Many of you may be familiar with the term uniformitarianism, a geological term that says that the present is the key to the past. It states that we interpret what went on in the past by looking at present observations and extrapolating backward in time. Uniformitarianism is extremely useful if we focus our attention upon physical and chemical processes. These are the kinds of processes that influence life in the way that Karl talked about. Chemical reactions occur in the past as in the present; physical constraints act on organisms in the past as in the present. However, if we apply uniformitarianism long enough, we begin to think that the past is the present. That the two are exactly the same. Then we fall into a trap. We look through our knowledge of the present world and we take this knowledge back in time and we think, oh the Mesozoic was just like today except they had dinosaurs instead of mammals as the dominant group. The Paleozoic was just like present day except it had amphibians instead of dinosaurs.

But this is incorrect. While the physical and chemical constraints and reactions have remained constant, their products have changed. The planet's life has not only evolved as individual lineages of organisms, it has also evolved as whole ecosystems. Again, we are going to focus on the terrestrial environment. That means a fairly limited period of time, because the terrestrial environment is probably inhabited in a serious manner only for about the last 400-450 million years The Earth, on the other hand, is approximately 4.5 billion years old. The oldest evidence for life is, depending on who you wish to hold a debate with, somewhere between 3.5 and 3.8 billion years old.

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