The seven kingdoms

Molecular data did allow us to reconstruct the primordial dichotomies that gave origin to eubacteria and archaebacteria (Figure 6-1 and 6-2), but so far have not revealed much about the other stages of cellular evolution. The eukaryotic cell is such a complex labyrinth that it is very hard to understand how its pieces were put together. It is even possible that its structures were so thoroughlly mixed that historical traces have been lost forever. It is also true, however, that molecular phylogeny is still a young science, and we cannot exclude that one day it may have some surprises in store for us.
For the time being, however, we can reconstruct only a few great events of cellular evolution on the basis of clearly visible morphological changes. More precisely, after the first three stages of cellular evolution we can clearly recognize only four other great stages (Figure 6-3):
(a) the appearence of a nucleus in some paleocells gave origin to paleokaryotes (step 4),
(b) some paleokaryotes acquired mitochondria and chloroplasts by symbiosis and generated the first protists (step 5),
(c) some protists went on developing new eukaryotic characters, and a few even started experimenting with multicellularity (step 6),
(d) some experiments in multicellularity had success and gave origin to the first progenitors of plants, fungi and animals (step 7).
The sequence of these steps gives us a reasonably accurate reconstruction, not only because any step needs the previous one as a biological prerequisite, but also because the whole sequence is in good agreement with the data from paleontology. We conclude therefore that there have been at least seven great steps in cellular evolution, and that such steps gave origin to the highest taxonomic groups that we call kingdoms (Figure 6-3).
It must be underlined that today there still is no general agreement not only about the names, but also about the number of the kingdoms. Some, for example, do not accept that paleokaryota (cells that did not acquire organelles by symbiosis) represent a kingdom.