Comparative genomics, minimal gene-sets and the last universal common ancestor.

Comparative genomics, using computational and experimental methods, enables the identification of a minimal set of genes that is necessary and sufficient for sustaining a functional cell. For most essential cellular functions, two or more unrelated or distantly related proteins have evolved; only about 60 proteins, primarily those involved in translation, are common to all cellular life. The reconstruction of ancestral life-forms is based on the principle of evolutionary parsimony, but the size and composition of the reconstructed ancestral gene-repertoires depend on relative rates of gene loss and horizontal gene-transfer. The present estimate suggests a simple last universal common ancestor with only 500-600 genes.