Archive for the ‘fossils’ 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
stuff on human ancestry 1: the australopithecines, mostly
All the evolution we know of proceeds from the vague to the definite
C S Pierce
I was in a bookshop yesterday, where I picked up a copy of Yuval Noah Harari’s book Sapiens and had a gander at the back cover. I read one sentence, which went something like ‘100,000 years ago there were at least six species of Homo sapiens, now there is only one.’ Or maybe it was just ‘six species of Homo‘. It resonated with me, because it’s been a while since I’ve researched and written about the ever-fascinating topic of human origins, a topic that resurfaced for me recently on reading an essay, ‘Lucy on the earth in stasis’ by Stephen Jay Gould in his 1996 collection Dinosaur in a haystack. The essay promoted his ‘punctuated equilibrium’ view of evolution, as it reported that Australopithecus afarensis appeared to be the only hominin type in existence for a period of almost a million years, from approximately 4.9 million years ago to 4 million years ago, after which there was a relatively rapid radiation of hominid species. I could only take the essay on trust, but I maintained the thought that I should investigate whether this claim still held, some twenty-three years later. And that, further, I should investigate whether we were any clearer about our descent, as the last surviving species of that apparent radiation.
And by the way, for my education’s sake I need to straighten out the difference between hominids and hominins. We humans are both, apparently. The hominids, or great apes, include four genera: Pongo, the orang-utangs, of which there are three extant species; Pan, of which there are two species, chimps and bonobos; Gorilla (two species), and Homo, of which there’s only one extant species, but many extinct ones including Neanderthals. The term ‘hominid’ has broadened over time. The term ‘hominin’ is more restrictive, referring only to those species ancestral or related to humans, since the split from the chimp and bonobo line. This explains, I hope, why we are both hominids and hominins. Clearly, though, I should stick to the term hominin for this post, or series of posts.
Anyway, I was surprised to read this claim about the state of human play 100,000 years ago. The old Bill Bryson question, How do they know that? came to mind, but I also felt skeptical, as I seemed to remember that the number was smaller – possibly dependent on whether you were a lumper or a splitter.
We know of course that our closest living relatives are (equally) chimps and bonobos, and the latest dating of our divergence from their line is 4 to 7 million years (according to Wikipedia, but Gould put it at 6 to 8 mya, and this video from the American Museum of Natural History gives it more ‘precisely’ at 7 mya, and another Wikipedia article gives the figure as 6.5 to 5.5 mya, so who knows?) There are a couple of possibilities for our last shared ancestor – Sahelanthropus tchadensis and Orrorin tugenensis – but their more or less competing claims are mired in uncertainty, due to the extreme sparsity of material. It may well be that neither of them fit the bill.
When they look at the evidence from early hominins, researchers are particularly interested in signs of bipedalism, which have been argued to exist in S tchadensis due to the placement of its foramen magnum (the hole in the skull through which the spinal cord passes) towards the back – though this placement has been disputed, quelle surprise. In any case, these earliest hominins evolved during the Pleiocene epoch into the definitely bipedal australopithecines. The bipedal adaptation is so important to the emergence of Homo sapiens that it has been the subject of a great deal of speculation, hypothesis and argumentation. It’s likely that there were a variety of converging factors that favoured this trait’s development. For example, it provided a wider visual field, especially on the ground; it left the hands free to grasp and carry food; it enabled long-distance running, and it reduced the expenditure of energy. However, bipedalism appears to have been a slow development, and early australopithecines such as A afarensis likely spent a lot of time in trees. This is supported by anatomical features such as longer arm-bones, curved fingers, a shallow rib-cage and strong clavicular anchors for brachiation (swinging from the arms among branches).
Over time there were anatomical changes favouring bipedality. These included greater robustness of the ankle and knee joints, and changed positioning of the foramen magnum, the femur and the spine, to support changes to the centre of gravity. But the changes which have had the most long-lasting, even at times dire effects, have been those to the pelvic region. The strengthening and widening of this region, including the ilium, ischium and pubis, to support an upright stance, has to a serious degree compromised the process of childbirth. It’s been observed that australopithecines share with modern humans a sexual dimorphism relating to the lumbar vertebrae, allowing the spinal curvature of females to become more pronounced during pregnancy, which helps to better distribute the weight of the unborn child and to reduce fatigue and maintain stability of posture for the mother. However, the changed shape of the pelvis and the consequent narrowing of the birth canal resulted in what has become known as ‘the obstetrical dilemma’. Unlike virtually every other mammalian species, humans face major difficulties and dangers in childbirth, which require others – midwives or other medical professionals – to assist in the process (for example, neonatal rotation is often necessary for safe delivery). A ‘solution’ to this dilemma, which appears to have evolved over time, is a comparatively short gestation period – the time spent in the womb – to give a greater opportunity for both mother and child to survive the birth. This of course leads to a longer period of childhood dependence as it develops outside the womb. Apparently, a modern human baby is born with approximately 25% of full brain development, compared to 45-50% in other primates. Brain size at birth is limited due to the obstetric dilemma, and greater neoteny is the result.
Encephalisation, which refers to a growth in brain size or mass relative to body size, is now seen as a later development in the human story than bipedalism. Brain size in general has become very questionable as a measure of complex evolutionary development – witness those smart corvids – and it’s worth noting that the Neanderthal brain is on average larger than ours. What’s important, though, is brain structure – something we can’t really look at vis-a-vis our ancestors. However it is reasonable to assume that our much larger brain size compared to australopithecines is largely due to growth in the temporal lobes and the prefrontal cortex. In fact all regions have grown, including the cerebellum, traditionally associated with fine motor control and balance, but more recently connected with cognitive function and language.
But let me return to the hunt for the hominin links from the other great apes to Homo sapiens. In the mid-nineties, two new examples of early hominins were discovered, Australopithicus anamensis and Ardipithecus ramidus. I’m guessing that Gould didn’t know about these discoveries when he wrote his essay, as they seem to have punctured his punctuated equilibrium thesis, at least as regards hominins. Anyway the A anamensis species is believed to have lived from about 4.2 to 3.8 million years ago, and the A ramidus specimens have been dated to around 4.4 million years ago, but interestingly, A afarensis, the principal subject of Gould’s essay, is now believed to have lived from 3.9 million years ago to 2.9 million years ago – that’s a million years after Gould’s stated range. The australopithecines first came to our attention in 1925 when Raymond Dart described Australopithecus africanus from specimens found in South Africa. A africanus is a more gracile type, and may well be in the direct line to humans, though there’s been a lot of dispute about the dating and classifying of different specimens. A africanus is generally thought to be a more recent species than A afarensis, another gracile type. So maybe we can link A africanus back to A afarensis, which in turn can be linked back to S tchadensis, with some intermediate missing links. But then there’s another recently discovered species, Australopithecus sediba, which has been dated to around 2 million years ago and is thought to be a transitional species between A africanus and either Homo habilis (which some prefer to describe as Australopithecus habilis) or Homo erectus. Another gracile species discovered in the nineties, A garhi, dating to about 2.5 million years ago, also seems to fit as a species connecting Australopithecus and Homo. From what I’m reading, the fragmentary nature of these finds, together with obvious questions as to whether particular specimens are typical of whole species (type specimens are often juveniles, which might not be such a good idea), are the main barriers to pinning down the precise lines of succession. That’s why every new discovery is such a treasure.
I haven’t mentioned Ardipithecus or Paranthropus as yet. In the nineties specimens were found in the Afar triangle in East Africa, and classified as Ardipithecus ramidus (around 4.4 million years ago, with uncertain evidence of bipedality, and some evidence of reduced sexual dimorphism) and Ardipithecus kadabba (about 5.6 mya, possibly an ancestor of A ramidus, but known from only a few teeth and bones – the type specimen being a bit of mandible with an attached molar). It’s possible, according to some researchers, that Ardipithecus, Orrorin, and Sahelanthropus all belong to the same genus.
I’ll have a look at Paranthropus, apparently a more robust distant cousin of ours, then move forward to the Homo genus, next time.
References
Australopithecus Evolution (video), by Henry the PaleoGuy, 2019
Seven million years of human evolution (video), American Museum of Natural History, 2018
https://en.wikipedia.org/wiki/Human_evolution
https://en.wikipedia.org/wiki/Australopithecus
https://en.wikipedia.org/wiki/Australopithecus_afarensis
https://en.wikipedia.org/wiki/Australopithecus_africanus
https://en.wikipedia.org/wiki/Australopithecus_sediba
https://en.wikipedia.org/wiki/Australopithecus_anamensis
https://en.wikipedia.org/wiki/Ardipithecus
http://humanorigins.si.edu/evidence/human-fossils/species/ardipithecus-ramidus