Relative Complexity and importance of Systems within Evolved Organizations

In any organization of systems that has adapted and survived as a result of selective pressure, can we assume that some systems are more complex or took more time to develop than others during co-evolution?

Is the modern human eyeball more or less complex, as a system, than the modern Central Nervous System? Which took longer to evolve, the modern eyeball or the modern Central Nervous System? Did their co-evolution make them inseparable to the point where the question is intractable?

If the goal is to replicate the function of an evolved system through planned engineering, and the sub-systems are going to be built in a decentralized manner, under the above assumption we can derive that some systems will be easier to build than others.

Should we then assume that the more complex systems will be more critical to the functioning of the whole system? Does it follow that the systems with a higher cost to develop are more important?