Paleoecology of Doggerland

[Music] Welcome to Science Sessions, the podcast of the proceedings of the National Academy of Sciences, where we connect you with Academy members, researchers, and policymakers. Join us as we explore the stories behind the science. I'm Paul Gabrielson. Thousands of years ago, when sea level was lower and water was locked in glaciers, the North Sea between Britain and Scandinavia was dry land. Scientists call it "doggerland." It's not easy to learn about the past ecology of a land buried under sea water. But DNA, preserved in the sea floor sediment, still holds clues about the species that lived there. In a recent PNAS study, Robin Allaby of the University of War at Cancolleagues, collected and studied DNA in sea floor sediment cores from doggerland. Their findings show a land more temperate than previously thought, and suggest a productive ecosystem that could have been a favorable habitat for paleolithic and mesolithic human cultures. Robin, where is or was doggerland? What do we need to know about its history? We've known about it for quite some time. It's named after a region of shallow sea in the North Sea called "doggerbank." We see references early as the 19th century, so even HG Wells, who wrote "Wall of the Worlds," famously, makes reference to the Tenze River running into the Rhine River's over a low undulating land. It was about 1913 that Clement Reed started discovering submerged forests around the side of Britain. In the mid-20th century, a harpoon was pulled up by a skipper of a fishing boat. Our godfather of palanology, the study of pollen grains, was a harry godwin, went to that site to get some samples from the bottom of the sea where it was actually peat, and he found evidence of oak forest there. So we knew by the mid-20th century there was this place, what we called doggerland, and it was forested, what we didn't know was anything about the timing of it. What we broke open in our study was really what was there and when it was there. When you're in a glaciation, all the waters locked up in the poles, doggerland will be dry then. So it would have appeared between glaciers regularly. It was there from the earliest point of our time study, and it would have been there for the last 100,000 years or so, because about as long as the glaciation was. So we see basically things switching in the environment from trees and grasslands and meadows to shallow marine environments as the water is coming up. It's around about something between six and seven thousand years ago that we really start losing the terrestrial signal entirely. Tell us about sedimentary DNA. What is it? And how is it useful in reconstructing paleoecology? So DNA is a remarkably stable polymer, probably one of the reasons that life uses it as our template of instructions. So not all environments are going to hold onto their DNA forever. One of the best places to get sedimentary DNA from is the bottom of the sea. Seas are really heavy, so the sediments at the bottom are really compacted, so there's not much movement. And at the bottom of the sea, it's really cold. It's about four degrees, which is almost perfect for preserving DNA. You've got a natural fridge system down there. Probably you can go for hundreds of thousands of years in terms of DNA preservation in that kind of environment. Tell us about how you collected your samples and conducted your study. You need to know where to look. The bottom of the North Sea is like a paleo landscape, but you need to use things like seismics to get underneath recent marine sediments. And when you do that, you can actually see old river systems. What we did in this project was to take cores from a boat. It's a boat that's got an engine on each corner, so you can get exact GPS positions. And basically, track this river system up. It took 60 cores in the end. You sink a tube into the ground, and then that tube comes up in the case in a plastic tube. And that doesn't get open until we're in a very forensic level, clean environment in our ancient DNA laboratory. We just sequenced absolutely everything because we wanted to have a very unbiased view of what was there. It's not trivial to identify fragments of DNA that are in my literature about 40 bases long. What organism they've come from. But then on top of that, we need to think about something called tephonomy. So tephonomy is basically the study of how things got to where they were. Does the DNA that we get out actually represent the environment in that exact location? This is a river system. Was this something brought down from a catchment somewhere else? It's quite complex, but it compares what we see in various different sediment types. We've got peets down there, we've got silts, we've got sands. We can see when those sediments are changing, when the environment is changing. So that tells us something about the dynamics of what's going on over time. What did you learn about the plant and animal life of Doggerland? Around 16,000 years ago, we're still in the last glaciation. The ice front is, we think, up around Scotland somewhere. The expectation is that it would be a tundra environment and it wasn't. We could see a range of trees that are not just trees that can survive in the cold. These are trees that like it warm. So things like oak, older, hazel, elm, over 14,000 years ago. Around that 16,000 year mark. Combined with that, we can see a bit of evidence of animals. Plants will leave about a thousand fold more DNA behind than animals, simply because there's more of them. But we could see little snippets of animals and we could see things like bears and boar. So together, this sort of looks like a forested environment that can support quite a lot of life. What other surprising results did you find? We got some evidence of a signal that looks something like a walnut. Now walnut was supposedly brought over by the Romans about 2000 years ago. On the face of it, that looks like contamination. So I dug a bit deeper and got all the data together. And then quite clearly we could see that actually it's not walnut. It's a relative called terracaria. And it used to be really common in this part of the world, but 400,000 years ago. So that was a surprise. We'd already established with our tephonomy that we didn't think this was the oldest sediment that had been reworked. And we looked at the amount of damage that was in the DNA and it looked not to be that old. It looked the same as everything else around. So that got us looking to see what other evidence might there be out there to back up or on the face of it seems quite an outlandish claim that there was terracaria. Could 400,000 years later than it should be. Once we looked into the pollen literature, we found that there is evidence of terracaria there. It's just been ignored because it's not supposed to be there. We look at the pollen profiles. We can see that it's actually restricted to the warmer periods between 14,000 and 13,000 years ago. There's a brief warm period towards the end of the last glaciation. And we see it popping up there. And then there are these stadial events within the last glaciation called Heinrich stadials. So a stadial is a brief warm phase in the cold of phase. And again, we see it popping up there in the pollen record. It's really highly convincing that terracaria really did hang on there for a couple of glaciations more than people thought. We never would have found this if we had gone in with an expectation of what we were going to find. What do these results say about the habitability of Daga land for mesolithic societies? So we're in the upper pay the lithic transitioning to the mesolithic period of that time. And most of our evidence and understanding of cultures comes from terrestrial evidence from northern Europe. So our early mesolithic cultures, they got names that were based around northern Germany, like the Arons Burgian. But this tells us actually they could have been much much further north in this environment. And interestingly, there's a death of mesolithic evidence at all in Britain or a few sites, but not many. But that could well be because they were ruled down in this dogaland area. The environment was clearly productive. There were animals to hunt. There was enough forest cover for those animals to be there. It was resource rich. The Arons Burgian culture really matches the timeline. So evidence currently is all in sort of northern Europe on mainland. Actually, you should find cultural artifacts much further north in dogaland. While our paper was in review, a paper was published finding almost exactly about. Somebody who found Arons Burgian technology much much earlier than expected, but really far north. So on the Isle of Sky, the deity wasn't precise, but it was around that late glacial period. As difficult as it is to prospect, archeologically under the sea, it's probably well worth it. What are the caveats or limitations of the study? We're always has to be careful to interpret sedimentary DNA. What is still difficult to do, I would say, nigh unimpossible to do really, is make a direct interpretation of how much of any particular organism there was there. So we see Elm and we see Oak. But that doesn't tell us how many trees there are, it just tells us presents really. It's tempting to say forests, but you can only really make the inference of forests once you see that there are animals there also that would live in forests. [Music]