An amoeba is observed to duplicate itself and at one point appears to be a two-celled living organism, however, that is just it's state prior to the split into two one-celled amoeba.
So, there is not an 'evolutionary' development, from a 'one-celled' organism to a 'two-celled' organism.
The next highest living organism is multicelled, as you say.
There are no 'missing links' between the two.
https://www.google.com/search?clien......0i71j0i67.mW5wjbNsOuM#imgrc=Zye3mLz8ENJ9kM:
That is ignoring the question of where they came from to start with and the Earth they are on, with the greater light to rule the day, and the lesser light to rule the night, hanging on nothing in The Universe.