In the Permian, we observed that the radiation of dicynodont herbivores spurred a radiation of carnivores. In a similar vein, a whole series of carnivores radiate with these new grassland herbivores. So a lot of exciting creatures, including our state animal, Smilodon, appeared. Neither of these is Smilodon. Being a sabertooth tiger is sort of a general morphology, a response by several cat-like lineages of pre-existing carnivores to a new food resource. California does not have a unique mascot.
The last climatic insult, the most recent climatic insult, that makes the world as we know it is the Pleistocene, when ice sheets spread, particularly in the Northern Hemisphere. Again, this refrigeration reflects alterations in oceanic currents caused by continental motions. It most affects the Northern Hemisphere since most land is in the Northern Hemisphere. It is this last cold period that sets up the present day ecosystems that we are familiar with.
We can look at the biomes of the world in the present day and see them as the product, either on a global scale or on a continental scale, of a long series of antecedent processes.They are the products of historical accident. Every one of these antecedent processes ultimately tracks its way back to plants - their evolution and their response to climatic change. Plants are the things that we define biomes by. Plants are what animals live with. If we look at the total history of land plants from the Silurian and Devonian to the present day, we find that we live in the most diverse flora that has ever existed. It has been assembled over a long period of time, through successive experiments. If we look at tetrapods on the one hand and insects on the other in terms of the history of their diversity we see that the present day is also the most diverse. Through time,
ecosystems have been becoming increasingly more complex. All of this has occasionally been overridden by extinction.
I pointed out that we passed through two major extinctions, the Permo-Triassic, and the Cretaceous Tertiary. Even in the face of these "disasters", diversity has continued to go up. The history of life is one of layers of change. I don't think extinction is necessarily "bad". It is a fact of life. I think our human problem with extinction is that, first, we are the animals that are most likely to drive ourselves extinct and we have a vested interest in that. And second, on a much more philosophical basis, when I started off I pointed out that we can comprehend the entire history of terrestrial life that I've just laid out before you. My personal feeling is that this understanding carries with it the responsibility to respect the product of this history, and not to treat it as our selfish plaything. We should not be afraid of extinction, but we should not allow ourselves to be its source. Thank you.