The Ediacaran: Flesh plants. Flesh plants! Great wobbling frisbee beasts! Flesh plants—
[img id] Digital cel-shaded art of a wibbly wobbly Dickinsonia, a weird wobbly lifeform from before the Cambrian era. It looks like a floppy frisbee with bilateral lines sectioning its body into many gelatinous ribs. It lives underwater surrounded by fleshy ‘plants’, including orange charnia with spots and neon green hylaecullulus. Since I’ve interpreted Dickinsonia as a transparent creature, the other life forms can be fuzzily seen through its body. This little scene is set on an isometric square chunk of sandy seafloor. [/id]
I simply don't have it in me right now to do napowrimo but reading others' poems reminded me I did finish a sonnet not too long ago that I'm still pretty happy with.
Mass extinctions are when a large sample of biological clades undergoes a geologically-brief period of die-off that is statistically higher than the background rate of extinction (Marshal, 2023). Most of the organisms that have ever lived died out through the regular trials of natural selection, but on occasion the Earth's conditions changed so rapidly that many different species could not adapt in time.
These extinction events were some of the driving forces that shaped the evolution of life, typically be "reshuffling the deck" or weeding out a few key taxa, thus paving the way for new life to flourish. So what are these mass extinctions and just how did they change the world?
In this post, I will be providing a brief account of our current understanding of each extinction event: when they occured, their causes, and the victims and survivors. As you'll come to discover, the pop-science view of "five big mass extinctions" is complicated by evidence suggesting that some singular events are best seen as two or three, while others have been neglected by mainstream coverage.
Great Oxidation Event
When? - ~2.4-2 Billion Years Ago (Paleoproterozoic Era)
Cause - When photosynthesizing bacteria evolved the Earth's atmosphere did not contain free oxygen, being mainly CO2 and nitrogen. The process of photosynthesis converts sunlight into sugars which takes in CO2 and expels oxygen. The abundance of prokaryotes engaged in this chemistry released oxygen in enormous quantities. While much of it initially was absorbed by dissolved iron on the seabed, the rest rose into the atmosphere. This increase in free oxygen changed the air content and triggered global cooling (perhaps spawning glacial periods).
Victims - It has been generally proposed that the newly oxygenated atmosphere had negative effects on the then-common anaerobic bacteria & archaea, who should have suffered a mass extinction due to oxygen poisoning. While direct evidence for this had been lacking for some time, recent geochemical work suggests "a rapid reduction in primary productivity of >80%" that "imply a collapse in primary productivity" (Hodgskiss, 2019).
Survivors - Aerobic prokaryotes would have flourished in the aftermath of the Oxidation Event, while anaerobic forms would have migrated and adapted into areas still free of oxygen. It has been argued that the evolution of eukaryotes was spurred by the changes, but so-far this is controversial (Fakhraee, 2013).
End-Ediacaran Extinction Event
When? - ~541 Million Years Ago
Cause - An event little-studied and sometimes doubted (MacLeod, 2015), recent work posits that Ediacaran communities were generally low-diversity and "quiet"-ecologically (Darroch, et al. 2015). The gradual evolution of new animal life (a.e. not an explosion as typically described) at the bridge of the Cambrian corresponds with ecosystem-engineering that simply pushed many of the Ediacaran species to extinction.
Victims - The mysterious "Ediacaran fauna": a collection of soft-bodied, basal-animals. Dipleurozoans (e.g. Dickinsonia), trilobozoans, and cephalozoans (e.g. Spriggina) went extinct.
Survivors - Petalonamids like Charnia, frond-shaped stem-eumetazoans (animals with tissues, nerves, and muscles) were the only classic Ediacaran forms to make it through. Genetic and fossil evidence shows that the ancestors of many living animal groups were around at the time: they likely contributed to the extinction in the first place. This event marks the transition towards the so-called "Cambrian Fauna" of marine invertebrate biodiversity that characterized the next 40 million years.
Archaeocyathids (Stanton F. Fink, CC BY 2.5)
End-Botomian Extinction Event
When? - ~513-509 Million Years Ago (Middle Cambrian Period)
Cause - Recent work points to roughly 4-pulses of the "Cambrian Explosion" (Benton, 2023). By 513 MYA, this had ceased, and marine invertebrate faunas were abundant in the world's oceans. Evidence from sulphur isotopes suggests that volcanic eruptions in present-day north Australia led to a rise in CO2 and anoxic conditions on the continental shelves (Hough, et al. 2006). Such shifts are prime conditions for disrupting and destroying reef ecosystems; the early Cambrian saw the rise of archaeocyathids, reef-building relatives of sponges, that supported marine communities.
Victims - The last of the petalonamids died out. Archaeocyathids and the reefs they built perished. There were losses among brachiopods (obolellates died out), trilobites (the spiny olenellids in particular), and it has been proposed that the halkieriids & Wiwaxia - potential stem-mollusks - were victims but there are some issues with stratigraphic-dating and there is tentative evidence they survived into the Ordovician.
Survivors - Paleontologists recognize a "Cambrian Dead Interval" following the Botomian in which marine biodiversity was low for a while (Brannen, 2017). The climate was fairly cool and geologists have found evidence of coastal ice within tropical zones that likely kept surviving invertebrates in check (Runkel, et al. 2010). For context, the Burgess Shale community is thought to have come into being following the extinction event.
Early Dresbachian & Franconian Extinction Events
When? - ~502 & 497 Million Years Ago (Late Cambrian Period)
Cause - A series of extinction pulses that are little known. Attention has been drawn to the Steptoean Positive Carbon Isotope Excursion or SPICE, a return to anoxic conditions due this time to coastal upwelling (Bond & Grasby, 2017).
Victims - Trilobites experienced particularly high losses, with the flat-bodied redlichiids dying out and the genera-records of North America and Australia lowering over time (Bond & Grasby, 2017).
Survivors - Marine invertebrate communities remained low in diversity compared to earlier times.
Cambrian-Ordovician Extinction Event
When? - ~485 Million Years Ago (Late Trempealeauan Age)
Cause - The culmination of punctuated Cambrian extinctions, with eruptions in southern Africa contributing to even-lower anoxic conditions in the oceans.
Victims - Among other invertebrate losses, trilobites were reduced in clade-diversity to such levels that they never recovered from their peak in the Cambrian.
Survivors - Despite the losses among trilobites, modelling suggests that a few clades may have entered symbiotic relationships with sulphur-bacteria, ensuring their survival in hard times (John & Walker, 2016). Brachiopods radiated into entirely new groups following the extinction event, while conodonts (lamprey-like early vertebrates) truly began to flourish afterwards. Evidence from Morocco and the United Kingdom show that some of the Cambrian marine fauna survived into the Ordovician (Botting, et al. 2023).
Anatomy of a Bryozoa zooid (SLP456, CC BY-SA 4.0)
Late Ordovician Extinction Events
When? - ~445-444 Million Years Ago (Mid & Late Ashgill Epochs)
Causes - Following the various Cambrian extinctions, marine invertebrate faunas underwent a radical shift called the Great Ordovician Biodiversification Event: the new "Paleozoic Fauna" constituted mainly of brachiopods, bryozoa, echinoderms, graptolites, and cephalopods and remained in place for roughly 230 million years. Two pulses of extinction are recognized at the end of the period, having commonly been attributed to the rise in the Gondwanan Ice Sheet in the southern hemisphere (a sort of proto-Antarctica), which caused a drop in coastal sea levels while simultaneously chilling the tropical oceans. Additional factors have been proposed, including a convergence in volcanic activity that reduced oxygen-levels in the seas and further cooled the climate. Recovery of black shales in North Africa and Arabia indicate that this rapid icehouse cooling was just as rapidly followed by severe greenhouse warming (Brannen, 2017). This is not consensus, however, and other proposals have been work-shopped including volcanic global warming (with no glaciation involved) and extraterrestrial events (Bond & Grasby, 2017, Benton 2023).
Victims - The first wave of extinctions primarily hit free-swimming and planktonic forms, and "multi-branched" graptolites were hit very hard. The second wave of extinctions was less severe, but overall both periods saw significant loses in sessile (fixed to the ocean floor) organisms (including crinoids, "inarticulate" brachiopods, and bryozoa). The iconic giant nautiloid cephalopods like Endoceras died out.
Survivors - Evidence points to a lessening in regional marine faunal diversity but a broadening of geographic ranges for particular organisms. Trilobites, brachiopods, and bryozoa did survive, but now they were a shell of their former variety. Perhaps the most remarkable post-catastrophe boost was in the early vertebrates: jawless fishes had evolved in the Cambrian, but following the Ordovician they radiated, developed paired fins, and gave rise to jawed fishes or gnathostomes.
Silurian Extinctions
When? - Between 432 and 420 Million Years Ago
Cause - A series of three little-known pulses of extinction during the Middle and Late Silurian Periods. The first (the Ireviken Event) seems to correspond to deep-ocean anoxia, while the second and final (the Mulde & Lau events) follow a drop in sea levels
Victims - In the strata in which these extinction events occurred, there were significant losses among trilobites, graptolites, and conodonts. There is evidence to suggest a fall in plantkonic-productivity (Bond & Grasby, 2017)
Survivors - Rugose and tabulate reef-builders remained unaffected
Cause - Five minor pulses of regional extinctions throughout the Early and Middle Devonian Periods, attributed to sea level rises and oceanic anoxia
Victims - Goniatites and agoniatitids, relatives of ammonites, suffered losses, as did trilobites, conodonts, brachiopods, bryozoa, and a little-known group of lophophorates called tentaculitids. An additional group of reef-builders - the stromatoporoid sponges - also experienced declines along with certain rugose and tabulate coral taxa
Survivors - While there were losses among marine invertebrate clades, no major groups died off altogether.
Devonian Reef, showcasing rugose & tabulate corals (James St. John, CC BY 2.0)
Late Devonian Extinctions
When? - Between 372.2-358.9 Million Years Ago
Cause - Long recognized as a singular extinction event (and one of the "Big Five", the Late Devonian is better understood to have encapsulated perhaps two pulses of extinction. The first was largest of the two, the Kellwasser Event, followed millions of years later by the Hangenerg Event which closed out the period. There have been issues with poorly-dated strata, and this has led to conflicting data suggesting times of global cooling and warming (Brannen, 2017). A leading contender for the two pulses had been the rise and spread of terrestrial plants: the evolution of root-systems lead to widespread weathering of rocks and soil run-off, which triggered a particularly deadly combination of ocean anoxia and poisonous planktonic blooms akin to red-tides (Smart, et al. 2023). There is also evidence to suggest volcanic activity played a role in at least the Kellwasser Event (Benton, 2023).
Victims - This was the peak for reef-building organisms during the Devonian, and these events led to such a great loss in rugose & tabulate corals, with the stromatoporoids going extinct, that these ecosystems never recovered. Overall marine life suffered tremendous extinctions across invertebrate and vertebrates groups. Trilobite diversity was cleaved once again, e.g. lichids, corynexochids, harpetids, odontopleuridans, and phacopids. Cystoid echinoderms went extinct. Pentamerid brachiopods went extinct. Most of the jawless fishes and all of the jawed and armored "placoderms" went extinct (think of Dunkleosteus & Bothriolepis).
Survivors - Trilobites only barely made it through the extinction event, with only the proetid clade surviving. Bivalve and gastropod mollusks - originally minor elements of the marine fauna - began to experience a rise and spread of variety. Fish diversity, though severely depleted, did eventually recover under new adaptive radiations, particularly among cartilaginous and lobe-finned groups. There is little evidence to suggest that land floras and faunas were effected by the changes.
Serpukhovian Extinction
When? - Between 330 and 325 Million Years Ago
Cause - A significant mass extinction at the end of the Mississippian or Early Carboniferous Period. Recent isotopic studies point to (you guessed it) deepwater anoxia spreading to shallow coastal waters (Hu, 2022).
Victims - The Mississippian seas were originally home to massive groves of crinoid forests supplemented by surviving rugose corals, but following the Serpukhovian Event these environments experienced a major turnover. Major brachiopod groups suffered losses, as did conodonts.
Survivors - Marine invertebrate faunas remained low throughout the rest of the Carboniferous Period.
Carboniferous Rainforest Collapse
When? - ~305 Million Years Ago
Cause - The famous coal forests full of giant arthropods and reptilian-esque stem-amphibians were only a Pennsylvanian or Later Carboniferous phenomena. The fossil record points to a widespread span of these forests across present-day Europe and North America, which at the time strode the equator (Benton, 2023). On the paleo-continent of Gondwana in the southern hemisphere, glaciers spread and facilitated a massive drop in global sea levels. The coal forests were situatied in low-lying wetlands, and so the great majority of these environments collapsed as the world cooled and dried.
Victims - Coal forests were primarily formed of lycopods, which suffered losses and declines. Within the forests, early tetrapods experienced a drop in species richness (Dunne, et al. 2018).
Survivors - There is evidence of a transition in flora between the coal forest lycopods and tree ferns, which in turn led to a shift in the development of "fixed-channel" floodplains and river systems (Davies & Gibling, 2011). Despite the declines, coal forests did survive in small pockets, with ecological members like Lepidodendron and the giant griffinflies making it to the end of the Permian Period.
Olson's Extinction
When? - 273 Million Years Ago
Cause - Recent research supports a small extinction event occuring at the end of the Early Permian Period (Brocklehurst, 2020). The causes are still being studied, although a rise in global temperatures has been implicated based on the taxa that were effected, whom primarily inhabited wetter environments.
Victims - Among the synapsid "protomammals", most of the early-branching forms went extinct, including edaphosaurids, ophiacodonts, and sphenacodontids (e.g. Dimetrodon). Reptiliomorphs and temnospondyls also experiences losses, as did the fern-dominated floras they relied upon.
Survivors - Therapsids, derived synapsids with more mammalian-traits survived and radiated into several new clades. Seed-bearing plants, including ancestral conifers and ginkgoes, survived and later flourished.
Carnivorous gorgonopsian preying on herbivorous pareiasaurs (Dmitry Bogdanov, CC BY 3.0)
The Great Dying or End Permian Extinctions
When? - Between 259 and 251 Million Years Ago
Causes - Statistically this was the largest mass extinction event in Earth's history, but recent work points to it unfolding in two massive pulses: the Capitanian Event and the Changhsingian Event (Bond & Grasby, 2017; Benton, 2023). An increase in research over the last decade has shed new light on the multifaceted causes of this extinction event. Both events were encapsulated by a spike in volcanic emissions, with the Capitanian perhaps centered on the Emeishan Traps eruptions of South China and the Changhsingian centered on the Siberian Traps eruptions of Russia. Such activity lead to spikes in ocean anoxia and acidification, and the Changhsingian in particular had additional baggage. The Siberian Traps emissions were enormous, on the order of 1.8 million cubic miles of lava flows which are preserved today as flood basaltic deposits 1,300-9,840 feet thick (MacLeod, 2015). Such devastating volcanic activity over several hundred million years pushed C02 levels in the atmosphere and blotted out the sun, triggering a runaway greenhouse effect. This global warming was assisted by the melting of deep ocean methane and underground salts, and the release of sulfates from the volcanoes (which chipped away at the ozone layer). The oceans became choked with acid and were heated to 93-104°F (Benton, 2023), and the land was boiling and suffocating under an sky filled with 2,500 ppm of CO2 (Wu, et al. 2021). The only organisms to really flourish were sulphur-eating bacteria.
Victims - Life across all clades experienced declines and total extinctions, hence the designation of "The Great Dying". In the seas, all the trilobites, rugose & tabulate corals, eurypterids ("water scorpions"), goniatites, and strophomenid & orthid brachiopods went extinct. Fusulinid foraminifera (forams are shelled amoeba-like microbes) went extinct. Receptaculitid marine algae went extinct, and there were sharp losses among land plants, including the Glossopteris forests. This is the only major extinction event to affect insects, and five clades died out including the paleodictyopterids and giant griffinflies (meganisopterans). Many groups of therapid protomammals went extinct, including gorgonopsians, dinocephalians, and biarmosuchians. The parareptiles - early diverging forms with the anapsid-skull condition - also mostly perished. Lepospondyls, amphibian-like stem reptiles, went extinct, while only some temnospondyl groups perished (e.g. the gharial-like archegosaurids).
Survivors - It is estimated that life took several million years to recover, and the earliest Mesozoic Era was for the most part an empty place for some time. Surviving marine invertebrates were all smaller than their ancestors, and many aquatic vertebrates may have survived the devastation by retreating to the deeper ocean depths. Overall, therapsids, reptiles, and temnospondyls survived and would jockey for available land and freshwater niches, the outcome of which would be determined by further catastrophies.
This survey will conclude with the Mesozoic and Cenozoic mass extinctions in the next post...
Book Citations
Michael J. Benton. Extinctions (Thames & Hudson, 2023)
Peter Brannen. The Ends of the World (Ecco, HarperCollins, 2017)
Norman MacLeod. The Great Extinctions (Firefly Books, 2015)
Paper Citations
David P. G. Bond & Stephen E. Grasby, 2017. On the causes of mass extinctions (Palaeogeography, Palaeoclimatology, Palaeoecology)
Joseph P. Botting, et al. 2023. A Middle Ordovician Burgess Shale-type fauna from Castle Bank, Wales (UK) (Nature Ecology and Evolution)
Neil Brocklehurst, 2020. Olson's Gap or Olson's Extinction? A Bayesian tip-dating approach to resolving stratigraphic uncertainty (PNAS)
Simon A. F. Darroch, et al. 2015. Biotic replacement and mass extinction of the Ediacaran biota (PNAS)
Neil S. Davies & Martin R. Gibling, 2011. Evolution of fixed-channel alluvial plains in response to Carboniferous vegetation (Nature Geoscience)
Emma M. Dunne, et al. 2018. Diversity change during the rise of tetrapods and the impact of the ‘Carboniferous rainforest collapse’ (PNAS)
Mojtaba Fakhraee, et al. 2023. Earth's surface oxygenation and the rise of eukaryotic life: Relationships to the Lomagundi positive carbon isotope excursion revisited (Earth-Science Reviews)
Ashley P. Gumsley, 2017. Timing and tempo of the Great Oxidation Event (PNAS)
Michelle Hough, et al. 2006. A major sulphur isotope event at c. 510 Ma: A possible anoxia-extinction-volcanism connection during the Early-Middle Cambrian transition? (Terra Nova)
Malcolm S. W. Hodgskiss, et al. 2019. A productivity collapse to end Earth's Great Oxidation (PNAS)
Dongping Hu, et al. 2022. Multiple S-isotope constraints on environmental changes during the Serpukhovian mass extinction (Earth and Planetary Science Letters)
Douglas L. John & Sally E. Walker, 2016. Testing symbiotic morphology in trilobites under dysoxic and oxic conditions from Cambrian to Early Ordovician Lagerstätten (Palaeogeography, Palaeoclimatology, Palaeoecology)
Charles R. Marshall, 2023. Forty years later: The status of the "Big Five" mass extinctions (Cambridge Prisms: Extinction)
Anthony Runkel, et al. 2010. Tropical shoreline ice in the late Cambrian: Implications for Earth’s climate between the Cambrian Explosion and the Great Ordovician Biodiversification Event (GSA Today)
Matthew S. Smart, et al. 2023. The expansion of land plants during the Late Devonian contributed to the marine mass extinction (Nature Communications Earth & Environment)
Yuyang Wu, et al. 2021. Six-fold increase of atmospheric pCO2 during the Permian–Triassic mass extinction (Nature Communications)
Eating fresh berries off the bush is really such a primal joy. Like not only is it delicious but you feel close to nature, and you’re part of this endless line back to your ancestors before they were even human. We really have always been grabbing delicious fruit to sustain ourselves. In the northern hemisphere now is the perfect time to go blackberrying and I highly recommend! They are p much impossible to confuse with anything inedible so it’s a very easy foraging intro
Deep time is not an easy thing to try and comprehend and it certainly doesn’t help when there’s all these crazy names being thrown at you. Let’s take a moment to break it down and really understand what we are seeing. The largest unit of geochronologic time is an eon. There are four formally defined eons: the Hadean, the Archean, the Proterozoic, and the Phanerozoic. The Hadean Eon is the oldest eon going from 4.6-4 Ga years ago. Its name is derived from the Greek god, Hades in reference to the still molten surface of the newly formed planet.
Following the Hadean Eon is the Archean Eon. This eon went from 4-2.5 Ga years ago. As mentioned in my Barberton Greenstone Belt video two weeks ago, this eon saw the creation of oceans and mountains as well as the appearance of the first living organisms known as stromatolites which would put oxygen into the atmosphere. Archean means “beginning or origin” which is fitting.
Next came the Proterozoic Eon. Lasted from 2.5 Ga-545 Ma. During this eon, the atmosphere became oxygenated, there were several glaciations, and the first eukaryotes evolved represented by the Ediacaran Biota. The name Proterozoic means “before life” because initially, fossils hadn’t been found until the Cambrian Period.
The last eon which we are currently in is the Phanerozoic Eon. It means “visible life” which makes sense as most living organisms have existed within this eon.
Okay, eons make sense? Excellent. Eons are broken down into eras. Now the Hadean is not broken into eras simply because the only evidence of this eon are detrital zircons in Australia.
The Phanerozoic Eon is the current eon and most of you are probably familiar with its eras: The Paleozoic Era, the Mesozoic Era, and the Cenozoic Era. These names mean “old life”, “middle life”, and “new life” respectively.
Eras are then split into periods. Like the Hadean before it, there isn’t enough evidence to split the Archean eras into periods. Again, we will use the Phanerozoic eras to show the break down starting with the Paleozoic.
The Paleozoic Era is split into six periods: the Cambrian Period,
the Ordovician Period,
the Silurian Period,
the Devonian Period,
the Carboniferous Period
and the Permian Period.
These will be discussed in more detail later on down the road. I have already made posts on the Cambrian and the Ordovician Periods and this month is all about the Silurian Period.
The Mesozoic Era is split into three periods of which most people know the names: The Triassic,
the Jurassic,
and the Cretaceous Periods.
Each of these will be discussed in detail further down the road as well.
The Cenozoic Era is also split into three periods: The Paleogene Period,
the Neogene Period,
and the Quaternary Period.
These will also be discussed in detail later.
Periods are further broken down into epochs. There are no formal epochs outside of the Phanerozoic Eon. Some early Paleozoic periods also do no have formal epochs. Many such as the Triassic and Jurassic Period are split into Early, Middle and Late. Strangely, the Cretaceous is only split into Early and Late though I think an argument could me made to create a middle.
The most well-known epochs are those of the Cenozoic Era. The Paleogene Period is split into three epochs: the Paleocene Epoch meaning “old dawn”,
the Eocene Epoch meaning “dawn”,
and the Oligocene Epoch meaning “few new”.
The Neogene Period is split into two epochs: the Miocene Epoch meaning “less new”
and the Pliocene Epoch meaning “more new/recent”.
The Quaternary Period is split into two epochs as well: the Pleistocene Epoch meaning “most new/recent”
and the Holocene Epoch meaning “whole new” and is the epoch we are currently in.
Epochs can then be broken down into still smaller categories called ages. For example, the Late Jurassic Epoch is broken up into three ages: Oxfordian Age, Kimmeridgian Age and Tithonian Age.
Now let’s put all these, eons, eras, periods, epochs, and ages into perspective. Let’s pretend Earth’s current lifespan is a clock. At 12:00, the Earth’s crust forms (Hadean Eon). At 2:10, the oldest rocks are preserved (Archean Eon). At 3:17, the first bacteria appear (Proterozoic Eon). At 11:52, the Cambrian Explosion occurs. At 11:53, first plants and fish. 11:54, first insects. 11:55, first reptiles and amphibians. 11:56, Mesozoic Era begins. 11:57, first dinosaurs, mammals, and birds. 11:59 Cenozoic Era.
Need a different visual? Try a calendar. The Hadean starts in January and ends halfway through February where the Archean picks up. The Archean lasts until halfway through June. From there, the Proterozoic starts and doesn’t end until about Thanksgiving. The Paleozoic Era then goes until about a week before Christmas. The Mesozoic covers Christmas to the December 30. New Year’s Eve represents the Cenozoic Era.
Thanks for coming to this lesson and make sure to tune in tomorrow for some fun trivia! Fossilize you later!
Nothing Can Ensure That We Will Meet Again - Cao Yu
"In this exhibition Cao explores gendered experiences of sexuality and motherhood; connections between life and the afterlife; links between species, and across aeons. Perhaps only in China, for example, could an artist procure a fossil from the Ice Age—a mammoth’s enormous leg bone unearthed in far north-eastern Heilongjiang Province—for an installation that examines profound human and post-human connections. In Nothing Can Ensure that We Will Meet Again (Ice Age - 2014), Cao Yu asks us to confront our deepest fears, and our deepest longings. She inserted the umbilical cord that once attached her to her firstborn child, frozen since 2014 for this precise purpose, into a space dug out of the bone and filled with resin.
"Inlaid and preserved like a prehistoric insect trapped in amber, the knotted cord will survive long past Cao’s own life span, and her son’s. It is a time capsule illustrating the powerful connection between a mother and her infant, but also a reminder of their inevitable separation and mortality."
(source)
How many living beings have there been on Earth -- ever?
It's a safe bet that the vast majority of them have been bacteria, to the point that counting bacteria alone should be enough; anything else vanishes into their rounding error.
Multiple sources suggest 10^30 individual bacteria are alive on Earth right now (that's a thousand billion billion billions, fwiw).
Biogeochemical evidence (Pearce &al 2018) place the origin of life around 3.8 billion years (Ga) ago, and bacteria or bacteria-like organisms must have been around since then. The earliest putative fossils don't look very different from modern bacteria in shape and size.
The life cycle of bacteria is quite variable, but a few hours seems typical for simple free-living bacteria (Lynch & Marinov 2017), and 10 hours seems the most common value outside a laboratory (Gibson &al 2018). Taking 10 h (1/877 of a year) as average, bacteria have been around for about 3.3*10^12 generations. Assuming a constant population, that implies 3.3*10^12*10^30 = 3*10^42 cells -- but not so fast.
Biomass on Earth has increased over time, with large jumps corresponding to major evolutionary advancements. Crockford &al 2023 helpfully estimate a 20-fold increase in global productivity correspoding to the appearance of photosynthesis around 3.5 Ga, a 10-fold increase with the appearence of oxygenic photosynthesis some 2.4 Ga (with a hypothetical overshoot and collapse that I'm going to ignore), and a 10-to-50-fold increase with the ascendance of multicellular life. This implies that the vast majority of life has lived just in the last half a billion years!
To simplify Fig. 1 in the cited paper, I'm making a model where global productivity compared to present is 0.0001 from 3.8 to 3.5 Ga, 0.002 from 3.5 to 2.4 Ga, 0.02 from 2.4 to 0.6 Ga, and then increasingly exponentially from 0.2 to 1 in the present time. From there I get 89% of all productivity ever in the last 0.6 Ga, and 99% in the last 2.4 Ga; since the margins of error are already very high, I can safely discount everything before 600 million years ago.
I'll be assuming that the number of bacteria on Earth is directly proportional to total global productivity in the last 600 million years, i.e. growing exponentially from 20% to 100% of the present number. In bins of 0.1 Ga, I get 0.6 Ga: 20%, 0.5 Ga: 26%, 0.4 Ga: 34%, 0.3 Ga: 45%, 0.2 Ga: 58%, 0.1 Ga: 76%, 0.0 Ga: 100%.
From there, I get 311*10^6 bacteria-years relative to the current biosphere.
Combined with the 10^30 estimate for today, that is 3.11*10^38 absolute bacteria-years. Since I already calculated a line of bacteria experiences 877 generations per year on average, I get 2.7*10^41 bacteria overall -- one order of magnitude less than the naive(r) estimate I got in point 3!
Since the errors on this number completely swamp the amount of multicellular organisms and -- most likely -- Precambrian bacteria, I estimate about 3*10^41 (300,000 billion billion billion billions) individual organisms have existed on Earth since the origin of Life.
