An answer to this puzzle is beginning to emerge. It looks like brain expansion began as an evolutionary accident and then led to changes that caused this growth to spiral. Surprisingly, the sorts of changes that drove this expansion could also explain a more recent 10 per cent reduction in human brain size. What's more, this suggests our brains may shrink further still ?" and might even cause humanity's demise. It is undeniable that hominin brains have grown larger through time. Sahelanthropus tchadensis, the oldest known hominin, which wandered northern Africa about 7 million years ago, had a brain volume of around 360 cubic centimetres. The average brain volume of a human today is almost four times that, at 1350 cubic centimetres. Granted, some of this expansion can be explained by the fact that we are larger than most of our hominin ancestors ?" for example, Lucy, a 3.2-million-year-old hominin, was just 1.1