For a long time men believed that each form of life appeared separately on earth and that none had ever changed. They also believed that the earth itself was about four thousand years old. And so there was hardly time for much change to have occurred.
Scientists claim they have shown that these beliefs are wrong. Living forms have undergone many changes in their long history. These changes came about very slowly. All of these changes are summed up in the term, evolution. Evolution describes the many changes that have taken place, and is also a theory developed to explain those changes.
The man who laid the foundations for the modern theory of evolution was Charles Darwin. Darwin said that all life had evolved, and that descendants of a species could become different from the parent forms. These ideas were not new or unusual. But Darwin suggested how evolution might have occurred.
He called the process natural selection. He said that nature “selected” those organisms best suited to survive in the “struggle for existence.” Each organism differed slightly from other organisms and had its own individual traits. Some had traits that made them better able to survive that other. Therefore they lived longer and had more offspring. In this way, “beneficial” traits were passed on to a larger number of descendants.
Little by little, living things became so different from their ancestors that biologists classified them as separate and distinct species. Darwin believed that this was how new species originated. His theory still forms the basis of modern thinking on evolution.
Biologists later discovered that certain small chemical units called genes determine the traits that are passed from one generation to another. The genes change, or mutate, from time to time. A changed gene causes a variation in a trait, and if the change is favourable to survival, it may be passed on to the next generation.