According to Greely, who has written a book about the implications of He’s project, the answer depends on what the edits do and how they’re inherited. “They might well just die out, or be overwhelmed by the vast sea of normal alleles and normal genetic variation,” he says. “Some people have this fear that if you make a change, eventually all humans are going to be carrying that change. That’s really unlikely unless the change is enormously, enormously beneficial.”
The latter is, of course, a possibility. Whether a mutation is generated via an editing blunder or natural errors as DNA is packaged up into sperm or egg cells, occasionally mutations are useful. Some experts have even suggested that the CCR5 babies may have had their brains inadvertently enhanced.
The argument stems from research that shows the wild version of the gene that most humans inherit – the kind the babies would have had – actually suppresses the brain’s “neuroplasticity”, or ability to grow and reorganise itself. Some studies have shown that people who lack a normal CCR5 may recover from strokes more quickly and they reportedly do better in school, while mice without a functional version of this gene have better memories.
However, there are some situations in which rare mutations can spread widely, whether they’re useful or not.
Take Huntington’s disease, a harrowing condition that gradually stops the brain from working normally, eventually causing death. It’s unusual for a genetic disease in that even if you have one healthy copy of the gene you will still develop it – meaning that you might expect it to eventually die out.
One is the fact that Huntington’s disease typically materialises when people are around 40 years old, which is after the age at which most people have children – and consequently, the illness is almost invisible to evolution, which primarily cares if an organism has survived to the age of reproduction.