What the evidence looks like
Brains do not fossilize. What scientists have instead are skulls, and the inside of a skull can be measured. The volume of the braincase gives an estimate of brain size, and natural or digital casts of the inside of the skull, called endocasts, sometimes preserve faint impressions of the brain’s surface. These clues are limited. Skull volume says little about internal wiring, and many fossils are fragmentary, so estimates carry real uncertainty.
Dates are similarly approximate. Figures given in millions of years are best read as rough ranges that get revised as new fossils and dating methods appear.
Starting from the primates
Primates as a group tend to have relatively large brains for their body size compared with many other mammals. Monkeys and apes generally show expanded visual areas and a large outer layer, the cortex, which is involved in perception, planning and social behavior. Living great apes such as chimpanzees have brains of roughly 350 to 400 cubic centimeters, which is a convenient reference point, though individual animals vary.
The human lineage and the chimpanzee lineage are generally thought to have split somewhere in the range of six to seven million years ago. Chimpanzees are close living relatives, but they are not our ancestors; both lineages have kept evolving since the split. Comparing the two is useful for suggesting what changed along the human line, while remembering that the common ancestor itself is unknown.
Early hominins
The earliest fossils usually placed on the human side of the split are poorly preserved, and their brain sizes are uncertain. Australopithecines, a group that lived in Africa millions of years ago, are often estimated to have had braincases in the neighborhood of 400 to 500 cubic centimeters. That is only modestly larger than a chimpanzee, once body size is considered. Their skeletons show they walked upright, which suggests that walking on two legs came well before large brains. This is a notable point: it argues against a simple story in which a growing brain drove every other change.
The genus Homo
Brain size increased more clearly in the genus Homo. Early members of the genus are often described as having larger braincases than australopithecines, and later forms such as Homo erectus are typically estimated at somewhere around 800 to 1,000 cubic centimeters, with considerable variation across fossils and regions. Modern humans average roughly 1,300 to 1,400 cubic centimeters, with healthy people ranging well above and below that. Neanderthals had braincases that were similar in size to ours, and in some estimates slightly larger on average, which is a reminder that size alone does not determine ability.
Not every lineage fits a steady upward trend. Fossils from the Indonesian island of Flores, assigned to Homo floresiensis, show a much smaller braincase than other members of Homo, and scientists continue to discuss how that species fits into the family tree. Researchers also debate whether brain growth in Homo was gradual, occurred in bursts, or progressed at different rates in separate populations.
Structure changed, not just size
Bigger is not the whole story. Studies of endocasts and comparative anatomy suggest changes in shape, including a rounder braincase in anatomically modern humans, and differences in the proportions of certain regions. The expansion of the cortex and its connections is a major theme, and the neuroscience of cortex expansion looks at it from the standpoint of living brains rather than fossils. Comparisons across living primates also point to differences in how neurons are packed and connected, which fossils cannot show directly.
Why did brains get bigger?
Several explanations have been proposed, and none is settled.
- Social complexity. Living in larger groups may have rewarded the ability to track relationships and cooperate.
- Ecological challenges. Finding and processing food, such as foraging for scattered resources or preparing it with tools or fire, may have favored better cognition.
- Cooking and diet. Some researchers suggest a higher-quality diet helped pay for an energy-hungry organ, since the brain uses a large share of the body’s energy.
- Culture and tools. Passing on skills may have created a feedback loop, with learning ability and culture reinforcing each other.
- Climate variability. Changing environments may have favored flexible behavior.
These ideas are not mutually exclusive, and the real picture probably involves several at once. Because experiments on the distant past are impossible, researchers rely on fossils, comparisons among living species, and genetics. Genetic work has identified differences between humans and other primates that might relate to brain development, but connecting a particular gene change to a particular ability is difficult and often unresolved.
Cautions
Popular accounts sometimes show a tidy ladder from ape to human. Evolution is better pictured as a branching bush, with several hominin species often living at the same time. Another caution concerns the word progress: a bigger brain is not simply better, since it is costly to build and run, and what evolved was a particular set of abilities in particular circumstances. Readers curious about the larger story of how our species arose can look at the discussion of human origins, and the way the modern brain keeps changing within a single lifetime is covered under brain plasticity.