, sexual selection advances with each generation. Sexual selection produces rapid evolutionary changes. Sexual selection advances evolution in stable environments. Sexual selection produces animals (especially males) less able to survive, with bigger, more colorful, or exaggerated features.
What’s striking about the cerebral cortex is how much of it is not dedicated to speciﬁc behaviors. The human cerebral cortex has billions of general-purpose neurons, capable of learning any new idea. If our cerebral cortex evolved via sexual selection, why were our ancestral mothers and fathers — unlike any other animals — sexually attracted to partners who could learn new ideas?
Paternal care is found in only 6% of mammal species, compared to 80% of birds. In other words, humans are among the few mammals in which fathers help raise their offspring. Among hunter-gatherers, children without fathers are more than twice as likely to die during childhood. Today children without fathers are also less likely to prosper and reproduce.
But two-parent families introduce a dilemma. If a woman chooses a man desired by other women, who might have wealth, power, athleticism, or good looks, he may leave her for other women. If a woman chooses a man who’ll stick around for the next twenty years, he likely isn’t a man whom other women desire.
This dilemma gave reproductive advantages to individuals who could deceive. Women’s fundamental lie is to mate with a “cad” but tell her “dad” partner that the child is his. Men’s fundamental lie is to tell a woman that he’ll stay with her “forever,” and then start a second family with another woman.
Catching lies also gives reproductive advantages. A woman who recognizes men’s lies can avoid “cads” and choose a faithful partner. A “dad” who figures out that he isn’t the father can leave his lying partner and start a family with a more honest woman.
Catching lies also evolved our concept of “other minds,” because catching lies requires conceiving that another person might be thinking something other than what he or she is telling us.
The result was an “arms race” between lying and catching lies that rapidly advanced cerebral cortex evolution.