Final answer:
Multicellular organisms originate from single-celled organisms that started to cooperate and form colonies over time. Red algae Bangiomorpha pubescens is the oldest known multicellular organism, fossilized in 1.2 billion-year-old rock.
Step-by-step explanation:
Multicellular organisms originate from single-celled organisms. The evolution of multicellularity occurred when single-celled organisms started to cooperate and form colonies. Over time, these colonies developed into complex, multicellular organisms. For example, red algae Bangiomorpha pubescens is the oldest known multicellular organism, fossilized in 1.2 billion-year-old rock.