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It is thought that the Cambrian explosion lasted for between 13 and 25 million years, and it was during this time that the majority of animal phyla (pretty well all animal forms) began appearing in the fossil record.
There are various theories about what prompted this rapid acceleration in evolution. It could have been the 2 billion years of photosynthesis which preceded it, and the resultant rise in the level of oxygen in the atmosphere. It could have been the development, around that time, of the ozone layer which protects life on earth from biologically lethal levels of UV radiation. There is also a theory called Snowball Earth which is less in favour nowadays than it has been. Various other theories concentrate on different phenomena of the day. It always amazes me how much information can be extracted from old rocks. Obviously there is a lot more to it than that, and many people have many differing views on what actually took place and when.
The very idea of evolution and particularly the complexity it has produced is still sometimes a hard concept to grasp (for me anyway), but harder than that is the origin of life itself. A lightning strike on a hot puddle of chemicals seems to be the best guess at this point, or perhaps something more gradual - but at some point an inanimate group of molecules must have (been) transformed into a group of molecules capable of reproducing themselves. Bada bing! Life!
And since that point, about 4 billion years ago, things have been moving along nicely. I have to say - I find these timescales quite difficult to comprehend. However you look at it, it's a very long time.
In order to make geological timescales easier to understand it can often be useful to compare a time in history with the relative point in time in the last twenty four hours. So the Cambrian explosion lasted roughly 20 million years, and the industrial period which has caused the current bout of global warming has been going for about 200 years - give or take a bit. If the Cambrian explosion lasted twenty four hours, the industrial age in which we now live started less than a second ago. You can probably see where I'm going with this: In geological terms, an event which lasted 20 million years has been described as an explosion. How then, shall we describe an event which is, so far, a hundred thousand times faster than that?
And even if we could describe it - how would we propose to control it, or even predict what its longer term effects might be? The human race has brought about such a rapid change in the composition of the troposphere, and the systems within it, that any attempt now to control the backlash might be futile. Or can it be fixed as quickly as it was broken?
