Dunbar said the increase in brain volume must have evolved relatively recently in human history.
While the aesthetics improved markedly throughout the 20th century, the mechanical underpinnings evolved relatively little.
There was a view that in the different regions an earlier species, Homo erectus, evolved relatively seamlessly to modern humans.
There is also another system that evolved relatively recently, the adaptive immune system, which only vertebrates have.
And much of what we know cannot be put into words, she explained, pointing out that language evolved relatively late in human development.
That implied "the existence of certain cognitive capacities and that these evolved relatively late in prehistory," the two researchers said, but they were probably not a consequence of a sudden genetic mutation.
Photograph: Eric Gay/AP My wise colleague Matthew Wells points out that the US spelling of centre has evolved relatively recently, as has the spelling of theatre on this side of the Atlantic.
However, after the dinosaur disappeared, the mammals evolved relatively rapidly into much larger creatures capable of exploiting a wide variety of ecological niches, from leaf-eating giant sloths to tundra-munching mammoths.
To Ms. Narvaez, "reasoning is something that develops from experience; it's a subset of what we really know". And much of what we know cannot be put into words, she explained, pointing out that language evolved relatively late in human development.
A Stone Age man who lived about 7,000 years ago and whose buried bones were discovered in 2006 has turned out to be the earliest known person with blue eyes, a physical trait that evolved relatively recently in human history, a study has found.
And a third part of the brain, an area called the anterior prefrontal cortex which lies just behind the forehead, evolved relatively recently and is thought to be unique to humans was involved in the complex, costly decisions when self-interest and moral beliefs were in conflict.
