This may, of course, sound like some freaked out theory, but as I said,
the similarity between, on one hand, proteobacteria and mitochondria, and
on the other, cyanobacteria and chloroplasts is striking.
And, we even have more evidence.
I said previously that all DNA in the eukaryotic cell was contained in
the nucleus, but it would be more correct to say almost all DNA.
Actually, we find a little DNA in two other organelles,
which are the mitochondria and the chloroplasts.
And if you sequence this DNA and compare it with DNA from other organisms,
I guess you can figure out where we find the greatest resemblance.
Mitochondrial DNA is very close to proteobacterial DNA,
whereas the chloroplast DNA resembles the DNA of the cyanobacteria.
So in other words, we have pretty good evidence supporting that cell
evolution through endosymbiosis actually took place.
And in fact, we can even show that it wasn't just a single event.
It has happened many times.