Since the last universal common ancestor—or LUCA—began to diversify, two clearly differentiated lineages have emerged . The lineage of bacteria , and that of archaea . Archaea are prokaryotic organisms —without a cell nucleus— but with unique characteristics that differentiate them from bacteria. So far, everything is pretty clear. But the next step in the process is already more complicated.
The origin of the eukaryotic cell— cells with a nucleus, like those of humans—has turned out to be one of the most contentious and difficult puzzles in evolutionary biology ; however, we know that they are closer to the archaeal lineage than to the bacterial one. Many studies have supported the idea that the earliest eukaryotes must have evolved from archaea, but the identity and nature of that archaean ancestor has been a particularly difficult question to answer.
Did eukaryotes separate from archaea before they diversified, or did they come from a specific group of archaea? If the correct solution is the first, then life is made up of three separate domains: bacteria, archaea, and eukaryotes (red, green, and blue in the left image). But, if the correct one is the alternative, life would only be made up of two real domains: bacteria and archaea (red and cyan in the right image), and eukaryotic organisms would only be a strange and particular form, evolved from archaea and therefore , belonging to that domain; just as birds are actually a group of dinosaurs. The answer to that question would not only solve a complex puzzle, but it is also part of the answer to the great question: “Where do we come from?”
Wisdom is needed by those who travel far, at home everything is easy.
Hávamál, gestaþáttr
During 2015, a group of researchers from Uppsala University, Sweden, discovered a new lineage of archaea at the bottom of the sea, near the emerging hydrothermal vents, between Norway and Greenland, at the northern end of the Atlantic ridge. ; a steep place at more than 2000 meters deep, very close to a formation that receives the name of Loki’s Castle . And that new bloodline turned out to be much more important than you might think at first.
Loki, Thor, Heimdall, and other archaea
In its discovery it was already observed that it was an exceptional group of archaea. Its genome encoded for a wide repertoire of proteins that, until then, were thought to be typical of eukaryotes, and it showed a great capacity to remodel their cell membranes. Could it be the answer to that great dilemma that gripped evolutionary biology?
They are organisms with ambiguous features, belonging to one group but with characteristics of another, like a giant posing as a god. Organisms capable of remodeling their membranes, like those who change their skin and acquire different aspects with their power of transformation. Key organisms in a controversial dilemma that has generated confusion among scientists. Organisms discovered near a region named Loki’s Castle.
Those researchers could find no more excuses to name the newly discovered lineage: Lokiarchaeota .
But it was the beginning. In a few years, more and more archaea closely related to those of Loki were found , new groups phylogenetically related to each other, with unique capacities and different from the rest of the archaea. They were called the ‘Asgard archaea’ , the home of the Norse gods. Other Aesir and Vanir received the honor of being the spokespersons for the new groups of archaea. Of course, Odin , All-Father, received his Odinarchaeota clade, closely related to the groups named after Baldr, Hermod, and Thor —three of Odin’s sons.
The giants Kari, Gerd and Jord – mother of Thor – Hodar, the blind god or Heimdall, the watcher, also have their groups of archaea, and even the terrifying goddess of the underworld, Hela, daughter of Loki, has received such an honor. Among the most basal groups in the phylogeny —that is, with more primitive features— we find a group in honor of Borr, father of Odin.
Recent studies have found even more bloodlines, whose proposed names bestow the honor on Sigyn, Loki’s wife; Njord, god of the sea —with beautiful feet, according to the myth—, Sif, wife of Thor, and Freyr, son of Njord and god of rain and fertility along with his sister Freyja.
Among all this pantheon of archaea with Norse names, however, infiltrates an unexpected visitor from another mythology: the Wukongarchaeota group, which is named in honor of the monkey king Sun Wukong, of the Chinese culture of the Tang dynasty. An attempt by the researchers who discovered the lineage, associated with Shenzhen University in China, to incorporate a bit of their mythology into the Norse latticework of Asgard’s archaea.
The importance of the arches of Asgard
The study of these archaea is not easy. They are difficult organisms to obtain. The first samples could not even be cultivable ; Genomic data were obtained from direct samples obtained from the environment.
Over time, new samples have been incorporated, and a kind of Loki archaea has even been cultivated . From the beginning, they provided researchers with strong support in the form of scientific evidence that the archaean ancestor of eukaryotic cells was probably part of the same group, that is, that all eukaryotes probably descended from —or rather, descended from— of the archaea of Asgard . Later discoveries have found evidence of the existence of condensed nucleoids in some archaea of Asgard that could be the precursors of the cell nucleus of eukaryotes. However, the conclusion is not as strong as the researchers would have liked.
I know I hung from the windswept tree, (…) from that
tree of which nobody knows the origin of its roots
Hávamál; runal
There is another hypothesis that can explain the apparent proximity between the characteristics of these archaea and eukaryotes. Horizontal gene transfer is a process by which an organism transmits genes to a different organism —either directly or indirectly, for example, through viruses—, causing what we could call natural transgenics. It is a process that until recently was believed to be marginal, but which has actually happened frequently in the history of evolution, and among unicellular prokaryotic organisms, such as bacteria or archaea, it can occur very easily.
There is a possibility that the ancestral archaea of eukaryotic cells, or even the first true eukaryotes, transferred their protein-coding genes to the archaea ancestors of Asgard , and that is why we find those genes today. in Thor, Loki and company.
This hypothesis is reinforced by the fact that the passage from the most complex of Asgard’s archaea to the simplest of eukaryotic cells involves many evolutionary transitions that have not yet been observed in nature. However, these processes must have necessarily occurred in some group, since eukaryotes exist, so this hypothesis only moves the problem to another unknown place.
Resolving this dilemma is very relevant in terms of the classification of living beings . If the horizontal transfer hypothesis is the correct answer, the origin of eukaryotes would remain unknown, and we would probably be in a three-domain scenario, where the lineage of bacteria splits off first, and from the second branch they diverge. two domains, the archaea on the one hand, and the eukaryotes on the other, we still don’t know how.
However, if indeed the archaea of Asgard are the true ancestors of eukaryotic organisms , we would be facing a classification with only two domains: bacteria and archaea. And all eukaryotic organisms would be extraordinarily evolved forms of archaea from Asgard.
As usual, the scientific debate continues, and much more research will be needed before solid conclusions are drawn. But if it turns out to be resolved in favor of the Asgardian ancestral hypothesis, we humans, as eukaryotic organisms, would be descendants of Asgard .
References:
Imachi, H. et al. 2020. Isolation of an archaeon at the prokaryote–eukaryote interface. Nature, 577(7791), 519-525. DOI: 10.1038/s41586-019-1916-6
Liu, Y. et al. 2021. Expanded diversity of Asgard archaea and their relationships with eukaryotes. Nature, 593(7860), 553-557. DOI: 10.1038/s41586-021-03494-3
Spang, A. et al. 2015. Complex archaea that bridge the gap between prokaryotes and eukaryotes. Nature, 521(7551), 173-179. DOI: 10.1038/nature14447
Xie, R. et al. 2022. Expanding Asgard members in the domain of Archaea sheds new light on the origin of eukaryotes. Science China Life Sciences, 65(4), 818-829. DOI: 10.1007/s11427-021-1969-6
