Question

What evolutionary milestone made it possible for more complex, multicellular organisms to exist?

Answer 1

The appearance of eukaryotic cells made it possible for more complex, multicellular organisms to exist.
The difference between the eukaryotic cells and the  prokaryotic cells  (bacteria and archaea) is the membrane-bound organelles, the nucleus, which contains the genetic material. The eukaryotes cells are responsible for forming multi-cellular organisms, which consist of many kinds of tissue made up of different cell types.