Biochemical changes do not seem , therefore, to be a main driving force in the diversification of living organisms.
The really creative part in biochemistry must have occured very early.For the biochemical unity that underlies
the living world makes sense only if most of the important molecular type found in organism , that is , most of the metabolic
pathways involved in the production of energy and in biosynthesis or degradation of the essentials building blocks
already existed in very primitive organisms such as bacteria.
Once this stage passed, biochemical evolution continued as more complex organisms appeared.
But it is not biochemical novelties that generate diversification of organisms.
In all likelihood , it worked the other way around. It is the selective pressure resulting form changes in behavior or in ecological niches
that led to biochemical adjustments and changes in molecular types.
What distinguishes a butterfly from a lion , a hen from a fly , or a worm from a whale is much less a difference in chemical constituents.
The few big steps of evolution required acquisition of new information.
But specialization and diversification occured by using differently the same structural information.Among neighboring groups , vertebrates for istance , chemistry is the same.
What makes one vertebrate different from another is a change in the time of expression and in the relative amounts of gene products rather than the small differences observed in the structure of these products
It is a matter of regulation rather than of structure