The Missing Link Between The Simple And The Complex

An image of Loki's Castle close to where the lokiarchaeota were collected

Life can be split into two broad groups: the prokaryotes and the eukaryotes. The prokaryotes are simple, single-celled organisms. The eukaryotes are far more complex, comprised of cells like our own replete with a variety of organelles designed to carry out vital metabolic processes.

Prokaryotes were undoubtedly the first life forms on Earth and how the complex eukaryotes evolved from them only became apparent in the 1960s when Lynn Margulis put forward the theory of endosymbiosis. This explained the origin of organelles, such as mitochondria or chloroplasts, as once free living bacteria. What was less clear was what the symbiont host cell was like.

In the 1970s the geneticist Carl Woese demonstrated that the prokaryotes could be split into two groups; the bacteria and the archaea. Since then it has become clear that the archaea are more closely related to the eukaryotes than are the bacteria. Recently, however, the group of archaea which bridges the prokaryote and eukaryote domains has been identified. The members of this new group were discovered living in a hydrothermal vent system known as Loki's Castle. Situated on the Mid Atlantic ridge between Norway and Greenland, this sunken kingdom is one the handful of known black smoker vent ecosystems.

A phylogram indicating the evolutionary relationships of the lokiarchaeota

Vents discharge plumes of water at 400 degrees Celsius laden with sulphide minerals. It is from these minerals that prokaryotes derive their energy. In turn the prokaryotes form the basis for the vent ecosystem, of which eukaryotes are often a part. Now an international team of researchers has found, living alongside the eukaryotes and the simpler prokaryotes, the archaean halfway house - the lokiarchaeota.

Genetic analysis of members of the group showed that they possessed a large number of genes previously thought to exist in eukaryotes only, demonstrating that the eukaryotes and the lokiarchaeota are closely related, considering their allegiances to separate domains.

'The puzzle of the origin of the eukaryotic cell is extremely complicated, as many pieces are still missing. We hoped that Loki would reveal a few more pieces of the puzzle, but when we obtained the first results, we couldn't believe our eyes. The data simply looked spectacular,' said Thijs Ettema at the Department of Cell and Molecular Biology, Uppsala University. 'By studying its genome, we found that Loki represents an intermediate form in-between the simple cells of microbes, and the complex cell types of eukaryotes'.

What remains is to find out precisely how two living cells can become one. Answers may even come from the lokiarchaeota and their bizarre genomes. Looking at the type of eukaryotic genes that the lokiarchaeota possess, it could be that it was at a stage where it developed a primitive way of taking up material via phagocytosis - the process required for endosymbiosis to occur. While it is a modern-day organism, it tells us about the steps that occurred during the origin of eukaryotes.