land, let alone evolved into trees and forests. With so much written by so many, what more can be said? With new and better estimates of the oxygen levels back then, the topic is ripe for reexamination.

OXYGEN AND THE CAMBRIAN EXPLOSION

The levels of carbon dioxide and oxygen postulated for the Cambrian Explosion interval are shown in the figure beginning this chapter, taken from the Berner curve introduced in the preceding chapter. According to this curve, oxygen levels soon after the start of the Cambrian Explosion 544 million years ago were about 13 percent (compared to 21 percent today) but then fluctuated. During this time carbon dioxide levels were far higher than they are today, tens of times higher in fact, and such high levels would have produced a greenhouse effect. Even with the drop in carbon dioxide levels at the end of the Cambrian around 495 million years ago, temperatures at this time would have been higher than in the present-day. Since less oxygen is dissolved in seawater with higher temperature, the already anoxic conditions of the oceans, due to the low atmospheric oxygen of that time, would have been exacerbated.

A CAMBRIAN DIVE

To explain the conditions, flora, and fauna of the Cambrian, imagine that we have journeyed back in time to Earth circa 522 million years ago. To really do this right we need not only a time machine but also a spaceship, in order to better see the position of the continents, for continental position and geological processes accruing from plate tectonic processes had a determining effect on subsequent biotic history.

The first thing we notice as we pass over the land surface is that there is so little vegetation. Low traces of green can be seen in the wetter areas but most of the land surface is bare rock. It looks like the areas around glaciers at high altitudes in our present-day world, but even as we cross the equatorial regions the starkness of the place is apparent. Plant life is limited to moss and vast slicks of plant-like, photosynthesizing bacteria. There is no rich organic soil. There are no trees, no



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement