THE UNIVERSAL ANCESTOR

A genetic annealing model for the universal ancestor of all extant lif e is presented; the name of the model derives. from its resemblance to physical annealing. The scenario pictured starts when ''genetic-tempe ratures'' were very high, cellular entities (progenotes) were very sim ple, and information processing systems were inaccurate. Initially, bo th mutation rate and lateral gene transfer levels were elevated. The l atter was pandemic and pervasive to the extent that it, not vertical i nheritance, defined the evolutionary dynamic. As increasingly complex and precise biological structures and processes evolved, both the muta tion rate and the scope and level of lateral gene transfer, i.e., evol utionary temperature, dropped, and the evolutionary dynamic gradually became that characteristic of modern cells. The various subsystems of the cell ''crystallized,'' i.e., became refractory to lateral gene tra nsfer, at different stages of ''cooling,'' with the translation appara tus probably crystallizing first. Organismal lineages, and so organism s as we know them, did not exist at these early stages. The universal phylogenetic tree, therefore, is not an organismal tree at its base bu t gradually becomes one as its peripheral branchings emerge. The unive rsal ancestor is not a discrete entity. It is, rather, a diverse commu nity of cells that Survives and evolves as a biological unit. This com munal ancestor has a physical history but not a genealogical one. Over time, this ancestor refined into a smaller number of increasingly com plex cell types with the ancestors of the three primary groupings of o rganisms arising as a result.