theory of evolution: can an organism evolve to an earlier level?

Answer

The answer to your question is yes and no.

Evolution works through a succession of random mutations. Many of those mutations are bad, but some give an organism an advantage in an environment. It is the organisms that have an advantage that are most likely to reproduce. Thus, subsequent generations will adopt that mutation more and more. If enough of these mutations follow each other, a new species can arise.

In this way, each species has completed an evolutionary trajectory with random steps. It is statistically impossible for a species to follow exactly the same trajectory and arrive at an earlier form or a primordial form.
It may be that a species returns to a form that may resemble it but is not quite the same.

An example. Life originated in the oceans. At a certain point you got fish and some of those fish went out on land, creating mammals in the very long term. However, some of those mammals returned to the sea. These are the dolphins and whales. Like fish, they are adapted to a ‘primordial’ environment, they have ‘fins’ and are streamlined. However, they still have mammalian features such as lungs and the structure of their fins.