The symmetry between aerobic carbohydrate respiration and photosynthesis is widely appreciated. In the presence of Oxygen, many living things (including photosynthesizers) burn carbohydrates, producing useful energy and CO2. Photosynthesizing creatures, using the energy of sunlight, manufacture carbohydrates from CO2 for their own use; and Oxygen. By outrageous good fortune, photosynthesis has been so wildly successful that has produced a surplus of carbohydrate and Oxygen sufficient to balance the evolution of hungry multicellular respirers; like ourselves.

In physics, this is called a symmetry, a conserved quantity. The reason the quantity is conserved is unclear and taken on faith as a fundamental property of the universe. For fun, we go out on a limb here and propose that physics can take a lesson from biology.

The basic idea of conserved quantities is that they remain constant amid the mayhem around them. When we say energy or baryon number is conserved, we mean only that at the end of a long day of collisions, phase transitions, etc., we get the same quantity we started out with. If we wish to take the lesson from biology, the continuous symmetry of aerobic respiration and photosynthesis means that all the powerful disruptive forces continue cancel out, leaving the relationship the same within the bounds of inevitable fluctuation.

Less well appreciated than the symmetry of aerobic respiration and photosynthesis, is the symmetry of aerobic and anaerobic metabolism. In the guts of hungry multicellular respirers from ourselves to termites, beneath the water surfaces, and as deep in the crust as we have yet drilled; lives a vast anaerobic world.

The vast anaerobic world is ancient and mostly microscopic Archaea, the first living things. The anaerobic world developed some internal symmetries of its own during a billion years or more of evolution before cyanobacteria got around to producing much Oxygen.

The primary modes of anaerobic metabolism are methanogenesis and fermentation. Fermentation produces CO2 and alcohol. Alcohol itself is valuable low entropy carbon (carbohydrate) used by other critters. Methanogenesis can occur  from carbohydrate, producing methane and CO2; or it can combine CO2 and Hydrogen, producing methane and water.

A few arcane critters use methane anaerobically, but methane from CO2 is essentially the final step in anaerobic metabolism. The resulting methane is waste to the Archaea, but an important gift of low entropy Carbon to the aerobic world.