70,000 years ago, a brown dwarf pair known as Scholz’s Star, right on the precipice of igniting hydrogen fusion in its core, passed through the Solar System’s Oort cloud. Stars, failed stars, and stellar remnants pass through our Solar System multiple times every million years. Both modern humans and Neanderthals were likely around to see this event. (JOSÉ A. PEÑAS/SINC)

What Was It Like When The First Humans Arose On Earth?

The cosmic story of us wasn’t inevitable, but the culmination of many chance events.

By the time our planet was four billion years old, the rise of large plants and animals was just beginning. Complexity exploded around that time, as the combination of multicellularity, sexual reproduction, and other genetic advances brought about the Cambrian explosion. Many evolutionary changes occurred over the next 500 million years, with extinction events and selection pressures paving the way for new forms of life to arise and develop.

65 million years ago, a catastrophic asteroid strike wiped out not only the dinosaurs, but practically every animal weighing over 25 kg (excepting leatherback sea turtles and some crocodiles). This was Earth’s most recent great mass extinction, and left a large number of niches unfilled in its wake. Mammals rose to prominence in the aftermath, with the first humans arising less than 1 million years ago. Here’s our story.

A planetoid colliding with Earth, analogous to (but larger and slower-moving) a potential impact between Swift-Tuttle and Earth. The asteroid that wiped out the dinosaurs had just 1/26th the energy that being hit by Comet Swift-Tuttle would carry, and that impact was enough to wipe out 75% of all species on Earth. (NASA / DON DAVIS)

65 million years ago, a massive asteroid somewhere between 5 and 10 kilometers in diameter struck our planet. It kicked up a layer of dust…