The origin of complex life forms is a topic of much scientific interest and research. There are many theories about how complex life forms evolved, but much of the evidence is still inconclusive.
One theory is that complex life forms, including animals and plants, evolved from simple, single-celled organisms over a period of billions of years. According to this theory, these simple organisms gradually became more complex through a process called evolution, which is driven by natural selection. Another theory is that complex life forms may have formed through the process of abiogenesis, in which life arises spontaneously from non-living matter. However, this theory is still highly speculative and has not yet been proven.
In any case, it is clear that the evolution of complex life forms is a fascinating and complex process that is still not fully understood. Further research and study will likely be necessary to shed more light on the origins of complex life forms.
Researchers have 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.
How did complex organisms evolve on Earth? This is one of biology’s major unanswered questions. A collaboration between Christa Schleper’s working group at the University of Vienna and Martin Pilhofer’s working group at ETH Zurich has brought us one step closer to an answer. Using microscopic methods, the researchers were able to cultivate and characterize a unique archaeon. This Asgard archaea member has distinct cellular characteristics and may be an evolutionary “missing link” to more complex life forms such as animals and plants. The findings were published in the journal Nature recently.
Christa Schleper
All life forms on earth are divided into three major domains: eukaryotes, bacteria, and archaea. Eukaryotes include groups of animals, plants, and fungi. Their cells are usually much larger and, at first glance, more complex than the cells 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. But how did the evolutionary leap to such complex eukaryotic cells come about?
Most current models assume that archaea and bacteria played an important role in the evolution of eukaryotes. A eukaryotic primordial cell is thought to have evolved from a close symbiotic relationship between archaea and bacteria about two billion years ago. In 2015, genomic studies of deep-sea environmental samples revealed the “Asgard archaea,” which are the closest relatives of eukaryotes in the tree of life. A Japanese group published the first images of Asgard cells derived from enrichment cultures in 2020.
Asgard archaea cultivated from marine sediments
Christa Schleper’s working group at the University of Vienna has now succeeded in cultivating a representative of this group in higher concentrations for the first time. It was discovered in marine sediments off the coast of Piran, Slovenia, but it is also found in Vienna, for example, in Danube bank sediments. This representative can be studied thoroughly due to its ability to grow to high cell densities. “It was extremely difficult and time-consuming to obtain this extremely sensitive organism in a stable culture in the laboratory,” says Thiago Rodrigues-Oliveira, a postdoc in the Archaea working group at the University of Vienna and one of the study’s first authors.
Asgard archaea have a complex cell shape with an extensive cytoskeleton
The remarkable success of the Viennese group to cultivate a highly enriched Asgard representative finally allowed a more detailed examination of the cells by microscopy. The ETH researchers in Martin Pilhofer’s group used a modern cryo-electron microscope to take pictures of shock-frozen cells. “This method enables a three-dimensional insight into the internal cellular structures,” explains Pilhofer.
“The cells consist of round cell bodies with thin, sometimes very long cell extensions. These tentacle-like structures sometimes even seem to connect different cell bodies with each other,” says Florian Wollweber, who spent months tracking down the cells under the microscope. The cells also contain 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 fuel evolutionary theories around this important and spectacular event in the history of life.
Future insights through the new model organism
“Our new organism, Lokiarchaeum ossiferum, has a lot of potentials to provide even more groundbreaking insights into the early evolution of eukaryotes,” says microbiologist Christa Schleper. “It took six long years to obtain a stable and highly enriched culture, but now we can use this experience to conduct many biochemical studies as well as cultivate other Asgard archaea.”
Furthermore, scientists can now use the new imaging methods developed at ETH to investigate the close interactions between Asgard archaea and their bacterial partners, for example. In the future, basic cell biological processes such as cell division will be studied in order to shed light on the evolutionary origins of these mechanisms in eukaryotes.
