lpetrich
Contributor
Turning to fi, we can look at the evolution of our planet's biota to get an idea of which steps are common and which are not.
The original source of energy was reactions like what methanogens use: CO2 + 4*H2 -> CH4 + 2*H2O
But that does not get much energy, and it is confined to certain kinds of environments, like hot springs and hydrothermal vents.
A good additional source of energy is sunlight, and capturing it evolved twice. Both times it was added into existing energy metabolism, but added in two different ways:
Water is the best-known of photosynthetic electron donors, but it was likely not the first, and some organisms use others, like hydrogen or sulfur or iron or various organic compounds.
Stripping electrons from water gives molecular oxygen and hydrogen ions, and O2 ends up accumulating in the atmosphere and in the ocean. This makes possible much more efficient decomposition of organic material, thus making animal life possible.
The original source of energy was reactions like what methanogens use: CO2 + 4*H2 -> CH4 + 2*H2O
But that does not get much energy, and it is confined to certain kinds of environments, like hot springs and hydrothermal vents.
A good additional source of energy is sunlight, and capturing it evolved twice. Both times it was added into existing energy metabolism, but added in two different ways:
- Retinal photosynthesis - membrane pumping - Haloarchaea
- Chlorophyll photosynthesis - electron transfer - a variety of bacteria, with chloroplasts descended from one of them
Water is the best-known of photosynthetic electron donors, but it was likely not the first, and some organisms use others, like hydrogen or sulfur or iron or various organic compounds.
Stripping electrons from water gives molecular oxygen and hydrogen ions, and O2 ends up accumulating in the atmosphere and in the ocean. This makes possible much more efficient decomposition of organic material, thus making animal life possible.