Was human evolution inevitable, or do we owe our existence to a once-in-a-universe stroke of luck? ... At first glance, everything that’s happened during the 3.8 billion-year history of life on our planet seems to have depended quite critically on all that came before. And Homo sapiens arrived on the scene only 200,000 years ago. The world got along just fine without us for billions of years. Gould didn’t mention chaos theory in his book, but he described it perfectly: ‘Little quirks at the outset, occurring for no particular reason, unleash cascades of consequences that make a particular future seem inevitable in retrospect,’ he wrote. ‘But the slightest early nudge contacts a different groove, and history veers into another plausible channel, diverging continually from its original pathway.’ ... One of the first lucky breaks in our story occurred at the dawn of biological complexity, when unicellular life evolved into multicellular. ... Throughout human prehistory, biological change and technological change ran in parallel. Brains were increasing in size – but this was not unique to our ancestors, and can be seen across multiple hominin species. Something very complicated was going on – a kind of arms race, Tattersall suggests, in which cognitive capacity and technology reinforced each other. At the same time, each branch of the human evolutionary tree was forced to adapt to an ever-changing climate.