Deep beneath the Atlantic Ocean between Greenland and Norway, scientists have found microorganisms they call a missing link connecting the simple cells that first populated Earth to the complex cellular life that emerged roughly two billion years ago.

The researchers said on Wednesday a group of microorganisms called Lokiarchaeota, or Loki for short, were retrieved from the inhospitable, frigid seabed about 2.35 kilometres under the ocean surface not too far from a hydrothermal vent system called Loki's Castle, named after a Norse mythological figure.

The discovery provides insight into how the larger, complex cell types that are the building blocks for fungi, plants and animals including people, a group called eukaryotes, evolved from small, simple microbes, they said.

"Humans have always been interested in trying to find an answer to the question, 'Where do we come from?' Well, now we know from what type of microbial ancestor we descend." - Thijs Ettema, study co-ordinator

The Lokiarchaeota are part of a group called Archaea that have relatively simple cells lacking internal structures such as a nucleus. But the researchers found the Lokiarchaeota share with eukaryotes a significant number of genes, many with functions related to the cell membrane.

These genes would have provided Lokiarchaeota "with a 'starter-kit' to support the development of cellular complexity," said evolutionary microbiologist Lionel Guy of Sweden's Uppsala University.

Archaea and bacteria, another microbial form, are together known as prokaryotes.

'A missing piece of the puzzle'

"Humans have always been interested in trying to find an answer to the question, 'Where do we come from?' Well, now we know from what type of microbial ancestor we descend," said Uppsala University evolutionary microbiologist Thijs Ettema, who co-ordinated the study.

Hydrothermal vent

Lokiarchaeota's location was desolate, pitch dark and around the freezing point. (R.B. Pedersen/University of Bergen, Norway Centre for Geobiology)

"Essentially, Lokiarchaeota represent a missing piece of the puzzle of the evolution from simple cells - bacteria and archaea, prokaryotes - to complex cells - eukaryotes, which includes us humans," Ettema added.

Earth's wide diversity of life would have been impossible without this transition from rudimentary cells into the more complicated ones seen in multicellular life. Microbial life originated about 3.5 billion years ago. The first complex cellular life came roughly two billion years ago.

How cellular complexity first developed has been one of the big puzzles of evolutionary biology, Guy said.

The Lokiarchaeota were retrieved from oxygen-starved sediment layers during voyages of a Norwegian research vessel, said microbiologist Steffen Jørgensen of Norway's University of Bergen.

While the Loki's Castle geothermal vents spew fluids reaching about 300 degrees C about 15 kilometres away, the Lokiarchaeota's locale was desolate, pitch dark and around the freezing point, Jørgensen added.

The research appears in the journal Nature.