There are four stages from the origin of life
until the appearance of oxygenic photosynthesis.
Stage 1
From no photosynthesis to the origin of chlorophyll
and the first reaction center protein.
There had to be a period in time before the origin of chlorophyll synthesis and photosynthesis. A series of proteins evolved to turn a porphyrin precursor into chlorophyll and bacteriochlorophyll. Enzymes with homology to nitrogenases were recruited for this purpose. Proteins capable of binding chlorophyll or its precursors had to evolve before photosynthesis. This include a membrane protein that would evolve into a reaction center capable of light-driven charge separation.
Stage 2
The first homodimeric reaction center diverges into
two classes presumably both homodimeric: a Type I and Type II reaction centers.
Stage 3
The primordial Type II reaction center protein diverges
into two distinct classes. One class of protein is the ancestral to the PufL
and PufM proteins in photosynthetic Proteobacteria and Chloroflexi. The other class is the
ancestral to D1 and D2 proteins in a lineage that would be the progenitors of Cyanobacteria.
 |
| Photosystem I (PDB ID: 1jb0) |
Stage 4
Starting with a homodimeric Type II reaction center,
in the lineage that would be a progenitor of the Cyanobacteria, a gene duplication
occurs that drives the divergence of D1 from D2. At some moment that could
have started even before D1 and D2 diverged, the capacity to oxidize water appeared.
The homodimeric Type II reaction center then becomes heterodimeric and gains
incredible complexity to evolve into the Photosystem II characteristic of
Cyanobacteria.
The last common cyanobacterial ancestor had
already a fully developed oxygenic photosynthetic machinery implying and unknown
biota of cyanobacteria, this is because the earliest branching cyanobacteria from
the genus Gloeobacter, already
possesses a fully developed Photosystem II and Photosystem I. This implies an
unknown biota preceding the last common cyanobacterial ancestor.
 |
| Photosystem II (PDB ID: 3arc) |
The key question is, how long it took from the origin of photosynthesis to the appearance of water oxidation. If we assume that the first water splitters originated around 2.7 to 3.0 billion years ago, and the origin of photosynthesis between 3.8 to 3.3 billion years ago, the completion of these four stages could have taken anything from 300 million to 1.1 billion years.
Knowing the speed at which bacteria evolves and the lengths of time we are talking about, hundreds of millions of years, each one of these stages should have given rise to a significant diversity. Most of which appears to have gone extinct.
