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.