Final answer:
Early eukaryotes containing mitochondria were selected during evolution due to the symbiotic relationship between the two organisms. The endosymbiotic theory explains that mitochondria were once free-living prokaryotes that were engulfed by larger prokaryotic cells, forming a mutually beneficial relationship.
Step-by-step explanation:
Scientists believe that early eukaryotic cells containing mitochondria were selected during evolution because of the symbiotic relationship between the two organisms. The endosymbiotic theory suggests that mitochondria were once free-living prokaryotes that were engulfed by larger prokaryotic cells. The larger cells provided a habitat for the smaller prokaryotes, while the smaller prokaryotes provided extra energy to the larger cells.
This relationship was mutually beneficial and allowed the early eukaryotes to obtain more energy, giving them a competitive advantage. Over time, these early eukaryotes with mitochondria were selected and became the ancestors of modern eukaryotic cells.