Paleontologist explores how mass extinction and evolution are intimately linked | CBC Radio

Ideas53:59Thomas Halliday: History of Extinct Ecosystems

The story of complex life on Earth is a story of evolution and extinction — of long-term continuity and massive disruption. 

British paleontologist and evolutionary biologist Thomas Halliday traces how this dance of evolution and extinction has played out over more than 500 million years in his book, Otherlands: Journeys Through Earth’s Extinct Ecosystems

Halliday vividly recreates scenes from ecosystems that flourished from the relatively recent past, such as the Pleistocene — better known as the Ice Age — all the way back to the Cambrian, which saw a remarkable explosion of biodiversity 500 million years ago. 

What they all have in common is that none of them have existed in a very long time — sometimes coming to cataclysmic ends in mass extinction events such as the asteroid strike that brought an end to the Cretaceous period 66 million years ago. 

In addition to celebrating the Earth’s rich and incredibly diverse history of life, Halliday also draws on the history of Earth’s five mass extinction events to warn of the perilous period of biodiversity loss and climate change humans are causing today in what many scientists call the sixth mass extinction.

Here’s an excerpt from IDEAS host Nahlah Ayed’s conversation with Thomas Halliday.

The asteroid, which of course, we all grew up hearing about, hit the earth just off the Yucatan Peninsula in what is now Mexico 66 million years ago. Can you talk about just how catastrophic it was? 

It has been described by many paleontologists as the worst day in the history of the earth, because unlike many other mass extinctions, which were these sort of long, prolonged periods of climatic stress, this is a sudden impact that changed everything within hours.

Ecosystems collapsed everywhere. Without enough food to eat, large herbivores, those dinosaurs that were eating plants, weren’t able to survive and they died. Without their herbivorous prey, those big carnivorous animals died. And so we see essentially nothing larger than about the size of a badger survived. It’s something which is an almost unimaginable sudden shift in ecology, and we see an immediate loss of the major parts of ecosystems of the Cretaceous. 

Was there anything that equipped certain species to be better able to survive this catastrophe than others? 

Yes, I mean, 25 per cent of things did survive. And we and everything that’s alive today are the descendants of those survivors. Being big is usually bad because you tend to not only require more food, but also reproduce more slowly, which means that you need to survive considerably longer between breeding events in order to maintain a substantial population.

So creatures that can reproduce quickly and consume what is available rapidly and have these population booms and busts all the time — they tend to be very resilient to ecosystem destruction, whether that’s at a local or global level. And being able to burrow seems to have been an important trait. It’s amazing the consistency of climate that you can have within a burrow. And we can see that in this kind of situation that would have been very beneficial, particularly for our ancestor mammals, who were largely occupying that kind of role at the time.

And being versatile in what you eat is a certain requirement. Because if you are very specific, say, for example, you’re an insect whose larvae can only reproduce by feeding on the leaf of one particular species, as is the case for so many butterflies. For example, if your host plant goes, then you go with it. Whereas if you’re versatile, then you can move on to something else. 

Is there any reason to think that humans would have evolved to become a dominant species had there not been an asteroid strike?

I think there is certainly reason to believe that, in the sense of humans being mammals and mammals not getting their chance to diversify. The Cretaceous mass extinction is fundamental to mammals being able to diversify. And we see this a lot with mass extinction events.

After the mass extinction event, some other group becomes extremely diverse, but it’s always some other group, and that’s what really makes us then concerned about implementing a mass extinction on Earth right now is that what comes back is never the same, right? As always, you lose what came before and what returns is different. 

So is it fair to say that you can’t have evolution without extinction?

Yeah, absolutely, that’s fair to say because evolution by natural selection requires there to be the death of those that have the disadvantageous traits in order for the species to become shaped into something else. And so death within the populations is a fundamental of there being change. And inevitably, that means that when you get speciation and species meeting one another again later on, you may have competition which excludes one of the other. So yeah, extinction at a background level is fundamental to evolution.

What you see at mass extinctions are these bizarre increases, firstly in the rate of extinction, and then afterwards you do get an increase in the rate of speciation. But it’s very much after the fact. It’s not that they come simultaneously and it takes millions of years to recover the same kind of diversity that you had before.

Q&A was edited for clarity and length. This episode was produced by Chris Wodskou.

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