Feature - Issue #8
A palaeontologist reflects on historic extinction events and what we might learn from them today.
One of the stranger things about being a palaeontologist is the insouciant way we trade in millions of years. For much of my working life I studied trilobites, a huge and varied group of extinct marine arthropods, those invertebrates with jointed appendages that include today’s insects and crustaceans. Trilobites thrived for more than 250m years. One genus of trilobite I know well lasted for 40m years - that is, about 20 times as long as our own hominine genus Homo. Yet when I talk about trilobites to audiences (at least those at literary festivals) I am often asked why my favourite animals “failed” to survive - as if to become extinct was somehow a weakness of character as much as an indication of lack of success in clearing life’s hurdles. My animals were not one of the winners in the cutthroat competition for survival - as the popular conception of evolution might express it. Alas, poor trilobites!
It is interesting how this idea of extinction as failure has even infiltrated our own lives. Businesses invoke resilience through a crisis as a case of adapt or die. “We must not be dinosaurs in a competitive world where the devil takes the hindmost,” they might say, thereby mixing metaphors to portray the mercilessness of the market. There is an element of triumphalism in surviving some financial crisis or other, and more than a dash of censoriousness directed at those less fortunate. Rather like the boxer who comes back to win after taking a pummeling, the resilience of a survivor is seen as virtue rather than providence.
I had plenty of reasons to ponder extinction and survival when I was commissioned two decades ago to write a narrative history of life on Earth. The vast expanses of geological time drove the narrative through 400 pages. To over-simplify, that was about 10,000,000 years per page. Of course, much of that book - Life: an Unauthorised Biography - was focused on the last 500m years or so, the Phanerozoic, the period when complex animals and plants arose and proliferated, and through the first half of which trilobites swarmed in the everlasting seas. The conquest of the land, and then of the air by animals that lived at the same time as my own favourite creatures were central parts of the story; but so also were the setbacks - the times when any temptation to portray life’s history as a series of unfolding triumphs for DNA were called into question. Life was stalled in its tracks by mass extinctions.
There were five mass extinctions in the last 500m years. A mass extinction is recognised when more than about 75% of species are reckoned to have died out within a very short time interval; some extinctions were probably multiple rapid events. Most life scientists believe that we are now in the sixth such extinction, one for which humans are overwhelmingly responsible. An inventory of creatures that will be resilient enough to survive this latest crisis is being assembled right now. What I want to examine here is what happened in the distant past, when life encountered serious brakes on its proliferation.
Past mass extinctions have attracted much serious research around the globe. There are some common features of these investigations, even though the conclusions drawn are disparate. An essential requirement is to discover rock sections in sedimentary rocks that were laid down throughout the crisis period in question. These are likely to record a blow-by-blow account of the events that led to the disappearance of animals and plants. The rocks tell tales. Field parties collect and identify all the fossils that betray a biological world in turmoil. No single rock section tells the whole story, as the extinction pattern may well be different at different parts of the world or various water depths. The rocks themselves can yield evidence of life support systems - things like the level of oxygenation, or the subtle signatures of aridity. Geologists and palaeontologists have vied to find the most revealing sedimentary sections, and claims of the rocky equivalent of the Rosetta Stone have made the pages of prestigious scientific journals.
There is no obvious periodicity in the major extinction events: the end Ordovician event at 442m years ago; the late Devonian event (373m); the end Permian event (252m); Triassic-Jurassic event (201m); and end Cretaceous (66m). The last is the event that has seeped into general consciousness: the end of the dinosaur giants that have thrilled so many children and fuelled many a blockbuster. Somehow the simultaneous extinction of the marine ammonites does not generate such a frisson, let alone the disappearance of so many tiny, single-celled foraminiferan species - but they all add up. To a trilobite specialist the fact that my group of animals surfed the end Ordovician and Devonian events (although both took a toll) is ample evidence of their resilience under pressure, but they did eventually all pass away at the end of the Permian, along with as much as 90% of their contemporaries, - the “mother of all mass extinctions” as Doug Erwin of the Smithsonian Institution described it. Just about everything in the ecology on land and sea was remade after this event. The forests that appeared later were different from those of the Carboniferous Period (359m to 299m years ago) that laid down the coal that fuelled the industrial revolution - and unwittingly started the climate crisis we are experiencing now. The coral reefs that thrived in the sea as the ruling reptiles of the Jurassic bestrode the land were different in kind from those that the trilobites would have known. So even after the greatest of mass extinctions the biological world was remade, but in many ways the ecologies after the extinction event as a whole were like those that preceded it. To use another famous metaphor: the cast changed but the play went on.
Each mass extinction had its own signature. The end of the dinosaurs was the only one associated with a major meteorite impact. The end Ordovician extinction probably began with a major glaciation. Several of the events were accompanied by huge eruptions of basaltic volcanic rocks and concomitant atmospheric challenges.
The Permian and Triassic events were a complex mess of volcanic activity, the influence of the supercontinent Pangaea, climate change and oceanic anoxia (a lack of dissolved oxygen). Particularly relevant to my story are the common features that typify the periods after the main extinction events. All extinctions seem to have been succeeded by a “recovery fauna”, living during a period when the rock record is dominated by huge numbers of just a handful of creatures - they might be humble clams or a few snails. Other survivors hung on in obscurity until conditions improved, but to a few opportunists the dismal world of acidified or oxygen deficient oceans were a paradise for proliferation. After this phase, life bounced back - new coral reefs grew, forests were made from newly-evolved trees, and ecological niches proliferated in the creative burst of evolution that followed. The most familiar examples are the “explosion” in birds and mammals that rebuilt ecology on land after the ruling reptiles, but similar tales could be told after the previous extinctions, and in all cases the recovery (in geological terms) was rapid (often less than 1m years). After the Ordovician extinction, my trilobites evolved the most fantastical carapaces bristling with spines, or with eyes like those of no other arthropod. In this sense extinctions were as creative as destructive.
I became interested in the animals and plants that seemed to slip through successive disasters. Trilobites were again my starting point. Many scientists believe that their closest living relative is Limulus (the horseshoe crab) that still abounds on the east coast of the US: that would be my starting point. Fossils easily recognisable as relatives of Limulus go all the way back to the Ordovician heyday of the trilobites. Yet they aren’t skulking inconspicuously today - they are thriving in their thousands, strikingly resilient for a creature that has hardly changed since the Jurassic. I traced the Ginkgo (maidenhair tree) to a remote mountain range in China, where it hung on thanks to the attention of Buddhist priests. Now, it is the “city tree” that can withstand very high levels of pollution. A range of Ginkgos greened the
Mesozoic (252m to 66m years ago) landscape, while pterodactyls soared above them. In New Zealand I discovered the velvet worm living in rotten logs - an animal that had survived since the Cambrian period (541m to 485m years ago), admittedly with some modifications. Darwin called these biological Methuselahs “living fossils”, and while some scientists rightly point out that nothing is truly unchanged through time, I still like the notion that I would immediately recognise a horseshoe crab if I were transported back 150m years in a time machine. Unlike, say, T. rex, the thread of time that ties these primitive animals to the present has not been severed. But is this mere luck, or some arcane virtue responsible for their longevity?
One striking fact about the survivors I studied was that they lived in habitats that were themselves resilient in times of crisis. Those horseshoe crabs could survive in an intertidal environment, and were tolerant of brackish water - they seemed almost indestructible. I found a tally of other “living fossils” that lived in the same environment. Could it be that those survivors were living in habitats that continued even as mass extinction happened elsewhere? I coined the term “time havens” in my book, Survivors, for places or habitats that favoured concentrations of living fossils. Even at continental level, Australia housed survivors of the monotremes (egg-laying mammals, echidna and platypus) that represent an early phase in the evolution of mammals, for all that they are so different in their habits today. In the same mountains of China where the Ginkgo tree survived, so, too, did magnolias - among the most primitive flowering plants - and several primitive conifers. Among all the dramas of climate change, plate tectonics, and extinctions, were there corners of the world where life forms could linger when turmoil prevailed elsewhere? Even in the oxygen-starved seas at the end of the Permian, the intertidal zone would be aerated by storms and tides, and maybe that was where the relatives of Limulus saw out the mother of all extinctions.
For all their resilience, those survivors of mass extinction were also conservative. They were well-adapted to their enduring habitat and did not need to change. Some were literally “stick in the muds” - like the little tongue-shaped brachiopod Lingula I dug out of the mud in the New Territories of Hong Kong. Relatives of this humble beast were skulking in the mud while early trilobites swarmed above them. Their muddy habitat sheltered them from momentous changes elsewhere in the oceans. However, it would have been a dull world if nothing but the long-term survivors were in charge. For it was the mass extinctions that cleared the stage for the evolutionary bursts that followed. Complex ecosystems with countless interrelationships between thousands of species of all sizes and habits were, in a sense, shaped by the mass extinctions that preceded them. There is statistical evidence that the number of species has been increasing through earth’s long history even if major extinctions periodically stalled the process of enrichment. The truly resilient phenomenon is life itself.
If the sixth mass extinction is happening now, the really long view might be that life will bounce back whatever we humans do to the biological world. If the coral reefs are poisoned or the rainforest destroyed - they will eventually return stocked with newly evolved corals or trees when our own greedy species has passed from the earth with the finality of the trilobites. But there is a difference this time. The global increase in Homo sapiens resembles the unrestrained proliferation of the “recovery fauna”, which happened after the previous mass extinctions - our billions alone are the ultimate cause of this extinction. We are a single taxonomic entity pushing thousands of other species to the brink. This is not like the previous five mass extinctions where external events drove the catastrophe. This one is internally driven, powered by exploitation of the energy stored in the earth’s geology. We just don’t know how this mass extinction is going to play out.
Life’s Resilience: the Perspective of Deep Time