Evolution means that animals and other organisms change over millions of years. They gradually change in their behaviour or appearance, and may even turn into a different species.
People have known about these changes since about 150 years ago when Victorian scientists started to find fossils of ancient extinct monsters. Some fossils show species gradually changing into other species – for example dinosaurs becoming smaller, growing feathers, and eventually turning into birds over millions of years. Most Victorian scientists agreed that evolution was happening but they had no idea how it worked.
Charles Darwin was a respectable Victorian gentleman who liked to breed fancy pigeons. He noticed that pigeon breeders picked out their favourite birds to breed, in the hope that they would pass their fine feathers and other characteristics on to their children. The same is true of dog breeders, who have created all sorts of breeds like Dalmatians, Chihuahuas, and Collies by selecting the spottiest, smallest or cleverest animals of each generation. This is called artificial selection, and it’s a way that human beings can cause evolution to happen.
In the wild though there is no breeder to pick out his favourite animals for reproducing. However, Darwin realised that there was a way that nature itself could pick the best animals. An animal that’s too slow to run away from predators, or too small to reach the best food, is likely to die. Those organisms that are the best at finding food, escaping predators and raising their young are most likely to survive and have lots of children. Hopefully their kids will be just as fit and good at surviving as their parents, and so will be able to have lots of fast/big/intelligent children of their own! Over millions of years, a group of animals might slowly all become bigger (or faster or whatever feature it was that helped them survive). This means that they have evolved, and the process it happens by is called Natural Selection.
Many Christians and Christian scientists believe that Natural Selection is probably how animals have evolved. However, certain groups refuse to believe this because of stories in the Bible which state that God created the world and all its creatures in seven days. Though some people believe these stories literally, others think that Natural Selection itself may have been created by God. Evolution remains a controversial issue with many strong opinions on both sides.