Self-evidently, the gravity on the surface of a planet limits the maximum size of large terrestrial organisms.
James Lovelock has pointed out that atmospheric levels of oxygen much above about 25 percent, let alone 30 percent, would cause raging conflagrations today.
As we have seen so far in this series, fire was an absolutely crucial component in humanity’s rise to civilization and technology.
The combustion of wood or coal may seem so familiar as to be unworthy of any comment.
The importance of metals, particularly iron, and the importance of the discovery of metallurgy can hardly be exaggerated.