Final answer:
The common ancestor of Bacteria, Archaea, and Eukarya likely had a cell membrane and ribosomes. Neither a nucleus nor mitochondria were present in this ancestor, with the latter arising through endosymbiosis in the eukaryotic lineage.
Step-by-step explanation:
The common ancestor of Bacteria, Archaea, and Eukarya was a simple cell that lacked several complex cellular structures that are found in modern eukaryotic cells. According to scientific research and comparisons of basic cellular features, the structures that were present in the common ancestor of these three domains include a cell membrane, ribosomes, and genetic material (DNA). The nucleus and mitochondria were not present in this common ancestor because these features evolved later in the lineage leading to eukaryotes.
Specifically, ribosomes are present in all three domains of life, and their general structure is indicative of a shared evolutionary origin. Ribosomes are essential for the synthesis of proteins and are found in the cells of all three domains. The cell membrane, also known as a cytoplasmic or plasma membrane, is another universal feature that functions to separate the cell interior from its environment.