Sarrita A. answered • 05/03/20
University of Cambridge Graduate with PhD in Biochemistry
It would require that animals make and express the appropriate photosynthetic organelles and enzymes. Further, it would require that animal gas exchange change such that we would take up CO2 and release O2. Such an extreme evolutionary change would require some actual selective pressure in the environment and that does not seem likely.
The endosymbiotic theory supposes that chloroplasts and mitochondria were once single-cell organisms that were taken up by some other organisms, and allowed for a division of labor in terms of cellular function. This theory presupposes that divergence to multicellular plants and multicellular animals occurred at this stage because plants and animals were able to limit the production of ATP to the membrane bound organelles. The ATP was used to allow cells to divide, and create cellular complexity and diversity.
The take up of chloroplasts and mitochondria into a cell allowed for plants to become more complex by obtaining their energy from the sun. Photosynthesis involves using the energy from the sun to fix carbon from the atmosphere. The carbon can then be used to make sugars etc.
Animals do not have chloroplasts, they only have mitochondria, as such they must consume energy in the form of carbohydrates (and fats) and break that down via a series of metabolic pathways which are distinct from those used in photosynthesis.
The enzyme, in plants, that fixes carbon is called RUBISCO, and it is the most abundant enzyme in the world. Once, energy is obtained from the sun that energy can then be used to fix carbon and make sugars for the plant, which we will consume as fruits and vegetables. Animals do not have the enzyme RUBISCO, so they have no capacity to fix carbon.
On another note, animals take in O2 and release CO2, plants do the exact opposite. So in order for animals to become autotrophs (which is what plants are) they would have to change the way they respire, to take in carbon and release oxygen, in addition they would need to have chloroplasts which contain the photosynthetic pigments that absorb high energy photons from the sun, which allows for the splitting of a water molecule. A further issue would likely be that if all organisms used a a combination of the sun and CO2 as an energy source we would likely deplete the atmosphere of CO2 and that would imperil survival.
Frankly, there are a whole host of reasons as to why in terms of evolution the proposed scenario would never occur. Finally, if all animals converted to photosynthesis they would be green, as the green pigment chlorophyll is optimal because it is the midpoint in the visible light spectrum. This means that plants reflect green light while absorbing red and blue light.
