Final answer:
A bacterium can be distinguished from the last eukaryotic common ancestor by its circular DNA, as bacteria have a single circular chromosome, while eukaryotes have multiple linear chromosomes within a nucleus.
Step-by-step explanation:
The observation that would distinguish a bacterium from the last eukaryotic common ancestor is the presence of circular DNA. Unlike eukaryotes, bacteria are prokaryotic organisms which lack a nucleus. Instead, their genetic material is in the form of a single circular chromosome located in an area of the cell called the nucleoid. Eukaryotic cells, on the other hand, have multiple linear chromosomes contained within a nucleus.
Other distinctive features of bacteria include the lack of membrane-bound organelles like mitochondria or chloroplasts. Instead, they may have structures such as flagella for movement and a cell wall, which can be made of different substances, including peptidoglycan, but not cellulose, which is found in plant cell walls. It is important to note that ribosomes, the sites of protein synthesis, are present in both prokaryotic and eukaryotic cells, and therefore, the lack of ribosomes would not distinguish a bacterium from a eukaryote.