Three main theories of the history of life:

1. Evolution. The idea that all living organisms are descended from a common ancestor.

2. Transformism. The theory that changes occur within a lineage of populations but that lineages do not split - there is no speciation - and lineages do not become extinct.

3. Separate creation. The theory that species have separate origins and never change after their origin.

It has sometimes been claimed that the theory of evolution is unfalsifiable and it is impossible to find evidence which either proves or disproves it. In fact, there are a number of lines of biological evidence that can be used to test between the three theories. Richard Dawkins explains what kind of evidence is studied.

Figure: Three thories of the history of life. (a) Evolution; (b) transformism and (c) separate creation. Each line represents a species in time. If the line moves up vertically the species is constant, if it deviates the species is changing in form.