|The tree of life|
Since Aristotle, living beings have been classified in kingdoms. At first there were two: plants, practically unmoving, and animals, capable of active movement.
When Antony van Leeuwenhoek discovered microorganisms, biologists tried to maintain this two-fold division, integrating some with animals (amoebas and Paramecium), others with plants (bacteria and microscopic algae and fungi). But at that level, the separation between animals and plants is blurry, and in the mid twentieth century a third kingdom was added to the other two: protists, unicellular living beings.
A little later, biologists came to the conclusion that the kingdom of plants should be divided into two: fungi at one side, all the other plats (metaphyta) at the other. By 1975, therefore, there were four different kingdoms.
In the eighties, a new way of classifying living beings was proposed: cladistics. The emphasis on molecular biology and DNA analysis changed how taxonomies were interpreted, making them subdivisions of the tree of life, a graphical depiction of evolution.
Some biologists have pushed the cladistic ideas to the extreme, requiring that every biological taxon must be a strict sub-tree of the tree of life (or a branch of the tree). This restriction, which at first light appears reasonable, would have serious consequences:
- Many classic taxons of animals and plants would disappear: all those which gave rise to another group at the same taxonomic level. For instance, bony fishes (because they are ancestors of terrestrial vertebrates); amphibians (ancestors of reptiles); reptiles (ancestors of birds and mammals). In strict cladistic classifications, those three groups would find no place.
- On the other hand, if animals, metaphyta and fungi are accepted as kingdoms, any other group of living beings located in independent branches of the tree of life that appeared before those kingdoms would also have to be a kingdom. There would thus be over fifty kingdoms of living beings, most of which one-celled, some with just a few species.
- Finally, fungi (and probably metaphyta) would be decomposed into several kingdoms, because they would be polyphyletic. The meaning of kingdom as a large subset of life would thus be lost.
A fundamental datum of evolution is forgotten here: the fact that life has gone through several critical thresholds which give rise to new levels of complexity. Passing through a critical threshold should be equivalent to changing kingdoms, whether the result is a sub-tree or not. In fact, usually it won’t be. If we apply this criterion, living beings would be classified into five kingdoms:
- Prokaryote (one-celled with no nuclei), further divided into two sub-kingdoms: archaea and bacteria.
- Eukaryote (one-celled with nuclei), which appeared from the symbiosis (common life) of bacteria and archaea.
- Fungi, mostly unmoving multi-cellular beings with no photosynthesis, which came from the symbiosis of a group of eukaryote.
- Metaphyta, mostly unmoving multi-cellular beings with photosynthesis, which came from the symbiosis of another group of eukaryote.
- Animals (or metazoa), mostly moving multi-cellular beings with no photosynthesis, which came from the symbiosis of a third group of eukaryote.
If the rules of the strictest cladists were applied, only the last would be considered a kingdom, and even this would be doubtful, as it could be polyphyletic, if sponges had an independent origin.