The Cambrian explosion:
what set it off?

The full narration of the video.

Life on Earth is more than three billion years old. But for most of that time, the sea held only things too small to see. Then, about five hundred and forty million years ago, everything changed. Within roughly twenty million years: the seas filled with creatures that had shells, legs and eyes. Almost every major group of animals alive today shows up for the first time right here.

Squeeze Earth's history into a single day, and those twenty million years take up just six minutes. In Chengjiang, in Yunnan, China: even soft little worms, right down to their guts, turned to stone. This is called the Cambrian explosion. So how did animals all show up at once?

Let's question the suspects one by one. Suspect number one: did the fossils fool us? Soft bodies rarely last: maybe animals only got photographed once they grew shells. That's partly right. But trails and burrows on the seafloor got more common and deeper at exactly this time. Those don't need shells. The explosion was real.

Suspect number two: oxygen. To move and grow, animals need to breathe. Shallow seas did get more oxygen back then. But in 2024, a team from more than fifty institutions ran the numbers: it was only a small rise. So oxygen was like opening a door a crack: it made complex animals possible, but didn't push them to change.

Suspect number three: genes. Animals carry an instruction manual for building a body: where the head goes, where the legs go. But tracing today's genes backwards, that manual was probably written long before. The ancestors were around too, just small and soft. The manual was ready, and the door was open. So what made them all rush out together?

The last suspect: eating. In rocks from Shaanxi, China, there are tiny tube shells with neat holes drilled right through. Just before the explosion, animals were already eating animals. Once there are hunters, the hunted need a plan: hard shells, sharp spines, digging into mud, running faster.

So the hunters level up too: eyes, and big grabbing claws. You make a move, I answer it. It's like a pot of popcorn. The kernels were already in the pot: that's the genes. The heat creeps up: that's oxygen, and more minerals in the sea. Then the first kernel pops, bumps the next, and the whole pot goes off. That's the chase between the eaters and the eaten.

So there's no single answer: several things lined up at once. Which one mattered most? Scientists are still arguing. So who lit the fire? Oxygen, or the very first hunter? Pick a side in the comments.