5. The Evolution of Metazoa

5.0 Introduction

The evolutive direction taken by the metazoa was improvements of energy utilization and refined perception of the internal as well as external world by the endocrine and nervous systems. The development of a nervous system with the accumulation of a … Continue reading

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5.1 The Phyletic Lines

  Figure 5.1. Geologic time chart. Ice Ages occurred 600 million years ago, 280 million years ago and 1 to 2 million years ago. They define the 3 geological great eras. Each era is subdivided in periods, themselves divided in … Continue reading

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5.2 Land Invasion by Chordates

The appearance of representatives of the chordate phylum has long been assumed to occur about 475 million years ago, during the Ordovician period, i.e. late in evolutive times. Finds in China (1999) show that at least two species of 2 … Continue reading

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5.3 The Amphibians

A period of severe drought prevailed during the Devonian period. This was precisely the time when the crossopterygian fishes were around, and their ability to use atmospheric air favored them. The lung was probably developed by bony fishes because they … Continue reading

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5.4 The Reptiles

Amphibians lay eggs in water. The development and hatching of these eggs depend on constant humidity and an optimal temperature. The hatched animal is still an embryo that can only live in water. Reptiles, on the other hand, protect their … Continue reading

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5.5 The Great Dying

Several mass extinctions have been recorded in geologic times, of which 5 were of momentous proportions. Two of these extinctions, i.e. the late Devonian 364 million years ago and the end Triassic of 200 million years ago lost the diversity … Continue reading

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5.6 The Mammals

The asteroid impact that wiped out the dinosaurs 65 million years ago allowed the mammals to inherit the earth. The emergence of the mammals was not due to the fact that they had a superior intelligence and were able to … Continue reading

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5.7 The Primates

The primitive insectivorous eutherian stock possessing as many as 44 teeth gave rise to six types of insectivores. Three lines specialized as gnawing animals and were later eliminated by the prolific rodents (rats, mice) that competed for the same ecological … Continue reading

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5.8 Contemporary Climate Stress

There are reasons to believe that a climate stress is on its way at the present time. Lakes disappear, rivers run dry and deserts appear. An accentuation of this drying will again wipe out large tropical homeotherms such as hippopotamuses, … Continue reading

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