Cambrian
From 570 to 500 million years ago
The Paleozoic Era ("of ancient life") begins about 570 million years ago; the period with which the era opens is called Cambrian. At that point in the story of life "the game is started". In the seas there are already multicellular vegetable organisms (capable of carrying out photosynthesis: they are algae) and multicellular organisms capable of eating them and eating each other (animals). The transition from unicellular organization (the cell is the whole individual) to the multicellular one (the cell is part of the individual) has taken place several times. For plants, perhaps, each large group of algae had an independent origin from a certain type of single-celled aggregation. The majority of scientists believe that the passage, for animals, has instead occurred only twice.
Once there were sponges, animals in which the numerous cells are not actually organized. They do not form tissues with well-defined characteristics or real organs (structures capable of performing a given function). In a distinct phase there was the transition to the common species of all the other animals (called eumetazoi). The transition to multicellular organization occurred only for eukaryotic cells (with defined nucleus).
Prokaryotic cells (without a defined nucleus) have always remained in the unicellular stage: these are blue algae and bacteria, organisms that still esist today. Bacteria are the "eating" prokaryotes (they are not photosynthesizers); they, by eating them, "dismantle" the most complex organisms: unicellular with true nucleus I (protists) and multicellular (both plant and animal). This "disassembly", due to the metabolic activity of bacteria, makes substances (molecules) simpler again available which can be "reassembled" by photosynthesizing organisms (blue and vegetable algae). Continental drift is now accepted as a fact but for a long time the theory has been opposed, in a cultural context that had been so reluctant to accept the changing of species. It was really difficult to even propose a continuous change of what appears more than solid. Among other things, it is worth remembering that geology (the study of the earth) has received considerable help from paleontology. The discovery of practically identical fossils on continents now separated by oceans has provided proof that, in the distant past, the continents in question were united and hosted the same fauna and flora. Evident traces of the drift movements occur on the continents themselves When in fact the clods come to "collide" against each other the edges rise and mountains are formed. Of the mountains that formed a long time ago, few traces remain but they are unmistakable: they appear as crumpled layers just like a pile of newspapers flattened on the sides.
