Eukaryotes often have symbiotic relationships with prokaryotes where the prokaryotes aid in metabolic paths, cell signaling, or other essential functions. Often the prokaryotes give the cells functions and enzymes they did not previously have access to and become organells in the eukaryotic cells. Mitochondria, which the majority of eukaryotes contain, most likely used to be its own organism before merging with eukaryotic cells. Because all eukaryotes have a mitochondria(or are descended from something that does), the partnering with the bacteria that became the mitochondria may have defined the evolution of eukaryotes. Chloroplasts also evolved from a type of bacteria called cynobacteria in too seperate occasions, one in plants and algea, and one in amobeas. There are also examples of eukaryotes acquiring other eukaryotes such as lichen where fungi acquire algae and use them as photosynthesis. In these the other cells are not organelles, but just have symbiosis. An example is nitrogen-fixing bacteria which convert air nitrogen into a usable form in some plants, protists, and even insects such as termites. There are some metabolic factors that are needed but we do not have, and bacteria produce the enzymes to metabolize certain nutrients animals do not have a natural ability to. One example is that bacteria are needed to break down celulose in plants. Sometimes the DNA from one organism even manages to naturally get transferred to another in a process called independent lateral transfer. Many times a signalling pathway becmoes dependent on a prokaryote which can be harmful if the host does not benefit from the same things as the bacteria.
