How has the human brain evolved over the years?

—Emma Schachner , Salt Lake City

John Hawks, a professor of anthropology at the University of Wisconsin–Madison, answers:

Humans are known for sporting big brains. On average, the size of primates' brains is nearly double what is expected for mammals of the same body size. Across nearly seven million years, the human brain has tripled in size, with most of this growth occurring in the past two million years.

Determining brain changes over time is tricky. We have no ancient brains to weigh on a scale. We can, however, measure the inside of ancient skulls, and a few rare fossils have preserved natural casts of the interior of skulls. Both approaches to looking at early skulls give us evidence about the volumes of ancient brains and some details about the relative sizes of major cerebral areas.

For the first two thirds of our history, the size of our ancestors' brains was within the range of those of other apes living today. The species of the famous Lucy fossil, Australopithecus afarensis, had skulls with internal volumes of between 400 and 550 milliliters, whereas chimpanzee skulls hold around 400 ml and gorillas between 500 and 700 ml. During this time, Australopithecine brains started to show subtle changes in structure and shape as compared with apes. For instance, the neocortex had begun to expand, reorganizing its functions away from visual processing toward other regions of the brain.

The final third of our evolution saw nearly all the action in brain size. Homo habilis, the first of our genus Homo who appeared 1.9 million years ago, saw a modest hop in brain size, including an expansion of a language-connected part of the frontal lobe called Broca's area. The first fossil skulls of Homo erectus, 1.8 million years ago, had brains averaging a bit larger than 600 ml.

From here the species embarked on a slow upward march, reaching more than 1,000 ml by 500,000 years ago. Early Homo sapiens had brains within the range of people today, averaging 1,200 ml or more. As our cultural and linguistic complexity, dietary needs and technological prowess took a significant leap forward at this stage, our brains grew to accommodate the changes. The shape changes we see accentuate the regions related to depth of planning, communication, problem solving and other more advanced cognitive functions.

With some evolutionary irony, the past 10,000 years of human existence actually shrank our brains. Limited nutrition in agricultural populations may have been an important driver of this trend. Industrial societies in the past 100 years, however, have seen brain size rebound, as childhood nutrition increased and disease declined. Although the past does not predict future evolution, a greater integration with technology and genetic engineering may catapult the human brain into the unknown.