The genetic mistakes that could shape our species

He Jiankui seemed nervous.

At the time, he was an obscure researcher working at the Southern University of Science and Technology in Shenzhen, China. But he had been working on a top-secret project for the last two years – and he was about to take to the podium at the International Summit on Human Genome Editing to announce the results. There was a general buzz of excitement in the air. The audience looked on anxiously. People started filming on their phones.

Jiankui had made the first genetically modified babies in the history of humankind. After 3.7 billion years of continuous, undisturbed evolution by natural selection, a life form had taken its innate biology into its own hands. The result was twin baby girls who were born with altered copies of a gene known as CCR5, which the scientist hoped would make them immune to HIV.

But things were not as they seemed.

"I was kind of drawn to him for the first five or six minutes, he seemed very candid," says Hank Greely, a professor of law at Stanford University and expert in...