Until recently it was widely accepted that the dynamic cytoskeletal matrix is exclusive
to the cytoplasm of eukaryotes, evolving before the emergence of the cell nucleus
to enable phagocytosis, cell motility and the sophisticated functioning of the endomembrane
system within the cytosol. The discovery of the existence of a prokaryotic cytoskeleton
has changed this picture significantly. As a result, the idea has taken shape that
the appearance of actin occurred in the very first cell; therefore, the emergence
of microfilaments precedes that of the eukaryotic cytoskeleton. The discovery of nuclear
actin opened new perspective on the field, suggesting that the nuclear activities
of actin reflect the functions of primordial actin-like proteins. In this paper, we
review the recent literature to explore the evolutionary origin of nuclear actin.
We conclude that both ancient and eukaryotic features of the actin world can be detected
in the nucleus today, which supports the idea that the cytoskeleton attained significant
eukaryotic innovations before the tandem evolution of the cytoskeleton and nucleus
occurred.
