An anonymous reader writes: By uncovering and comparing other fossil skulls and endocasts, paleontologists documented one of the most dramatic transitions in human evolution. We might call it the Brain Boom. Humans, chimps and bonobos split from their last common ancestor between 6 and 8 million years ago. For the next few million years, the brains of early hominins did not grow much larger than those of our ape ancestors and cousins. Starting around 3 million years ago, however, the hominin brain began a massive expansion. By the time our species, Homo sapiens, emerged about 200,000 years ago, the human brain had swelled from about 350 grams to more than 1,300 grams. In that 3-million-year sprint, the human brain almost quadrupled the size its predecessors had attained over the previous 60 million years of primate evolution.
Fossils established the Brain Boom as fact. But they tell us next to nothing about how and why the human brain grew so large so quickly. There are plenty of theories, of course, especially regarding why: increasingly complex social networks, a culture built around tool use and collaboration, the challenge of adapting to a mercurial and often harsh climate — any or all of these evolutionary pressures could have selected for bigger brains.
Although these possibilities are fascinating, they are extremely difficult to test. In the last eight years, however, scientists have started to answer the “how” of human brain expansion — that is, the question of how the supersizing happened on a cellular level and how human physiology reconfigured itself to accommodate a dramatically enlarged and energy-guzzling brain. “It was all speculation up until now, but we finally have the tools to really get some traction,” said Gregory Wray, an evolutionary biologist at Duke University. “What kinds of mutations occurred, and what did they do? We’re starting to get answers and a deeper appreciation for just how complicated this process was.”