 Imagine what it would be like if you were the last living person on earth. Imagine yourself as Martha, the last passenger pigeon who came from a species that just less than a century earlier flocked in the billions. Martha lived the last years of her life alone in a cage in the Cincinnati Zoo and died in 1914. Lonesome George, the last pinta island giant tortoise, he lived alone the last of his kind for more than 40 years of his 100 or so year life. Imagine yourselves as Najin or Fatu, the two last northern white rhinos. Still alive, but a mother-daughter pair. Extinction is a part of evolution. There are more species that are extinct than are alive today. And for most of these, we have no idea how it happened, whether it was fast or slow, driven by over-predation or starvation, or perhaps something catastrophic like a giant storm or a volcanic eruption. For a few, though, we know exactly how it happened. And today, I'm going to tell you one of those extinction stories, that of the mammoth, the large, cold-adapted elephant that, until rather recently, was found across all of the continents in the northern hemisphere. And not only am I going to tell you why the mammoth went extinct, but I'm going to tell you how we figured it out, and also what understanding the mammoth story can do to help us to come up with more innovative ways and more scientific ways of stopping to have to tell stories about the last living individual of any sort of species. So I'm a paleogeneticist. I don't expect you to know what that means, because it's not a real thing. I'm both a paleontologist. That means I study plants and animals that used to be alive, and a geneticist. That means not only do I take pictures and measure and look at the bones and other remains, but I also take a little piece of that and grind it up and extract their DNA. And using that DNA, I can reconstruct the history of life on Earth. I work mostly in a part of the world known as Beringia. During ice ages, the sea level was a lot lower than it is today, because much of the ocean's water was taken up as glaciers sitting on top of the continents. This exposed the shallower areas of the ocean, like these in light colors here, and in this part of the world created a land bridge that connected Asia and North America. This land bridge was too dry to become glaciated itself, but sufficiently rainy to produce a community of grasses and flowers and shrubs that supported much of the animals that we have come to associate with the ice age. Mammoths, woolly rhinos, different species of cat, lots of different species of horses, for example. Today, Beringia looks more like this. The diversity is gone. But, crucially, it's still cold. And as any of us with a fridge and a freezer know, cold is very good for the preservation of organic remains. When an organism dies, its cells, its bodies, and the DNA within its cells is in really good condition. Imagine long streamer-like strands of DNA. But over time, things like UV light, freezing and thawing, and critically consumption by microbes, chop those long strands of DNA down into really short, tiny little confetti-like fragments. And those are no good for genetic reconstruction. In Beringia, though, because it's cold, this process is slower. And we can go out and we can find incredibly well-preserved specimens like this baby mammoth tooth that one of my students who's holding it found a few years ago in Canada. And this fantastically well-preserved wolf pup, mummified wolf pup that we know is more than 50,000 years old that we also found in the Canadian Arctic a few years ago. So we go up, out into the Arctic, and we stay in five-star accommodation. And for any of you who have not been to the Arctic during the summer, those are mosquitoes, yes? It's lovely. And we look for these frozen remains everywhere this permafrost. The frozen dirt is melting. This is actually at an active placer mine in Canada's Yukon. And the gold miners are washing away this frozen dirt to get to the gold-bearing gravels underneath. And when we do this, thousands of bones come washing out of that dirt. And we wander around and collect them. It's surprising even how many of these remains we can find. And these are mostly bison and horses and some mammoth. Just to point out, this one here, here's a mammoth femur that we found in northern Alaska one year. So we measure these bones and photograph them. And we take them back to the lab. And we wear a funny suit to protect the samples from our own DNA. And we grind that bit up and we extract the DNA. And we can collect this information from animals that live all across the northern hemisphere and see how they differ from each other genetically. And from that information, we can learn when populations were growing, when they were shrinking, when individuals might have been moving across long distances, and when that connectivity stopped. And connectivity, we will see, is a crucial part of many of these species' extinction stories. So one of the first species to have been studied using this approach was mammoths. Mammoths, for much of the last million years or so, were distributed across the northern hemisphere. Here's the extent of the woolly mammoth range in brown. But over the course of time, that range slowly shrunk, smaller and smaller, as populations disappeared, until around 8,000 years ago, when only two populations of mammoths remained, one on Wrangel Island, about 150 kilometers off the coast of Siberia. And this population went extinct around 3,200 years ago. And the other on St. Paul Island, here in the Bering Sea. And this population was the second-last population to go extinct, but little was known about this. So a few years ago, I became part of an expedition to go out to St. Paul to try to figure out exactly when and why mammoths went extinct. St. Paul is small. This is a plot that shows how big the island actually has been over time. The present-day land area here is about 100 square kilometers. It's very small, but it hasn't always been this small. By 13,500 years ago, when the sea level was lower because of the glaciers, St. Paul was actually connected to the Alaskan mainland. And then over time, the sea level rose and St. Paul became smaller and smaller and more and more isolated. Mammoths, it seems, were the only large mammal that have become isolated on St. Paul. There were no bears. There were no lions. There were no people. People didn't arrive until a couple hundred years ago. So there were no predators. It was a mammoth utopia. And yet, they became extinct. Why? So to answer this question, we were going to have to collect some mammoth DNA. So we went out to the island and we called on all the local people and we said, bring us your mammoth bones. Come on, bring us these things. And they came out, came together. We went scouring around the island and we came up with about a dozen mammoth bones. And this isn't very much. It's not really enough to learn about the evolutionary history. So we would have to do something different. And crucially, there is another really interesting thing on this island. Right here, Lake Hill. This is a hill with a lake in it. It's actually the cone of a collapsed volcano. Looks like this today. And the children on the island use it for swimming during the summertime. But lakes are actually really brilliant for ancient DNA because they are kind of sink for genetic material. During the summer, the wind blows in the plants and the pollen and the leaves. There are things living in the ocean. It is the only source of fresh water on St. Paul. So all the animals that are there will come up to the island and wander in to drink that fresh water and in doing so will deposit their own DNA. And every year that DNA will sink to the very bottom of the lake and then it freezes and then the same thing happens the next year. There is an accumulation, like a stratigraphy of layer upon layer of everyone that was present on the island from the past to the present day. If we could get this copy, a copy of this, we could figure out who was there, when, and with whom. So during the winter, we went out into the middle of Lake Hill on a boat on a sled and we drilled a long core through the lake, through the dirt all the way to the gravels at the bottom and we sucked that long column out of the lake, back to the lab and split it in half. And then we went through and from the very bottom which we learned later was about 17,000 years ago, we took tiny little plugs of DNA all the way to the top, to the present DNA, present day. And we looked in those tiny plugs of DNA for mammoth DNA. If mammoth DNA was present, mammoths were there. If it wasn't present, mammoths weren't there. We looked at the vegetation community to ask whether the plants were changing over time. We looked at some components of the lake itself. These are tiny little microscopic algae and microscopic animals that can tell us whether the lake was salty or not, how shallow the lake was, for example. And we looked for this other kind of cool thing which is called spore myela. It's a type of fungus that only grows on the poops of large mammals. So in case we couldn't find mammoth's DNA, if we found this, it would tell us that mammoths were probably there. They were the only large mammal on the island. And with all of these data, we actually solved the mystery. We found that mammoth DNA was present all the way from the bottom until around 5,600 years ago. And spore myela, the dung fungus, the same, also until about 5,600 years ago. And then gone until about 250 years ago when the dung fungus comes back at the same time as we know that Russian fur traders were bringing caribou to the island. We asked, so this is exactly when they went extinct, 5,600 years ago. But why? I looked at the vegetation, nothing changed. So they didn't run out of food. But everything else about the lake changed. The water chemistry changed, the rate of sediment accumulation changed, and that community of microorganisms completely turned over from a community that really likes to live in clear, deep, fresh water to a community that prefers to live in very shallow, cloudy and slightly salty water. Together, these data tell us what happened. Around 5,600 years ago, St. Paul Island experienced a severe weather event, a drought. The lake, the only source of fresh water on the island, started to dry up. Sea salt infiltrated, and with no new rainwater to balance it out, it became salty. Mammoths died on St. Paul because they ran out of fresh water. Had this happened 13,500 years ago, Mammoths would have had another option. They could have left. They could have wandered on to the mainland and looked for another source of fresh water. But they couldn't because they were on an island, completely isolated, cut off from the mainland, stuck. And so they became extinct. Mammoths are, of course, not the only wide-ranging taxon to have their survival threatened by the islandization of their habitat. Today, islandization takes different forms, where the habitats that we've chosen to protect are surrounded not by water, but by other things, like farms and agriculture. By roads and highways and freeways and by cities of all sizes. This places the plants and animals that live in these islandized habitats in a precarious situation. Any extreme weather event, the introduction of a predator or a disease, any of these things can upset the balance of interactions taking place within these island habitats, potentially leading to extinction. We've learned from the past, not just from Mammoths, but also from our studies of woolly rhinos and arctic horses and the different lion species that connectivity is key. In each of these species, as they declined toward extinction, the populations that remained became increasingly isolated from each other, both geographically and genetically, with each of these island populations functioning as their own tiny, isolated thing more at risk of extinction. The same is no doubt true today. And as we work to protect these island habitats, which is a crucial part of any plant to protect and preserve endangered species, we also have to remember that plants and animals need to move between them. To escape, yes, but maybe also to find new habitat as climate shifts or to find a mate. We need to come up with ways to try to minimize the isolating effect of islandization. This could mean building highway overpasses where animals can cross these potentially death-inducing large highways that we now are crisscrossing habitat with or at a larger scale creating linked-up corridors where animals can move, like the Pantera Project has done for cats across the world. Or this newer initiative, the Yellowstone Yukon Initiative, is working to connect Yellowstone National Park to the Yukon where I do much of my research. Perhaps we need to deliberately move individuals between island habitats like US Fish and Wildlife did in the 1990s when they took Texas Panthers and put them into a Florida Panther population and saved Florida Panthers from becoming extinct. We could create green ways, green roofs, city parks, green corridors along rivers and roads, and we could just not build walls or borders or barriers or fences of any kind that just further fragmented this already fragmented landscape. A sustainable future of biodiversity will require creativity but it will also require collaboration. The good news is, we can do this. Some of the most successful conservation legislation that we have, both in North America and in Europe includes protection for species whose ranges span borders. Mammoths, woolly rhinos, ice aged lions, they're all gone. We can't bring them back. We still have elephants. We still have some rhinos. We still have horses. We still have Florida Panthers. We still have polar bears and long-eared bats. Emma works on bats. We don't have to write the story of the last living individual of any of these species. Thank you.