Author: Massimo Sandal

The song of ice and diatoms

In a previous post on this lethargic blog, I briefly touched the Antarctic extinction, the mass extinction of the fauna and flora inhabiting a whole continent, reduced to a desert of ice.

As is too often happens, I worried myself only about the macroscopic biota, and lazily forgot the microscopic lifeforms. Luckly, scientists are smarter than I am. Eveline Pinseel and coworkers have now described, in a paper on Science Advances, what happened to some of the most iconic microbial taxons – diatoms – that inhabited Antarctica (I have to thank Sylvie Coyaud for bringing this to my attention). Diatoms are special for many reasons, but mainly as being an example of microorganism with incredibly beautiful and unique shells, that readily fossilize and can be classified into prehistoric species and genera, something that it is hardly possible with many other unicellular beings (a notable exception are foraminiferans).

Extant Southern hemisphere diatoms and their extinct relatives, from Eveline et al. 2021

The study shows that in the Miocene (14-15 millions of years ago) Antarctic lakes had a rich diatom flora, with unique species so far unknown to science, but strongly related to those now present in New Zealand, Tasmania; and was overall not unlike the flora present in the contemporary Arctic. Temperatures, now hovering around -12° C, were around +5 °C at the time.

Then, the ice came. In the Pleistocene, about 150.000 years ago, the climate of Antarctica was much similar to today and, correspondingly, diatoms were much less diverse than in the Miocene, but still more diverse than today. The last glacial period was the final hammer for the Antarctic diatom flora: more diatom species were wiped out, not unlike the mammoths, or were confined to sub-Antarctic realms. The diatoms of today’s Antarctica land are a relict of what was once a diverse flora, adapted to some of the harshest conditions on Earth.

According to the authors

Although there are multiple records of regional extinctions and species turnover of lacustrine diatoms in Quaternary paleorecords of the Northern (60) and Southern (61) Hemisphere, the scale of the extinction of diatoms in continental Antarctica since the mid-Miocene is, both at the species- and genus-level, beyond anything reported in the literature thus far.

In other words, the Antarctic extinction was also the most profound mass extinction of diatoms known stodayo far. Their fate was most probably shared by large parts of the Antarctic microflora, of which we cannot say anything only because they didn’t leave shells to fossilize. Mass extinctions ,therefore, events reshape biodiversity at all levels. This is not news per se, we know that for example the end-Cretaceous extinction led to the extinction of numerous foraminiferan species. But it is a somber reminder of how much biodiversity has been lost forever, how much is going lost now, without us even noticing, without us being even able to interpret what such a loss would mean for other lifeforms.

The paper is: Eveline Pinseel et al. “Extinction of austral diatoms in response to large-scale climate dynamics in Antarctica”, Science Advances, Volume 7, Issue 38, 15 September 2021, https://doi.org/10.1126/sciadv.abh3233

Blood of birds, disintegrating butterflies, two end-Permians, innocent volcanism

So, this blog has a loooooong backlog of stuff I wanted to talk about but I didn’t. Old stuff, like, one year old or more. Alas, I cannot hope to make a complete post on all that, but I don’t want to let it rot. Let’s do a bit of quick catching up:

  • Bird blood on our hands. In 2019 a couple of papers pointed directly the fingers at humans for the extinction of the great auk and the Carolina parakeet, two once-widespread, iconic species of birds that went extinct in the XIX century. Both papers analyzed paleogenomes (is it right to use the ‘paleo’ prefix when it’s a couple centuries ago?) and found out that both species populations had a vibrant genetic diversity until their numbers fell abruptly to zero. Which means: no, they weren’t already fragile, declining species that we gently pushed off a cliff they would have met anyway. We systematically exterminated two robust, healthy bird species in the space of a few decades or centuries. Not exactly unexpected, but now there’s more proof.

Singapore Coney Island Butterfly - Free photo on Pixabay

  • Butterflies that we will never know. In Singapore, 46% of the butterfly species disappeared (locally) in a mere 160 years, according to a paper of February 2020. Interestingly enough, the study accounts for extirpations of undetected species, using a model. I’m in no position to comment on the math, but the very idea is intriguing and melancholic: about a hundred of species would have gone extinct before we ever discovered them. Of these, some could have well been endemic species: ghosts, of which now we have nothing else than numbers in a statistical analysis.  “14.9% of the species discovered before 1900 also were extirpated before 1900. These high early observed extirpation rates, during a period where many species remained to be discovered, suggest that a high number of species were never detected before they were extirpated”

    The Karoo Basin, in South Africa, where the best deposits on terrestrial end-Permian/early-Triassic fauna are preserved.
  • A tale of two end-Permians. Discerning a single event that happened 252 millions of years ago is incredibly hard; discerning a complex interplay of events even harder. Compared to the relative simplicity of the Chicxulub impact, the Permian extinction is a maddening puzzle, muddled by its remoteness in time. There were always hints of multiple extinction events or at least multiple “hits” that led to the Permian catastrophe, but now a paper of March 2020 seems to imply that the extinction on the sea was different from the extinction on the land. It seems that whatever happened on the continents, leading to the demise of most terrestrial fauna and the temporary dominance of Lystrosaurus, happened 300.000 years before the extinction in the oceans: “Instead of the currently favored paradigm of calamitous and globally synchronous turnover in ecosystems, the reported terrestrial turnover in Gondwana occurred hundreds of thousands of years before the marine one and, therefore, marine and terrestrial responses likely had different extinction mechanisms.“. We have to see if and how it will be confirmed, but if so, it seems that the end-Permian extinction is truly two extinctions, above and below water. It will be extremely interesting to grasp how did one influence the other, and how does it translate to our current situation.
Balaur bondoc, an avialan dinosaur that lived at the end of the Cretaceous
  • Innocent volcano. In January 2020, Pincelli Hull and coworkers put another nail in the coffin of the volcanic hypothesis for the K/T extinction. The K/T event has the distinction of having two competing or possibly synergic explanations: the well known Chicxulub asteroid impact, and the Deccan traps, a major volcanic event. For decades scientists have fought on what of these events was most important, and even if the impact seemed more and more clearly the culprit, the Deccan enthusiasts didn’t lose their grip. However, if the study is correct, it seems that 1)Deccan outgassing isn’t chronologically correlated to the extinction, but the impact is, and 2)the Deccan volcanism simply wasn’t generating enough gas to trigger an extinction, since similar events didn’t alter the biosphere so much. Another paper a few months later even argued that, if the Deccan volcanism had any effect, it was mitigating the extinction effects.

The continent that died

Antarctica is now the closest thing on Earth we have to another world: a barren continent almost entirely covered in miles-thick ice, with temperatures going below -80° and practically devoid of macroscopic life once you go beyond the coast.

It wasn’t always so.

Yesterday night I was reading the a bit outdated but still amazing The Origin and Evolution of Mammals by T.S.Kemp and my jaw dropped to the floor when I stumbled on the following sentence:

“Needless to say” but oh dear, that’s when the obvious slaps you in the face, and it needs to be said. For tens or hundreds of millions of years Antarctica was a living, flourishing continent like the others, covered in forests, teeming with life. Right before the ice came, Antarctica looked like this:

Picture from Reguero et al. “Antarctic Peninsula and South America (Patagonia) Paleogene terrestrial faunas and environments: biogeographic relationships” Palaeogeography, Palaeoclimatology, Palaeoecology 179:3-4 (2002) https://doi.org/10.1016/S0031-0182(01)00417-5

I will have to dig deeper into this subject, that I suspect deserving a book (if one isn’t already there), and now I have no time to detail why and how it happened. But just let that sink in: The death of Antarctica is one of the main tragedies of the biosphere in the last 66 million years. Imagine if tomorrow Europe or South America, with all their life forms, their forests, their rivers, the singing of birds and the buzzing of bees, if all of that simply disappeared. That is what happened between 45 and 34 millions of years ago. Ice started to build up in the middle Eocene, and by the end of the period it was a frozen desert.

Imagine the slow death: a cap of ice every year slowly crawling from the core of the continent towards the coasts, animals and plants pushed to the edges, the winters every year more freezing, the summers every year shorter, until the ice reached the sea and there isn’t anywhere else to go. The penguins are basically the only survivors of this tragedy, relicts of a rich ecosystem.

What happened to Antarctica was an extinction of major proportions, but confined to a specific continent. Was it a mass extinction? Perhaps we need more categories, we need to start a taxonomy of extinction events. I will think about it. But for now, just remember: every time you see the beautiful glaciers and icebergs of Antarctica, you are witnessing the grim burial of an entire continent full of life, that was and now is gone.

 

Diese Liebe macht krank

One of the things probably most people do not suspect about Germany is that it has a vibrant gangsta rap scene, whose main leaders is the infamous crew 187 Strassenbande from Hamburg. And, especially, one rapper: Gzuz.

As he himself sings, “the guy is real, whether he comes from Compton or Altona“. He’s indeed a character: He did, among other things, two or three years in jail for armed robbery, slapped a swan, and celebrated New Year with automatic weapons. His over the top, so-real-it’s-unreal videos shocked hip hop fans in Usa.

Why are we talking of gangsta rappers here? One of the best known Gzuz hits is CL500. A celebration of his car, symbol of the wealth he got by rapping, it’s about as deliciously Asozial (a term that in Germany is about the same as chav in England) as it can get:


Why are we discussing German gangsta rap here? I was intrigued by how, in the song, Gzuz is proud to show how much he loves to waste energy and fuel. He loves to enjoy his car even when it does not make sense:

Die Straßen hoch und runter, auch wenn ich keinen Grund hab’

(«Up and down the streets, even when I have no reason to»)

And in the end:

Und wir blubbern, blubbern, blubbern
Blubbern, blubbern, blubbern, blubbern
Wir blubbern, blubbern, fünf Euro weg
Zehn Euro weg

(«And we gurgle, gurgle gurgle, we gurgle, five Euro away, ten Euro away.», referring to the engine roaring and easily burning five, ten Euros of fuel)

This sounds funny, a kind of childish boast: but is it so different from what happens actually all the time in our civilization or, perhaps, in every civilization that has been blessed with cheap energy? It’s the Jevons paradox: the more energy is cheap and available, the more energy consumption is efficient, the more we use it. But there’s more: the CL500 of Gzuz is actually, as far as I can read, quite fuel consuming. He’s not just consuming more fuel because he can, he loves that waste. It’s more of a potlatch, a waste of valuables to show off wealth.

It’s hardly the most pressing issue in the whole problem of climate change, but it’s interesting to see that there are, indeed, cultural barriers to reduce energy and fossil fuel consumption. We are in love with the sheer ability to use cheap energy whenever we can, however we can. This very blog post is inessential: it consumes energy when I write it, when you read it, when it is stored on a server. But oh, how easily inessential things today become essential tomorrow; how necessary is luxury after a while.

Alles ist entspannt wenn er rollt, rollt
Digga, erst der CL und Erfolg folgt

(«Everything is easy when it rolls, rolls
Dude, first the CL and then success follows»)

It rolled for a while. Western civilization has had an intense love story with fossil fuels, which as many love stories first inebriated us, then intoxicated us. They gave us an unprecedented freedom, but at a price. Getting out of toxic relationships is a hard job, and painful. But we have to, because, as Gzuz says, when describing his addiction to the smell of gasoline:

Diese Liebe macht krank

«This love makes you sick».

 

Stealing iron from a snail

What is an island? One thinks of a patch of land circled by the ocean, but islands are concepts. From a biological point of view, whenever you are stranded on a place with no hope to escape, you are on an island.

Hydrothermal vents, among the weirdest and most fascinating environment on Earth, are insular environments. Their hot, peculiar and extreme conditions quickly give space to the usual cold peace of the abyss, and creatures which live on a vent rarely can venture elsewhere. They are submerged islands: patches of an environment surrounded by something entirely else, its creatures prisoners depending on their prison and, as such, fragile.

Deepsea Scaly foot gastropod (Crysomallon squamiferum ) from Dragon vent field, Indian Ocean.

The reality of the fragile vent ecosystems has been put forward last year with the debut, in the IUCN Red List, of the scaly foot snail (Chrysomallon squamiferum), the «scaly foot snail» or «pangolin snail».

I name it the iron snail: since it’s the only one metazoan which naturally uses iron (in the form of iron sulfide) to build its armor. As it often happens, islands allow the tree of life to explore avenues it couldn’t elsewhere, in relative peace. The iron snails are little, parallel experiments of evolution, running since tens of millions of years; current hydrothermal vent species started to diverge after the K/T event and the Paleocene/Eocene thermal maximum.

The iron snail however reverberates, in its unsightly scales, the much more ancient history of its lineage. An analysis of its genome reveals that its sclerites are formed by repurposing an ancient biomineralization genetic machinery, perhaps more ancient than mollusks themselves. Distant cousins of mollusks which teemed in the Cambrian seas, such as Halkieria, were covered of a chain armor of sclerites as well. «It is unclear whether the evolution of the sclerites in the Scaly-foot Snail should be interpreted as a recurring ancestral phenome, or a recently derived adaptive novelty.», states the paper. Whatever the truth, it means that covering themselves with scales is a recurring option in the evolutionary toolkit of mollusks and mollusk-like organisms. The iron snail, in the blind warm depths of the oceans, tells us a story much larger than itself.

So far, the iron snail does not know anything about being in danger. But it is threatened by deep sea mining, since two of its three known hydrothermal vent habitats are now considered for exploitation. Ironically enough, they could be mined for iron, among other metals. The iron snail evolved millions of years to exploit the metals of a deep ocean. Until a few thousands of years ago no other animal on Earth knew how to use them. How weird would it be for the snails, if they could know, that a species that breathes air and evolved in the savannahs now wants to steal their metal, the metal that for millions of years was theirs, and theirs alone.

Paper: Sun, J., Chen, C., Miyamoto, N. et al. “The Scaly-foot Snail genome and implications for the origins of biomineralised armour.” Nat Commun 11, 1657 (2020). https://doi.org/10.1038/s41467-020-15522-3

Improbable ashes

The Copernican Principle states that we are not privileged observers of the Universe. It is a restatement of the mediocrity principle: there is nothing exceptional about us, about the planet, about our history and so on.

Copernicus is also the name of one of the most prominent and iconic lunar craters, testimony of a massive impact roughly 800 million years old. And while impacts are, per se, relatively common events -one needs only to look at the Moon to see how many happened in the history of the Solar Systems, not all impacts are the same. And some, maybe, are exceptional: so much to make our own history exceptional.

A new paper by the Expedition 364 to the Chicxulub crater has compared the actual geological makeup of the crater to various impact simulations and found something interesting. Most asteroid impacts are oblique, following highly inclined trajectories:

Only one quarter of impacts occur at angles between 60∘ and the vertical and only 1 in 15 impacts is steeper than 75∘.

But, once again, Chicxulub is exceptional. It seems it most resembles craters from steep impacts, about 45-60 degrees. The Chicxulub impactor was steep but not quite vertical hit, zooming right on Earth from the north-east:

Comparison of these observations with our simulation results suggests that the observed configuration is most similar to the 60∘ impact simulations (or possibly the 45∘ impact simulation at 20 km/s; Fig. 5).

And this was very bad news for the end of the Cretaceous, because they are exactly the conditions that lead to maximum disaster:

Impacts that occur at a steep angle of incidence are more efficient at excavating material and driving open a large cavity in the crust than shallow incidence impacts5,19. Our preferred impact angle of ca. 60° is close to the most efficient, vertical scenario[…]

Impact angle has an important influence on the mass of sedimentary target rocks vaporised by the Chicxulub impact37. […] a trajectory angle of 30–60° constitutes the worst-case scenario for the high-speed ejection of CO2 and sulfur by the Chicxulub impact37. At this range of impact angles, the ejected mass of CO2 is a factor of two-to-three times greater than in a vertical impact and approximately an order of magnitude greater than a very shallow-angle (15∘) scenario37. An absence of evaporites in the IODP-ICDP Expedition 364 drill core is consistent with highly efficient vaporisation of sedimentary rocks at Chicxulub27. Our simulations therefore suggest that the Chicxulub impact produced a near-symmetric distribution of ejecta and was among the worst-case scenarios for the lethality of the impact by the production of climate-changing gases.

So much for the Copernican principle, this further reinforces that the Phanerozoic history is punctuated by improbable events. A giant 10-20 km asteroid such as the Chicxulub impactor hits every few hundred million years. We know of no other impact causing a mass extinction. And Chicxulub happened in the worst possible spot (a shallow sea with bottom rocks rich in sulfur and carbon), in the worst possible moment (terrestrial ecosystems dominated by very large -and thus vulnerable- vertebrates; Chicxulub would have been far less important in the Cambrian or the Silurian!) and, now we know, probably with the worst possible geometry.

Here it is, from the paper: in twenty second, a 30-km deep wound was dug, and vaporized rock was thrown beyond 20 km high in the sky. We are now on Earth because of this event. If the asteroid were a few minutes early, a few minutes late, a few hundred km somewhere else, the history of life would have been completely different. Sometimes the Copernican principle is a principle, not a law. It is a useful guidance, but after all, it would be very weird if no rare, strange events at all happened. We are all improbable ashes of a fateful day that started 66 millions of years ago and still has not ended.

The paper is: Collins, G.S., Patel, N., Davison, T.M. et al. A steeply-inclined trajectory for the Chicxulub impact. Nat Commun 11, 1480 (2020). https://doi.org/10.1038/s41467-020-15269-x

Darwin of a billion years

Is there any chance of predicting the history of life? On the footsteps of Gould, that would seem a foolish quest. Rewind the tape and replay it, he said, and you will see a different movie every time: infinite possible worlds lost in the tiniest of chances. And yet it is all too tempting to search for general rules, and sometimes we are rewarded. What if we could predict what creatures can survive multiple extinctions, on the scale of half a billion years?

This apparently absurd proposition is what a recent study by Matthew L. Knope, Jonathan L. Payne et al. has shown at the end of this year’s February. They analyze a huge amount of living and fossil marine genera -30,074 and 19,992 respectively, another proof of the power of big data when dealing with the history of life- and try to correlate the taxonomic diversity to the ecological diversity over time. That is, the number of species of a clade versus the ecological strategies exploited by members of that clade.

One could find it obvious that there is some correlation -after all, if you have lots of genera, you can imagine there are more chances they evolved to different niches- but it is not necessarily always so. And this is the main finding of Knope et al.: this correlation evolves over geological time.

The graph on the left shows the (log-log) correlations between the richness of genera on the x axis and the ecological diversity on the y axis, divided by geological period. The “rays” must be interpreted dynamically. The dark lines laying low are the correlation as it were in the Paleozoic, starting from the Silurian and Devonian. Going forward the correlation shifts: turquoise and green lines, corresponding to the Mesozoic, grow steeper, until in the Tertiary -yellow lines- it becomes almost double than the Paleozoic one. In other words, the more you go forward in time, the more ecological diversity correlates with taxonomic diversity.

Now, this is nice but hard to interpret. Now see the graph on the right: the slopes of the lines on the left are plotted against time. You can see the vertical dashed lines separating mass extinctions. It is not always convincing, but there is some hint that mass extinctions did something. After each extinction, ecological diversity becomes more tightly related to taxonomic ones.

What is going on? We are seeing how extinctions shape the composition of life on the scale of the Phanerozoic. It is not that ecologically diverse clades are better at diversifying and evolving: actually what they find is that, at least in the Tertiary, they diversify less than ecologically poor clades. Each ecological crisis instead tends to wipe out clades that were not flexible enough, forced to live within a few ecological modes. Clades that instead could exploit more ecological niches were able to find at least some of their members surviving a crisis. They diversify and repopulate the oceans for the simple reason they are the ones surviving, and they survive because they have the strategies to do so.

The classes that had high taxonomic richness and low ecological differentiation during the Paleozoic, such as rhychonelliform brachiopods and crinoids, consisted largely or entirely of nonmotile suspension feeders that mostly cannot occupy infaunal or pelagic habitat tiers. In contrast, the classes and phyla that are genus-rich in the Cenozoic (e.g., mollusks, arthropods, and vertebrates) are generally motile, feed in a variety of ways, live across many habitat tiers, have more control over gas exchange with the environment, and have weathered mass extinctions well.

Let’s take it in what this means. First, this means that we have some hope of predicting what animals groups would survive extinction better than others. Animals who are more motile, more versatile, that exploit more environments and more strategies will have better changes. Brachiopods were dominant in the Paleozoic, but a glance at their physiology compared to that of bivalves, in hindsight, shows us they didn’t have a chance. They were simply too rigidly adapted to a single mode of life to fare well after a catastrophe such as the end-Permian.

Second: Our seas are not different from the ones of the Paleozoic just because we have different animals. Our ecosystems have been selected. The animals we have today in the sea are quicker, more mobile, smarter at exploiting different niches, have better metabolisms – and they are like that because they are the hardened survivors of multiple ecological crises. As Knope explains:

The oceans we see today are filled with a dizzying array of species in groups like fishes, arthropods, and mollusks, not because they had higher origination rates than groups that are less common, but because they had lower extinction rates over very long intervals of time.

We are seeing natural selection acting not on the scale of the individual or of the species: we are seeing it acting on entire ecosystems on the scale of half a billion years. Knope et al. show that the shadow of Darwin is longer than we imagined, as long as the tape of Gould, shaping not only single species but the entire composition of life along hundreds of millions of years.

Paper: Knope et al., “Ecologically diverse clades dominate the oceans via extinction resistance“, Science 367, 1035–1038 (2020)

Songs of the rising sun: I

It is particularly poignant, now that we are in the middle of the covid crisis (that also stopped this blog), to reflect on what happens after the catastrophe. We have no idea of what we world we live will be, only that it will be different, that we will miss the past, that we will see something new unfold.

The same holds for mass extinctions. I’m working on a feature on extinction recovery, something I curiously forgot discussing when writing my book, and so I will drop some reading notes here. As it often happens, one misses the best papers right after publishing something, and I don’t know how I managed to miss the brilliant review by Pincelli Hull on life in the aftermath of mass extinctions. I mean, just the sentence “Rather than a plodding tortoise, extinction is a hare – racing in fits and starts” would have been perfect in my book. Oh well, it’s never too late.

The first point of Hull is that extinction recovery and species radiation takes place in a strange world. The world right after Chicxulub or after the Permian is not business as usual, with just some players missing. It is a bizarro world of precarious ecosystems with entire parts missing, of still recovering geochemical cycles. Disaster taxa dominate, entire ranges of body size are gone, the atmosphere is still recovers. It is in that arena that the destinies of evolutionary lineages are played.  Take it after the Permian catastrophe:

Coral and metazoan reef systems were replaced by microbial carbonate mounds for up to six million years [16]. Key marine functional types including macroalgae, metazoan suspension feeders, mobile predators and deposit feeders were lost or rare for at least the first million years [16]. Complex burrowing of benthic sediments remained relatively rare for yet another ~4 million years […] In the aftermath of the PT extinction, certain
features were preserved in rocks that had not occurred since
abundant metazoans fully colonized the soft sediments of the
seafloor [55]. These features, and other anachronistic structures
[56], are important because they imply that certain ecological
strategies were so rare (or even absent) that they no longer
had a readily observable effect on ecosystems.

Something that comes up from different lines of evidence is that recovery is never a monolith: different environments and groups recover at different speed:

Although ammonites suffered very high levels
of extinction during the end-Permian, they diversified within
the first 2 million years after the extinction […] Benthic taxa, by contrast, generally took more than 5 million years to fully recover pre-extinction levels of diversity and community complexity […] Regardless of the cause, variation in diversification rates amongst taxa is typical of mass extinction aftermaths more generally. On land, some ecological strategies lost at the extinction boundary re evolved immediately while others, like small-bodied insectivory and large bodied herbivory, took more than 15 million years to reappear [64], resulting in distinct Early Triassic food-web structures in the meantime [65].

However there is an act of balance between the intrinsic factor, such as different competition or niches opening and closing, and instead extrinsic factors such as the quenching of volcanism after the main extinction event (Hull reminds that volcanic outgassing and anoxia were still present for millions of years in the Triassic), or the carbon cycle stabilization. Ammonite diversification in the Triassic seems coupled to the carbon cycle, for example. Post-extinction ecosystems seem weirdly unstable; for example various dynasties of dominance and decline follow each other for coccolithophores in the Paleocene, with no apparent reason.

Hull here posits a theory, that these two factors cannot be taken in isolation. Geochemistry affects ecosystems but ecosystems affect geochemistry as well:

On a global scale, the structure and function of ecosystems affects key earth-system processes, such as rates of weathering, soil formation, organic carbon sequestration, nutrient availability and recycling, and the availability of key substrates such as soils and reefs [78, 79, 80]. These processes move various elements between earth system reservoirs, for instance from the ocean to the atmosphere or from soils to streams. On a global scale, the time it takes for various elements to move, on average, between reservoirs can be quite long (100 years to 10 million years) [81]. As various earth-system reservoirs and fluxes change, they have the scope to, in turn, affect ecosystems on a global scale, because they alter the prevailing nutrient availability and environmental conditions

This reciprocal feedback results in what Hull calls earth system successions: not only biomes are different after mass extinctions, but the whole functioning of the planet. The extinction is thus followed by a state of instability while the systems comes back to a novel equilibrium. The waves of disaster ripple for millions of years before settling down.

Hull also reminds us that mass extinctions are not the only game in town when it comes to major evolutionary upheavals. The Paleocene-Eocene warming event was a minor extinction event but a major turnover in megafauna (modern whales and pinnipeds arise there). The rise of flower plants during the Cretaceous was a massive evolutionary event in the history of Earth but it did not follow any major extinction, as far as we know.

What Hull teaches us is that mass extinctions are not point events; they are ripples of enduring change that interweave with other ever changing conditions:

In the prolonged aftermath, ecosystem change across the globe exerts an evolutionary influence distinct from the extinction itself, with a timing characteristic of the earth system (i.e., earth system succession). As such, mass extinctions should not be considered as macroevolutionary point events, but rather as prolonged intervals of varying selection spanning the mass death and subsequent radiation of taxa.

A deep, sobering perspective. And I want also to note: I believe such insight can come only from the awe and respect Hull has for the monumental task of reconstructing Earth’s history, something that we should all consider. Speaking of the Permian:

“To step back for a moment, just consider how remarkable
this hypothesis is — to argue that we can trace the trigger of
mass extinction a quarter of billion years ago to a single pulse
of volcanism in Siberia and its cascade of environmental effects.
It is astounding and relies on dramatic improvements in our
ability to precisely date the geological time scale, to discern
the relative amount of time captured in very thin layers of rock
and to measure various aspects of the environment with
geochemistry.”

It is marvellous to live in the age of humanity when this history unfolds under our eyes.  To see such a majestic landscape in time and be moved by it, is what makes it possible for us to appreciate its complexity.

Paper: Hull, P. (2015). Life in the aftermath of mass extinctions. Current Biology, 25(19), R941-R952. https://doi.org/10.1016/j.cub.2015.08.053

Sick of biodiversity?

The new Chinese coronavirus is the current talk of the town. As for every emerging zoonotic disease, the relationship between environment exploitation, biodiversity and spillover casts its shadow. Therefore, just as a side note, I am driven to reread this review that appeared in December on Nature Ecology and Evolution. The main message is relatively technical, and it is that the relationship between biodiversity and disease is not linear, but depends on the geographics scale we’re looking at. But to my simple mind, a few words are useful as a remind of our grim, conflictual relationship with the living world: ecosystems are both richness and danger.

Ecosystems regularly pose a threat of disease to humans and wildlife,

Which does not mean that wiping out forests is a good way of managing zoonosis. On the contrary:

targeting conservation toward protecting ecosystems that are not currently posing a major threat of problematic disease to humans or wildlife might prevent increases in disease

and

preservation of intact, functioning ecosystems and finding sustainable, equitable interventions that discourage human incursions into those ecosystems (for example, for logging and bush-meat hunting), could reduce the risk of transmission of multiple pathogens, even if these interventions are not the single most efficient control method for individual diseases. Thus, they could represent win–win scenarios for conservation and disease control.

There is hope, then. But the whole review is interesting for whoever wants to see how little we know and how many the nuances. Ecosystems bring beauty and disaster. Despite what the term “ecosystem service” brings to mind, they’re not there to serve us. They are features of nature, and should be treated with the circumspection and respect this implies.

Charting the tides of life: a fine grained history of Paleozoic biodiversity

Is there a view grander than this?

We speak of evolution in the history of Earth, of the millions of years, of entire groups of living beings waxing and waning, but our view on the actual timeline has always been tragically blurry. Deep time does not let things escape its clutch with ease. Fossils are always few and far between, and we can never be certain of having sampled everything. If we see a fossil species popping out, say, 400 millions of years ago, who’s to say it was not there 450 millions of years ago too, but we didn’t find it? If we see it dying 300 millions of years ago, who’s to say it didn’t survive somewhere undocumented? (The formal term for all this is the Signor-Lipps effect) That is what happened to the coelacanth, for example: considered dead for 66 millions of years, until Marjorie Courtney-Latimer found a fresh one in a fish market.

But we slowly have more and more data, and we have more and more computer power to devise their meaning from sophisticated probabilistic model. That is what the breathtaking paper from Jun-xuan Fan et al. published on Science is all about. Using a huge amount of data on marine fossils from Chinese strata, spanning from 538.85 (Early Cambrian) up to 244.41 (Early Triassic), and processing it on one of the most powerful Chinese supercomputers, the Tianhe II, they managed to chart the rise and fall of marine biodiversity (pardon: marine mostly animal biodiversity: as usual non-animal groups are forgotten, fusulinids being an exception) in the Paleozoic, with a time resolution of about 26.000 years. Limiting to China is not a problem as big as it seems: today’s China is a puzzle of geological units that once span from North to South pole, all around the world.

It might seem that a resolution of 26.000 years is fuzzy -a ‘pixel’ of their timeline is almost four times longer than the history of civilization- but it’s geological time we are talking here. A span of 294 millions of years, the most recent 244.41 millions of years ago. Split into 11326 intervals. This is truly high resolution, and the result is magnificent.

Look at the image above. On top you see the global genus and species biodiversity (note the two different scales left and right), on the bottom the number of species split into different taxonomic categories. The coloured time intervals are  1, Great Ordovician Biodiversification Event; 2, end-Ordovican mass extinction; 3, early Silurian radiation; 4, Middle to Late Devonian diversity decline; 5, late Carboniferous–early Permian biodiversification event; 6, end-Permian mass extinction.

Extinctions happen on all variety of timescales. One plot, and we can finally end forever the war between ‘instant mass extinction’ catastrophism and Lyellian gradualism. There is, simply, a spectrum of all timescales. The razor-sharp vertical cliff at the end of Permian is clearly visible: in a few tens of thousands years, life on Earth was all but wiped out. On the other hand, in the Devonian the number of animal species halved slowly but steadily between about 400 to 370 millions of years ago, and only started to recover about 330 millions of years ago. Is that a mass extinction, only as long as the time interval separating us from the dinosaurs? How do we conceptualize this sluggish but constant loss of biological diversity? Recovery events are also variable: sometimes biodiversity immediately bounces after a decline, sometimes there are millions of years of stasis before life diversifies again.

It is also interesting to my former-scientist eye how many phenomena we believed in, such as the end-Guadalupian extinction, could have been nothing more than artifacts. Past charts of biodiversity history were much more coarse grained, and counted genera and families into million years-long bins. A poorly appreciated fact is that lowering the resolution does not only hide details, it also creates false details, by lumping and restructuring the information. The illusory canals on Mars appeared to XIX-early XX century observers due exactly to that.

We often imagine paleontology as a single-fossil discipline. One digs and finds an Archaeopteryx, a Sinosauropteryx, a Taung Child, a Hallucigenia, a key set of bones and stones that makes or breaks an evolutionary hypothesis. Sometimes it is true, but to appreciate the river of life, one must look beyond the individual drops that make it. Palaeontology is a lot about statistics, and it is amazing how the progress in searching, cataloging and analyzing big data on past biodiversity is yielding a picture of our complex past.

Ernst Rutherford (maybe) said that science is either physics or stamp collecting. I hope it was just a silly joke; a stupider take on science is hard to find. Here we see the epitome of stamp collecting: thousands of shells, traces on rocks, legs, bones being lovingly removed from their hundred-of-million-years graves, described and pinpointed on the history of life. The result is nothing less than the irregular heartbeat of the tides of life, the endless tree of forms growing and wilting, under ever diverse, endless forces, in a time incomprehensibly long and remote, and yet no less real.

The paper is: Fan, Jun-Xuan et al. A high-resolution summary of Cambrian to Early Triassic marine invertebrate biodiversity Science, 17 Jan 2020 : 272-277

(Also read the Perspectives on Science and Nature)