Biochemists assume that the three cellular kingdoms evolved from a single common ancestor, because the alternative of supposing an independent origin of life two or more times presents still greater difficulties. The common ancestor is merely hypothetical, as are the numerous transitional intermediate forms that would have to connect such enormously different groups to the ancestor. From a Darwinist viewpoint all these hypothetical creatures are a logical necessity, but there is no empirical confirmation that they existed.
Phillip E. Johnson, Darwin on Trial
biology, creation-science, creationism, evolution, science