But there is another way to trace the journey of humans across the planet: looking at when mammals went extinct. Wherever we see large mammals dying out, we find our ancestors’ footsteps not far ahead. Not long after humans reached Australia, species of giant kangaroo were killed off. When we reached North America, the American mastodon went extinct. Our arrival in South America was the end of the ground sloths. This wave of mammal extinctions stretched across the globe from around 52,000 to 9,000 BC in an event called the Quaternary Megafauna Extinction.
