Evolution is change over time. Under this broad definition, evolution can refer to a variety of changes that occur over time—the uplifting of mountains, the wandering of riverbeds, or the creation of new species. To understand the history of life on Earth though, we need to be more specific about what kinds of changes over time we're talking about. That's where the term biological evolution comes in.
Biological evolution refers to the changes over time that occur in living organisms. An understanding of biological evolution—how and why living organisms change over time—enables us to understand the history of life on Earth.
They key to understanding biological evolution lies in a concept known as as descent with modification. Living things pass on their traits from one generation to the next. Offspring inherit a set of genetic blueprints from their parents. But those blueprints are never copied exactly from one generation to the next. Little changes occur with each passing generation and as those changes accumulate, organisms change more and more over time. Descent with modification reshapes living things over time, and biological evolution takes place.
All life on Earth shares a common ancestor. Another important concept relating to biological evolution is that all life on Earth shares a common ancestor. This means that all living things on our planet are descended from a single organism. Scientists estimate that this common ancestor lived between 3.5 and 3.8 billion years ago and that all living things that have ever inhabited our planet could theoretically be traced back to this ancestor. The implications of sharing a common ancestor are quite remarkable and mean that we're all cousins—humans, green turtles, chimpanzees, monarch butterflies, sugar maples, parasol mushrooms and blue whales.
Biological evolution occurs on different scales. The scales on which evolution occurs can be grouped, roughly, into two categories: small-scale biological evolution and broad-scale biological evolution. Small-scale biological evolution, better known as microevolution, is the change in gene frequencies within a population of organisms changes from one generation to the next. Broad-scale biological evolution, commonly referred to as macroevolution, refers to the progression of species from a common ancestor to descendent species over the course of numerous generations.