Oxygen And The Rise Of Humans

The change in oxygen levels over time seems to have provoked major changes in evolution. What about one of the biggest of all changes and the most important to us? Can any aspect of the changing atmosphere be interpreted in a new way so as to explain the evolution of our own species? In one sense it can. Johns Hopkins paleontologist Steve Stanley has dubbed us "Children of the Ice Ages," and many paleoanthro-pologists think that the rise of high intelligence and culture was a means of dealing with the challenges imposed by the glaciation of the past 2 million years, the recent ice ages. But what about oxygen levels? That angle has never been examined.

Almost all models of the atmosphere for the past 10 million years indicate that oxygen levels were higher than now, with atmospheric oxygen levels as high as 28 percent even 5 million years ago. Could it be that higher oxygen levels stimulated, or made easier, the transition to larger brains? Nervous system tissue needs more oxygen than any other kind of tissue, and the rapid enlargement of brain volume perhaps outstripped the circulation system in the brain necessary to sustain all the new neurons. Higher oxygen would give some cushion for error, it seems to me. But this is still sheer speculation.

THE END OF HISTORY?

We have now finished our journey through time. The 540-million-year trip is over with this chapter. We have seen evidence of major changes in oxygen levels through time. Will oxygen levels continue to change in the future? That is the subject of the final chapter.

SHOULD WE FEATURE?

e have come to the end of history, if history can be counted as something that has already happened. In this last chapter, let's gaze into the future. In keeping with the rest of this book, we will simply ask: can oxygen levels be expected to stay the same, or will they undergo wild swings as they have for the past 540 million years?

The future stretches before us not as one long dark tunnel but as a series of vignettes of variable clarity, like a long avenue punctuated by streetlights of differing luminosity. This century and at least the next will continue to be a time of warming from greenhouse gases produced by humanity. It would be nice if the volcanoes of our world would politely stop outgassing carbon dioxide as well, at least until we humans get our act together and curtail our carbon dioxide production. But the volcanoes just keep spewing this gas into the air, as they have since Earth began. It is our addition to that natural input that is the problem.

We are warming our world, rapidly. A hundred years from now the planet will have returned to its atmospheric condition during the Late Cretaceous through Eocene. Happily, those were times when oxygen was at about its present level, or was even slightly higher. But can we foresee a time farther in the future, when oxygen levels might change? If they do, will they be higher or lower? The fate of oxygen is fixed by the rates at which organic compounds and sulfur-containing compounds and minerals (such as pyrite) are buried, or not, and the rate at which they are weathered or not. Many things control burial and weathering rates. One of these is continental position, and it is this that can best be predicted for the future.

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