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| 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.
