ages, and the how of life on earth – some thoughts
Stromatolites, Shark Bay
I’ve written before about my difficulties with Oliver Morton’s Eating the sun, a complex book about photosynthesis, its beginnings and development, its components and combinations, carbon dioxide, oxygen, chloroplasts and chlorophyll, rubisco, stomata, grasses and trees, the ages of the Earth, Gaia and so much more. I’ve never felt more overwhelmed. So to clarify and comprehend I want first of all to get my head around timelines for the Earth, starting with the pre-life period.
So, without being too precise, Earth began its existence as a more or less defined planet about four and a half billion years ago. The Hadean is recognised as the first geologic era, lasting about half a billion years, and has been described as hellish, as the name suggests. So, from Wikipedia:
Earth in the early Hadean had a very thick hydride-rich atmosphere whose composition likely resembled the solar nebula and the gas giants, with mostly water vapor, methane and ammonia.
Understandably, there’s quite a lot of uncertainty about this end-of-planet-formation period. So a ‘hydride-rich atmosphere’ suggests hydrogen, I reckon. Hydrides are the anions of hydrogen (H-), having two electrons to give them a negative charge, I think (there will be lots of uncertainties in this piece). So a cation has a positive charge – fewer electrons than protons, which is to say, for hydrogen, no electrons at all.
So the water vapour condensed as the planet cooled over time, creating an ocean surface, but it was likely still being bombarded by asteroids and wee planetesimals, and this with upwelling gases from vulcanism led to a new environment, or period, or eon to use the standard term, known as the Archaean, dating from just over 4 billion years ago to 2.5 billion. During this time the surface gradually cooled, continents began to take shape, and the first life forms appeared. And of course it’s life that I’m most interested in. So, just to be clear, the Earth’s history is divided by the cognoscenti into four geologic eons: Hadean, Archaean, Proterozoic (2.5 billion to 539 million, approximately) and Phanerozoic (up to the present).
It’s generally believed, but not quite certain apparently, that an event called the late heavy bombardment marked the beginning of the Archaean (or the end of the Hadean). Much of the evidence for this comes from a period of intense lunar bombardment dating to around 3.9 billion years ago. Presumably, if this is verified, Earth would have been bombarded in the same period, but it’s believed that our planet was covered in water at this time. Wikipedia again:
The Earth during the Archean was mostly a water world: there was continental crust, but much of it was under a super-ocean deeper than today’s oceans. Except for some rare relict crystals (Hadean zircon), today’s oldest continental crust dates back to the Archean. Much of the geological detail of the Archean has been destroyed by subsequent tectonic activity. The Earth’s atmosphere was also vastly different in composition from today’s: the prebiotic atmosphere was a reducing atmosphere rich in methane [CH4] and lacking free oxygen.
I’ve removed the links to reduce my level of discombobulation. So what about these life forms in the Archaean? There were microbial mats in the shallow waters, also known as stromatolites – or to be I think more accurate, the stromatolites formed microbial mats. Or maybe not – stromatolites are layered microbial formations (famously found in Shark Bay in Western Australia), which date back to around 3.5 million years ago, and they were built by cyanobacteria.
So, were these the first life forms, and how did they come to live? Via photosynthesis apparently. So, cyanobacteria are autotrophic gram-negative bacteria that ‘can obtain biological energy via oxygenic photosynthesis’ (Wikipedia). Is there any other kind of photosynthesis? Apparently so, but the oxygenic type is ‘by far the most common type of photosynthesis used by living organisms’. An autotrophic organism makes its food out of stuff that isn’t food, or not as we know it Jim. That is, from mineral elements, CO2 and maybe unknown other stuff.
It might seem that I’m getting away from the Earth’s ages here, But I’m particularly interested in how life forms shaped our atmosphere, our biosphere and such, over time. Cyanobacteria and other microbial life-forms, beginning in the Archaean, began to oxygenate the atmosphere. This was the so-called Great Oxygenation Event. Or maybe not, that was a bit later…
The earliest life forms may date back as far as 4.1 million years ago, based on a tiny piece of ‘biologically fractionated graphite inside a single zircon grain in the Jack Hills range of Australia’. So the early Archaean marks life’s starting point, quite early in Earth’s history. We’ve found evidence in Greenland (the Issua Supracrustal Belt), and again in Australia’s Pilbara region, all between 3.5 and 4 billion years ago.
So the ‘Great Oxygenation Event’ (GOE) occurred near the beginning of the Proterozoic eon, about 2.4 million years ago, long after the beginnings of life, but that earlier life was anaerobic, I think. Meaning it didn’t require oxygen. So what did it require? Information from various sources tells me that this first life was single-celled, sans nucleus, just a bag of chemicals, more or less. And genes. As there was no oxygen, or very little, and the Earth at this time was covered in water, they would have congregated mostly at ocean vents, living off the heat and assorted carbon-based chemical compounds. So this was the situation up to about 3 billion years ago. It was a watery world with very little oxygen, mostly methane and CO2 in the atmosphere. The land, when it emerged, was void of green, or even soil. Life, essentially bacterial, lived on the border of land and sea, but mostly sea. Sunlight was weaker then, but there was no ozone layer to shield the surface from ultraviolet radiation.
So that was the situation for more than a billion years up to the aforementioned GOE. Cyanobacteria are it seems the culprit regarding the mass production of oxygen. Lots of cyanobacteria, presumably. They basically learned to split H2O via sunlight, releasing oxygen as a waste product. That process, or set of processes, is of course the subject of Oliver Morton’s book, and it’s fiendishly complex, and took decades to fully comprehend.
Anyway, let’s get back to cyanobacteria. I’ve described briefly what they are, but how did they come to be? After all, it seems they lay claim to kick-starting the abundance and variety of Earthly life, mainly due to their oxygen production. They’re often known as ‘blue-green algae’, but then, what are algae? The answer is that they’re not yet quite plants but they require water as well as light. So, on surfaces or edges. They can be eukaryotic or bacterial, as the earlier forms were. And of course they’re endlessly fascinating as probably the first life forms – the connection between life and – chemistry. Or how about the bridge between inorganic and organic chemistry? Chemical building blocks coming together over thousands, million of years until, somehow…
And it seems photosynthesis was the trick that flicked chemistry into biology. Light was the energy source, oxygen the waste product that in turn, and really quite bizarrely, became the next great life source. So how did these barely living entities effect this without which not transformation?
Well, it wasn’t their intention, and it certainly didn’t happen overnight. They used some kind of chemistry, based on light, to create cells. But no, they already had cells, as bacteria. Organic molecules, whatever that means, preceded cells. Here’s something from AI:
Simple organic molecules formed in Earth’s early environment, eventually enclosed by lipid membranes to create the first primitive cells. These simple structures developed metabolic energy, complex DNA replication, internal organelles, and eventually multicellular cooperation.
That’s a helluva lot of development in one tiny paragraph. And again, what exactly is the difference between an organic molecule and a mere molecule? The internet tells me that they ‘must’ be based on carbon, though I know there has been speculation that other base elements, such as silicon, might work, if the environment was right. That’s to say, nothing like that of Earth. Carbon can form strong chemical bonds with up to four other elements, as well as itself, in long chains. Think CO2, hydrocarbons, methyl groups and such. Do I know what I’m talking about? Not really, except that complexity is key. Life somehow emerges from complexity. Carbon-based complexity, due to this flexibility, this love of bonding, this affinity with other elements. But I’m still looking for that boundary, that spark, that difference that made all the difference. But then, it’s also reasonable to assume, or understand, that we will never locate that precise place or time when that entity we recognise as ‘alive’ came into being. It may have come, and then died, many times before a generational link occurred, and a chain of being came to persist. Time, after all, was on its side.
So I suppose I should try to satisfy myself with what we know. The above AI quote mentioned ‘complex DNA replication’. DNA?! Out of nowhere? Maybe that’s the next thing to explore…
References
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