People often say we are like our parents. We often look like them, sound like them or do the same kinds of things.

We are not exactly the same as our parents, though. There are always little changes.

The same things happens to animals. Baby animals are quite like their parents but not exactly the same.

The way humans and animals change like this is called evolution.

Over millions of years, all the little changes can add up into big changes. After a long enough time, the changes can be so big that one type of animal can evolve into a whole new type of animal.

Natural selection

Everybody is good at something, and we all have things we find difficult, too. Luckily, most of these things don't really matter - it's nice if you can run fast, but it won't kill you if you can't.

If you were a cheetah, running fast would be important because you would need to catch antelopes. A cheetah that could not run fast enough would not be able to catch any food and would die. Occasionally a cheetah will be born which can't run fast enough, and it probably will die. It is sad to think about it but that is how nature works.

Because only fast cheetahs can survive, and because baby cheetahs will be quite like their parents, most baby cheetahs grow up to run quickly. It can seem like nature has "chosen" (or selected) the fastest cheetahs (but of course nobody did the choosing - it is just that only the fastest survived). This choosing by nature is sometimes called survival of the fittest but its proper name is natural selection.


When evolution and natural selection work together, it can seem like animals or plants have changed themselves (or been designed) to do a particular job well. This is called adaptation - we say the animals or plants have adapted to survive in their environments.

It is not only speed which help animals survive. Tortoises are slow but over millions of years, they have adapted by evolving hard shells to protect them. Some animals evolved feathers on their arms and small bodies until they were light enough to fly - we call them birds. Giraffes have long necks so they can reach the highest leaves on trees when other animals have eaten all the lower leaves. Some animals (including humans) have adapted to do more complicated things by evolving opposable thumbs which enables them to grip things. (Imagine trying to hold something with your toes - it's much harder than using your hand!)

It works for plants, too: Trees are tall so that they can grow above all the others plants to reach the sunlight they need to survive. In fact, some trees have adapted by growing thin trunks very quickly so that they "get there first" (but might be snapped by strong winds) whereas other trees have evolved to grow very thick, strong trunks (but they take longer to grow so they might never see enough sunlight). Nettles have evolved stings to stop animals eating them or treading on them. Other plants have evolved ingenious ways of spreading their seeds far and wide so that more of them will grow.

What about humans?

We don't know every detail of our history but scientists and historians think they have worked out most of it.

We think life started in the sea. Some chemicals mixed together and made something that could copy itself. You might think this sounds pretty unlikely - and you'd be right - but it only needed to happen once, and we think it was about 4,000,000,000 (4 billion) years ago.

Over millions of years, that life became more complicated. Bacteria evolved into fish, some fish evolved fins and some of those fins became longer and stronger. Most fish can only breathe under water but some evolved the ability to breathe air. Some of the animals that could breathe air started moving around on land, out of the water and their fins evolved into legs. Some of these animals grew stronger back legs and lighter front legs until they were able to stand upright. Some animals evolved fingers and toes so they could climb trees. Some animals grew larger brains so that they could solve problems in order to survive - that's us!