For billions of years life on Earth was basically just slime.
One day some mad cyanobacteria started a war for a reason nobody can remember. With no Geneva Convention, attacks were made with poison gas, on and on year after year, until the conflict finally ended in a massive Cambrian explosion. Plants colonised the land (triffids, I expect). Someone invented bones. Dinosaurs decided they wanted to be top dog and ate anyone who disagreed.
Eventually some mammals turned up with very firm views on ending their short lives as dinosaur poop. They killed all the dinosaurs and blamed it on a meteor in case of a Geneva Convention with back-dated powers. Everyone took LSD to celebrate so things got really weird for a bit. Horses the size of dogs. Carnivorous pigs the size of horses. Cows turned into fish. A handful of dinosaurs who had survived the "meteor" got so fried they thought they could fly.
The craziness would not end until some weird, hairless apes appeared and freaked everyone out by walking on two legs. They vowed never to take LSD again.
Soon, however, the weird hairless apes and their stubborn commitment to bipedalism became very popular, especially in the carnivore community. Short-faced bears liked nothing better than to stuff their short faces with weird, hairless apes. They got very fat but it didn't matter because the apes couldn't run very fast.
Understandably, the weird, hairless apes got really pissed and made a vow to kill all life on Earth, starting with the most delicious. When they realised they couldn't simply eat a whole planet they remembered the first great war of the cyanobacteria and started poisoning the atmosphere instead. And - finally - they made a Geneva Convention so history would remember them as the good guys.
But ultimately we're all descended from slime. And the slime itself is the product of a freak collision between two planets which dumped a lot of iron on the surface of the Earth where it could catalyse the first self-replicating chunks of RNA.
It's a funny old world.