Asklemmy

The answer is egg, because egg-laying creatures predate the chicken.

If we count it as a chicken egg only, then it depends on if you describe a chicken egg as "an egg laid by a chicken" or "an egg that could hatch into a chicken".

the egg that hatched the first chicken, obviously, laid by something that was not quite what we’d consider a chicken.

Fish, arthropods, etc, had eggs millions of years before chickens.

_ /\ _

The concept of "eggs" is way, way older then chickens. Fish lay eggs. So in this case, it's definitly the egg that came first.

You can dig deeper, but eventually you end up on the what is a "chicken egg" and a "chicken" .. which means you have to deal with taxonomy. And well, it's just made up... so that doesn't really lead anywhere.

So I say it's the egg. final answer.

I'm genuinely curious OP how you could even consider the chicken coming first? How did you imagine that scenario going down?

I can’t really remember the study or whatever, but the answer is egg. You’d need the mutation in DNA in the laid egg before you could get the chicken. And then propagation after until chickens are everywhere of course.

This is a simplified version of what I read, but that’s basically it.

To answer your other question, yes there are still single-cell organisms evolving into new species all the time, in the ocean and elsewhere. That includes new multi-cellular species evolving from single cells all the time. But it takes a long time to develop from cell, to clump of slime, to something with legs. So you might not notice the changes if you aren't super patient.

Or were those separate questions? Are you asking if chickens descended from single-cell organisms? Yes they did. With a lot of steps in between.

Egg

to answer your edit, yes all creatures started out as single celled-organisms many billions of years ago and gradually evolved. this process is still happening today but takes millions of years, not something you would observe in a human lifetime