Why life does not run out

If plants and animals didn't reproduce, life would slowly vanish from the Earth. Reproduction makes sure each kind of organism carries on: it keeps up the continuity of life.

We expect animals to have young that look like them: cows have calves, cats have kittens. That happens because parents pass on instructions to their young about how to develop from a single cell.

These instructions are called genetic material, or genes, and they are stored inside every cell of a living thing. Think of genes as an instruction manual in each cell: some instructions say how to make blood, others how to build bones, muscles or skin. Together they make sure a calf grows into a cow and a kitten into a cat.

But reproduction does more than make more of the same. It also lets small changes creep into the instructions passed from parents to young.

Sometimes a change helps a plant or animal survive better in a new place. Over a long time, camels developed humps to store fat and survive in deserts. Even microbes change: as you learnt in the chapter on health, some bacteria have become resistant to antibiotics, which helps them survive. Over many generations, such changes can lead to new features, or even completely new kinds of living things.

But how can one process give both similarity (a cow gives birth to a cow) and variation (individuals differing in colour or height)? There are two kinds of reproduction: in one, the young are almost exactly like the parent; in the other, they look a bit different.

In sexual reproduction the instructions from two parents combine. This mixing keeps useful features while letting new ones appear, and over many generations small differences add up to big ones, and even to new life forms.