Macrointerval B: The Second Step; The Birth of Animals (the Metazoans)

In the previous section, we described a scenario that allows us to explain the emergence of the eukaryotic cell as a symbiotic association between prokaryotes. The new organism is not better or more evolved than the previous ones, but represents a higher level of complexity achieved by the different distribution of activities among distinct centers within the cell. For example, the mitochondrion is entrusted with energy production, being the organelle exempt from other major tasks. So, what will be the next step? It is possible to recognize, in the kingdom of life, an increase in complexity by considering step-by-step the biological entities obtained from the combination of those from the previous level. Let us explain this concept better: we have moved from level I, that of prokaryotes, to level II (the eukaryotic cell), by combining elements of the previous level (prokaryotic cells united in a symbiotic association). In the same way, it is possible to move to a further level, level III, by combining various elements of the previous level II, and so on [76]. This trend has already been rigorously suggested by Daniel W. McShea and Jean-Pierre Rospars [93]. For us, therefore, the next step will be sought in multicellular eukaryotes and, in particular, in the animal kingdom. In fact, in no other eukaryotic group or kingdom is it possible to encounter “intelligent” activities as in that of animals. Plants, evolutionarily speaking, while not at all inferior to their animal cousins, have chosen different solutions more suitable to their existence as immobile beings “rooted” in the soil. The same goes for fungi and other groups of eukaryotes.