Wikijunior:Biology/Origin of Life
All living things live in a shared environment. They need to use certain things (called resources) from their environment, like food, water, and a place to live. These resources are limited, so when more than one organism tries to use the same resource, they end up in competition. When two living things compete for a common resource, one of them will eventually win and consume (or use) that resource. When something about a thing's body makes it better at competing for resources, we call that special feature an adaptation. Since these adaptations can be passed from parents onto their children, as time goes on, these adaptations become more common within a population, or group of similar living things living together. This is called Natural Selection, or Evolution.
When a small group of living things (a small population) gets separated from the main population that they came from (like if they move over a mountain range or a river, or if they move to a new island so that they can't easily move back) they will often find themselves in a different environment than they were in before. This new environment has different resources and different competitors, so the new population will need different features or adaptations to be a strong competitor than what they had needed before. The original population hasn't changed at all, they still need the same adaptations as before. Over time, as the new population begins to adapt to their new environment, they start to look less and less like the other population. Eventually, after thousands or even millions of years, the two populations will look so different that they can't be called the same species. We call this process speciation, which just means the formation of new species. Speciation is an unavoidable consequence and a very important part of evolution.
The Earth itself was born 4 and a half billion years ago. At first, it was just a bunch of rock and water. There were no living things. But then, about 3.8 billion years ago, the first life was formed in the oceans. It was no bigger than a single cell, but that single cell was able to copy itself and form more and more cells. Over billions of years, as that one cell evolved, it became more and more complex. Eventually, about 1 billion years ago, the first living things with more than one cell were born. Many of the kinds of things that lived that long ago can't be found living in the world any more, because newer things have been better competitors and forced the older things to extinction, but we know they existed because we can find their fossils, which are traces of ancient living things buried deep in the rocks.