Scientists from the Swiss Federal Institute of Technology Zurich (ETH Zurich) revealed key details about the evolution of life on Earth by studying microorganisms known as Asgard archaea. The study was published in the journal Cell.
Ten years ago, nobody knew that Asgard archaea even existed. In 2015, researchers examining deep-sea sediments near the underwater volcano Loki discovered gene fragments indicating a new and previously undiscovered form of microbes, which were later classified as Asgard archaea. With computer assistance, the researchers assembled gene fragments like puzzle pieces to compile the entire genome of Asgard archaea.
The name Asgard archaea is taken from the heavenly realm in Norse mythology. The name references the initial discovery of Asgard archaea close to Loki's Castle, a black smoker on the mid-Atlantic ridge between Norway and Svalbard. After further searches, microbiologists identified the corresponding organisms, described them, and classified them as a separate archaeal subgroup called Asgard archaea. Like bacteria, archaea are single-celled organisms. Genetically, there are differences between bacteria and archaea, especially regarding their cell envelopes and metabolic processes.
Microtubules are important for transport processes within a cell and the segregation of chromosomes during cell division. In eukaryotes, microtubules are responsible for transport processes within the cell, and in Asgard archaea, they may have played a key role in the evolution of these microorganisms.
In their study, the researchers combined different microscopy techniques to demonstrate that a protein found in Asgard archaea—called Lokiactin—forms filamentous structures resembling the cytoskeleton of eukaryotic cells, especially in the microbes' numerous tentacle-like protrusions. Unlike actin, tubulin proteins only appear in very few species of Asgard archaea.Eukaryotes are organisms whose cells contain a nucleus, such as plants and animals.
The origin of microtubules had been unclear until now, but the research sheds light on their evolutionary significance. The researchers are confident that the cytoskeleton was an important step in the evolution of eukaryotes. "This remarkable cytoskeleton was probably at the beginning of this development. It could have enabled Asgard archaea to form appendages, thereby allowing them to interact with, and then seize and engulf a bacterium," said Pilhofer.
In the course of evolution, the bacterium developed into a mitochondrion, which serves as the powerhouse of modern cells. This step could have occurred aeons ago when an Asgard archaeon entwined a bacterium with its appendages. Over time, the nucleus and other compartments evolved, and the eukaryotic cell was born.
There is growing evidence that eukaryotes may have evolved from Asgard archaea. Some researchers proposed regarding eukaryotes as a group within Asgard archaea due to their unique genetic and cellular features, which indicate a close relationship. The division of all living organisms into the three domains of bacteria, archaea, and eukaryotes did not hold up to this surprising discovery, which suggests a reduction to two domains: archaea (including eukaryotes) and bacteria.
Pilhofer and his team are fascinated by Asgard archaea and examined these microbes for several years. "We still have a lot of unanswered questions about Asgard archaea, especially regarding their relation to eukaryotes and their unusual cell biology. Tracking down the secrets of these microbes is fascinating," said Pilhofer. Researchers from the fields of microbiology, biochemistry, cell biology, and structural biology collaborated closely on the study.
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