Origins:
On the Origin of Eukaryotes
Carl Zimmer
In the eighth essay in Science's series in honor of the Year of Darwin, Carl Zimmer describes one of the most important transitions in the history of life: the origin of cells with a nucleus, which gave rise to every multicellular form of life. The fossil record doesn't tell us much: The earliest fossils that have been proposed to be eukaryotes are only about 2 billion years old, and paleontologists have not yet discovered any transitional forms. Fortunately, living eukaryotes and prokaryotes, cells that lack a nucleus, still retain some clues to the transition, both in their cell biology and in their genomes. By studying both, researchers have made tremendous advances in the past 20 years in understanding how eukaryotes first emerged. A key step in their evolution, for example, was the acquisition of bacterial passengers, which eventually became the mitochondria of eukaryote cells. But some scientists now argue that the genes of these bacteria also helped give rise to other important features of the eukaryote cell, including the nucleus.
