The question of the evolution of eukaryotic cells from prokaryotic ones has long been a topic of heated discussion in the scientific literature. It is generally thought that eukaryotes arose by some prokaryotic cells being engulfed and assimilated by other prokaryotic cells. Called endosymbiotic theory, there is some empirical basis for this. For example, mitochondria contain a single circular genome, carry out transcription and translation within its compartment, use bacteria-like enzymes/components, and replicate independently of host cell division and in a manner akin to bacterial binary fission.
Despite such evidence, however, when assessing the causal sufficiency of unguided processes, they — predictably — come up short. After all, it is all-too-easy to lapse into a long-discredited Lamarckian “inheritance-of-acquired-characteristics” mentality. It is important to bear in mind that, even if a cooperative assemblage of prokaryotes did by some fluke of luck arise, such an arrangement is of no evolutionary significance unless there is a genetic basis to ensure its propagation.
A second problem with this scenario is that mitochondria use a slight variation on the conventional genetic code (for example, the codon UGA is a stop codon in the conventional code, but encodes for Tryptophan in mitochondria). This implicates that the genes of the ingested prokaryotes would need to have been recoded on their way to the nucleus. The situation becomes even worse when one considers that, in eukaryotic cells, a mitochondrial protein is coded with an extra length of polypeptide which acts as a “tag” to ensure that the relevant protein is recognised as being mitochondrial and dispatched accordingly. The significant number of specific co-ordinated modifications which would be required to facilitate such a transition, therefore, arguably make it exhibitive of irreducible complexity. Read More>>>