Dr. David Gwynn
speaker
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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Appearances
And that extends from the western edge of the Rocky Mountains down to the Pacific coast. So you've walked across from Asia, Siberia into Alaska, but then you're going to have to stop because you've got those two big ice sheets between you and the rest of North America.
Yes, exactly. Good. Thank you. I should have clarified that. Yeah. Alaska is a cul-de-sac. as it were, right? It's a dead end. You can get into Alaska because Alaska was glaciated along the Aleutian Islands and in Southern Alaska, it was glaciated on top of the Brooks Range in sort of Northern Alaska, but otherwise it was ice-free, okay?
So you walk across the land bridge, you get into Alaska and you're thinking, you know, you're checking your watch and you're thinking, I could be in Miami, Florida if I just keep moving, right? But no, you can't because the glacial conditions that produce that land bridge by reducing the sea level at the same time are an obstruction.
Now, what that means is that it sort of sets some general parameters for when people could have come in, because it basically means that if we're assuming that they walked across, and at this time period, that's the most likely scenario, they could either come in after the glaciers have started to expand and sea level has dropped at least 52 meters, and before it has, the ice sheets, that is, have completely obstructed the route south,
Or they have to wait until after the ice sheets begin to melt. Because when the ice sheets begin to melt as the world is warming at the end of the ice age, you've got a couple of routes that will open up that will take you from that cul-de-sac where you've been parked for however many centuries or millennia down into what we refer to here in the States as the lower 48, right? The lower 48 states.
One of those routes is along the Pacific coast and it appears to be ice free starting around 17,000 years ago, but probably for sure by around 16,000 years ago. And then there's the so-called ice free corridor, which opens when the Laurentide and the Cordilleran ice sheets begin to melt and there opens between them.
an unglaciated route down to the south, an unglaciated corridor down to the south. This is like a corridor, isn't it? A narrow route almost.
The width of the route obviously grows over time and it opens in kind of a funny way. You know those winter coats that we wear that have a zipper that zips up from the bottom and zips down from the top? Well, this was the ice-free corridor. So it opens, it's pretty wide at the top, it's pretty wide at the bottom. And then in the middle, it still hasn't quite opened up yet.
So people basically have to wait for the whole thing to be open. But, and this is an important point, and this is something that we found with ancient environmental DNA, just because the corridor is open doesn't mean it's a passage. Because when the ice retreats, you've got nothing there yet. at least immediately after the ice retreats, except mud water flats, lakes, nothing's growing there.
And so at the end of the Pleistocene, when the corridor opens up, you can't pack a lunch in Fairbanks and say, you know, if I'm really careful and I don't eat too much on the first day and the second day, I can make that thousand kilometer trek with my sack lunch.
No, you've got to wait for animals and well, you've got to wait for plants and then animals to basically make that corridor biologically viable. You've got to have something to eat in there, right? And so...
One of the things that we found, and this is again work with Willerslev's group, in particular Mikkel Pedersen, who's one of the sort of specialists in that group on ancient environmental DNA, we had cores from that center of the zipper, as it were. It was one of the last places within that corridor to open up. And what we found was that, again,
With ancient environmental DNA, you know when the plants show up. You know when the animals showed up.
Sorry about that. Yeah, we go to a lake, we drill, we get a core of sediment, this tube of sediment, and you slice it up like a salami, right? And then you look at what the DNA is in the different slices that are of different ages. And what we found was that that corridor was not biologically viable until around 13,000 to 12,500 years ago.
And of course, we've had people in the Americas already for a few thousand years. So what that tells us is that the ice-free corridor was not the initial route into the Americas because it didn't become a route and available until well after people had already been here, which tells us they must have come down the coast.
Yeah, exactly right. This was a very, very different landscape at the end of the Pleistocene. The land bridge itself, I mean, look, for all intents and purposes, if you walked from Northeast Asia and Siberia into Alaska, you wouldn't have noticed the difference in terms of the environment. It was cold. It was dry. It was a grassland.
The tundra, as we think of it today, is a much more geologically recent phenomena. And it has to do with warming and increased precipitation and that kind of thing. So during the Pleistocene in Alaska and across the entire land bridge, you had horses, you had mammoth, you had giant bison. And these are animals that needed to be dry underfoot. right?
These are not animals that do very well on tundra. Caribou do great on tundra, but they're really not around during this time in the numbers that mammoth and horse and bison are.
Well, exactly right. These were diverse landscapes. You've got everything from vast grasslands to deep forest and forests that vary tremendously in terms of the nature of the vegetation. You've got boreal forests, you've got deciduous forests, you've got, as you go further south, of course, you're getting into tropical stuff.
And so one of the key things that's really important that we still haven't quite got our finger on is the whole issue of landscape learning. How do you figure out what to do if nobody's been there before, if the landscape is unfamiliar to you? How do you identify resources that will help you, that will cure you, that might hurt you, or that might even try and kill you, right?
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