From Chemistry to Biology
One conceptual difficulty in the likely sequence described in the left panel is how we can define
that a complex chemical becomes a living thing. Is there an instant, before which
it is complex chemicals and after which it is living? Or is it more like the famous Escher
woodcut shown in the
left panel,
where we clearly transform from fish to bird but no point can be called "the" transition
point?
Notice that we deal with a related issue in everyday life. How do we define when a human fetus becomes a human?
Intelligent people have very strongly held and divergent opinions on the answer.