Did Complex Life Begin in a Natural Nuclear Reactor?

Two unique events in world history are believed to have both taken place around 1.7 billion years ago.  Might their coincidence in time explain why life as we know it exists? 

The Emergence of Eukaryotes.  

For more than 2 billion years, the only life forms that existed were the two domains of prokaryotes, the bacteria and the (slightly larger) archaea.  Since their origin almost 4 billion years ago, these simple cells became so successful and adaptable that they haven't evolved greater size or complexity even to this day.  Then all of a sudden, geologically speaking, there appeared about 1.7 billion years ago much larger and more complex single cells known as eukaryotes, the third domain of life (Ref. 1).  Eukaryotes include every complex single-cell species and every multi-cellular species known today, characterized by much more complex biochemistry and cellular structure than those of the prokaryotic domains.  

In Reference 1, author Nick Lane argues that the first eukaryotes evolved from the coalescence of an archaeon ("host") and bacterium inside it ("parasite"), a very rare event which almost always kills the host, but in this case formed a chimeral cell that lived and reproduced.  Over succeeding generations, the parasites became "endosymbionts", losing most of their DNA and retaining only what they needed for energy production.  They became the sub-cellular organelles known as mitochondria.  The host's DNA concurrently expanded to include most of the genes for synthesizing many of the proteins and enzymes that the mitochondria needed to function and reproduce.  This one (and perhaps only) successful prokaryotic chimera to emerge in world history has evolved into all the branches and twigs of the eukaryotic domain: all algae, fungi, plants and animals.

The Only Known Natural Nuclear Reactors.

The other unique event in world history that took place about 1.7 billion years ago was the onset of natural self-sustaining nuclear reactions at 16 sites in Oklo, Gabon (Ref. 2).  It is believed to be a unique event in world history because the decay products of nuclear fission fragments have been found nowhere else in rock strata that predate the atomic era.  These nuclear reactions took place in a uranium layer embedded in sandstone.  When it became inundated with ground water, the layer moderated (slowed down) neutrons enough to sustain a fission chain reaction.  The energy imparted to fission fragments heated the water and boiled it off, shutting down the reaction until the layer cooled enough for water to flow through again and restart it.  This cycle repeated every 3 hours for a few hundred thousand years, until there was too little fissionable uranium left to sustain the chain reaction. 

A nuclear chain reaction, natural or otherwise, is only possible when at least 3% of uranium atoms are the fissionable isotope U-235.  They can no longer occur in nature because the decay rate of U-235 is much faster than the dominant isotope’s (U-238).  In the last 1.7 billion years, the natural abundance of U-235 fell from 3% to only 0.72%, much too low to sustain a chain reaction. 

How might these unique events be causally connected? 

To understand how, consider that adenosine triphosphate (ATP), the energy-storing molecule common to all life, is produced in very thin membranes (6 nanometers thick) which are porous to the flow of protons in one direction but not the other. The "respiratory chain" (oxidizing food, hydrogen sulfide, or even ferrous iron!) drives protons in the porous direction until the voltage drop across the membrane exceeds 150 millivolts.  While this sounds low, such voltage across such a thin membrane creates lightning-bolt-high electric fields (25-30 million volts per meter), strong enough to pull protons back through the membrane in the non-porous direction. This reverse flow of protons powers the production of ATP by ATPsynthase molecules, exquisitely complex protein nanomotors embedded in the membrane.  Every 10 protons rotate a nanomotor one complete turn, producing 3 ATP molecules from 3 adenosine diphosphate molecules (ADP) and 3 phosphate ions(Pi).  The ATPsynthase kicks the new ATPs off the membrane to float around the cell until they encounter proteins that split them back into ADP and Pi, releasing the stored energy to carry out the cell's work.  The ADP and Pi continue floating until they encounter other membranes, whose ATPsynthases grab them to make more ATP.  The main consequence of this mechanism is that the energy available to a cell is proportional to the total area of its energy-producing membranes.

Bacteria and archaea are both limited in their size and complexity by the limited area of their only membrane, the one that encloses most of the cell just inside the cell wall. Given this constraint, these simple cells lack the energy they would need to grow larger or more complex.  Lane believes this constraint was overcome when an archaeon somehow acquired a bacterium and survived.  Archaea are larger than bacteria, but the latter reproduce faster, so an archaeon with bacterial parasites multiplying within is most likely to burst.  But in a sheltered, energy-and-mineral rich environment, such as a porous mineral matrix through which hot water flows, there's a chance that the archaeon and its endobacteria could mutate by sharing DNA, giving the archaeon the ability to expand and accommodate the growing number of bacteria within.  This would enable it to survive and reproduce as a chimera, evolving in size and complexity.  Thus evolved the first eukaryotic cells, 15,000 times the chimera’s original size.  Its thousands of mitochondria, descendents of the original bacterium, contain densely folded membranes that collectively crank out millions of times more ATP than any prokaryote.  

Lane proposed an alkaline hydrothermal vent as the site where this took place.  But such vents are numerous on the ocean floor, and have been around for billions of years, yet no other multicellular domains of life emerged from them.  Why did this only happen once in 4 billion years, giving rise to only one domain of complex life?

Any of the natural nuclear reactors at Oklo might have provided a similar environment, exactly when the earliest eukaryotes emerged.  Uranium in the absence of oxygen is insoluble, and before that time there was not enough oxygen for uranium to dissolve in water and percolate through sandstone to form the uranium ore layer.  Shortly after that time the U-235 in the ore became too depleted to sustain fission.  So if complex life had been kick-started by nuclear power, it would only have happened in a very narrow window of time (0.0004 billion years wide), 1.7 billion years ago.  Translated into an a priori probability per unit time, that’s one domain of complex life per 400,000 years.  Comparing to the a priori probability per unit time of it happening in alkaline hydrothermal vents (once per 4 billion years), it’s 10,000 times as many domains of complex life per unit time! 

While these a priori probabilities are scientifically meaningless because they’re based on an event that happened only once anywhere, they suggest that the Oklo sites are well worth investigating as a possible birthplace of complex life as we know it.  Is there an Evolutionary Biochemist in the House who agrees?  

References

1.  Nick Lane, The Vital Question (Norton & Co. 2015).

2.  https://en.wikipedia.org/wiki/Natural_nuclear_fission_reactor