Why did we ever think that there was a common ancestor of all living things on Earth? Why did we entertain even such exaggerated myths that there was a single original cell? We told stories such as the following: we shed millions of dead skin cells every day, each of which is a straight-line descendant of the first cell that ever lived. This claim has been proved to be absurd by a recent article by W. Ford Doolittle in Scientific American, February 2000, pp. 90-95.

We knew that bacteria freely swap genes, rather than following the strictly vertical-descent lines of sexual reproduction. But it took detailed DNA-comparison studies to show definitely that in the bacterial superkingdom there are complex genetic networks, not vertical trees of genetic descent. Species are ill-defined in this realm; kinship links are horizontal as well as vertical, with diagonal crosslinks as well. That is the substructure, the bottom “mush”, under the proper Eukaryotic “trees of life”. Sex was a belated invention of the Protists and Fungi, carrying on then to Plants and Animals. The tree is supported by a crosslinked net of roots.

Lateral gene transfer is like lateral thinking. Why did living forms give it up for mere sex? It could have been even more productive of novelty.