The advent of eukaryotic cells represents one of the great, epoch-making occurrences in the history of life on our planet. Through this event, the biosphere, which had long remained composed exclusively of simple prokaryotes, became enriched with a wealth of new microorganisms considerably more complex than the first ones and, eventually, with all plants, fungi, and animals; without it, we wouldn't be around.

When this fateful event took place is not precisely known. As we saw in the preceding chapter, there is evidence, provided largely by ribosomal RNA, that the eukaryotic line started diverging from the prokaryotic ones early after LUCA days – that is, 3.5 billion years ago – or perhaps even before. But the time of appearance of the first typical eukaryotic cells remains highly uncertain. The oldest clearly identified eukaryotic fossils are dated 1.5 billion years ago (Knoll, 2003). Earlier traces have been described, but their eukaryotic nature has been disputed. An origin going back to as early as 2.7 billion years ago is suggested by the presence, in terrains of that age, of steranes, the parent group of cholesterol, steroid hormones, and other derivatives, which are universal constituents of eukaryotes, though not entirely exclusive ones (Brocks et al., 1999). This thus leaves, between the LUCA and the earliest known eukaryotes, a gap of at least 800 million years on which the fossil record is entirely silent. All we have to fill this gap is the information extant organisms can offer on their history.