Then, the ability to split water and use it as an electron provider evolved. This was a breakthrough due to
water’s iniquitousness. Because this oxidation of water is so unfavourable (water has a highly negative
oxidation potential) two photons are needed; the electrons must be promoted twice in two separate
photosystems to allow them to reach a point where they can reduce the terminal electron acceptor. This led to
the evolution of cyanobacteria.
The splitting of water produces oxygen. When this evolved, oxygen was released into Earth’s atmosphere in
large quantities for the first time. It initially reacted with iron, especially oxidising Fe(II) ions to Fe(III) ions.
However, once the majority of metals were oxidised, oxygen began to accumulate in the atmosphere.
Eventually this led to a selective pressure to develop an aerobic method of ATP production; not just because
of oxygen’s prevalence but also because many cofactors used in anaerobic metabolic processes are oxidised
and so damaged by oxygen.
This led to the evolution of ETC that uses oxygen as the terminal electron acceptor, also known as ‘aerobic
respiration’; organisms that use these are a subset of the chemoheterotrophs.
At some point in the Earth’s history (the actual period is disputed), a fermenting primitive prokaryote
engulfed an aerobically respiring prokaryote, becoming the ancestor of all eukaryotic cells, and all
mitochondria descend from that aerobically respiring prokaryote. Later, a eukaryote engulfed a
cyanobacterium; this cyanobacterium then became the ancestor of all chloroplasts.
Many of the evolutionary steps mentioned
involved the re-appearance of previously
used structures, for example the
repurposing of the ATP H+ pump. The
unique ability of prokaryotes to share
genes between different linages (lateral
gene transfer) allowed this to occur, as
more advanced organisms could re-uptake
genes for those previous structure.