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A recent report from the laboratory of Margaret Dayhoff and her colleagues at the National Institutes of Health ( U.S.) shows that the respiratory protein cytochrome c and the small, iron-containing proteins, the ferredoxins, as well as the 5Sribosomal RNA molecules, all show homologous similarities among monerans and protistans. Of particular interest is the fact that in the eukaryotes the cytochromes are located in the mitochondria and the ferredoxins are obtained from plastids (they are involved in photosynthesis). Similarly other workers have compared the RNA from the plastids of Euglena with moneran RNA and have come up with good evidence of homology. All these molecular studies are based on point-topoint similarities of amino acid sequences in proteins and of nucleotide sequences in RNA.

This kind of evidence strongly supports what is now called the serial endosymbiont theory of protistan origins. This theory was first advanced in the nineteenth century and given its modern form by Lynn Margulis, of Boston University, in 1970. This theory argues that protists are symbiotic associations of monerans. Or, stated more exactly, prokaryotes enter into cooperative associations to form eukaryotes. Margulis and others differ somewhat on the exact details of how this occurred, and those differences are being investigated. Margulis proposes that plastids, mitochondria, and the microtubular structures of eukaryotes are derived from different prokaryotes. For example, a host cell engulfed a blue-green alga that evolved into a plastid. An engulfed aerobic prokaryote could have provided a mitochondrion. And association with a motile prokaryote like a spirochaete could have given rise to flagella, kinetosomes, and other microtubular elements, in short, the primoridal kinetide. In fact, Margulis has proposed that a colorless ameboid prokaryote was the host cell and phagocytosed other prokaryotes of the sort just mentioned. This point of view suggests that protophyta and protozoa, both, are products of serial endosymbiosis.

For reasons given above regarding the homologies between protozoa and colorless protophyta, it seems unnecessary to suggest that the protozoa originated by endosymbiosis. But the suggestion that the protophyta originated by endosymbiosis is attractive and is supported by the molecular evidence given above.