ABSTRACT
Figure 20.1 shows the phylogenetic tree constructed from the small subunit rRNA genes of living organisms reported by Woese eta/. (1990), in which the root was placed on the basis of the results presented by Gogarten eta/. (1989) and lwabe eta/. (1989). The species, which is a group of living organisms with sufficient gene interaction to constitute a single gene-pool, has divided into two species at the point Commonote. The species can be called the last and the latest common ancestor of all the living organisms on the earth. We have suggested that the last common ancestor must have already had a rigid genetic system with circular chromosomal DNA, based on the circularity of chromosomal DNAs of eubacteria and archaebacteria (Kondo et al., 1993; Yamagishi and Oshima, 1995). In the first half of the chapter we will discuss the characteristics of the last common ancestor.
