The reasons why there are no silicon cows or sand based pigs around are subtle and tricky to answer. It's the obverse of the question 'Why is life carbon based?'. In chemistry class, you generally learn that carbon forms chains and rings readily while silicon does not. However, this is not quite true, and merely restates the question as 'Why does life require rings and chains?'

In general terms, life is CHNOP based - that is, Carbon, Hydrogen, Nitrogen, Oxygen and Phosphorous. You cannot form the equivalent range of SiliconHNOP small molecules, because it does not make the right sort of bonds with them. The problem really is that Si will not make the types of structures that we recognise as parts of life; fatty acids and proteins must rely on the peculiar properties of carbon. On the other hand, Silicon compounds do make good crystals (although glass is, by definition, not crystalline). Therefore, if there was a form of Silicon Life it would have to be constructed very differently from what we know. A SciFi story I remember described a planet that was wired with silicon, forming a gigantic intelligence. Although this is a lifeform, an individual, it is not a form of life. That is, it cannot breed or evolve.

What would silicon life look like? Well, although many carbon based life forms use silicon (nettles' needles, plankton skeletons or even our bones and teeth) this is only peripheral. To be silicon based would involve some form of crystalline life - maybe using carbon as a skeleton or defense material? Indeed, the world may have started like this...

Another problem that has been pointed out to me* is energy. Most organisms use an oxidised compound (such as, indeed, oxygen) to break down food - eventually expelling the waste as carbon dioxide. If silicon life was to do the same, the waste product would be solid silicon dioxide. This is more difficult to remove than CO2 (you can't breathe it out!).

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