‘Missing link’: Scientists may have just found the source of all complex life forms
One of the big mysteries in the field of biology is how the complex organisms that roam the Earth first arose. Now, a collaboration between the working groups of Christa Schleper at the University of Vienna and Martin Pilhofer at ETH Zurich may have found the answer, according to a press release by the first institution published on Wednesday.
“The researchers succeeded in cultivating a special archaeon and characterizing it more precisely using microscopic methods. This member of the Asgard archaea exhibits unique cellular characteristics and may represent an evolutionary "missing link" to more complex life forms such as animals and plants,” stated the press release.
Three major domains
On Earth, all living things are divided into three major domains: eukaryotes, bacteria, and archaea. Animals, plants, and fungi are all eukaryotes featuring cells that are usually much larger and more complex than those of bacteria and archaea.
“The genetic material of eukaryotes, for example, is packaged in a cell nucleus and the cells also have a large number of other compartments. Cell shape and transport within the eukaryotic cell are also based on an extensive cytoskeleton,” explained the statement.
Scientists today may understand how these complex cells operate but they still can't pinpoint how the evolutionary leap that led to them came about.
Most current theories speculate that archaea and bacteria played a central role in this process. It is largely believed that it was a close symbiosis between archaea and bacteria about two billion years ago that led to the evolution of a eukaryotic primordial cell.
“In 2015, genomic studies of deep-sea environmental samples discovered the group of the so-called "Asgard archaea", which in the tree of life represent the closest relatives of eukaryotes. The first images of Asgard cells were published in 2020 from enrichment cultures by a Japanese group,” further stated the press release.
Schleper's working group has now succeeded for the first time in cultivating a representative of this group in higher concentrations from marine sediments on the coast of Piran, Slovenia. The ETH researchers in Pilhofer's group then used a modern cryo-electron microscope to take pictures of shock-frozen cells from the group and found that they contained an extensive network of actin filaments thought to be unique to eukaryotic cells.
“This suggests that extensive cytoskeletal structures arose in archaea before the appearance of the first eukaryotes and fuels evolutionary theories around this important and spectacular event in the history of life,” further stated the press release.
A newly-discovered organism
The researchers called the newly-found organism Lokiarchaeum ossiferum.
Now, the scientists hope to use the new imaging methods to further investigate the interactions between Asgard archaea and their bacterial partners as well as basic cell biological processes such as cell division.
"It has taken six long years to obtain a stable and highly enriched culture, but now we can use this experience to perform many biochemical studies and to cultivate other Asgard archaea as well,” concluded Schleper.
The study was published in the journal Nature.
Asgard archaea are considered to be the closest known relatives of eukaryotes. Their genomes contain hundreds of eukaryotic signature proteins (ESPs), which inspired hypotheses on the evolution of the eukaryotic cell. A role of ESPs in the formation of an elaborate cytoskeleton and complex cellular structures has been postulated, but never visualized. Here we describe a highly enriched culture of ‘Candidatus Lokiarchaeum ossiferum’, a member of the Asgard phylum, which thrives anaerobically at 20 °C on organic carbon sources. It divides every 7–14 days, reaches cell densities of up to 5 × 107 cells per ml and has a significantly larger genome compared with the single previously cultivated Asgard strain7. ESPs represent 5% of its protein-coding genes, including four actin homologues. We imaged the enrichment culture using cryo-electron tomography, identifying ‘Ca. L. ossiferum’ cells on the basis of characteristic expansion segments of their ribosomes. Cells exhibited coccoid cell bodies and a network of branched protrusions with frequent constrictions. The cell envelope consists of a single membrane and complex surface structures. A long-range cytoskeleton extends throughout the cell bodies, protrusions and constrictions. The twisted double-stranded architecture of the filaments is consistent with F-actin. Immunostaining indicates that the filaments comprise Lokiactin—one of the most highly conserved ESPs in Asgard archaea. We propose that a complex actin-based cytoskeleton predated the emergence of the first eukaryotes and was a crucial feature in the evolution of the Asgard phylum by scaffolding elaborate cellular structures.
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