The first organisms on earth appeared approximately 3.5 billion years ago and were all unicellular. multicellular organisms have only existed for about one billion years. what evolutionary milestone made it possible for more complex, multicellular organisms to exist? appearance of heterotrophic cells appearance of autotrophic cells appearance of stem cells appearance of eukaryotic cells

The answer is "appearance of eukaryotic cells".
There are also membrane bounded organelles that are present in eukaryotic cells for example mitochondria and the Golgi apparatus, chloroplast is present in plants and algae.
Dissimilar to unicellular archaea and microorganisms, eukaryotes may likewise be multicellular and incorporate organism comprising of numerous sorts of tissue and types of cells.

Amight be the answer