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How did life leap from simple microbial cells to the complex, structured cells that make up animals, plants, and fungi? A in The EMBO Journal by researchers at the Indian Institute of Science (IISc) offers fresh insight into this deep evolutionary mystery. It sheds light on how the cytoskeleton, the cell's internal scaffold, may have evolved from far simpler beginnings in ancient microbes.

The cytoskeleton is a dynamic network of protein filaments that gives modern eukaryotic cells their shape, powers their movement, helps organize their internal components, and controls cell division.

In humans and other complex organisms, this network is built of the thin filaments called actin, the thicker tubular filaments called microtubules, and intermediate filaments forming sophisticated molecular machines that are supported by a host of other helper proteins. But the origins of the cytoskeletal proteins themselves stretch far back into the microbial world.

Modern evolutionary biology points to microbes belonging to the Asgard archaea, discovered in extreme environments like deep-ocean sediments, as the closest living relatives of all eukaryotes. These archaea carry proteins resembling those seen today in the cytoskeleton of a modern eukaryote, hinting at an intermediate stage in the evolution of present-day cytoskeletal networks.

In the new work, IISc scientists teamed up with research groups at IISER Pune, NCBS, and NISER, and focused on an Asgard species called Odinarchaeota yellowstonii, named after the Norse god Odin, which was isolated from the Yellowstone Park in the U.S.

They examined two proteins, FtsZ1 and FtsZ2, from the FtsZ family, which are the ancient relatives of tubulin and form the building block of microtubules in modern cells.

Using biochemical analysis and cryo-electron microscopy, the team discovered that the two proteins behave very differently. OdinFtsZ1 forms curved single filaments, similar to those seen in the rings formed in bacteria by FtsZ during cell division. OdinFtsZ2 assembles into stacked spiral rings, giving the appearance of a tubule, a formation that may resemble primitive microtubule‑like structures.

Remarkably, the proteins also anchor themselves to the cell membrane in distinct ways, one directly via a helical tail, and the other using an adaptor protein. This suggests an early form of "division of labor" among structural proteins, foreshadowing the functional specialization seen in modern cytoskeletons.

The modern cytoskeleton's complexity likely arose through gene duplication, specialization, and cooperative interaction among filament systems. The findings show that this process may have already begun in Asgard archaea. These twin proteins could capture a pivotal point where simple filaments diversified into multifunctional networks—a critical step towards the intricate inner framework of eukaryotic cells.

The team now aims to culture Asgard archaea in the lab, enabling direct cell biology experiments. Observing these proteins in living cells could provide unprecedented insight into how early cytoskeletal systems functioned and how they set the stage for the emergence of complex life.

"We believe that these proteins preserve a snapshot of an ancient transition," says Saravanan Palani, Assistant Professor in the Department of Biochemistry, IISc, and corresponding author of the study.

"They connect the threads of history between the simplest microbial filaments and the dynamic scaffolds that sustain all higher organisms."