Archive for the ‘Early Earth’ Category
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
water on Earth – no problemo
So, as described in my last post, H2O in its various forms is plentiful in our solar system as well as beyond it. But, being more or less scientifically illiterate – despite decades of reading stuff on science – I can’t quite work out how liquid water is so abundant on the Earth’s surface. The story has long been told of water-iced asteroids in the time of the heavy bombardment being responsible, with the major proof being that these carbonaceous chondrite asteroids have, or had, the same signature of heavy (deuterium-rich) water as the water we find on Earth. While this seems a strong argument to me, how did the Earth manage to hold on to that water during those super-heated days?
I’ve looked at this in a previous post, sort of, but I’m still not clear on the atmospheric conditions that brought about our soggy planet (much more soggy during the Mesozoic though). In any case, I’ve recently read that bonafide researchers on this topic have also been mystified about the sheer volume of water on Earth.
Enter a new (to me) hypothesis, published in the Journal of Geophysical Research: Planets a little over a year ago. It argues – and other astrophysicists appear to be impressed by the reasoning and the detailed analysis in the paper – that the water came not only from asteroids but also from the solar nebula.
Solar nebula? Never heard of it, but apparently the concept has a long history. The so-called nebular hypothesis for the formation of our solar system was first proposed by Emanuel Swedenborg in the 1730s, and further elaborated by such luminaries as Immanuel Kant and Pierre-Simon Laplace later in the 18th century. Surprisingly for such an early contention, it has stood the test of time and survives today, though the details are still argued, and there are a few competing hypotheses. In any case, without going into too much detail, a nebula of dust and gas began to form around 4.6 billion years ago, and collapsed in on itself due to gravitational forces, spinning around a newly-formed sun. Out of this material, protoplanets gradually formed.
Water in the Earth’s oceans has approximately the same D/H (deuterium to hydrogen) ratio as that of the above-mentioned asteroidal carbonaceous chondrites, so it has always seemed a safe bet that most if not all water came from those asteroids. Yet the sheer volume of water was still a problem. Jun Wu, the lead author of the recent paper, had this to say about the theoretical situation:
The solar nebula has been given the least attention among existing theories, although it was the predominant reservoir of hydrogen in our early solar system.
What has apparently added credence to the new hypothesis is that samples of hydrogen near the core of the Earth have significantly less deuterium and may fit better with the ratio of hydrogen in the solar nebula. Also the isotopic signatures of the noble gases helium and neon found in the Earth’s mantle fit the signatures of these gases from the time of the solar nebula. The explanation of how the lighter hydrogen found itself drawn to the Earth’s centre, in a process called isotropic fractionation, is provided in the paper, apparently. It’s a very interesting story, if true, and it may have implications for liquid water on habitable-zone exoplanets. That’s to say, there’s no reason for it not to be quite common. Here, to finish, are a couple of thought-provoking comments from members of the research team.
… there’s another way to think about sources of water in the solar system’s formative days. Because water is hydrogen plus oxygen, and oxygen is abundant, any source of hydrogen could have served as the origin of Earth’s water.
Our results suggest that forming water is likely inevitable on sufficiently large rocky planets in extrasolar systems.
References
How did Earth get its water?
https://www.britannica.com/science/solar-nebula
a little about the chemistry of water and its presence on Earth
Earth before life: more skeptico-romantic chitchat
The early Earth – more cracks than facade?
Canto: So we’ve talked all too briefly about Earth’s probable formation and how its moon was formed some fifty million years later, and I’m not sure whether I want to go back further in time to try to answer some big questions about the solar system in general or the solar nebula, or forward to consider how life emerged from inanimate matter on this seething-hot, volatile planetary surface…
Jacinta: Well since we’re the blind leading the blind, it doesn’t much matter which direction we go. Let’s choose life.
Canto: Okay, but we’ll have a way to travel before we get there.
Jacinta: Well most of us learned at school that the Earth has a crust, a mantle and a core, and that the core is of iron and it’s really hot down there, and the crust is formed of plates that move around and go under each other, and that the atmosphere above the crust consists of layers, like the stratosphere and the ionosphere, and the atmosphere around us is around three-quarters nitrogen and a quarter oxygen with traces of other gases, and if it wasn’t like that we wouldn’t be here. But it wasn’t anything like that when the first life appeared.
Canto: Yes, it was very different, and it seems there’s more that we don’t know about the period between 4.5 and 4 billion BP than there is that we do know, if you know what I mean.
Jacinta: BP?
Canto: Before the Present. I got that from the excellent Stuff You Should Know podcast, and I’m going to use it from now on.
Jacinta: D’accord. So yes, we know that the early Earth was incredibly hot, reaching temperatures of 2000 celsius or more, but there’s also evidence from ancient amphibolite rocks and banded iron formations that there was water on the Earth, and plenty of it, 4.3 billion years ago. Which suggests an extraordinarily fast cooling down period, and where did all that water come from?
Canto: Yes I think we really need to look at this period, or what we know of it, to try and make sense of it, because it doesn’t quite make sense to me. A hot magma world, melted fom the inside out, but also bombarded from the outside by meteorites, then after the bombardment suddenly cooling from the outside in, and flowing with water. All in a couple of hundred million years?
Really?
Jacinta: That’s a long time actually. We’re hoping to live for a hundred years for some strange reason – a two millionth of our time-frame, if we’re very lucky.
Canto: Well it’s all relative, but where did this water come from? Some say it must’ve come from space, because that’s all that happened, meteors from out there crashing into here. Where else could it come from?
Jacinta: How do you trap water here when the surface temperature is so high? Water boils at 100c, right?
Canto: Under ‘normal’ atmospheric pressure. The early Earth was anything but normal.
Jacinta: Anyway it just doesn’t seem possible to get so much water from rocks crashing into us. There’s another alternative – the water was already here. So the original bits and pieces that formed the Earth – carbonaceous chondrites or whatever – contained water and this water somehow made its way to the surface.
Canto: Somehow. Leaving aside the rising-to-the-surface problem, carbon-rich chondrites are found in asteroids today, and they have apparently a similar water-plus-impurities ratio to our oceanic water, and that’s obviously very suggestive.
Jacinta: Yes and the isotopic ratios pretty well match, but they don’t for comets. Scientists have been able to measure the isotopic ratios in comets such as Halley and Hale-Bopp, and they don’t have anything like the proportions found in our oceans. I’m talking heavy water here, deuterium, but also protium which is another isotope of hydrogen.
Canto: NASA also launched a spacecraft, Deep Impact, to probe the constituents of a comet, Tempel1, and the results were negatory for its candidature as feeder of the Earth’s water, had it ever landed here, but of course not nugatory for astronomical research generally. But then, what comet is ever typical? Anyway, there’s a just-so story, sort of, that I watched on video recently, which explained the oceans, sort of. It told us that the planetesimals that created the Earth contained water locked inside, and that years of later volcanic activity released that water to the surface as steam, which condensed in the cool upper atmosphere and fell as rain. And the rain it rainèd every day.
Jacinta: So the Bible was right then?
Canto: More than forty days and nights – thousands of years, they claimed. But that made up only half the world’s oceans. The rest came from comets, they said. Now that seems unlikely, but replace comets with the right sorts of asteroids, and the recipe still works.
Jacinta: Well here’s another story, which is meant to explain how that heat-creating heavy bombardment came to an end. The Earth’s bombarded surface was extremely hot, melting everything, even the rocks, and in this state the heavier elements such as iron sank to the centre, forming our core, which was vital in protecting us from the notorious solar wind – that incredibly strong force that has blown away the atmosphere of Mars.
Canto: Yeah, they say it kind of magnetised the Earth, and that was like a shield of steel.
Jacinta: Aka the magnetosphere, but I’m afraid that electromagnetism was a subject that transformed me into a gibbering mass of incomprehension at school.
Canto: I can’t say I understand it myself, but the magnetosphere works to almost perfectly preserve our atmosphere. We do lose a percentage to the solar wind every year but it’s so tiny that it’s not a problem. Another anthropic circumstance that proves the existence of God.
Jacinta: Hallelujah. So did this magnetosphere form before or after the formation of the moon?
Canto: God knows.
Jacinta: Goddess.
Canto: Sorry princess.
Jacinta: Princess, goddess, actress, countess, diminutives. They diminish.
Canto: Seamstress.
Jacinta: Temptress.
Canto: Watercress. Anyway it probably happened around the same time. The great crash that probably created the moon has been nicely computer-simulated by Robin Canup of the Southwest Research Institute – it’s well worth a look. The theory goes that this great glancing blow tilted the Earth and gave us our seasons, probably vital to life as we know and love it.
Jacinta: Yes but it would’ve heated up the planet even more, so I’m interested in the problem of the shift from this to our amphibolite rocks under water from nearly 4.3 billion years ago. Where the eff did that water come from? It steamed up from beneath the surface? Not likely. And from asteroids? Really?
Canto: Possibly. But according to this excellent Naked Science video, the best-preserved meteorites ever recovered came from a landfall in British Columbia in 2000. And when they investigated this meteorite material they found that it was made up of 20% water by weight, and that’s pretty significant…
Jacinta: Because water isn’t dense like rock is it, so that sounds like a lot of water. We’re learning a lot from this video, such as that meteorites don’t cause great fireballs or anything like that, because they’ve been tumbling about in cold space for eons, and their entry into the Earth’s atmosphere only heats up a few millimetres of the outer surface, and then only for a very brief period, so they pretty well instantly go cold again.
Canto: Right and maybe that explains something else; that a heavy bombardment of these big wet boulders – and apparently they’ve found that the further they are from us, the more water they contain – would’ve cooled the planet.
Jacinta: Interesting idea, which I’m sure someone’s thought of and maybe even computer modelled. Certainly it would help to explain the apparent speed with which the oceans were formed. So… I’m not really convinced, but in lieu of a better explanation I’ll take it on trust that the oceans were created in little more than a million years or so by a hailstorm of asteroids, together with water steamed up from below the surface. So now we have a somewhat cooler Earth, ready at last for some kind of life, but not as we experience it.
Canto: Right, we’re talking about an atmosphere containing virtually no oxygen. Made up mostly of nitrogen, carbon dioxide and methane.
Jacinta: And how do they know that? I’ve also heard hydrogen sulphide mentioned.
Canto: Yeah, upwellings from volcanic activity I believe.
Jacinta: So the stage is set for some sort of proto-life, with RNA or some precursor. And so the fun begins, if it hasn’t already.
Canto: Indeed it does. So that’s what we’ll be exploring next. I’ve even heard some researchers claim that water isn’t necessary for basic life to get started. Now there’s heresy for you.
Jacinta: That’s the fun of heresy these days, you don’t get burned alive for it, no more than a bit of gentle ribbing. I’m looking forward to the next post.