Question

What are we referring to? When we say 'simple organisms' or 'complex organisms'.

Answer 1

When we say simple or complex organisms we are talking in terms of evolutionary history of an Organism. A ‘simple organism’ is considered to be primitive and has simple thallus organisation; The level of complexity of metabolism is also low. On the other hand a ‘complex organism’ refers to a more evolved form forming higher levels of structural and functional complexities. They are believed to have arisen from simple organisms.