Interpreting the universal phylogenetic tree

The universal phylogenetic tree not only spans all extant life, but its roo t and earliest branchings represent stages in the evolutionary process befo re modern cell types had come into being. The evolution of the cell is an i nterplay between vertically derived and horizontally acquired variation. Pr imitive cellular entities were necessarily simpler and more modular in desi gn than are modern cells. Consequently. horizontal gene transfer early on w as pervasive. dominating the evolutionary dynamic. The root of the universa l phylogenetic tree represents the first stage in cellular evolution when t he evolving cell became sufficiently integrated and stable to the erosive e ffects of horizontal gene transfer that true organismal lineages could exis t.