Dr. David Gwynn

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906 appearances 4 recordings 1 series first heard Nov 2024 last heard 21 Dec

Dr. David Gwynn’s voice in public audio — every appearance, attributed to the second.

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The Ancients · Ice Age America · 21 Nov 2024 podcast

It's been a game changer.

Well, absolutely right. Not only can you, of course, extract DNA from animal bones, you can extract DNA from sediment. A gram of sediment, and this is just mind-blowing stuff, a gram of sediment contains literally billions, that's billions with a B, DNA fragments in just a gram of sediment. And in those fragments, you've got the ecosystem.

So you can trace what plants are growing on the landscape at that instant, what animals are passing over and shedding skin or doing their business, right? Every time a mammoth would drop one of those, well, gigantic mammoth poops, that DNA gets worked into the sediment.

And so there have been some really phenomenal studies, and this is even more recent than a lot of the human DNA, in which, well, in one particular study, for example, we were able to track the last 50,000 years of Arctic vegetation and animal communities. And among the things that came out of that work

And this is work that was done in concert with and under the direction of my collaborator at the University of Copenhagen, Eski Willerslev. We were able to show, for example, that mammoth, woolly mammoth, right, survived up until... around 10 or 11,000 years ago across most of the Arctic region, right? We had samples that were literally, you know, sort of circle the Northern hemisphere.

We had samples from North America. We had samples from Northern Europe. We had samples from Northern Asia. And one of the just astonishing finds that came out of that work was that mammoth, as the Pleistocene is coming to an end, of course, are going extinct across virtually the entire Arctic, except in far northern Siberia.

That's the exact correct term, because what happened is, and we know this from the vegetation record that we have in that sediment DNA, is that for all intents and purposes, in terms of the vegetation... The Pleistocene didn't end until 4,000 years ago.

Now, let me just add something because I suspect there'll be savvy listeners out there who will say, well, we've always known that mammoths survived on these islands in the Arctic seas. And that's true, right? But these were mammoths that survived on the mainland, right? The mainland of what's now the Timur Peninsula up until around 4,000 years ago when the vegetation finally...

changed from that sort of mammoth step these animals favored to the vegetation that we have in that region today. So again, DNA in all its forms, ancient forms, whether from sediment or from bone, has been just a sea change.

Well, so the Ice Age proper, for all those geologists out there listening, it goes back 2 million years. Wow. But it was never sort of frozen the entire time. It was never that we had ice sheets. The Ice Age goes in cycles, where you have glaciers advancing, glaciers retreating.

In terms of our story, the story of the Americas and people coming in and animals going extinct, we're really talking about the last, say, 30,000 years. Because starting around 30,000 years ago, we start to see glaciers building up over what's now Canada and expanding. And they will expand basically in all directions and ultimately will reach what's known as the last glacial maximum.

That's the period of which ice was at its greatest extent on the landscape. And that last glacial maximum The dates vary depending on who you ask, but we're talking about, say, oh, 24,000 years ago up until around 18,000, 19,000 years ago. We don't need to be terribly precise about it. But that was the window of time when the ice was at its maximum. It was coldest.

globally, as well as in North America, this was also the time when sea levels were at their lowest worldwide. And we know why that is. If you're building a glacier over North America, and at the same time building one over Northern Europe, the so-called Fenoscandian ice sheet, basically what you're doing is you're interrupting the hydrological cycle.

The hydrological cycle, just put it in very simplistic terms, you've got evaporation from the ocean, clouds move over the land, they rain, the water drains into rivers, it goes back into the ocean. Okay? Real simple. Well, simplistic actually, but for our purposes, that'll work, right? If you stop that water by freezing it on land, it doesn't get back into the ocean. What happens?

Ocean levels go down. And the amount that they've gone down is going to vary geographically. because of a bunch of things that are related to gravitational forces, the pull of an ice sheet, continental shelves, mass. But on average, it's going to be about 130 meters lower than it is today, okay?

What that does, of course, is it exposes shallow continental shelf, continental margins, and those become land, right? areas that are now underwater or land. And that includes, of course, the continental shelf that lies underneath the Bering Sea. So that's northwest? Indeed, indeed. That's Alaska area. It's only, you know, if you go out to the Bering Sea today, it's only about 52 meters.

That sea level is about 52 meters above what's the continental shelf below it. So all you have to do is drop sea level 52 meters and you can walk from Asia to To America. You can walk from Siberia to Alaska, okay? All right.

So we assume, and there's no reason to doubt, that the way in which humans got here, and in fact, the way in which animals over time may have come to the Americas, is that during these episodes where you have a lot of ice on land, there were these bridges, these land bridges. And so people came across. But at the same time, let me just give you a quick... geography of the ice sheets themselves.

There's two major ice sheets in North America. There's the Laurentide ice sheet, which extends from what's present-day northern North America down to Central Ohio. I don't know if that's going to work for all of your audience, but basically past the Canadian-US border. Yes.

And it goes from Newfoundland and Labrador all the way across Canada and laps up against the eastern flank of the Rocky Mountains. That's one big ice sheet, and it's literally kilometers thick. On the other side of the Rocky Mountain spine, you've got the Cordilleran Ice Sheet. And these are a series of mountain glaciers that come down from high elevation and coalesce to form an ice sheet.

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