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.
The origin of the eukaryote cell was a milestone in the evolution of life, since they include all complex cells and almost all multi-cellular organisms. The timing of this series of events is hard to determine; Knoll (2006) suggests they developed approximately 1.6 - 2 billion years ago. Some acritarchs are known from at least 1650 million years ago, and the possible alga Grypania has been found as far back as 2100 million years ago. Fossils that are clearly related to modern groups start appearing around 1.2 billion years ago, in the form of a red alga, though recent work suggests the existence of fossilized filamentous algae in the Vindhya basin dating back to 1.6 to 1.7 billion years ago. However, biomarkers suggest that at least stem eukaryotes arose even earlier. The presence of steranes in Australian shales indicates that eukaryotes were present 2.7 billion years ago.
RNA trees constructed during the 1980s and 1990s left most eukaryotes in an unresolved "crown" group (not technically a true crown), which was usually divided by the form of the mitochondrial cristae; see crown eukaryotes. The few groups that lack mitochondria branched separately, and so the absence was believed to be primitive; but this is now considered an artifact of long-branch attraction, and they are known to have lost them secondarily.