Let’s return to Lake’s new hypothesis about the origin of double-membrane bacteria:
Here, by analysing the flows of protein families, I present evidence that the double-membrane, Gram-negative prokaryotes were formed as the result of a symbiosis between an ancient actinobacterium and an ancient clostridium.
Since actinobacteria and clostridia might represent cells very similar to the original cells, let’s have a look at them. First, consider actinobacteria.
Symbiogenesis is a theory of evolution. It argues that symbiosis is a primary force of evolution, because acquisition and accumulation of random mutations or genetic drift are not sufficient to explain how new inherited variations occur. According to this theory, new cell organelles, new bodies, new organs and new species arise from symbiosis, in which independent organisms merge to form composites. This challenges some standard textbook ideas of how evolutionary change occurs. To some degree, Darwin emphasized competition as the primary driving process of evolution, symbiogenesis emphasizes that co-operation can also be important to the process of evolution.
It is now very reasonable to propose four major events of symbiogenesis:
1. The origin of double-membrane (gm neg) bacteria.
2. The origin of the eukaryotic cell.
3. The origin of mitochondria.
4. The origin of chloroplasts.
James Lake was recently proposed and supported a fascinating new hypothesis:
Lake has discovered the first exclusively prokaryote endosymbiosis. All other known endosymbioses have involved a eukaryote — a cell that contains a nucleus. Eukaryotes are found in all multicellular forms of life, including humans, animals and plants.
In the Nature paper, Lake reports that two groups of prokaryotes — actinobacteria and clostridia — came together and produced “double-membrane” prokaryotes.
“Higher life would not have happened without this event,” Lake said. “These are very important organisms. At the time these two early prokaryotes were evolving, there was no oxygen in the Earth’s atmosphere. Humans could not live. No oxygen-breathing organisms could live.”
Here is the abstract for the paper:
Endosymbioses have dramatically altered eukaryotic life, but are thought to have negligibly affected prokaryotic evolution. Here, by analysing the flows of protein families, I present evidence that the double-membrane, Gram-negative prokaryotes were formed as the result of a symbiosis between an ancient actinobacterium and an ancient clostridium. The resulting taxon has been extraordinarily successful, and has profoundly altered the evolution of life by providing endosymbionts necessary for the emergence of eukaryotes and by generating Earth’s oxygen atmosphere. Their double-membrane architecture and the observed genome flows into them suggest a common evolutionary mechanism for their origin: an endosymbiosis between a clostridium and actinobacterium.
This hypothesis is fascinating whether we look backward or forward from the proposed event.