GETTING TO THE ROOT OF THINGS: Charles Darwin drew this famous tree on page 36 in his Notebook B (1837-38) to illustrate his ideas about the relationship between living (branches with perpendicular tips) and extinct (branches with no tip) organisms that descended from a common ancestor (circled 1 at base of tree).© CAMBRIDGE UNIVERSITY LIBRARYAll known organisms share a number of fundamental features that, taken together, point to a common evolutionary history: DNA as the chief molecule of genetic inheritance, proteins as the primary functional molecules, and RNA as an informational intermediate between the two. The simplest explanation for why organisms share these common features is that they are inherited from a last universal common ancestor (LUCA), which sits at the root of the tree of life. Most studies of gene duplications that occurred prior to the first branch on the tree place LUCA in between the Bacteria and the common ancestor of the Archaea and Eukarya, the three taxonomic domains of cellular life.
The availability of the genome sequences from so many species across the tree of life has made it possible to look for common genomic traits that were most likely inherited from LUCA. The methods employed to identify these common genomic traits can vary greatly, however, and as a result lead to very different predictions. Some studies have estimated there to be fewer than 100 LUCA-derived gene families, while others count more than 1,000, depending on how conservatively the methods rule out genes on suspicion of horizontal gene transfer or how liberally they include genes that appear to have been present in LUCA, but subsequently lost. Despite the conflicting results, the new data are yielding insight into ancient life on Earth.
The majority of ancient gene ...
