What is life? The question sounds simple enough, but for centuries, scientists and philosophers have struggled to find a precise answer. Carl Zimmer’s engaging and informative book surveys a wide range of suggestions across the ages.

For ancient Romans, a baby’s life began with its first breath, so herbally induced abortion wasn’t considered infanticide. Christian theologians maintained that foetuses acquired a soul while still in the womb, but couldn’t agree when. In the 16th century, Italian legislators decided that life began forty days after conception; an 18th-century British judge said it was when the unborn child first moved.

Some would say that life begins at the moment of conception, but Zimmer argues that conception is a process, not a moment. He notes that a single fertilised egg can develop into a pair of identical twins: ‘If we must believe that a fertilized egg immediately becomes a person, then we’re left to wonder where that person went when it became two people.’ More rarely, a pair of non-identical embryos can merge to produce a chimera, a person with two distinct genomes. ‘If every fertilized egg is a single person with all the rights that a single person is entitled to, does a chimera get to have two votes?’

The question of when life ends is complicated too. Cessation of heartbeat was the standard sign until the 1960s, when brain death began to be accepted – but how can you tell exactly when a brain has died? Zimmer relates the heart-rending story of Jahi McMath, a thirteen-year-old