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| Eras of Life, from PosterPlus (Sorry I don't know who the artist is) |
Except, of course, that this picture is misleading - and at worst, a misrepresentation of how evolutionary succession actually occurs.
In a broad-strokes, kiddie-science kind of way (see this Kurtzgesagt episode about "Lies to Children"), the whole "Ages of Life" thing is correct; non-avian dinosaurs, for instance, had a roughly 200 million-year span of dominance on land before being wiped out, and mammals came to prominence. The problem is that this succession didn't occur in the nice, orderly fashion we like to imagine. There was no carnival barker dropping the curtain on each Age, then opening it to reveal a new and astounding tableau: "And then it was...the PLEISTOCENE!" (cue Mammoths and snow). Instead the reality was much messier, more subtle, and in many ways overwhelming to modern scientists as they try to sort things out. The fossil record can only really show us large-scale trends, without explaining the how or the why: we know this fossil appears on this continent within this time frame, but disappears two million years later. It's only when we combine the fossil data with climactic, oceanic, and tectonic records from that same time that we begin to piece together the circumstances that caused the appearance and disappearance of a certain fossil from a certain geologic age.
1. It's All About Rocks
This, in fact, is a crucial point that is often missed when paleontological evidence is presented to the public. Within the past hundred years we have managed to place a very strong theoretical schema around the rocks we have found - i.e, "This bone-shaped rock represents the femur of an Amargasaurus from the braided-river delta of the La Amarga formation in Argentina, an herbivorous sauropod dinosaur of the diplodociod clan that ate such-and-such plants and was preyed upon by such-and-such predators in these-and-those climactic conditions, etc. etc. etc." - but that entire gigantic edifice ultimately has its base in lifeless, dumb, minerals. See folks, no matter how many times somebody says "We found dinosaur bones", they actually mean, "We found rocks that replaced dinosaur bones." Because that's what fossils are. Even the scientific names, in reality, merely describe a collection of rocks. Some fossils may miraculously contain biological tissue, but in the end, there's nothing living left, just a ghostly concrete remnant.*
And as for the whole "Ages" thing? That means, "Layer of Sediment". The famous Jurassic Period, for instance, is named for the Jura Mountains in Switzerland, which displayed a rather striking band of sediment full of seashells and other marine fossils. This lithological unit is then subdivided into three smaller bands, the Upper, Middle, and Lower Jurassic. Their distinguishing features are the kinds of fossils (i.e., rocks) present or absent from their strata, as well as the chemical makeup of each layer. Talk to an oil-prospecting geologist, for instance, and they'll be intimately familiar with the Jurassic and Cretaceous...but only as rocky layers that affect oil retention and drilling. Unless they're also a paleo-nut, it's unlikely they'll know much about the biological implications of these layers.
As always, this explanation is a bit simplistic; there is a technical difference between an actual Era (a time when certain life forms existed and others did not) and the stratigraphic layers themselves - but in broad-brush terms my point is clear: the rocks are the basis. Everything else is an extrapolation.
But it's a damn good extrapolation. By looking at these rock-ghosts we call fossils, we can compare ancient life-forms to those in our modern times, and draw such strong conclusions that the system has become self-referencing: we now compare the fossils of one extinct species to another extinct species in order to determine where it falls on the tree of life. Dinosauria - "Terrible Lizards" - garnered their name because Sir Richard Owen thought their fossils looked more like modern reptile bones than modern mammal bones. But after a few decades of shuffling (and some odd reconstructions), scientists began to reference dinosaur bones against each other more than against any modern analogues. With the basics down, we could then extrapolate outwards to include the types of plants and animals that inhabited ancient ecosystems alongside the dinosaurs, as well as climate and topography of a given place. We have gotten so good at this, it's possible to theorize, with at least some degree of accuracy, a typical March day at Hell's Creek formation during the Cretaceous Period, 66 million years ago.
2. Deep Time Doesn't Stay Still
But as scientists gather more and more data, things become...complicated. The Ages of Life are in constant flux. True, the broad strokes remain relatively consistent - but we're talking very, very broad here. It turns out that one Age of Life often leaks into another, and stories we thought were concluded start popping up for a reboot, over and over again.
Consider the Triassic.
The Triassic is considered the first period in the Mesozoic (the so-called "Age of Reptiles"), when Archosaurs - a group that includes pseudosuchians, pterosaurs, marine reptiles not affiliated with mososaurs, and dinosaurs - came to dominate the globe. Before this, the dominant land species were the synapsids, the group that eventually led to mammals. The vast majority of the synapsids (and a huge chunk of earth's organisms) died off in the Permian/Triassic Extinction, a million-years-long event so cataclysmic that it is known as the Great Dying. For a long time it was assumed that, aside from a few straggler species (and the little pre-mammals), the curtain had dropped on the synapsids with a huge thud.
Then, in 2006, a bombshell dropped - a new species of synapsid had been discovered, dated to the end Triassic. This was the dicynodont Lisowicia bojani, and it was a behemoth - about the size of an elephant, and considering its stout frame, probably even heavier. Dicynodonts (literally, "Double-Dog-Tooth") were a peculiar type of synapsid, strongly associated with water, with skulls featuring flat beaks and a pair of hanging tusks; it was known they had hung on through the end-Permian extinction, but it was assumed they were a dwindling last gasp. But with this new discovery, it seems that dicynodonts were not merely hanging on into the Age of Reptiles, but thriving.
The clue is in the size of Lisowicia. As a general rule, organisms don't reach elephant-sized bulk unless certain factors are met, including an availability of food and space, the toughness to resist predatory pressure, the sociability to live in large herds, and the aggression to horn in on resources. At this time, the world was dominated by crocodilian relatives known as pseudosucians (dinosaurs had only recently stepped onto the scene), and so the fight for survival was intense. The dicynodont relics from another age managed to hold their own several tens of millions of years longer than the rest of their kind, surviving at least until the Triassic-Jurassic extinction.
The frustrating fact is, we don't know what kind of phylogenic context Lisowicia evolved in. Were there dozens of other dicynodont species surviving at this time? What about other non-mammalian synapsids? And what does this mean for the Great Dying - was it really not as "Great" as it appeared? We know that synapsids called Tritylodonts survived into the Early Cretaceous, living alongside a healthy population of placental, marsupial, and multituberculate mammals...it's bewilderingly anachronistic, like having tribes of Australophithecus wandering around New York City, interacting with humans like it's no big deal.
The Case of the Wandering Synapsids isn't the only example of organisms popping up long after they were supposed to be extinct; for instance, Cambrian animals showing up in the Ordivician, tens of millions of years after they should have died out. And these alien invertebrates weren't "locked in time", but had evolved significantly from their Cambrian ancestors. The takeaway here is, it's very, very difficult to kill an organism off entirely (just ask any exterminator), and while it's reasonable to assume that a particular Age was devoid of a particular organism, there's always the possibility that a new discovery will show them surviving long past their expiration date.
3. There Are No Such Things As Intermediates
Wait wait wait, put down your pitchforks: I'm not saying "There are no intermediate/transitional fossils" - that is, no "links" between one form of life and another. I'm saying the concept is stupid: the idea that one life form is somehow waiting or striving to become something else. Yes, I know scientists don't mean it that way, but that's inevitably how it's presented: life as a ladder, a set of rungs to be achieved. Fish just can't wait to get out of the water; amphibians just can't wait to develop scaly skin and hard eggs; apes are striving to leave the trees and walk upright. Et cetera. And inevitably, the poor creatures stuck between the golden ideals of FISH and FROG are doing all the grunt work, changing and wriggling and forcing out all those toes, oh wait that's too many toes, good now shrink that tail fin, very good aaaaaand PRESTO! The Amphibian. Cue applause.
It's all rather insulting. We talk a lot about "transitional organisms", so much so that we completely miss the point: we're all transitional. We're all evolving toward something. And not just in the personal growth sort of way, either: the creature we call Modern Man, while functionally the same as when we first evolved 200,000 years ago, is still noticeably different. Nobody could put Clovis Man side-by-side with Frank from Accounting and fail to tell that something has changed, even if the general outline is similar. And 200,000 years from now, a Homo martius, i.e. a Mars-adapted human, will look back at Homo sapiens and say, "I came from that?" And will he think Frank from Accounting, a complete human being with his own hopes, dreams, drives, and fears, is nothing more than a transitional subspecies between himself and Clovis Man?
This attitude we have, in which any creatures evolutionarily "between" two Golden Ideals (ie. Bird, Fish, Mammal, etc.) is nothing more than a "transition", has larger implications for our understanding of life. Remember how I mentioned the weird idea that life somehow "stops" between Ages, as though one form of life is clearing out of the way so another can enter? The fact is, this never happens. Even after mass extinctions - the most mass of mass extinctions - life finds its footing fairly rapidly. The leftovers don't just kind of quietly slink around, they get busy.
You see, the history of life isn't so much the kind of conquest wars we like to imagine, where organisms are set up in pitched battles and "triumph" over one another; instead, it's more like a huge, eons-long marathon, with no holds barred, and random natural disasters periodically wipe out 50% of the contestants. If those at the front of the race suddenly disappear, the runner-ups immediately jostle their way to the front, exploiting every possible advantage. Being human, we try to divide up the leg of each race by which particular group of runners disappears or gains the lead, but often the old runners are mixed in with the new, and form a respectable group of their own despite being far past their heyday. We also have the problem, as observers of this marathon, that we can't see the whole race at once - we can get "snapshots" at certain points, but these only capture small moments in the contest, and still images don't tell you anything about what comes before or after a certain point until you arrange them all in a sequence.
This "snapshot" problem is deeply troubling to Paleontologists, regardless of how much they adore the "mystery" of it all. See, for all the millions of fossils we do have, representing hundreds of thousands of organisms, we only have access to a minute fraction of the planet's total fossil load (which, incidentally, only represents a tiny fraction of the organisms that ever existed). It's very simply a case of being able to get at some, but not being able to get at the rest of them. The western United States is heavily represented in the fossil record because a) the original conditions were good for fossilization (i.e., heavy silt deposits, quick burials, large numbers of animals), and b) the land is now arid and eroded, exposing fossils on the surface. When we think of dinosaurs, we immediately think of Cretaceous Western North American dinosaurs - Tyrannosaurus, Triceratops, Deinonychus, Hadrosaurus - despite the fact that this area was isolated during much of the Cretaceous, essentially a long, thin island, full of relatively weird dinosaurs descended from northeast Asian ancestors.
5. Evolution happens fast...then stalls
We have an understanding of evolution as a sort of slow, gradual process; by human standards, of course, it's pretty slow. Organisms take several million years to show any kind of significant physiological change. But from a view of the fossil record, we see something perplexing: instead of "transitional" species popping up in nice, orderly rows every million years or so, we instead see explosions of new species arising immediately when conditions become ideal (usually when an extinction clears many niches, but other factors can allow this too), followed by long periods of little change occurring at all. The most powerful example of this is the Cambrian Explosion, in which a sharp rise in oxygen levels in the oceans allowed the rapid evolution of hundreds of new complex organisms from the relatively simple animals which preceded them. So sudden is this appearance in the fossil record, that some posited a meteorite had brought the creatures to earth. While we have found enough transitional fossils for a "proof of concept", there's still an embarrassing paucity of links between organisms in the fossil record.
To explain this weird stopping and starting of evolution, Niles Eldredge and Stephen Jay Gould proposed the theory of Punctuated Equilibrium. Essentially it states that, given the right conditions, a relatively small population of organisms will experience a burst of adaptive radiation until they fill all possible open niches, at which point their numbers increase so much that they "dilute" most of the occurring mutations, and remain the same until either a) their population shrinks and allows mutations to propagate again, or b) they go extinct and allow a new set of organisms to fill their niche and repeat the process.
The finches of the Galapagos Islands, known collectively as Darwin's finches, are cited as an example of adaptive radiation - an organism diversifying to fill all available niches - but they are also a great example of Punctuated Equilibrium. Studies have shown that they are no longer evolving, or at least their evolution has slowed to a point where they are no longer diversifying. They've filled all the niches they can. They might get better and better at fulfilling their particular role, but unless something drastic happens, they aren't going to change much for the next several million years. And the fact is, we'll probably never know what the original finch looked like, because chances are it never fossilized. The only birds in the fossil record on Galapagos will be the already-diversified finch species. Their adaptive radiation happened in an evolutionary blink of an eye, and now they've settled down into their comfortable niches.
4. Extinction
a) Extinction is Constant, and Climate-Driven
This last point deserves an entire post to itself. Something we often fail to grasp, is that each and every geologic division is punctuated by extinction. We hear a lot about the Asteroid What Killed the Dinosaurs (the K-T mass extinction), and we definitely hear a lot about the Anthropocene Extinction - the vast numbers of organisms going extinct on our watch and usually because of our activities. We might even hear about some other Mass Extinctions as well, especially the Permian-Triassic Extinction, perhaps the deadliest such event in earth's history. There were many, many more mass extinctions before life flopped out onto dry land, which we may have a hazy acknowledgment of. But what about the smaller extinctions? What about the Triassic-Jurassic Extinction? Or the Jurassic-Cretaceous Extinction? Or the fact that each of these three period of the Mesozoic are further divided into Upper, Middle, and Lower strata, each defined by yet another extinction? Or that even these subdivisions are further subdivided? And even these sub-subdivisions could be divided even further. Because, as we've mentioned, each stratigraphic layer is defined by the presence of certain rocks (fossils) and the absence of others. Such-and-such a shell appears in such-and-such a layer; in the subsequent layer, it is no longer present. In one layer, the organism was living, dying, and becoming fossilized; a layer later, it was not.
This may seem like a ridiculous amount of extinction, until you consider the time scales we're dealing with here. A quiet period of even a measly 1 million years is still 5 times as long as modern Homo sapiens sapiens has existed in the world. By that measure, even a "short lived" species is fantastically successful.
But through all these extinctions, one question keeps popping up: what actually causes the extinctions? I mean, an asteroid is terrifying and all, but would a single explosion, no matter how devastating, be powerful enough to wipe out 75% of species, including 99.99% of the most successful dominant land animals the world has ever seen (non-avian dinosaurs)?
Once again, we turn to the rocks to get an answer. After every major extinction event, there are not only different fossils present, but also a change in the chemistry of the strata, over and above the usual "lava flow/sediment/forest" distinctions of the particular area. Oxygen is one key indicator of a healthy biosphere; the more O2 trapped in the rocks, the more was available in the atmosphere and oceans. Many of the largest mass extinctions, especially the Great Dying, show a catastrophic drop in O2 levels.
And what is the biggest single cause of fluctuating O2 levels? Say it with me: Climate Change! That's right, folks. While there's a multitude of factors that go along with climate change, one of the most important is that the warmer the oceans, the less oxygen they hold. And since we're talking about gas dissolved in liquid, anoxia can happen very, very fast - think of how quickly gas escapes when you open a soda can. We're talking massive die-offs here. When climate changes, the oceans are affected first and most catastrophically; we see this reflected in the fossil record, where land organisms tend to fair better in extinctions than their sea-living counterparts.
Of course, climate change is so much more than just O2 levels. Even a minor drop in temperature, as occurred from the 13th-19th centuries, is enough to generate mass chaos. There is evidence that the "Little Ice Age", as it was called, was responsible for the collapse of the Mayan and Cambodian empires due to water shortages; the 17th century saw massive conflict and famine break out across Europe as crops failed. I'm not going to say that all the incidence of mayhem in human history was directly caused by climate change, but there is a strong correlation between global temperature change and the rise and fall of empires. Consider the Bronze Age Collapse of 1200 BCE: it's strongly correlated with the explosion of Santorini, a megavolcanic event which, through the spread of ash in the upper atmosphere, may have partially blocked sunlight enough to cause crop failure in the Mediterranean region. Could this have pushed the "Sea Peoples" to migrate, leading to opportunistic attacks against the already weakened Bronze Age Empires, resulting in a near-literal Dark Age as the civilizations collapsed?
b) The "Weak Empire" hypothesis
I wanted to address one particular thing that always nagged me about the "competition between species" and "survival of the fittest" narrative of pop-evolutionary science: do species, on a level playing field, really "outcompete" one another? Is there really a sort of Capitalist struggle going on, as though each species is a company trying to consume each other's market share? If we really take a look at life in the broad angle, we tend to see a lot more commensalism than competition. Think about the Amazon rainforest: how can such a (relatively) small geographic realm hold more species than the rest of the world put together? Wouldn't they all "outcompete" each other?
The fact is, on the whole, species trend toward the avoidance of competition. Competition takes energy away from the more important task of acquiring resources and mates. And so long as there is an abundance of resources, organisms seem relatively happy (or at least tolerant) of sharing them - many predators, for instance, can enjoy the same prey species, so long as they don't get in each others' way. Life is not necessarily a zero-sum game. That's why, when I hear things like, "The arrival of the canines and felids into North America doomed the borophagines ("bone-crushing dogs") to extinction" or whatever, I get annoyed: life doesn't work like that.
Let's take the example of those cats and bone-crushing dogs. If you know anything about the Miocene, you'd know that North America was teeming with megaherbivores, choice fare for both native and invading predators. The borophagines initially had the continent to themselves, as the Bering Strait divided NA and Siberia at that time (as it does today). However, when global temperatures wobbled and the sea level dropped, a whole new influx of mammals entered North America over the Bering Land Bridge. With the abundance of megaherbivores, the new and old predators should have coexisted. And in fact they probably did for a while. But eventually the bone-crushers disappeared from the fossil record, while the true cats and dogs dominated until the present day.
So what happened? Was it a case of direct competition? Was it one of those playground "Who would win in a fight" arguments, like Animal Face-Off? Did all the cats and dogs steal all the carcasses from all the bone-crushers, or at least enough times that the bone-crushers were steadily beaten back?
I'm going to posit that, no, direct competition didn't lead to extinction - because that's stupid. Monumentally stupid. Look at hyenas and lions on the Serengeti, for instance: animals described as "Eternal Enemies", whose brawls are epic and tantamount to pitched battles. Hyenas are even a great analogue to the Bone-Crushing Dogs, which exhibited the same heavy builds and feeding styles. And yet they do not outcompete each other. Lions and hyenas share the same environment and same set of prey species, and are arguably evenly matched in terms of predatory capability...and yet, though they've coexisted for millions of years, neither of them are responsible for the dwindling numbers of the other (humans are! Go us). Now, one example doesn't prove a rule, but I think it's major evidence that the whole "fight to the death" thing doesn't quite add up.
Okay, maybe it was because the cats and dogs were an invasive species? We've had thousands of examples in our modern world of invaders destroying biospheres by outcompeting, consuming, or even changing the environments of native creatures in their lust for conquest. Look at Florida, or Hawaii, or Guam: ecological disaster areas. Hell, look at domestic house cats, which gobble up all manner of small tetrapods by the billions, or ship rats, which contributed to the demise of the poor doomed dodo and other flightless birds. Invasive species are fascinating because they don't come into an area and just install themselves in their accustomed niche, but change their biology to invade as many niches as they possibly can. Surely a plague of cats and dogs was enough to overwhelm the poor doomed bone-crushers?
This is another narrative that simply falls flat upon closer examination. It's true that invasive species are enormously destructive: introduce mammalian predators to an environment where there were none before, for instance, and you can kiss those beautiful birds goodbye. But we have to remember that North America already had a robust predator-prey dynamic when the invasive predators arrived - this was no virgin landscape full of innocent fauna, ripe for the plucking. Invasive species aren't super-organisms (at least at first); they still have to be able to establish themselves with enough resources and territory to produce a viable, self-sustaining population. One might argue that, upon arrival, invasives are subject to more evolutionary pressure than the native species, since they have to adapt quickly to a new environment or simply fade out of existence. In order for an invasive species to truly take off, there needs to be another factor at play: the local ecosystem must be under stress.
If we examine any ecosystem where invasives have gained a strong foothold - hell, even a suburban backyard - we realize that the environment was under a certain amount of stress to begin with. Some niche needs to be either unfilled, or filled by a species which, a) has experienced a dearth of a certain resource, or b) is having trouble due to a climatic shift (and I'm not even talking Big Climate here - I'm talking one bad winter or dry summer). If we take the suburban backyard as an example, maybe it was once filled with Creeping Charlie, but suddenly yellow clover has taken root. Did the yellow clover just horn in unnannounced, becoming the Most Fit Organism? Or did a sudden heat wave bake the earth, starving the Creeping Charlie of water and nutrients and allowing the hardier yellow clover to spread into new territory? I'm putting my money on the latter scenario. Yellow clover and Creeping Charlie might have coexisted side-by-side for years, until a sudden - even subtle - shift weakened the territorial holdings of one and allowed the other to spread.
You see how climate snuck back in? Climate. Is. Everything. Let's tie this all back into the cats & dogs vs. borophagine scenario. In order for the Bering Land Bridge to open up, giving the new predators access to North America, the climate had to cool enough for glaciers to advance and siphon up a lot of ocean water. The resulting colder, dryer air meant that the warm woodland conditions which existed for most of the bone-crushers' time on earth began to transform into grassland. As the plant life changed over to more dry-adapted flora, the bone-crushers' prey began to disappear from their accustomed latitudes. Prey became more scarce. Meanwhile new, strange herbivores, to which the bone-crushers were little accustomed (and may not even registered as "prey"), began to infiltrate these environments, changing them even further.** The bone-crushers' population began to fragment, and with fragmentation came genetic stress.
From the north, following the Siberian prey, came the cats and dogs. It may have taken hundreds of thousands of years for them to gain a foothold, or it may have taken a few decades. Maybe the new predators and the borophagines really did come into individual contact, just as lions and hyenas do today. Maybe they really did fight over carcasses. Would one organism always triumph in a fight? Probably not. It wouldn't matter anyway. The climatic conditions were such that the bone-crushers, even if they did hold their ground, ultimately ceded territory to the true cats and dogs.
At the risk of falling into the very martial framework I wish to destroy, I call this the "Weakened Empire" hypothesis. It comes from the evidence of human history, wherein empires don't simply crumble against an attacking horde of barbarians, but fall due to two separate yet related factors: their own internal weakness, and the vast movement of another group of humans into their territory. Both of these conditions are met when climate change occurs and resources become scarce. Empires run on their stomachs; a heavily-networked, stationary empire (the Romans, for example) are fairly invulnerable to barbarian attack so long as crops are plentiful. Conversely, nomadic peoples tend to stay independent of one another and in their own territories, so long as fodder is plentiful for their flocks and (especially) horses. But change the balance of nature even slightly, and nomadic pasturelands become less conducive to their flocks. They need to move in order to survive. This movement leads to banding together, and opportunistic raiding against sedentary villages, and even political conglomeration. Waves of "barbarians" begin to slam against the frontiers of the Empire. Under these conditions, even a slight hiccup in the food supply of an empire can cause disruptions in the military along the borders; the empire cedes territory in order to consolidate, and the nomads settle in to the newly-opened territory. If the climatic changes continue, further eroding the food supply, the empire will eventually collapse under external pressure and internal disorder; since the "barbarians" are not an established population, but waves of different peoples, they will eventually wash over the former imperial territory and settle in to divide up the resources.
Now, you might wonder, how does this follow in terms of native and invasive species? Consider that the native species are already adapted. Under the related theories of Adaptive Radiation, and Punctuated Equilibrium, already-adapted species are in equilibrium due to the large sizes of their population; i.e. they don't evolve much. When rapid climate change occurs, they are, behaviorally and genetically-speaking, caught off guard. But invasive species have the upper hand, since they are moving with a climate they are already adapted to; meanwhile they are adapting as they foray into their new environment. They have much more flexibility, both genetically and behaviorally, than the natives. If you think of the native species as an empire, and the invasives as "barbarians", the invasives can only take over the natives' territory once the empire is weakened. In terms of human empires we tend to think of martial prowess and strategy, and that's certainly a factor; but the reality is that the conditions of weak natives and adaptable invaders must be met.
*This also explains why "ichnofossils" - fossilized trackways and other traces - are given scientific names as well. Until there is some degree of certainty that "The organism represented by x bone-rocks produced y fossilized trackway", the track itself is classified only by its characteristics, given hand-wavey names like Megapus ornithoides ("Giant Birdlike Foot"), or Velocipus mongoliensis ("Swift Foot of Mongolia").
This threw me for a weird loop when I first heard it. It brought up a whole new, interesting point about the Linnaean binomial classification system: it's not meant to specifically classify organisms. rather, it classifies things, some of which happen to be organisms.
This last point deserves an entire post to itself. Something we often fail to grasp, is that each and every geologic division is punctuated by extinction. We hear a lot about the Asteroid What Killed the Dinosaurs (the K-T mass extinction), and we definitely hear a lot about the Anthropocene Extinction - the vast numbers of organisms going extinct on our watch and usually because of our activities. We might even hear about some other Mass Extinctions as well, especially the Permian-Triassic Extinction, perhaps the deadliest such event in earth's history. There were many, many more mass extinctions before life flopped out onto dry land, which we may have a hazy acknowledgment of. But what about the smaller extinctions? What about the Triassic-Jurassic Extinction? Or the Jurassic-Cretaceous Extinction? Or the fact that each of these three period of the Mesozoic are further divided into Upper, Middle, and Lower strata, each defined by yet another extinction? Or that even these subdivisions are further subdivided? And even these sub-subdivisions could be divided even further. Because, as we've mentioned, each stratigraphic layer is defined by the presence of certain rocks (fossils) and the absence of others. Such-and-such a shell appears in such-and-such a layer; in the subsequent layer, it is no longer present. In one layer, the organism was living, dying, and becoming fossilized; a layer later, it was not.
This may seem like a ridiculous amount of extinction, until you consider the time scales we're dealing with here. A quiet period of even a measly 1 million years is still 5 times as long as modern Homo sapiens sapiens has existed in the world. By that measure, even a "short lived" species is fantastically successful.
But through all these extinctions, one question keeps popping up: what actually causes the extinctions? I mean, an asteroid is terrifying and all, but would a single explosion, no matter how devastating, be powerful enough to wipe out 75% of species, including 99.99% of the most successful dominant land animals the world has ever seen (non-avian dinosaurs)?
Once again, we turn to the rocks to get an answer. After every major extinction event, there are not only different fossils present, but also a change in the chemistry of the strata, over and above the usual "lava flow/sediment/forest" distinctions of the particular area. Oxygen is one key indicator of a healthy biosphere; the more O2 trapped in the rocks, the more was available in the atmosphere and oceans. Many of the largest mass extinctions, especially the Great Dying, show a catastrophic drop in O2 levels.
And what is the biggest single cause of fluctuating O2 levels? Say it with me: Climate Change! That's right, folks. While there's a multitude of factors that go along with climate change, one of the most important is that the warmer the oceans, the less oxygen they hold. And since we're talking about gas dissolved in liquid, anoxia can happen very, very fast - think of how quickly gas escapes when you open a soda can. We're talking massive die-offs here. When climate changes, the oceans are affected first and most catastrophically; we see this reflected in the fossil record, where land organisms tend to fair better in extinctions than their sea-living counterparts.
Of course, climate change is so much more than just O2 levels. Even a minor drop in temperature, as occurred from the 13th-19th centuries, is enough to generate mass chaos. There is evidence that the "Little Ice Age", as it was called, was responsible for the collapse of the Mayan and Cambodian empires due to water shortages; the 17th century saw massive conflict and famine break out across Europe as crops failed. I'm not going to say that all the incidence of mayhem in human history was directly caused by climate change, but there is a strong correlation between global temperature change and the rise and fall of empires. Consider the Bronze Age Collapse of 1200 BCE: it's strongly correlated with the explosion of Santorini, a megavolcanic event which, through the spread of ash in the upper atmosphere, may have partially blocked sunlight enough to cause crop failure in the Mediterranean region. Could this have pushed the "Sea Peoples" to migrate, leading to opportunistic attacks against the already weakened Bronze Age Empires, resulting in a near-literal Dark Age as the civilizations collapsed?
b) The "Weak Empire" hypothesis
I wanted to address one particular thing that always nagged me about the "competition between species" and "survival of the fittest" narrative of pop-evolutionary science: do species, on a level playing field, really "outcompete" one another? Is there really a sort of Capitalist struggle going on, as though each species is a company trying to consume each other's market share? If we really take a look at life in the broad angle, we tend to see a lot more commensalism than competition. Think about the Amazon rainforest: how can such a (relatively) small geographic realm hold more species than the rest of the world put together? Wouldn't they all "outcompete" each other?
The fact is, on the whole, species trend toward the avoidance of competition. Competition takes energy away from the more important task of acquiring resources and mates. And so long as there is an abundance of resources, organisms seem relatively happy (or at least tolerant) of sharing them - many predators, for instance, can enjoy the same prey species, so long as they don't get in each others' way. Life is not necessarily a zero-sum game. That's why, when I hear things like, "The arrival of the canines and felids into North America doomed the borophagines ("bone-crushing dogs") to extinction" or whatever, I get annoyed: life doesn't work like that.
Let's take the example of those cats and bone-crushing dogs. If you know anything about the Miocene, you'd know that North America was teeming with megaherbivores, choice fare for both native and invading predators. The borophagines initially had the continent to themselves, as the Bering Strait divided NA and Siberia at that time (as it does today). However, when global temperatures wobbled and the sea level dropped, a whole new influx of mammals entered North America over the Bering Land Bridge. With the abundance of megaherbivores, the new and old predators should have coexisted. And in fact they probably did for a while. But eventually the bone-crushers disappeared from the fossil record, while the true cats and dogs dominated until the present day.
So what happened? Was it a case of direct competition? Was it one of those playground "Who would win in a fight" arguments, like Animal Face-Off? Did all the cats and dogs steal all the carcasses from all the bone-crushers, or at least enough times that the bone-crushers were steadily beaten back?
I'm going to posit that, no, direct competition didn't lead to extinction - because that's stupid. Monumentally stupid. Look at hyenas and lions on the Serengeti, for instance: animals described as "Eternal Enemies", whose brawls are epic and tantamount to pitched battles. Hyenas are even a great analogue to the Bone-Crushing Dogs, which exhibited the same heavy builds and feeding styles. And yet they do not outcompete each other. Lions and hyenas share the same environment and same set of prey species, and are arguably evenly matched in terms of predatory capability...and yet, though they've coexisted for millions of years, neither of them are responsible for the dwindling numbers of the other (humans are! Go us). Now, one example doesn't prove a rule, but I think it's major evidence that the whole "fight to the death" thing doesn't quite add up.
Okay, maybe it was because the cats and dogs were an invasive species? We've had thousands of examples in our modern world of invaders destroying biospheres by outcompeting, consuming, or even changing the environments of native creatures in their lust for conquest. Look at Florida, or Hawaii, or Guam: ecological disaster areas. Hell, look at domestic house cats, which gobble up all manner of small tetrapods by the billions, or ship rats, which contributed to the demise of the poor doomed dodo and other flightless birds. Invasive species are fascinating because they don't come into an area and just install themselves in their accustomed niche, but change their biology to invade as many niches as they possibly can. Surely a plague of cats and dogs was enough to overwhelm the poor doomed bone-crushers?
This is another narrative that simply falls flat upon closer examination. It's true that invasive species are enormously destructive: introduce mammalian predators to an environment where there were none before, for instance, and you can kiss those beautiful birds goodbye. But we have to remember that North America already had a robust predator-prey dynamic when the invasive predators arrived - this was no virgin landscape full of innocent fauna, ripe for the plucking. Invasive species aren't super-organisms (at least at first); they still have to be able to establish themselves with enough resources and territory to produce a viable, self-sustaining population. One might argue that, upon arrival, invasives are subject to more evolutionary pressure than the native species, since they have to adapt quickly to a new environment or simply fade out of existence. In order for an invasive species to truly take off, there needs to be another factor at play: the local ecosystem must be under stress.
If we examine any ecosystem where invasives have gained a strong foothold - hell, even a suburban backyard - we realize that the environment was under a certain amount of stress to begin with. Some niche needs to be either unfilled, or filled by a species which, a) has experienced a dearth of a certain resource, or b) is having trouble due to a climatic shift (and I'm not even talking Big Climate here - I'm talking one bad winter or dry summer). If we take the suburban backyard as an example, maybe it was once filled with Creeping Charlie, but suddenly yellow clover has taken root. Did the yellow clover just horn in unnannounced, becoming the Most Fit Organism? Or did a sudden heat wave bake the earth, starving the Creeping Charlie of water and nutrients and allowing the hardier yellow clover to spread into new territory? I'm putting my money on the latter scenario. Yellow clover and Creeping Charlie might have coexisted side-by-side for years, until a sudden - even subtle - shift weakened the territorial holdings of one and allowed the other to spread.
You see how climate snuck back in? Climate. Is. Everything. Let's tie this all back into the cats & dogs vs. borophagine scenario. In order for the Bering Land Bridge to open up, giving the new predators access to North America, the climate had to cool enough for glaciers to advance and siphon up a lot of ocean water. The resulting colder, dryer air meant that the warm woodland conditions which existed for most of the bone-crushers' time on earth began to transform into grassland. As the plant life changed over to more dry-adapted flora, the bone-crushers' prey began to disappear from their accustomed latitudes. Prey became more scarce. Meanwhile new, strange herbivores, to which the bone-crushers were little accustomed (and may not even registered as "prey"), began to infiltrate these environments, changing them even further.** The bone-crushers' population began to fragment, and with fragmentation came genetic stress.
From the north, following the Siberian prey, came the cats and dogs. It may have taken hundreds of thousands of years for them to gain a foothold, or it may have taken a few decades. Maybe the new predators and the borophagines really did come into individual contact, just as lions and hyenas do today. Maybe they really did fight over carcasses. Would one organism always triumph in a fight? Probably not. It wouldn't matter anyway. The climatic conditions were such that the bone-crushers, even if they did hold their ground, ultimately ceded territory to the true cats and dogs.
At the risk of falling into the very martial framework I wish to destroy, I call this the "Weakened Empire" hypothesis. It comes from the evidence of human history, wherein empires don't simply crumble against an attacking horde of barbarians, but fall due to two separate yet related factors: their own internal weakness, and the vast movement of another group of humans into their territory. Both of these conditions are met when climate change occurs and resources become scarce. Empires run on their stomachs; a heavily-networked, stationary empire (the Romans, for example) are fairly invulnerable to barbarian attack so long as crops are plentiful. Conversely, nomadic peoples tend to stay independent of one another and in their own territories, so long as fodder is plentiful for their flocks and (especially) horses. But change the balance of nature even slightly, and nomadic pasturelands become less conducive to their flocks. They need to move in order to survive. This movement leads to banding together, and opportunistic raiding against sedentary villages, and even political conglomeration. Waves of "barbarians" begin to slam against the frontiers of the Empire. Under these conditions, even a slight hiccup in the food supply of an empire can cause disruptions in the military along the borders; the empire cedes territory in order to consolidate, and the nomads settle in to the newly-opened territory. If the climatic changes continue, further eroding the food supply, the empire will eventually collapse under external pressure and internal disorder; since the "barbarians" are not an established population, but waves of different peoples, they will eventually wash over the former imperial territory and settle in to divide up the resources.
Now, you might wonder, how does this follow in terms of native and invasive species? Consider that the native species are already adapted. Under the related theories of Adaptive Radiation, and Punctuated Equilibrium, already-adapted species are in equilibrium due to the large sizes of their population; i.e. they don't evolve much. When rapid climate change occurs, they are, behaviorally and genetically-speaking, caught off guard. But invasive species have the upper hand, since they are moving with a climate they are already adapted to; meanwhile they are adapting as they foray into their new environment. They have much more flexibility, both genetically and behaviorally, than the natives. If you think of the native species as an empire, and the invasives as "barbarians", the invasives can only take over the natives' territory once the empire is weakened. In terms of human empires we tend to think of martial prowess and strategy, and that's certainly a factor; but the reality is that the conditions of weak natives and adaptable invaders must be met.
There's so much more to discuss about the Eras of Life, and naturally I'm not doing the subject justice. I think my main focus is the weirdness of it all, the sort of "left of center" things that don't make it into the usual discussions. I'll try to cover more of this stuff in future posts.
Rick Out.
*This also explains why "ichnofossils" - fossilized trackways and other traces - are given scientific names as well. Until there is some degree of certainty that "The organism represented by x bone-rocks produced y fossilized trackway", the track itself is classified only by its characteristics, given hand-wavey names like Megapus ornithoides ("Giant Birdlike Foot"), or Velocipus mongoliensis ("Swift Foot of Mongolia").
This threw me for a weird loop when I first heard it. It brought up a whole new, interesting point about the Linnaean binomial classification system: it's not meant to specifically classify organisms. rather, it classifies things, some of which happen to be organisms.
**Something not often mentioned here, is that open-range prey species act differently than woodland prey species: herds are much larger, and herding behavior is tighter and more coordinated. While woodland herbivores - even very large ones - are more likely to freeze and hide in the dense undergrowth, grassland animals have no choice but to either run or fight. It is much easier for a predator to hide next to a forest game trail and ambush a passing animal, than to try and separate an individual out of a large, watchful grassland herd. Borophagine dogs were evolved to ambush large herbivores in relatively wooded environments, not to run down prey like modern canids. They might have been able to adapt to their changing environment and prey fauna eventually...but the invading predators didn't give them the chance.
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