Earth's first traces of life probably showed up around 3.5 billion years ago, a billion years or so after our planet formed. Just when these simple, single-celled organisms—classified as “prokaryotes” due to their lack of a nucleus—evolved into multicellular, nucleated forms called “eukaryotes” is a matter of debate. read more
Prokaryotic cells are believed to have arisen in the Archean Eon about 2500-3800 million years ago. There is fossilized evidence of prokaryotes approximately 3.5 billion years ago, during the first billion years after the formation of the Earth. read more
All these events appear to have preceded the oldest fossil stromatolites. So the eukaryotic lineage appears to be very ancient, about as ancient as the two prokaryotic lineages. "The key unanswered question here concerns when on the eukaryotic line the eukaryotic type of cell formed. read more