Evolution And Oxygen

Chapter 1 posed the iconic question of what determined the body plans that we observe in the fossil record and on Earth today. Evolution has created each of the body plans, but what specific factors were involved? Today, we can observe natural selection in action among microbes and among organisms with short generation times. Organisms are adapted to their environments and undergo evolutionary change if changes in their environment, either physical or biotic, affect their survival rates. Increasing temperature or pH are examples of physical environmental changes, while examples of biotic changes might be increased, or a new source of, competition or increased, or a new source of, predation. Evolution can occur in response to new opportunities and resources or as a defense against some new and deleterious condition. If we are to understand why the first animals recorded in the fossil record during the Cambrian period evolved the shapes and morphology that they did, we will have to have an understanding not only of the physical and biological characteristics of their Cambrian environments but of the changes to those environments as well. The history of life subsequent to the Cambrian Explosion can likewise be understood if we have a reasonable understanding of the physical and environmental changes that have occurred since the Cambrian. Evolution of animals can thus be understood as being caused by two different effects: modernization, where body plans increase in fitness through increases in morphological efficiency, and changes in response to environmental change, either physical or biological.

What have been the environmental factors that have changed the most since the Cambrian, thus stimulating evolutionary change? Most stocks of organisms have increased their efficiency of design over time and have found new ways of utilizing new resources, such as the vast stock of plant material on land following the evolution of land plants, and have responded to new types of predation, such as the Mesozoic Marine Revolution, proposed some years ago by University of California biologist Gary Vermeij. His hypothesis supposes that there was a great increase in predation during the Mesozoic Era, compared to the Paleozoic Era, brought about by the evolution of shell-breaking or -boring adaptations in many separate groups of animals. These adap tations caused a resultant evolution toward more shell defense in the prey organisms. But of the various nonbiological factors affecting life, changing oxygen levels were highly significant and the only factor that was global rather than regional.

In addition to changing oxygen levels, the significant nonbiological factors affecting evolution would include global temperature, the chemical makeup of seawater (such as pH and salinity), and the amount of sunlight hitting Earth. Of these, the swings in oxygen levels have been relatively greater than the changes in the others. While global temperatures have swung from relatively hothouse conditions during the Cambrian and Triassic through Eocene to glacial conditions during the Ordovician, early Permian, and Pleistocene, in reality there was never a time where some part of Earth, even during the most extreme temperature swings, did not maintain temperatures not only suitable but also favorable for animal life. Unlike the Snowball Earth (when the planet seemed to have undergone a very cold episode, perhaps freezing the oceans), episodes of 2.7 billion and 0.6 billion years ago did not involve pole-to-pole ice cover; there were still tropics at low latitudes.

In similar fashion, the hottest periods were never so hot as to threaten the existence of animal life. Swings in ocean salinity and pH have been even relatively less extreme. It is only oxygen content that has been both a global phenomenon and a parameter undergoing swings wide enough to affect not only life but also the evolutionary history of life. This factor perhaps has been rivaled only by biotic interactions of competition and predation in producing the makeup of animal body plans and their changes through time.

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