Every animal on Earth runs on the same metabolism. The combustion of sugar plus oxygen yields carbon dioxide plus water plus energy, and that energy — captured in the molecular currency of ATP — powers everything from a hummingbird's wings to a thought. Cellular respiration is the same chemistry as fire, but slow, controlled, and routed through cellular machinery that extracts energy in stages rather than releasing it all at once. The process is so universal that the same enzymes appear in bacteria and in human muscle cells, evidence of common ancestry going back about 3.5 billion years.
The full pathway is one of biochemistry's masterpieces. Glycolysis (in the cytoplasm) splits a six-carbon glucose into two three-carbon pyruvates, netting two ATP. The citric acid cycle (in mitochondria) oxidizes pyruvate, generating electron carriers (NADH, FADH₂). The electron transport chain (on the inner mitochondrial membrane) uses those carriers to pump protons across a membrane, building an electrochemical gradient. ATP synthase — a remarkable rotary enzyme — lets the protons flow back through it, using the gradient to phosphorylate ADP into ATP. Total yield: about 32 ATP per glucose. The mitochondria themselves are descended from ancient bacteria engulfed by a larger cell approximately two billion years ago — the endosymbiotic origin of mitochondria (Lynn Margulis, 1967) is now accepted. Mitochondria retain their own DNA (inherited maternally in animals), their own ribosomes, and their own membrane lipids. Cellular respiration is evolutionarily old: the pathway in E. coli and the pathway in your liver cell share most of the same enzymes, and the ATP synthase rotor is found essentially unchanged across the tree of life.
Mitochondrial diseases — disorders of cellular respiration — are an active area of medicine; many neurodegenerative conditions have a mitochondrial component. Mitochondrial DNA analysis is a major tool in human population genetics (mitochondrial Eve dates to about 200,000 years ago). Aerobic exercise is, biochemically, training the mitochondria to be more numerous and efficient. The deep connection between metabolism, ageing, and disease — through ROS, NAD⁺ depletion, mTOR signaling — is one of the hottest areas of biomedical research, and the prospect of metabolic interventions (dietary restriction, rapamycin, metformin) extending healthspan is a serious experimental programme even if the public-facing claims often outrun the data.