The outer arm dynein molecule consists of two heavy chains (tow-headed) in animals as observed in sea urchin and mammalian spermatozoa, while that of protists such as Chlamydomonas and Tetrahymena consists of three heavy chains (three-headed).  It is likely that duplication of a certain gene (OADβ family) would bring about the third heavy chain in protists.  It is possible to judge whether the outer arm is two-headed or three headed by observing cross-sections of flagella or cilia.  Our early investigations revealed that not only Deuterostomia but also Protostomia as well as Mesozoa and choanoflagellate have two-headed outer arm dynein.  Examination of the flagella of an amitochondriate protist, Giardia intestinalis, indicated that it has two-headed outer arms.  Among the Giardia genome, however, there is a pseudogene corresponding to one of the three heavy chains of Chlamydomonas.  It is possible that one of the once duplicated genes of the outer arm dynein in Giardia had secondarily lost as in the case of mitochondria.  Gain or loss of dynein family members is an eventual feature of the dynein history in eukaryotes.  As far as the outer arm dynein is concerned, two-headed structure would be the prototype found in Opistokonta or unikonts, including animals, choanoflagellates and fungi.  On the other hand, in bikonts, such as Archaeplastida including plants and Chlamydomonas, and Alveolata including Tetrahymena and Paramecium, the duplicaton of OADβ family gene took place at a certain point of evolution, resulting in three-headed outer arms.  Recently we have also found that calaxin, Ca²⁺dependent modulator of outer arm dynein, is present only in Opisthokonta.  In contrast a dynein light chain LC4 is a component of outer arm dynein in Archaeplastida and Alveolata.  Thus it is conceivable that a critical event had taken place in the structure of outer arm dynein at the onset of diversificaton.