Evolution of eukaryotes

For roughly the fist 1.7 billion years of life on Earth consisted of prokaryotes, nucleus-less cells grouped into bacteria and archea. Then 2 billion years, two prokaryotes symbiotically joined to form the more complex eukaryotic cells that make up all animals, plants, and fungi.

Starting in the 1960s, the biologist Lynn Margulis did much to clarify and make mainstream the theory of the symbiotic origin of eukaryotes. Margulis writes with her co-author (and son) Dorion Sagan, "The creative force of symbiosis produced eukaryotic cells from bacteria. Hence all larger organisms--protocoists, fungi, animals, and plants--originated symbiogenetically."

There are a number of reasons to believe that components of eukaryotic cells originated as independent prokaryotes. The DNA of a eukaryote's component parts, specifically, mitochondria and plastids, is unlike the DNA of the nucleus. Mitochondria and plastids make proteins in ways that prokaryotes do and are affected by drugs that affect prokaryotes. One way that the symbiosis may have occurred is that the host cell may have preyed on and digested the bacteria. A second way is that the host cell and the bacteria may have lived side by side, producing substances for each other, until the host took the other cell inside itself.

Science journalist David Quamm explains, "[E]xperts agree nowadays that endosymbiosis played an essential role: somehow a bacterium got captured and domesticated inside another cell, a host, where it became a mitochondrion."

In 2015, however, a group of scientists led by Thijs Ettema proposed that the host cell was a type of archea (not bacteria) they found at the bottom of the Atlantic Ocean between Greenland and Norway. They called the it Lokiarchaeum and found it to be closely related to eukaryotes.