The common ancestry of eukaryotes, archaebacteria and eubacteria is well demonstrated by amino acid sequence comparisons of numerous proteins that are common to all three groups. On the other hand, there are a few proteins, like ubiquitin, that are common to eukaryotes and archaebacteria and which have yet to be observed in eubacteria. Some proteins appear to be wholly restricted to eukaryotes; this is especially true of cytoskeletal proteins. Recently, actin has been found by crystallography to be homologous with an ATP-binding domain found in a heat shock protein and several other proteins common to all three urkingdoms. This observation is puzzling on several counts. Most cytoskeletal proteins like actin and tubulin are very slow changing and must have been so for a very long time. How is it, then, that no sequence resemblance can be discerned with their alledged prokaryotic antecedents? The question is addressed by considering two bacterial fts proteins which appear to be related to actin, on the one hand, and tubulin, on the other. One answer may be that the rate of change of these proteins changed dramatically at a key point in their history. Another possibility is that eukaryotes are much older than some of their other proteins indicate.