Scientists have found the missing link in the evolution of complex life forms

(ORDO NEWS) — The way in which the first truly complex organisms appeared on Earth is one of the main open questions in modern biology.

In a new study, an international team of scientists has studied in detail and for the first time visualized the internal structure of the “archaea of ​​Asgard” – organisms recognized as an intermediate stage in the evolution of complex life forms.

All life forms found on Earth today are divided into three main domains in scientific systematics: bacteria, archaea and eukaryotes.

Moreover, it is in this order that the complexity of the work and internal structure of these organisms increases.

Thus, eukaryotic cells are usually much larger and, at first glance, much more complex than bacterial and archaeal cells.

At the same time, archaea have many similarities with both bacteria and eukaryotes.

They are related to the former by the size and shape of cells, the way genetic information is stored, and to the latter by certain genes and metabolic pathways.

From this, the assumption arises that the evolution of complex life forms followed precisely this path – from bacteria to archaea, and then to eukaryotes.

However, there is no understanding of how this evolutionary leap took place. Most current models suggest that the primary eukaryotic cell arose from a close symbiosis of archaea and bacteria about two billion years ago.

To better understand this issue, a group of researchers from Austria, Switzerland and Germany studied in detail the closest relatives of eukaryotes – Asgardian archaea, namely Lokiarchaeum ossiferum , named after the Scandinavian god Loki.

Previously, scientists were aware of the existence in the archaeal genome of the so-called eukaryotic signature proteins (ESPs), a group of genes encoding proteins commonly found in eukaryotes.

Some of these proteins must form the cytoskeleton of the cell, responsible for its shape and participating in the internal transport of molecules.

But the exact role played by ESBs in Asgardian archaea remained unclear.

Scientists have found the missing link in the evolution of complex life forms
Cryoelectron micrographs and 3D graphical representation of cells from the Asgardian archaea Lokiarchaeum ossiferum . The cell membrane is indicated in blue, ribosomes in gray, cytoskeleton filaments in orange

A group of scientists led by microbiologist Christa Schleper ( Christa Schleper ) from the University of Vienna for the first time managed to accumulate a sufficient number of archaeal cells, extremely sensitive to the slightest changes in external conditions, to study their structure under a cryoelectron microscope.

By capturing 3D images of internal cell structures, the researchers found that archaeal cells are composed of round cell bodies with thin, sometimes very long processes, and also contain an extensive network of actin filaments (parts of the cytoskeleton) that were previously thought to be unique to eukaryotic cells.

This means that the extensive structures of the cytoskeleton arose in archaea even before the appearance of the first eukaryotes.

“Our new organism, named Lokiarchaeum ossiferum , has great potential for further research into the early evolution of eukaryotes,” said lead author Christa Schleper.

“It took six long years to get a culture that grew steadily to high concentrations, but now we can use this experience for many biochemical studies, as well as for the cultivation of other Asgardian archaea.”

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