You might be thinking of the genesis of eukaryotes, which is thought to be from a specific event where one archaeon incorporated a bacterium, and all eukaryotic

You might be thinking of the genesis of eukaryotes, which is thought to be from a specific event where one archaeon incorporated a bacterium, and all eukaryotic organisms are descended from the resulting symbiotic arrangement, with our nuclear DNA descending from the archaeon, and our mitochondria descending from the bacterium. All multicellular life is eukaryotic, but not all eukaryotes are multicellular, e.g. amoebae.