The brains of Neanderthals and humans were similar at birth but developed differently in the first year of life, according to a German study published in the journal Current Biology.
Brains of newborn human babies and Neanderthals, who became extinct about 28,000 years ago, were about the same size and appear almost identical at first, the researchers say.
But after birth and particularly during the first year of life there are stark differences in development, says lead author Philipp Gunz of the Max Planck Institute for Evolutionary Anthropology in Germany.
"There was a huge difference in the way they grew their brain compared to modern humans in the first one-and-a-half and two years," Gunz says.
To compare the two brains, scientists assembled a virtual Neanderthal brain by scanning skull fragments and comparing the computer models at different stages of growth to the human baby brain.
Neanderthal had a huge brain
The human brain began much more activity in neural circuitry in the first year of life, which may have helped early Homo sapiens survive in the process of natural selection, the study says.
"The interesting thing is within modern humans, the size of the brain correlates only very weakly with any measure of intelligence," Gunz says. "It's more the internal structure of the brain that is important."
"And the Neanderthal, they were smart because they had a huge brain, but we think that internal structures must have been different because they grew differently, so we don't think the Neanderthal saw the world as we do."
Neanderthals are believed to be modern humans' closest ancestor, and some scientists view both as the same species.
In May a landmark genome analysis determined that humans most likely interbred with Neanderthals, and that as much as four percent of the modern human genome seems to be from Neanderthals.