That plastids were once free-living cyanobacteria is now taken for granted by many, and for good reasons, for there is a wealth of data – in particular from the comparison of plastid and cyanobacterial genomes – that support this view. There is currently no seriously entertained alternative hypothesis to the view that plastids descend from cyanobacteria. But that was not always the case. Well into the 1970s there was a generally favoured alternative hypothesis, namely that early in evolution plastids arose de novo from within a non-plastid bearing cell (an autogenous origin) rather than through invasion by a cyanobacterium into a non-plastid-bearing cell with subsequent intracellular coexistence and reduction to an organelle (an endosymbiotic origin). Interestingly, the shift from autogenous to endosymbiotic hypotheses during the 1970s was a reversal of state for during the first two decades of this century, the endosymbiont hypothesis for the origins of plastids (and mitochondria, which will not be further discussed here) was very popular among biologists. It fell into disfavour shortly after the First World War, for reasons that are very difficult to summarize briefly, and remained scorned for 50 years (see Sapp, 1994, for an historical account in English, and Höxtermann, 1998, for a succinct historical account in German). So where did the first version of the endosymbiont hypothesis come from? In a nutshell, it came from Konstantin Sergejewiz Merezkovskij (usually written as Constantin Mereschkowsky), a Russian botanist of little standing who worked at a rather small and by no means prominent university in Kasan and who published a very remarkable paper in 1905. We are not aware of any true precedent for his paper, which draws upon three lines of evidence known at the time.
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