Our home planet is very old. Do you know how old? Well it's four thousand six hundred million years old. That's 4,600,000,000 years! And simple life has been around for most of that period -- at least 3,500,000 years. That's plenty of time to try out a system for making new life to replace old, and this works just about perfectly. What's more, very simple life forms - just single cells - long ago began to join together to make more complicated bodies with thousands or millions of cells. All these new types of life were natural experiments to see what worked and what didn't. Many didn't work and died out - they became extinct.

Some worked for hundreds of millions of years like the dinosaurs, but still ended up extinct. Humans have been around for just a few hundred thousand years at the most, and have only begun to mess up our planet in the last hundred or so.

Did you know penguins have been around for over 40 million years and haven't messed anything up?

New types of bodies - organisms - generally work when they happen to be in a part of the planet where there's plenty of food and few enemies. Too little food, nasty climate or vicious enemies spells disaster and means extinction unless an organism can adapt in some way. This is where genes come in.

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