Molecular fossils show that photosystem II, a crucial part of the biochemistry behind photosynthesis, is at least 3.5 billion years old. Where does life come from?
Billions of years old biochemical evidence for Photosystem II
Three billion years is a long time, especially for biological material. The researchers therefore used an indirect method. Namely, biochemical fossils in modern plants, algae and cyanobacteria. These all descend from an ancestor who lived billions of years ago. The greater the difference between proteins in descendants of this oldest cell, the longer ago they separated. And so the older the ancestor with that protein.
Until now, biologists thought that photosystem I, which does not split water, was the primal form of photosynthesis. And that the production of oxygen only evolved billions later. New measurements of the mutation rate showed that the ancestor with Photosystem II must have lived billions of years earlier than thought.  And that photo system II, must have been about as old as life on earth itself. And with it the production of oxygen. At least: the system that is now in use for the production of oxygen. But it is always possible that this system used to have a different purpose. That wouldn't be unique in evolution. Think of the flippers of penguins with which their distant ancestors flew.
This increases the likelihood that life does not originate from Earth, but from elsewhere in the universe. LUCA, the Last Universal Common Ancestor of known life was a pretty complex cell that was little inferior to simple bacteria now. The earliest estimates are 4.5 billion years ago. That is only 100 million years after the creation of the earth, when it had barely cooled down. The chance is of course quite small that a complicated system like LUCA will develop so extremely quickly.
Panspermia, the theory that life can spread between planets and between stars seems more and more promising. Comets and other objects from the rest of the Milky Way regularly arrive, according to the discovery of objects such as' Oumuamua and 2I / Borisov. The clues accumulate in such a way that life on earth comes from elsewhere. From Mars, perhaps, or even further away.
What is Photosystem II?
A crucial part of photosynthesis is the conversion of light into chemical energy. The first step is to split water into oxygen, hydrogen ions and electrons. This is what Photosystem II does. Chlorophyll molecules, a type of solar panel molecules, catch light particles and transport their energy to a reaction point, where this energy splits a water molecule. These electrons and hydrogen come in handy later in the production of temporary energy carriers. In photosystem II this is done via the enzyme P680, which transfers the electrons plus hydrogen ions to so-called plastoquinones. The dark response elsewhere in the cell strips it high-energy hydrogen-bearing plastoquinones of their hydrogen. Other photosystems exist, such as photosystem I, but they do not produce oxygen.
Thomas Oliver, Patricia Sánchez-Baracaldo, Anthony W. Larkum, A. William Rutherford, Tanai Cardona. Time-resolved comparative molecular evolution of oxygenic photosynthesis. Biochimica et Biophysica Acta (BBA) - Bioenergetics, 2021; 1862 (6): 148400 DOI: 10.1016 / j.bbabio.2021.148400