The Paleozoic era lasted from about 538.8 million years to 251.9 million years ago ⁸⁾. It corresponds to approximatelly half of the Phanerozoic which is the histroy of the earth since the precambrian to today ^{4), 7)}.

The Paleozoic era is divided into six geological periods, Cambrian, Ordovician, Silurina, Devonian, Carboniferous (in the US, Mississipian and Pennsylvanian), Permian.

During the Paleozoic era, almost all ancestors of the extant animals emerged with the explosive diversification of multicellular organisms. At the beginning, gigantic continent separated to large one and smaller continents. Then, the life landed on, some insects flied the sky. At the ending, the continents assembled again into supercontinent, about 90 % of species extinceted and the Paleozoic era closed ⁷⁾.

There is an idea that the increasing of dissolved oxygen level had accelerated the explosive diversification of the animals at the early Paleozoic era ^{1), 5)}.

The Ediacaran animals, that is, the animals before the Cambrian had been grazing on the microbes or directly been getting organic detritus from the sediments, they had no need to move around. There were no predator-prey relationship yet ¹⁾.

Cyanobacteria which inventated the photosynthesis released the oxygen as wastes, then the oxygen accumlated into the water and reached up to several percents of modern oxygen level, although it is the level that modern fishes would faint and die soon. Afterwards, the animals which get enough nourishment by eating other animals as preys, namely, predators appeared. Predator-prey relationship has emerged in the ecosystem of Earth ^{1), 5)}.

Both predators and preys require locomotor systems and precise infomation processing to control them, which consume a huge amount of energy. Adequate energy production by respiration requires a certain level of oxygen. Research using modern oxygen minimum zones as an analog for Proterozoic oceans shows that more complex food webs requires higher level of oxygen ⁶⁾. The advent of carnivores accelerated the arms races.

Animals invented the body plan which guanrantee their predatory functions. Joints and muscles to capture other animals as preys, nerve system with central to enable cooperative actions, sense organs to provide the information of outer world to central nervous system are developed. Such sensors and nerveus system are assembled into head for efficiency. Mouth-opening placed on the head, gut arranged along the body axis, anus as another opening separated from mouth to defecate while eating or moving, all of these are efficient design for predators ³⁾.

This is a invention of bilateral symmetry that is extremely common body plan in the extant animals.

The morphogenesis of these Bilaterian animals would have not enable to accomplish even though the environmental or ecological factors, such as oxygen level change, sea level change, mineral concentration change and so on, have been fulfilled the requirements. They would required a genetic network in place to build such stuctures in their body ³⁾. But, it would have not be so difficult. That is, there were no need to create a number of new genes.

Ancestor type of the homeobox gene family which plays a critical role in the morphogenesis of modern multicellular organims already had arose before opening of the Paleozoic era ^{2), 3)}. These regulatory genes groups had been duplicated, had mutated, and co-opted for novel functions, then had enabled the animals to develop diverse morphology ^{2), 3)}. The weird wonder animals of the Cambrian share a fundamental body plan with modern animals, such as bilateral symmetry, segmentated body, digestive tract with mouth and anus, and sense organs concentrated to anterior end of body.