<P> A widespread current model of the evolution of the first living organisms is that these were some form of prokaryotes, which may have evolved out of protocells, while the eukaryotes evolved later in the history of life . Some authors have questioned this conclusion, arguing that the current set of prokaryotic species may have evolved from more complex eukaryotic ancestors through a process of simplification . Others have argued that the three domains of life arose simultaneously, from a set of varied cells that formed a single gene pool . This controversy was summarized in 2005: </P> <P> There is no consensus among biologists concerning the position of the eukaryotes in the overall scheme of cell evolution . Current opinions on the origin and position of eukaryotes span a broad spectrum including the views that eukaryotes arose first in evolution and that prokaryotes descend from them, that eukaryotes arose contemporaneously with eubacteria and archeabacteria and hence represent a primary line of descent of equal age and rank as the prokaryotes, that eukaryotes arose through a symbiotic event entailing an endosymbiotic origin of the nucleus, that eukaryotes arose without endosymbiosis, and that eukaryotes arose through a symbiotic event entailing a simultaneous endosymbiotic origin of the flagellum and the nucleus, in addition to many other models, which have been reviewed and summarized elsewhere . </P> <P> The oldest known fossilized prokaryotes were laid down approximately 3.5 billion years ago, only about 1 billion years after the formation of the Earth's crust . Eukaryotes only appear in the fossil record later, and may have formed from endosymbiosis of multiple prokaryote ancestors . The oldest known fossil eukaryotes are about 1.7 billion years old . However, some genetic evidence suggests eukaryotes appeared as early as 3 billion years ago . </P> <P> While Earth is the only place in the universe where life is known to exist, some have suggested that there is evidence on Mars of fossil or living prokaryotes . However, this possibility remains the subject of considerable debate and skepticism . </P>

When did prokaryotic cells first appear on earth