the time of Darwin, science has accepted the principle that species
decended from an ever more limited number of common ancestors, back
to a single common ancestor. This all inclusive hierarchy has formed
the basis for phylogenic classification. Modern phylogenetics is
conducted at the molecular level using nucleotide (DNA and RNA)
and amino acid (protein) sequencing. To an extent, a hierarchical
tree of life has been derived using molecular phylogenetics (Woese,
PNAS, 87 4576, 1990). More recently, Doolittle, in a review (Doolittle,
Science, Vol 284, p. 2124, 1999) has discussed evidence that most
archaeal and bacterial genomes, and by inference the ancestral eukaryote
genome, contain genes from multiple sources. Such lateral gene transfer,
if extensive, would deny the hypothesis of a single universal tree.
If so, the so-called tree of life would comprise multiple, superimposed
WF. Phylogenetic classification and the universal tree. Science.
1999 Nov 19;286(5444):1443 [PubMed]