[P.O.E.]

I’d’ve thought the first animals would be single-celled.

IIRC, an animal does not have chloroplasts and needs to eat other things for energy (heterotrophic, with extremophiles aside).

[Moonman62]

From Wiki:

Animals are multicellular eukaryotic organisms that form the biological kingdom Animalia.

[Moonman62]

https://en.wikipedia.org/wiki/Phagocytosis

Eating or ingestion of organic material was a very important evolutionary development. Without it there never would have been endosymbiosis or the eukaryotic cell.

It predates the evolution of animals by billions of years.