Within deep hydrothermal vents beneath the Arctic Ocean, scientists have discovered an organism that may be the evolutionary link between simple and complex cellular life. The microbe belongs to a family known as Lokiarchaeota, reported in the journal Nature. They are strange single-celled microorganisms with traits that link them to eukaryotic cells – the type of cell that makes up all complex lifeforms in Earth’s “tree of life” from fungi to humans.
The work, led by Anja Spang of Uppsala University in Sweden, represents human’s closest known simple-celled relative. “This technically outstanding paper has far-reaching implications for how we view early eukaryotic evolution, including our own deep ancestry,” T. Martin Embley and Tom A. Williams at Newcastle University in England wrote in a commentary on the study.
Since the late 1980’s, all cellular life on Earth has been split into two categories: prokaryotes (which include bacteria and archaea) and eukaryotes (everything else). Eukaryotic cells are distinguished in particular by a nucleus filled with DNA, mitochondria to power the cell and the presence of many membrane-bound organelles. However, it has been questioned whether eukaryotes shared a common ancestor with archaea, or if eukaryotes sprung into existence as a subset of archaea.
Deciphering how eukaryotes materialised from prokaryotes has been a mystery because of very few organisms alive today of which have lineage that helps to bridge the ancient evolutionary gap. The study authors wrote, “The origin of the eukaryotic cell remains one of the most contentious puzzles in modern biology.”
The researchers discovered Lokiarchaeum (or Loki for short) in a 7.5 gram chunk of sediment taken from a 2-metre-long core removed from an active venting site along the Arctic Mid-Ocean Ridge 10,771 feet below sea level, near a site named Loki’s Castle. Upon analysing the DNA in the tiny sample, Thijs Ettema of Uppsala University and his colleagues were able to observe evidence that the microbe is an archaean prokaryote. However, one microbe shared a number of similar genetic features with eukaryotes. Such features include a dynamic protein cytoskeleton and proteins involved in phagocytosis that allow it to engulf and consume other cells – an ability that may have enabled them to swallow mitochondria.
“Our results provide strong support for hypotheses in which the eukaryotic host evolved from a bona fide archaeon, and demonstrate that many components that underpin eukaryote-specific features were already present in that ancestor,” the authors explained. “This provided the host with a rich genomic ‘starter-kit’ to support the increase in the cellular and genomic complexity that is characteristic of eukaryotes.”
The team aims to facilitate results in which this finding might raise the possibility of unearthing other living examples of microorganisms that fit the prokaryote-eukaryote puzzle. This discovery is exactly what researchers in the field strive to find – something that may explain how cells developed from simplicity to something more complex. The team believes that, “The identification of Lokiarchaeota so early in the history of this nascent field suggests that more-closely related archaeal relatives of eukaryotes [might] soon be discovered.” The genomic and cellular features of these relatives might give a more detailed picture of the most recent common ancestor of eukaryotes and prokaryotes. Environmental surveys have already revealed a number of uncultured archaeal lineages, some of which might be even closer relatives of eukaryotes.
What other interesting transitional fossils might still be uncovered in the evolutionary web?