During the long history of the evolution of life on earth, boundaries between geologic eras represent time of transition, when character and composition of the biosphere changed more rapidly and markedly than at intersystemic boundaries within eras. Although many causes have been suggested to explain these changes, most workers have attempted to find one common cause for all. More detailed analysis of the nature of the biologic changes that occur at era boundaries, however, support the conclusion that a common cause does not exist.

At the transition from the Precambrian to the Paleozoic Era, life changed from a little-known state in the Precambrian to a state that at least in part resembled life as we know it now. The emergence of Paleozoic life took place during Cambrian and Early Ordovician time, a span of 100 million years. At the Paleozoic-Mesozoic boundary, mass extinctions were common among benthonic marine life. Life on land, both plant and animal, was not so affected. After the mass extinctions it took 20-30 million years until a well-balanced and diversified benthonic fauna was reestablished in the seas.

At the Mesozoic-Cenozoic boundary, mass extinctions were widespread among terrestrial animals, especially reptiles, but not among plants. In the sea spectacular extinctions occurred among planktonic organisms such as foraminifers and coccoliths, but not nearly to the same extent among benthonic organisms, including benthonic foraminifers. On the land it took many millions of years to fill the ecologic niches left vacant by the disappearance of the reptiles. In the sea replacement of the extinct forms of planktonic organisms by other types was almost instantaneous.

It seems that the biologic changes that took place at and around the three era boundaries were so different in kind that no single cause can be invoked to explain them all.

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