Whereas chimpanzees at this age have already formed over 90% of their brains, Selam had formed less than 90% of the adult brain size of her species when she died. This might point to a relatively slow brain growth in Australopithecus afarensis, similar to the brain growth pattern of modern humans, rather than that of chimps; this may point to a possible behavioral shift in Selam's species 3.5 million years ago and the emergence of the delayed pattern of brain development and maturity that we know of as human “childhood”.
The passage argues that Selam's brain development pattern, which was slower than chimpanzees at her age, might indicate a shift in behavior for Australopithecus afarensis 3.5 million years ago and the beginning of human "childhood".