Quite a good overview of evolution, in my opinion, considering the rather short length of the chapter. Here are some points of interest I found:
1. The definition of "life".
Life is generally characterised as the ability to reproduce and use energy. Of course, then we are left with all kinds of problems concerning things which we do not normally think of as alive but which nonetheless seem to have these qualities.
For instance, is fire alive? It certainly replicates itself (ever seen a fire spread?), and it also seems to acquire and use energy, grow, and even move. Could we consider fire a living thing? I think that perhaps fire can give us an idea of what "life" could be like for non carbon-based or non DNA-based lifeforms that may exist elsewhere in the universe.
Then there is Scott's own example of a virus. It seems to be non-life until it becomes parasitic upon something already alive. Is it non-life up until it attaches to a living cell and replicates, or is it somehow in-between, or what?
As with Mad's discussion of the concept of "species", I think the boundary between life and non-life is arguably quite arbitrary. Because life almost certainly arose from non-life, we have the same sort of gradation that we see in speciation. With species it becomes nearly impossible to determine when speciation occurs--you can only tell at the extreme boundaries of a lineage. For example, the genetic differences between a parent and its offspring are minimal--but the differences between a parent and its great-great-great-great-great-great-great-great offspring are much--I don't mean to be redundant--greater
. Scott discusses how clay may have had a part in the creation of life, and this can be seen as non-living semi-life, then we get viruses and simple protein chains, and then we get DNA-based organisms--and it seems incredibly difficult and arbitrary to just decide that at one of these points life occurs whereas everything before it is not alive.
In the end, such distinctions are purely for conceptual value, and aren't really important. It doesn't matter how we define species or life, so long as we recognize that the reality shows a gradation of different characteristics. It seems similar to the problem Wittgenstein noted when trying to determine the essential characteristic of the word "game"--it seems there isn't one, but there appear to be "family resemblances", meaning there are a cluster of basic characteristics and any thing that has a particular number of these may be described as a "game".