But we mammals did survive. The characteristics we now equate with “mammalness” are the consequence of these near-disasters during times of low-oxygen atmosphere, and also from times when oxygen was far higher than now. For most of our history we were the size of rats, sluggishly gasping in a low-oxygen world as we avoided the active dinosaurian overlords.

The thesis of the pages to come is deceptively simple: The history of atmospheric (and hence oceanic) oxygen levels through time has been the most important factor in determining the nature of animal life on Earth—its morphology and basic body plans, physiology, evolutionary history, and diversity. This hypothesis means that the level of oxygen influenced every large-scale evolutionary adaptation or innovation that is the history of animal life on Earth, that oxygen levels dictated evolutionary originations, extinctions, and the architecture of animal body plans. Support for this hypothesis will make up the chapters that follow.


We begin a very brief version of the standard history of animal life on Earth with Charles Darwin. Darwin hoped that the history written in the fossil record would sooner or later support his contention about his then (1859) new theory of evolution: that change came about in small increments. In his On the Origin of Species he wrote:

From the beginning of life on earth there was a connection so close and intimate that, if the entire record could be obtained, a perfect chain of life from the lowest organism to the highest would be obtained.

But this was hope, not history. A new understanding came a half-century later from Charles Wolcott, discoverer of the Burgess Shale, a Canadian rock deposit that gives our best look at the nature of what is known as the Cambrian Explosion, a short time more than 500 million years ago when most animal body plans rather suddenly appeared in the fossil record:

In early times the Cephalopoda ruled, later on the Crustacea came to the fore, then probably fishes took the lead but were speedily out powered by the Saurians, the Land and Sea Reptiles then prevailed until Mammalia appeared upon the scene when it doubtless became a struggle for supremacy until Man was created.

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