Charles Darwin's theory of evolution by natural selection, published in 1859, explains that all of life on Earth today evolved over the eons from common ancestors. And the essence of this grand theory can be captured in a single sentence:

Within a species there's always slight random variation in the hereditary traits of its members, and given the competition to survive and reproduce, those variations that happen to be most conducive to doing so will naturally tend to become widespread in the species.

So the process of evolution by natural selection involves the non-random 'selection' by nature of particular random variations, which become established in the species. The variation - the fuel of this process - is ever-present due to the continuous occurrence of random gene mutations. These have several causes, including gene copying errors during cell division.

When two populations of a single species become separated - perhaps by a mountain range or an ocean, or through migration - and face different environments, natural selection leads them to evolve in different directions. And they may eventually diverge so much that they can no longer interbreed, thereby becoming separate species.

Thus, the steady accumulation of innumerable small changes over billions of years has led to the extraordinary complexity and diversity of today's plants and animals.

This beautifully simple but powerful theory, so often misunderstood and misrepresented, explains how the appearance of design in living things arose from a mindless process that's as natural as water flowing downhill.