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.