Final answer:
The mitochondria are the organelles that are believed to have originated from symbiotic bacteria in ancient cells according to the endosymbiotic theory.
Step-by-step explanation:
The organelle believed to have been a symbiont of an ancient cell is the mitochondria. This belief is based on the endosymbiotic theory, which was first proposed by Lynn Margulis and is now widely accepted by the scientific community.
Mitochondria contain their own DNA and ribosomes, which are similar to those found in bacteria, suggesting they originated from free-living prokaryotes that were engulfed by early eukaryotic cells. They physically resemble bacteria and have a double membrane characteristic of gram-negative bacteria, further supporting this theory. Over time, the engulfed prokaryotes are believed to have forged a symbiotic relationship with their host cells. This endosymbiotic relationship provided the host cell with additional metabolic capabilities, such as aerobic respiration, which was advantageous to the survival of both entities.