Photosynthesis seems to have evolved from a single endosymbiotic event in the eukaryotes.  Those taxa in this supergroup (Viridiplantae, Rhodophytae, and Glaucophyta) are the primary endosymbionts.  All other photosynthetic eukaryotes evolved from secondary endosymbiotic events (Cavalier-Smith, 2002 and Keeling, 2004).  Recent supertree analyses (Baldauf, 2003; Keeling, 2004; and Nikolaev et al., 2004) confirm the monophyly of the three groups.  The Rhodophytes and Viridiplantae seem to be sister groups while the Glaucophytes are outgroups within the Plant clade.

In 1993 Baldauf and Palmer demonstrated the phylogenetic relationship between the animals and fungi by using sequences of 25 proteins.  This followed a suggestion by Cavalier-Smith (1987) that animals and fungi evolved from a choanoflagellate-like line.  Cavalier-Smith and Chao (1995) and Cavalier-Smith (1996) strengthened the link and called the group, the Opisthokonts in reference to the posteriorly-directed flagellum in motile cells.  Since then, other work (e.g. van de Peer et al., 2000) confirmed a sister group relationship between the Amoebozoae and the Opisthokonts.  Tentatively, this new group (Opidsthokonts + Amoebozoae) is called the Unikonta, in reference to the occurrence of single flagella in their motile cells.  The Unikont Hypothesis, a derivative of the discovery of the supergroup, was proposed by Stechmann and Cavalier-Smith (2003) who claim that the fundamental separation in the eukaryotes was a divergence between uniflagellated taxa (unikonts) and biflagellated taxa (bikonts).  Thus, the line of the unikonts could have emerged near the root of the eukaryotes, which would make this clade as old as the last common nucleus-bearing ancestor.

Literature Cited for the Eukaryotes

This page is maintained by Jack R. Holt: 03/01/2005