The Bonsai theory of microcephaly is at least 100 years old. In colonial times, British health officials fulminated against "this barbaric practice". Their concern has modern echoes. Every few years, some globetrotting reporter or public health official learns of the chuas and calls for their manufacture to be stamped out. While the sentiment may be admirable, its premise is almost certainly false.

Microcephaly is a rare disorder in Britain. No one seems to know precisely how common it is in the Asian community of north England, but it was common enough to attract the attention of Geoff Woods, a geneticist working at Leeds University. He found that it ran in families. That implied that its cause was genetic; it was caused by a mutation. Or, more precisely, several. By the late 1990s, the disorder had been mapped to deficiencies in at least six different genes.

The discovery of the microcephaly genes was important. It instantly told us something about how the human brain grows. But the true beauty of this work is that it has told something even more profound: how the human brain has evolved.