JAMIE BARRETT
Thousands of metres beneath the waves of the Norwegian Sea, on a throne of iron and sulphur, a strange family of microbes thrives among 300°C waters. Their very existence may force us to rewrite the tree of life.
Since the discovery of archaea in the 1970s, the predominant picture of life was split into three distinct domains: bacteria, archaea (a type of single-celled organism without a nucleus), and eukaryotes. James Lake at the University of California noticed that eocytes, a type of archaea discovered in the 1980s, had cellular features that were similar to those of eukaryotes. He argued that these similarities suggested a more recent evolutionary connection between archaea and eukaryotes. He proposed that eukaryotes are a branch within archaea, rather than a separate domain.
Lake’s idea - known as the eocyte hypothesis - was largely ignored. However, a decade ago, Martin Embley and his team at Newcastle University attempted to see how bacteria, archaea, and eukaryotes interrelate by analysing genes that occur in the three groups. Embley’s results supported the two-domain tree of the eocyte hypothesis, which then became more popular, but many still found the hypothesis hard to believe.
Loki’s Castle, named after the shapeshifting god due to the area’s shifting heat signature, is a group of hydrothermal vents on the Mid-Atlantic Ridge that were discovered in 2005. Three years later, a team of researchers based in Uppsala University, Sweden, discovered a lineage of archaea, named the Lokis, that seemed to share many properties of eukaryotes.
Thijs Ettema and his colleagues at Wageningen University in the Netherlands sequenced DNA from sediments near Loki’s Castle and found dozens of genes thought to be unique to eukaryotes. Just like the feathered dinosaur fossils that convinced doubters of the bird-dino link, this finally gave strong evidence that eukaryotes originated from archaea.
In particular, the Loki genome contains genes that help eukaryotic cells engulf other microbes, which can lead to endosymbiosis, thought to be the origin of mitochondria. Despite lacking a nucleus, a defining feature of modern eukaryotes, the genome also includes genes that code for the compartments that are needed to build this organelle. Since 2015, Loki-like microbes - collectively dubbed the Asgards - have been discovered all over the world.
However, it is still possible that the three-domain tree of life is correct. Early eukaryotes would have had characteristics similar to archaea, so it is difficult to establish whether the Asgards are archaea with eukaryotic genes or eukaryotes that look like archaea. Moreover, evolutionary history is often contentious, and some argue that the presence of eukaryotic genes is entirely due to contamination.
Asgards were first identified by analysing and piecing together fragments of DNA, but to study the microbes in enough detail to conclusively determine whether they are archaea or not, a living Asgard microbe would be required. Now, the focus is to attempt to culture one in the lab, but the slow metabolism of many archaea means this is proving difficult to achieve.
More work is needed before we can be sure whether the two-domain or three-domain tree is correct, but even if the Lokis turn out not to be archaea, the newly discovered organisms are still shedding light on the origins of complex life.
Bibliography
ScienceDaily (2008), Scientists Break Record By Finding Northernmost Hydrothermal Vent Field. Available at: https://www.sciencedaily.com/releases/2008/07/080724153941.htm (Accessed: 11 May 2020)
TheRegister (2015), Weird ARCHAEAN LIFE FORM Found at ‘Loki’s Castle’ DEEP beneath Arctic Circle. Available at: https://www.theregister.co.uk/2015/05/07/boffins_bridge_the_gap_in_cellular_evolution_from_the_bottom_of_the_ocean/ (Accessed: 11 May 2020)
NewScientist (2020), The mysterious microbes shifting humanity’s place in the tree of life. Available at: https://www.newscientist.com/article/mg24532670-900-the-mysterious-microbes-shifting-humanitys-place-in-the-tree-of-life/ (Accessed: 11 May 2020)
Nature (2019), The trickster microbes that are shaking up the tree of life. Available at: https://www.nature.com/articles/d41586-019-01496-w (Accessed: 11 May 2020)