Answer: The structure that evolved in cells approximately one billion years ago that allowed for the forming of more complex, multicellular organisms is the nucleus.
Explanation: The evolution of the nucleus, a membrane-bound organelle, marked a significant milestone in the development of eukaryotic cells. Prior to the emergence of the nucleus, prokaryotic cells lacked a defined nucleus and their genetic material was scattered throughout the cell. The formation of the nucleus allowed for the separation of genetic material from the rest of the cell, which enabled the development of more complex cellular structures and functions. This, in turn, paved the way for the evolution of multicellular organisms, as cells could now specialize in different functions and work together to form complex tissues and organisms.