The tree of life, with tangled roots

New data support the proposition that two ancient forms of life merged to create the first complex cell. Relatives of the two rudimentary organisms, called prokaryotes, survive as bacteria and related microbes. But they also at some point combined, producing eukaryotes, some scientists assert. The eukaryotic trunk of the tree of life sprouted branches leading to organisms ranging from yeasts to humans.

Several theories compete to explain how eukaryotes arose. The cells might have descended from a simpler organism or acquired their complexity by snatching genes from other microbes.

James A. Lake and Maria C. Rivera of the University of California, Los Angeles favor a third explanation: A mutually beneficial microbial relationship culminated in the engulfment of one simple organism by another, they contend in the Sept. 9 Nature.

The investigators compared the genomes of two yeast species, which are among the most primordial eukaryotes, and 30 prokaryote genomes. They found that the yeasts share many genes with two classes of prokaryotes—eocytes and proteobacteria—that aren’t closely related to each other. The best explanation for the unusual evolutionary relationship among three organisms is that the two lesser-related ones are common ancestors of the third, Lake and Rivera contend. That would make eukaryotes the shared descendants of the two prokaryotes.