‘Earth saw five mass extinctions due to natural change — the sixth is driven by human actions’

Paul Wignallteaches palaeontology and sedimentology at the University of Leeds. Speaking to Times Evoke, he explains past — and present — trajectories of huge species losses on Earth.
What is the core of your research?
I focus on investigating mass extinctions. There have been five major ones and around a dozen lesser extinctions. The one event that stands out from the rest took place at the end of the Cretaceous period, around 66 million years ago, associated with the sudden loss of three-quarters of plant and animal species on Earth, from ammonoids or marine molluscs to non-avian dinosaurs.

What distinguishes a mass extinction from a regular event of a species going extinct?
There are always background extinctions happening — but mass extinctions are when extinction rates peak over a very short period of time. This also happens globally, so we see high levels of the loss of life everywhere on the planet at the same time.

Picture courtesy: iStock
Which have been the most important mass extinctions on Earth?
The biggest one occurred at the end of the Permian era. We think this killed about 95% of all species, including animals, insects and beings in the sea. It took place around 250 million years ago and saw many plants as well, from conifers to ferns, disappear. Among animals, the species which went extinct included ones entirely unfamiliar to us today, like placodermi, which were jawed, armoured fish, Moschops, a species described as a mammal-like reptile despite being neither and pelycosaurs, fin-backed reptiles. This event, like all mass extinctions, coincided with a giant scale of vulcanism and the climatic and environmental changes which followed. In most extinctions, we find greenhouse gas emissions, typically carbon dioxide (CO2), with very rapid phases of global warming and its consequential effects.

What materials illuminate these past events?
From geology, we use the fossil record which we study by collecting fossils and seeing where and when species emerge and start to disappear, indicating extinction rising. Rocks also contain chemical clues and we look at carbon isotopes (which means different kinds of carbon), nitrogen isotopes and concentrations of trace metals in rocks — it has become increasingly possible to use rocks to learn how much oxygen there was in the oceans or catch past temperature fluctuations.
What were the aftermaths of extinctions?
The aftermath varied but generally, once a lot of species had perished and there was a good deal of empty habitat, that allowed both recoveries and the development of new species. The fossil record shows mass extinctions were often followed by new groups appearing. The example of mammals is most famous — these could appear, proliferate and radiate once dinosaurs had disappeared. Dinosaurs themselves owed their success to a mass extinction about 200 million years ago which took out the major land animals — dinosaurs survived this, spread and dominated the world for another 100 million years.

SURPRISE: An extinction helped dinosaurs. Picture courtesy: iStock
You have written about how life on Earth survived ‘the worst of times’ — how did certain species develop such resilience?
It’s interesting that often, in the most severe mass extinction, a few groups end up proliferating because they could adapt to those conditions. These are usually very opportunistic forms that reproduce faster like little weeds or marine species tolerant of low oxygen levels in the sea such as certain clams. They actually benefit from a crisis — however, once things get back to normal, many tended to not be so successful and were often replaced as new groups evolved.
Do past mass extinctions tell us how fast evolution works?
The response of evolution to some mass extinctions has tended to be quick — but that depends on the kind of evolution while ‘quick’ here means in geological time which could be thousands of years when new species emerge. In a geological timescale, anything that happens in 1,00,000 years is quick — evolution has been known to move fast therein.
Is Earth experiencing a mass extinction now?
Yes. Currently, lots of species are going extinct on the planet — this is not comparable to past extinction events yet but the rate of extinctions is very high now. This is happening mainly because of habitat destruction and the loss of natural areas to farmlands and urban construction. In the future, as past patterns show us, global warming will cause many more extinctions. The crisis at the moment is caused by humanity’s over-exploitation of natural habitats, over-fishing in the seas, etc. — there are no parallels in the past of that.

BYE-CATCH: This could end life in the seas. Picture courtesy: iStock
What links the loss of non-human species to the quality of human life?
If we get to the level of extinctions which happened at the end of the Permian era, human life would be in trouble. Then, essentially, the collapse of terrestrial ecosystems unfolded — if we get that far, it would be catastrophic for life on Earth. That takes thousands of years but there are faster potential impacts too. As ecosystems are lost with species, there will be much more climate unpredictability — this will cause more food and water insecurity. Sea level rise is another challenge for human societies. These are all early signs of a crisis.

What can the ongoing COP28 summit do to address the current extinction crisis?
The best mitigation would be delivering on meaningful reductions in greenhouse gas emissions — the need to reduce such warming emissions and transition to clean energy and sustainable economies is entirely fundamental now. Meanwhile, the world must accept the fact that it will grow warmer and we need to prepare for this reality. I am most concerned today about the incredibly rich biodiversity found in tropical rainforests and warm tropical seas which are at huge risk. To preserve the greatest number of species on Earth now, these habitats need the most protection — COP28 would do well to focus on that.