Final answer:
Eukaryotic cells, which evolved around 2 billion years ago, developed a nucleus and internal organelles due to endosymbiotic events, leading to complex structures and multicellularity.
Step-by-step explanation:
Basic changes in eukaryotic cell structure and function that occurred a long time ago include the development of a nucleus and internal membrane-bound organelles, as evidenced by fossils dating to about 2 billion years old. These changes, explained by the endosymbiotic theory, were driven by larger cells engulfing smaller ones, which then evolved into organelles like mitochondria and chloroplasts. As eukaryotes became more complex, multicellularity evolved allowing for specialized functions. The last common ancestor of today's Eukarya also contained DNA associated with histones, a cytoskeleton, and underwent mitosis and sexual reproduction. Characteristics like cell walls, while present in prokaryote precursors, are less understood in terms of their development and homology in eukaryotes.