Final answer:
Ancestral eukaryotes became aerobic by consuming aerobic bacteria through endosymbiosis, which evolved into mitochondria and allowed the use of oxygen for energy extraction.
Step-by-step explanation:
Ancestral eukaryotes became aerobic through a process known as endosymbiosis. In a first endosymbiotic event, the ancestral eukaryote consumed aerobic bacteria that evolved into mitochondria, the powerhouses of eukaryotic cells.
Over time, these aerobic bacteria became integral to the eukaryotic cells, eventually losing the ability to live independently and becoming the mitochondria we know today. This was a crucial adaptation that allowed the ancestral eukaryotes to use oxygen to extract energy from organic compounds, which is a more efficient method compared to anaerobic respiration.
Photosynthesis was another significant biological innovation that arose from endosymbiosis, wherein photosynthesizing cyanobacteria were included inside a eukaryote cell that already contained a mitochondrion, giving rise to eukaryotic cells with chloroplasts capable of photosynthesis.
The advent of photosynthesis transformed Earth's ecosystems by consuming carbon dioxide and releasing oxygen as a byproduct, dramatically altering the atmosphere's composition and favoring aerobic organisms.