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.
Manuel Alfonseca
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