Posts Tagged ‘first life’
oxidised by hydrogen escape…? Chasing the origins of life…
first life? something tiny and green?
I’ve been intrigued and mystified by this phrase (among many others) in Oliver Morton’s book on photosynthesis, the chemistry of which I’m really struggling with. I first read the book a decade or so ago, and in this second reading I’m really beginning to understand how little I’ve understood about the process so central to life on Earth. So, as I too often do, I’ll start with AI (never lies):
Oxidation by hydrogen escape is a planetary process where light hydrogen atoms permanently drift into space from a planet’s upper atmosphere. Because hydrogen is lost, leftover oxygen atoms and other heavy molecules chemically bind to crust and mantle rocks. This leaves the planet’s surface and interior permanently oxidized over geologic time…
I don’t fully understand this. Light hydrogen atoms are also known as protium, which is the most common and lightest isotope of hydrogen (AI again). It’s just a proton in a nucleus. So this drifting off of light hydrogen in an atmosphere, leaving oxygen, etc behind, suggests that the atmosphere must have contained water vapour, inter alia?
So there are further explanations. Ultraviolet sunlight breaks apart water vapour in the upper atmosphere, as well as methane CH4), The hydrogen, being light and fast-moving, escapes the atmosphere, while the heavier elements, such as oxygen, drop down to the planetary surface, and, for example react with iron in the crust.
So what’s with this oxidising thing? It’s a bonding in particular with iron and manganese and appears to have a connection with the Great Oxidation Event (GOE). According to Science Direct, ‘oxygenic photosynthesis appears to be necessary for an oxygen-rich atmosphere like Earth’s’. Are there other kinds of photosynthesis? Apparently so – anoxygenic photosynthesis is used by some bacteria, with hydrogen sulphide being the likely electron donor, but let’s not get side-tracked…
Atmospheric oxygen today is at around 21% concentration, and this is known as PAL, the present atmospheric level by volume. The GOE has been calculated as a 200 million-year transformation occurring about 2.4 billion years ago, bringing about a relatively rapid rise in atmospheric O2…
So, our atmosphere is layered: nearest the ground and rising about 12 kilometres is the troposphere, and above that is the stratosphere (up to 50 kms), then the mesosphere (85), the thermosphere (600) and finally the exosphere, where it thins out into space and where satellites are mostly situated. But to complicate matters we also have the homosphere, and at its upper boundary the turbopause. This ‘sphere’ is so called due to its more or less uniformity. To quote AI it’s ‘the lower layer of the atmosphere where constant mixing by wind and turbulence keeps the chemical composition uniform’.
So when did our atmosphere stabilise, assuming it ever has? Again, I find this is divided into periods, the first one being ‘primordial outgassing’, some 4.5 to 4 billion years go, ‘As early Earth cooled, volcanic outgassing created a thick, toxic precursor atmosphere mostly made of water vapour, carbon dioxide, and nitrogen’. And the Smithsonian Environmental Research Centre goes on, re the Archaean Eon, during which:
methane droplets in the air shrouded the young Earth in a global haze. There was no oxygen gas on Earth. Oxygen was only in compounds such as water. Complex chemical reactions in the young oceans transformed carbon-containing molecules into simple, living cells that did not need oxygen to live. Instead they made energy out of sulphur and other elements.
It also points out that in this early phase of our planet’s life the sun was only around three quarters as bright as it is now – it has gotten brighter over time. The Earth was prevented from freezing over by the greenhouse gases, mostly CO2 and CH4 (methane). But over time, cyanobacteria began to flourish in the oceans, and they were photosynthesising, making free oxygen gas from CO2, H2O and the sun’s light, and so creating something like the atmosphere we know today.
So what exactly are cyanobacteria and why did they come to exist, and then flourish? As to when – likely not before 3 billion years ago, at least not in great numbers. As to where – in the oceans, which were more plentiful, a ‘water world’, according to you know what. And as to how, that’s of course more complex and uncertain. AI gives this not-so-satisfactory explanation:
- Ancestral Microbes: Early Earth had an anoxic (oxygen-free) atmosphere filled with methane and carbon dioxide.
- New Biochemistry: Single-celled anaerobic bacteria in the oceans evolved specialised cell membrane structures.
- Splitting Water: These organisms harnessed sunlight to split water molecules (H2O) and carbon dioxide into sugars for energy.
- Oxygen Waste: The process released oxygen gas (O2) as a waste product, which built up over time and triggered the [GOE]
So, it’s all about ‘ancestral microbes’ and ‘anaerobic [meaning ‘sans oxygen’] bacteria’. So, the ‘life from non-life’ problem presumably remains unsolved. Understandable, I suppose.
References
https://www.sciencedirect.com/science/article/pii/S0009254113003513
https://en.wikipedia.org/wiki/Anoxygenic_photosynthesis
https://arxiv.org/html/2512.09844v1
abiogenesis – some amateur explorations
woteva
One of the greatest mysteries and challenges we face, as living beings – if we’re interested – is how living beings came to be. And we’re the only form of living beings, that we know of, asking this question. Hans Castorp, the central character of Thomas Mann’s The Magic Mountain, pondered the matter in his loggia while taking the cure in an alpine sanatorium. He even went further than the What is life question, asking What is matter? Why is there something rather than nothing?
It was a novel that changed my life. From that reading experience I turned, quite abruptly, to science. I bought Scientific American every month, until I switched to New Scientist, and started reading books by Richard Dawkins, Peter Atkins et al. Of course I’ve never undertaken any formal studies in science, and I’ve always preferred the informal to the formal, and not being subject to authorities telling me what to learn or know. That’s why Hans Castorp, reading and musing in his loggia, so appealed to me.
So what do we know on this subject? When did life begin on Earth, and how? It could have been close to 4 billion years ago, only half a billion years(!) after our planet was fully formed. We don’t have solid evidence, though. The earliest accepted evidence goes back 3.5 billion years, of ‘bacteria-like organisms’. That sounds pretty complex already, and presumably the ‘ingredients’, the intracellular material that sustained and motivated these beings, were around long before. Complexifying chains of molecules, formed out of the ‘primordial soup’, to use an unhelpful term. We think RNA and DNA of course, or at least nucleic acid chains. But what are nucleic acids, and what are the parts thereof? Other essential components include proteins and lipids, with the latter being essential to create more or less permeable boundaries between the organic and the inorganic (or proto-organic?). Lipid molecules, as the Arvin Ash video referenced below tells us, consist of a hydrophilic body, of sorts, and a hydrophobic tail. These molecules tend to come together to form spheres, with the outer, bulkier, hydrophilic ends joining together to protect or insulate the hydrophobic tails from the watery outer environment.
So there’s always a ‘what came before’ question. Where did these lipid molecules spring from, not to mention the other bits and bobs of life? Well, on lipids, I’m relying, for now, on the same video. Carbon monoxide (CO), hydrogen and minerals found in the Earth’s crust can combine to form lipids. All of these components can be found in the hydrothermal vents so recently found in the Pacific depths. But lipid structures break down in the presence of salt or magnesium ions, and these ions are essential for cellular and RNA development. Big problem, as the primeval oceans are believed to be more salty than those of today – though apparently we’re far from being certain about this. In any case, a 2019 paper from the University of Washington showed that lipid spheres remained intact in the presence of amino acids, the building blocks of protein molecules. To quote from the video,
The enclosing of amino acids within cell walls allows them to concentrate within those walls and interact with each other to form proteins, which are part of the ‘trinity’, one of the essential components of life.
So lipid cell walls and proteins, both of course non-living, require each other to survive in salty or iron-rich water. But what about the nucleic acids, DNA and RNA? These are the self-replicating molecules, the genetic material, or precursor genetic material. Today we know that RNA is created from DNA to build proteins according to DNA’s code, but the fact that RNA is the simpler of the two genetic materials suggests to most analysts that it came first. So there’s a hypothesis called the ‘RNA world’, which is generally well accepted by those in the field, but unfortunately we’ve made little progress in working out how RNA came to be formed.
RNA is made up of three chemical components – ribose (a sugar), the nucleobases, and phosphate. A ribose-base-phosphate unit links with other such units to form RNA polymer. But it’s not well understood how these links were formed, and they haven’t been successfully replicated in human experiments. The ribose-base link has proved particularly problematic. As Arvin Ash describes it, ‘this is because cells in your body require complex enzymes to bring RNA building blocks together before they combine to form polymers’. He describes one study, however, which found that today’s RNA could have formed on the surface of clays ‘which act like a catalyst to bring RNA bases together’. A later study showed that the building blocks of RNA could have polymerised in the early Earth, using organic molecules from meteorites and interplanetary dust in shallow ponds, where wet/dry cycles would have been conducive to such polymerisation. They considered that these polymers were probably present on Earth shortly after its formation.
So Ash describes a trinity – RNA, lipids and proteins. What about the proteins? We can go back to the Miller-Urey experiments of the 1950s, which showed that amino acids, the essential components of proteins, as well as other organic compounds, could be produced under particular atmospheric conditions, which they were able to replicate in the laboratory.
So, all these precursors might be explained, but they still need to combine for life as we know it, however basic. This is the big question that still needs to be answered. We haven’t discovered any precise mechanism, but oodles of time, and incremental steps are probably required, and there is surely a possibility of this in the first billions of our planet’s existence, wherein trillions of molecular interactions may have taken place. It’s something of a numbers game, something that many earlier theorists, and today’s creationists, have not taken sufficient account of. It’s also probable that the earliest life forms, those sparks, were so basic that they were quickly improved upon and rendered obsolete by – evolution. But that’s another story…
Needless to say, this piece was more or less wholly reliant on Arvin Ash’s excellent video, which I highly recommend.
References
Why is the Ocean So Salty?