Blackwell Publishing
Precambrian Paleozoic Mesozoic Cainozoic

The other kind of cell - the eukaryotic cell - appears on the fossil record 1,500 mya. They are larger than prokaryotic cells, and many of them photosynthesized - introducing free, uncombined oxygen into the atmosphere for the first time.

The long time from the origin of prokaryotes to the origin of eukaryotic cells suggests it was an evolutionarily difficult transition.

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