Apparently, the rate of the evolution of life accelerated in the [[Cambrian]] period (542-488 million years ago). The sudden origin of many new species, [[phylum|phyla]], and forms in this period is called the Cambrian Explosion. The biological formenting in the Cambrian Explosion was unpreceded before and since that time. Whereas the Ediacaran life forms appear yet primitive and not easy to put in any modern group, at the end of the Cambrian most modern phyla were already present. Because of the development of hard body parts such as shells, [[skeleton]]s or [[exoskeleton]]s in animals like [[mollusc]]s, [[echinoderm]]s, [[crinoid]]s and [[arthropod]]s (a well-known group of arthropods from the lower Paleozoic are the [[trilobite]]s) made the preservation and [[fossilisation]] of such life forms easier than those of their Proterozoic ancestors. For this reason, much more is known about life in and after the Cambrian than about that of older periods. The boundary between the Cambrian and [[Ordovician]] (the following period, 488-444 million years ago) is characterized by a large mass-extinction, in which some of the new groups disappeared altogether. Some of these Cambrian groups appear complex but totally different compared to modern life, examples are "[[Anomalocaris]]" and "[[Haikouichthys]]".

During the Cambrian, the first [[vertebrate]] animals, among them the first [[fishes]], had appeared. A creature that could have been the ancestor of the fishes or was probably closely related to it, was "[[Pikaia]]". It had a primitive [[notochord]], a structure that could have developed into a [[vertebral column]] later. The first fishes with [[jaw]]s ([[Gnathostomata]]) appeared during the Ordovician. The colonisation of new [[Ecological niche|niche]]s resulted in gigantic body sizes. In this way, fishes with increasing sizes evolved in the course of the early Paleozoic, such as the gigantic [[placoderm]] "[[Dunkleosteus]]", which could become 7 meters long.

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अलीकडील घटना
Proterozoic eon