The Evolutionary Trade-offs of Human Physiology

Key Insight

Humans possess extraordinarily large brains; modern Sapiens brains average 1200–1400 cubic centimetres, significantly surpassing the 200 cubic centimetres typical for a 60-kilogram mammal or the 600 cubic centimetres of early humans 2.5 million years ago. Neanderthals even had larger brains. This cognitive power comes at a high metabolic cost: the brain, though only 2–3 percent of total body weight, consumes 25 percent of the body's energy at rest, compared to 8 percent for other ape brains. This necessitated humans to spend more time foraging for food and resulted in muscle atrophy, essentially diverting energy from 'biceps to neurons.'

Walking upright on two legs, another unique human trait, freed the hands for tasks like throwing stones, signaling, and, critically, making and using sophisticated tools, with the earliest evidence of tool production dating back 2.5 million years. However, this upright posture had its downsides. The human skeleton, evolved over millions of years for quadrupedal movement and a smaller head, struggled to support an extra-large cranium, leading to common ailments such as backaches and stiff necks.

For women, an upright gait imposed an additional burden by requiring narrower hips, constricting the birth canal at a time when babies' heads were growing larger. This made childbirth a major hazard, consequently favoring earlier births. As a result, human babies are born prematurely, with many vital systems underdeveloped, leading to prolonged dependency on elders for sustenance, protection, and education. This extended period of helplessness significantly contributed to humankind's extraordinary social abilities, requiring constant help from family and neighbors, and fostering the development of strong social ties, while also allowing for unparalleled educability and socialization.