Did the first living cell evolve suddenly in a single step or gradually as the result of many intermediate steps?

Gradually as the result of many intermediate steps is my answer because as life on Earth for example started off with simple atoms making molecules, and as the time went past more complex compounds were be produced. And from there later on unicellular organisms started appearing. Nothing in evolution that I know of happens in a short time, at least from the human perspective.

organisms that lack symmetry are called asymmetrical and belong to phylum porifera/sponges.

organisms with specialized stinging cells belong to phylum cnidaria. stinging cells are called cnidoblast cells.

cnidarians show two morphological forms : polyp (sessile) and medusa (mobile)

filter feeding is observed in porifera members.  

alternation of generation is observed in coelenterata or cnidaria. there is alternation of asexual polyp form and sexual medusoid form. it is also referred as metagenesis.

it’s lava came from an explosive eruption