a bonobo humanity?

According to Morton, and planetary science in general, there are four periods, or eons, in our planetary history, the Hadean, the Archaean, the Proterozoic and the Phanerozoic. That’s in order from the earliest to the current. The Hadean largely covers Earth’s formation, from around 4.6 billion years ago to 4 billion. The Archaean takes us from 4 to 2.5 billion, with the planet cooling and the first life-forms. The Proterozoic, which was the longest eon (so far), took us from 2.5 billion to about 539 million years ago, and its defining event for us was the great oxygenation and the snowball earth in the early part of that period. The Phanerozoic kicked off or was kicked off by the Cambrian explosion and the emergence of complex life. 

So, to the long Proterozoic, which has been dubbed the ‘boring billion’, it’s been argued that the most important of the atmospheric changes in this period has to do with sulphur:

The oxidised surface of the planet would have provided the oceans with a greatly increased supply of sulphate, which microbes in the oxygen-free depths of the ocean would reduce into sulphides (Morton, p212). 

Most of this stuff isn’t particularly comprehensible to me, so I’ll try to understand. First, something about ‘nitrogen-fixing’:

To be useful to life, nitrogen has to be fixed from the gaseous form in the atmosphere to a reduced form, the ammonium ion. No eukaryotes have the energy-intensive electron transfer chains and associated enzymes needed to pump the necessary electrons into the nitrogen; but various bacteria, including some cyanobacteria, do (ibid. p212). 

So I’ll have to do more research into nitrogen fixation. Nitrogen gas is inert, which is to say, unreactive. It’s different, though, from the chemically inert gases of the periodic table (helium, argon, neon – the ‘noble gases’). As a diatomic molecule, N2, it’s an extremely stable, triple bonded element. 

So, nitrogen, sulphur (sulphate and sulphides), ammonium and cyanobacteria.. But let me go to another source, Wikipedia, on nitrogen fixation.

Nitrogen fixation is a chemical process by which the abundant but relatively inert molecular dinitrogen (N₂) is converted into bioavailable nitrogen compounds such as ammonia (NH₃) and nitrates (NO₃).^[1] It occurs both biologically and abiologically, the latter either naturally through weather phenomena (e.g. lightning) or artificially by chemical industries. Biological nitrogen fixation or diazotrophy is catalyzed by enzymes called nitrogenases,^[2] produced by microorganisms such as cyanobacteria and rhizobia. These enzyme complexes are encoded by the Nif genes (or Nif homologs) and contain iron, often with a second metal (usually molybdenum, but sometimes vanadium).^[3]

So … where does sulphur come in, if anywhere? 

Sulphur is essential for biological nitrogen fixation. Without enough sulphur, legume-rhizobium symbioses form fewer and smaller root nodules, produce less nitrogenase enzyme, and struggle to convert atmospheric nitrogen into plant-usable forms. Sulphur acts as a key metabolic partner to nitrogen in building proteins and chlorophyll. 

Somehow I can’t quite picture this, though I’m sure it’s all true. So how do you add an elegant sufficiency of sulphur? Well, I’ve found a website that explains not only why sulphur is essential but how nitrogen fixation works. It’s referenced below, but I’m going to try describe the process in my own words. 

So, in North America, where wheat and corn are grown, legumes (plants in the pea family Fabaceae) are used as a rotational crop, as they increase nitrogen levels in the soil, reducing the need for nitrogen fertiliser. There’s a common bacterium in soil called rhizobia, which likes to inhabit the root nodules of legumes. It produces enzymes called nitrogenases, which are in the habit of converting atmospheric nitrogen into ammonia. I know this was explained, sort of, in the Wikipedia quote, but my skull is thick. Plants convert ammonia into ammonium (NH4+) somehow, which they then absorb through their roots to build important/essential planty stuff, like amino acids, proteins and chlorophyll. They also have special enzymes – e.g. glutamine synthetase – which change the ammonia, which is toxic in large amounts, into, er, something less toxic. 

So now back to sulphur, for some reason. It’s apparently essential for nitrogen fixation in legumes. It’s absorbed from the soil as sulphate ions, by the roots, and it works with nitrogen ‘to convert raw nutrients into usable plant proteins’.

And all of this, perhaps, is related to the emergence of complex eukaryotic life…

References

https://en.wikipedia.org/wiki/Nitrogen_fixation

4 Reasons Sulphur Is Necessary for Nitrogen Fixation in Legumes.