Between Greenland and Norway in the mid-Atlantic ocean, 7,546ft (2300m) below the surface, there is a black and smoky region where hot water spouts up from the sea bed. This strange place is called “Loki’s castle” after the shape-shifting Norse God Loki: the one Tom Hiddleston played in The Avengers.

Close to Loki’s castle, there lives a microbe like no other on Earth.

In 2015, a team of scientists led by microbiologist Thijs Ettemaof Uppsala University in Sweden reported that they had discovered a new kind of micro-organism. They found genetic traces of this single-celled microbe in sea-floor sediments 9 miles (15km) from Loki’s castle.

Writing in the journal Nature, the team called the new microbe “Lokiarchaeota“. This quickly got shortened to “Loki”.

This mysterious microbe turned out to be the closest living relative of the eukaryotes, the group that includes all complex living organisms; from plants and fungi to insects and humans. That means Loki could help us understand how the eukaryotes first came into being. In other words, Loki may help explain why people – and all other complex life – exist.

Loki the god has been described as “a staggeringly complex, confusing, and ambivalent figure”. The same is true of eukaryotes. How eukaryotes first evolved is a puzzle that continues to flummox scientists the world over.

If the eukaryotes had never formed, neither would we

Until a few decades ago, biologists thought that there were essentially two kinds of life on this planet: eukaryotes and prokaryotes. While the eukaryotes include both single-celled organisms and more complex ones like mushrooms and chimpanzees, the prokaryotes are all single-celled. The most famous prokaryotes are the bacteria.

The key difference between prokaryotes and eukaryotes is the complexity of the cells. Prokaryotes are simple cells, with only a few internal structures, whereas the cells of eukaryotes are much more intricate. Eukaryotic cells also tend to be bigger, often ten times as large as prokaryotic cells.

It is the complexity of eukaryotic cells, and the resulting extra abilities, that allow them to bolt together to form multicellular organisms like us. Prokaryotes cannot do this. So if the eukaryotes had never formed, neither would we.

This straightforward split between simple and complex held sway for decades, until one scientist showed that there was more to it.

In the late 1960s, American microbiologist Carl Woese decided to look at organisms’ DNA sequences to figure out how they were all related.

Woese identified a gene that all organisms carry, and compared all the different versions of it. Species that had more similar versions of the gene were probably closely related. In this way he built up a family tree of all the known living organisms.

Instead of two domains of life, there were three

Woese’s analysis revealed that there were two kinds of prokaryote. As well as the bacteria, there was an entirely new group that he called “archaea”.

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