One of the most exciting and enduring obscurities of biology lies in the early stages of the evolution of “our” eukaryotic cells (Figure 1). The endosymbiotic theory accounts well for the present existence of the mitochondrial and chloroplast organelles of eukaryotes. Although there is evidence for present day inter-bacterial endosymbiosis (also see here and here), the details of the route leading to the establishment of organelles remain enigmatic.
Even murkier is the question regarding the origin of the nucleus (Figure 2). While prokaryotic cells with organelles arguably exist and scientists have identified Planctomycetes that enclose their DNA with internal membrane continuous with the cell membrane, a truly independent membrane-bound nucleus prevails as the defining hallmark of the eukaryotes. (A recent report even calls this textbook difference into question! However, another study calls these conclusions into question.)
Unlike the clear bacterial origin of the mitochondria and chloroplast, it turns out that no single existing model has received broad acceptance to explain the existence of the nucleus. Scientists have speculated that it could have arisen from an endosymbiotic event between an archaeon and a bacterium (though who engulfed whom is also uncertain). Some have proposed that an autonomous internal gathering of cell membrane as in Planctomycetes formed the eventual independent nuclear organelle, and yet others have even suggested infection by an enveloped virus as playing a role.
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