So, although I suspect that the photosynthetic radiation has gone up over time, it's very difficult to get an idea of what's happened with UV on the face of the earth. However, we do also know that the oxygen, the amount of oxygen in the atmosphere has gone from essentially zero during that time up to modern levels way up here. And when you don't have oxygen, you don't have an ozone shield. As I always like to say, you think we've got ozone problems now. The Earth was one of giant ozone holes in this period of time. So organisms should have been very severely affected.

I should point out, although this isn't actually related to UV, that at the same time we think that there's been quite a change of carbon dioxide in the atmosphere. The reason is, if you only have 71% of the current level of radiation back there, you would imagine the earth would be frozen over. But there isn't good evidence that the Earth was frozen over during this period of time. So there had to be something in the atmosphere that acted as a greenhouse gas to keep the earth from freezing. One of the major components that people have settled on is carbon dioxide. Modelers also thought of some other things. So, the world has changed in a very major way, major environmental ways over time. There are all sorts of other environmental changes that occurred but here are two major ones that people don't tend to think about.

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