How old is evolution? How conservative? Try this: the proteins that an amoeba (ameba) uses to move are essentially the same as those that make up the muscle fibres in a vertebrate such as ourselves.
Though it appears to have nothing in common with muscular movement, ameboid movement probably depends on contractile components and mechanisms surprisingly similar to those in the muscle cells of animals. …[T]he cytoplasm of an ameba is found to contain thick and thin microfilaments similar in appearance and dimensions to thick (myosin) and thin (actin) microfilaments of striated muscle. Extracts of muscle myosin appear to cross-react with the thin filaments from amebas. And cytoplasm from amebas uses the nucleotide adenosine triposphate (ATP) as an energy source for movement, as muscle does.
From Living Invertebrates, page 27 (chapter on “Protozoans”), by Vicki Pearse, John Pearse, Mildred Buschbaum, & Ralph Buschbaum.