EVEN the gods struggled to cope with Loki, the trickster of Norse mythology. So it may have been foolhardy to beckon the notorious schemer into the world of modern science 鈥� but that鈥檚 what a team of researchers did in 2008. They had struggled to find a group of hydrothermal chimneys at the bottom of the Norwegian Sea because the heat signature seemed to keep shifting. When they finally tracked down the rocky spires, they thought it would be apt to in reference to Loki鈥檚 ability to confound those around him by shape-shifting.
The castle鈥檚 smallest residents soon began stirring up trouble too. Strange microbes living there (inevitably dubbed the Lokis) are shedding light on one of evolution鈥檚 biggest mysteries: the origin of complex life. What is more, they have reignited an argument about the shape of the tree of life, one of biology鈥檚 most fundamental ways of describing the rise of life on Earth, with implications for all of us. The discovery of the Lokis may leave humanity lumped together with a group of weird single-celled organisms called archaea, dramatically redefining our species.
Textbooks will tell you that shortly after biological cells appeared on Earth more than 3.5 billion years ago, there was a parting of the ways that . One led to bacteria 鈥� single-celled organisms only visible through a microscope. A second led to similarly simple but biologically distinct microbes called archaea. The final branch led to complex organisms called eukaryotes, a group that includes humans, trees and fungi. These differ from their simpler cousins at the cellular level, containing intricate internal structures.
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Long before the Lokis were found, doubts about this picture of life had emerged. They began with the discovery of an entirely new type of archaea, first identified in an Italian sulphur hot spring in the 1980s. These organisms are called the Crenarchaeota, also known as eocytes, but seemed to share cellular features with eukaryotes, which was odd. Until then, the two had been considered distinct.
These similarities suggested a close evolutionary connection between archaea and eukaryotes and led James Lake at the University of California, Los Angeles, to suggest an . He argued that cellular life actually began with a two-way split, meaning there are just two great domains of life: bacteria and archaea.
One implication of Lake鈥檚 idea is that the eukaryotes would then be an evolutionary branch within the archaea 鈥� in much the same way that biologists now accept that birds are an evolutionary branch within the dinosaurs. A sparrow might not look like a Brachiosaurus, but it is technically a dinosaur. Likewise, a William Shakespeare or an Albert Einstein might seem to have little in common with methane-generating microbes living in a muddy bog 鈥� but in Lake鈥檚 scenario all are, technically, archaea.
Lake鈥檚 idea became known as the eocyte hypothesis. But you are unlikely to find this mentioned in textbooks. 鈥淚t鈥檚 largely been ignored,鈥� says Martin Embley at Newcastle University, UK. 鈥淚 never really understood why.鈥�
A decade ago, Embley and his colleagues . One of the problems facing research on the early origins of life is that the organisms that existed billions of years ago have long since disappeared, which means progress relies on understanding the biology of present-day organisms.
So Embley and his team analysed dozens of genes that occur in existing bacteria, archaea and eukaryotes to work out how the three groups interrelate. To their surprise, the results strongly favoured the eocyte hypothesis鈥檚 two-domain tree.
Not everyone was ready for such a shake-up. 鈥淚鈥檝e never had such a difficult review process as the one surrounding that paper,鈥� says Embley. 鈥淲hat was strange to me was people didn鈥檛 show we were wrong for any particular reason. They simply didn鈥檛 believe our results.鈥�
To be fair, says Embley, reconstructing evolutionary events that probably occurred more than 3.5 billion years ago isn鈥檛 easy. The task is made harder because bits of DNA are commonly swapped 鈥渉orizontally鈥� 鈥� directly absorbed rather than inherited 鈥� between organisms in the different domains. Embley says we can get a better sense of the tree of life鈥檚 shape by focusing on a subset of genes that seem particularly difficult to swap horizontally. Some enzymes, for instance, interact with such a wide variety of other molecules inside the cell that microbes can鈥檛 easily swap their version for one carried by an unrelated microbe. As a result, the genes that produce these enzymes resist horizontal transfer. As researchers have begun taking this more nuanced approach, Embley says the has become more popular. 鈥淧eople are now beginning to think the two-domain tree is the better supported hypothesis,鈥� he says.
But to really hammer home the idea, two-domain advocates needed a symbolic discovery 鈥� something equivalent to the feathered dinosaur fossils that came to light during the 1990s and that finally convinced many doubters of the bird-dino link. This is where the Lokis enter the story.
Thijs Ettema, now at Wageningen University in the Netherlands, and his colleagues sequenced DNA collected from the muddy sea floor near Loki鈥檚 Castle. Some of it turned out to belong to microbes that appeared to be archaea, but that also carried dozens of genes we had previously assumed to be unique to eukaryotes. They were the archaea-eukaryote equivalent of a dinosaur with feathers 鈥� in other words, the Lokis were a missing link that showed how some archaea had evolved to be extra complex and turn into the first eukaryotes. 鈥淲e didn鈥檛 dare to predict we would make such a discovery,鈥� says Ettema. 鈥淚t鈥檚 super-exciting.鈥�
鈥淭hey were the microbial equivalent of a dinosaur with feathers鈥�
The Lokis, more officially known as the Lokiarchaeota, have versions of the genes that help eukaryotes build membrane-enclosed compartments inside their cells. Without those compartments, eukaryotic cells would lack their most dramatic feature, the nucleus.
Also present within the typical Loki genome are versions of the genes that help eukaryotic cells engulf smaller microbes. That might be another important finding. Many biologists think that eukaryotes became large and complex only because they carry cellular powerhouses called mitochondria, and a leading idea is that
Collectively, such discoveries fit with the idea that the eukaryotes evolved from, and are part of, the archaeal domain. 鈥淢aybe we have to get used to the idea that we are some weird group of archaea,鈥� says Ettema.
Since 2015, researchers have found Loki-like microbes all over the world 鈥� in Romanian lakes, Australian microbial mats and in hydrothermal vents in New Zealand and Yellowstone National Park in the US. Each group is named in honour of another character from Norse mythology and they are , in reference to the mythological realm.
Even some researchers who have been sceptical of the two-domain idea are willing to accept that the Lokis 鈥� and the Asgards more broadly 鈥� are closely related to the eukaryotes. They include Gregory Fournier at the Massachusetts Institute of Technology. However, Fournier says it is important to note that this Asgard-eukaryote connection doesn鈥檛 necessarily mean the tree of life needs to be pruned down from three to two domains.
After billions of years of evolution, eukaryotes are dramatically more biologically complex than archaea or bacteria. But the very earliest microbes on the branch leading to eukaryotes would have been so biologically simple that they probably looked a lot like archaea. This means it is possible that the Asgards could have an archaea-like appearance while actually on the path to eukaryotes. Under that scenario, the Asgards would still fit within the traditional three-domain tree.
Studying the Asgards in more detail might help to establish beyond doubt whether they really are archaea. These microbes were first identified by piecing together fragments of their DNA, but a living Asgard microbe had never been seen. So the focus was on working out how to culture one in the lab to get a proper look at their biology and behaviour.
Tentacle discovery
Last year, Hiroyuki Imachi at the Japan Agency for Marine-Earth Science and Technology and his colleagues became the first to do just that. They announced that they had isolated and grown a type of Asgard collected from the seabed off the south coast of Japan. Bucking the trend, they named it Prometheoarchaeum syntrophicum, after a character from Greek rather than Norse mythology.
The microbe is so difficult (and slow) to grow in the lab that Imachi鈥檚 team actually began their experiment 14 years ago, two years before Loki鈥檚 castle was discovered 鈥� and several more years before Ettema and his colleagues found the Lokis living there. Despite the topsy turvy timeline, P. syntrophicum is still the first cultured Asgard. Imachi鈥檚 paper, which was published last month, has been widely praised by microbiologists. 鈥淚t鈥檚 really exciting to finally have a cultured Loki,鈥� says Fournier.
The study shows that P. syntrophicum has very odd tentacle-like projections. Imachi鈥檚 team speculates that these projections could have allowed ancient Asgard-like microbes to surround bacteria and, eventually, incorporate them into their cells as mitochondria, becoming the first eukaryotes in the process. The team also showed that these microbes don鈥檛 live on their own, and could only grow in conjunction with a little community of other microbes they rely on to survive. This close partnership might also have led to Asgard species incorporating helper microbes.
The P. syntrophicum discovery helps strengthen the idea that the Asgards are closely related to eukaryotes, but it doesn鈥檛 really help us decide whether the Asgard microbes really are archaea, says Fournier. For one thing, no known archaeon has tentacle-like projections quite like P. syntrophicum鈥榮. The problem is that it is a relatively specialised Asgard microbe. There is clearly more work to be done to settle the matter. Fournier says we now need to culture Asgard species that more closely represent the earliest stages of the group鈥檚 evolution because it will be easier to judge if these primitive Asgards are archaea.
Ultimately, the Lokis and the Asgards may encourage biologists to reject the three-domain tree in favour of a two-domain version. But even if they don鈥檛 lead to such a radical change, these newly discovered organisms are shedding light on the origins of complex life forms such as ourselves. The Norse god of mischief would surely approve of the shake-up his namesake microbes are provoking.
