07 April 2015

Oxygen-producing photosynthesis as a Critical Step in the evolution of life on Earth

Below is a slightly edited excerpt from The Revolutions that Made the Earth by Watson & Lenton (pp. 88-89). They are making the case here that the evolution of oxygen-producing photosynthesis is one of what they refer to as critical steps in the evolution of life on Earth, i.e., events so unlikely that if you were to 'play the tape over' as Stephen Jay Gould described in his 1989 book Wonderful Life, they would likely not happen again. (If you think about it, that implies that most planets where life might have evolved based on initial conditions will not have experienced the critical steps described). 

[W]e think the case for [oxygenic photosynthesis’] being a critical step is very strong. Photosynthesis powers the planet, and we’ll describe [later in the book] the evolution of this remarkable mechanism, and just how profoundly it has transformed the Earth system. Here we simply want to establish reasons for believing it may have been one of our critical steps, using the two criteria of uniqueness and timing by which we hope to recognize such transitions.
The bacterium that first produced oxygen by splitting water with sunlight coupled together to earlier-evolved photosynthetic pathways, called photosystems I and II. In addition it possessed a unique enzyme containing four manganese atoms, called the water-splitting complex. The biochemistry of oxygenic photosynthesis is staggeringly beautiful and complex, involving the absorption of a total of eight protons in the process of splitting two water molecules to release one molecule of oxygen. Among all the prokaryotes, only the cyanobacteria evolved this ability, and later, they became the ancestors of all the chloroplasts in all the eukaryote algae and plants [on Earth].
The evidence is consistent with the original invention having occurred just once, with no sign that it evolved separately ever again, and the water-splitting complex has remained essentially unchanged over billions of years. This is significant, because oxygenic photosynthesis is an amazingly useful trick for any organism to possess, since it enables the uptake of carbon and the production of energy using just light, carbon dioxide and water — three of the most abundant resources on the surface of the planet. An organism that can do it is well equipped to make a living in most habitats on Earth, and, therefore, you might suppose that if it were easy in evolutionary terms, it might have happened more than once. Since it did not, we can assume that it was no everyday event: this was a red-letter day for life on Earth.

I just love this stuff. What's really amazing is that they make a pretty good case, based on statistical reasoning pioneered by Brandon Carter in the 1980s, that the critical steps they argue occurred on Earth (evolution of the genetic code, this one, emergence of endosymbiotic eukaryotes, and the evolution of language-using observer sophonts (i.e., us)), are all so unlikely that they would not be expected to occur in the 4 billion years or so that Earth has been habitable (even if stretched to add in the 100 to 500 million years its habitability has yet to go). In fact, all four are far more unlikely even than the origin of life itself, which actually occurred just about as quickly as it was physically possible for it to have done. (Setting aside the possibility of panspermia, i.e., extraterrestrial origin of life on Earth, or miracles; the first being difficult to assess but generally considered unlikely and the second being beyond the realm of science). Similarly, the evolution of macrobiota, meaning visible multicellular plants and animals, which is usually front and center in any history of life on Earth, is almost certainly not a critical step, as there is good evidence that it happened independently several times. But the upshot is that at least four entirely unique, and very, very unlikely evolutionary developments, each of which required the ones before to have happened, and in that sequence, all happened in the evolution of life on Earth. (Think about that: without the genetic code, photosynthesizing bacteria could not have evolved; without photosynthesis, there would be no oxygen atmosphere, and eukaryotes could not have evolved; without eukaryotes, intelligent animals, which are necessarily eukaryotic, could not have evolved; so the sequential order is necessary). The assessment of improbability is based on timing and uniqueness. The chance of all four occurring on any given potentially habitable planet, in order, and during the time the planet remains habitable (that is, before its star gets too hot to allow life there, which is the universal fate of habitable planets)... is truly miniscule. Of course, what that doesn't address is the likelihood that other, unknown and potentially equally remarkable, critical steps could be occurring or have occurred elsewhere, leading to unimaginably different biospheres. I suspect if and when humanity encounters life that did not originate on Earth, we will be simply amazed by how different the initial potentials can turn out.

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