字幕列表 影片播放 列印英文字幕 We want to thank Google’s Science Journal App for supporting PBS Digital Studios In 1873, a geologist named Robert Stearns reported a rather odd find on the Channel Islands off the coast of southern California. He had discovered a single tooth from an animal that wasn’t known to have lived on the islands: a mammoth. The tooth was was a bit on the small side, but it was obviously from a mammoth. And it was enough for Stearns to conclude that the Channel Islands must have once been connected to the mainland. Otherwise, he thought, how could the mammoth have gotten there? Well, Stearns’ hypothesis turned out to be wrong; the islands weren’t ever part of the mainland. But that tooth would eventually be recognized as evidence for something even more unexpected. By 1928, enough mammoth fossils had been found on the Channel Islands that paleontologists realized that many of them were unusually small, around 2 meters tall at the shoulder. The mammoths were so small that they came to be seen as their own species, commonly known as the Channel Islands Pygmy Mammoth. These animals aren’t known anywhere else in the world. Which means that they must have evolved in place on the Channel Islands. But they’re also extremely similar to the mammoths that lived just a few dozen kilometers away, on the California mainland: the 4 meter tall Columbian Mammoth, one of the largest mammals to ever walk North America. So if the islands were never connected to the mainland, then how did these little mammoths get there? Were they related to the giant Columbian mammoths? And how -- and why -- were they so small? The answers can be found in the way species respond to isolated environments with limited resources. It’s a phenomenon that’s been observed all over the world. And the evolutionary forces that drive it are slow and gradual, but powerful. Powerful enough to shrink a mammoth. The oldest mammoth fossil ever found on the Channel Islands is a tusk found on Santa Rosa Island in 2014. Uranium-series dating of nearby corals showed it to be about 80,000 years old, from the late Pleistocene Epoch. And even though the tusk was discovered more than 140 years after Stearns found that first tooth, both fossils raise the same question: How did mammoths manage to get to these islands, some of which are more than 40 kilometers from the mainland? To solve that puzzle, researchers began by studying what southern California was like in the Pleistocene. Back then, coastal California was often cooler and drier than it is today. And southern California was covered with grasses and conifer trees, which fed herds of enormous Columbian mammoths, as well as horses, and giant ground sloths. And in turn, these herbivores were stalked by skilled predators like saber-tooth cats and dire wolves. But the Pleistocene was a time of tumultuous change. The planet cooled, then warmed, then cooled again, over and over. And those changes dramatically altered the coastline of California, as well as the size of the Channel Islands themselves. During warmer spells, as nearby glaciers retreated, portions of the Earth’s crust underneath slowly rose back up. But every time the climate turned colder, sea levels dropped, as more water became locked up in glaciers and ice caps. So as the seas dropped, more of the islands’ land was exposed, making them bigger and bigger until some of them eventually became connected. The four northernmost islands were joined together into a single island that geologists call Santarosae. And during these times, the surface of Santarosae was much closer to the mainland than than the islands had been. Sometimes, when the sea level was really low, it was just over 7 kilometers away. So, that’s a shorter distance; but it still doesn’t explain how mammoths crossed the water. Well, the thinking among paleontologists is: They swam! Believe it or not, swimming comes easily to elephants. They’re strong, buoyant, and their trunks can act like snorkels. In rare instances, African elephants have been known to swim as much as 48 kilometers, so it seems at least plausible that a Columbian mammoth could paddle less than a quarter of that distance to Santarosae when sea levels were low. And even though the oldest definitive mammoth fossil on the island is 80,000 years old, some experts think that the first mammoths made their historic journey as much 150,000 or even either 250,000 years ago, when sea levels were especially low. But the next question you might be wondering is: Why would these animals even want to swim out there to begin with? I mean, southern California is nice, right? Well, for much of the Pleistocene, mainland California was an ideal habitat for Columbian mammoths, but in some ways, the islands were even better. Pollen samples from the Late Pleistocene show us that Santarosae was covered with vegetation, like pines, firs, grasses, and sage. And although they mostly ate grasses and sedges, mammoths also sometimes ate woody plants. Additionally some researchers think that Columbian mammoths knew that food was there, because not only could they see this nice, lush island, they may have even been able to smell it. Research in 2014 found that modern elephants have twice as many genes dedicated to smell as dogs do, and they can smell food and water from considerable distances. No matter what drew them to the island, once they got there, the mammoths found another benefit to their new home: there were no big predators. There’s no fossil evidence of saber-toothed cats, dire wolves or any other large carnivores on the island. It’s not clear why; maybe they just didn’t want to get their feet wet. But even in the absence of predators, those pioneering Columbian mammoths eventually ran into trouble in their new home. Because, the same climate changes that made the glaciers grow also sometimes made them shrink, dumping fresh water into the ocean and rising sea levels worldwide. This happened a lot in the Pleistocene; the most recent period of rising sea levels began around 20,000 years ago. And when the seas rose, much of the terrain flooded, including some of the grasslands that grew the mammoths’ preferred food. When seas were at their highest, only Santarosae’s highest points were above water, and in fact those high points form the four northern islands that we see today. This new island environment was totally different from the one the mammoths experienced when sea levels were lower. It was also very different from the mainland habitats that their mammoth ancestors had adapted to originally. And one of the most important adaptations that the ancestral Columbian mammoths had acquired was their enormous size. Back on the mainland, the mammoths’ size gave them a big advantage: helping to deter the saber-toothed cats, American lions, and dire wolves that shared their ranges. And big animals typically need more food, and there was plenty of that on the mainland. But on Santarosae, there were no large predators, but there was a limited food supply. So, suddenly, being big wasn’t helpful; it was detrimental. And this is when a fascinating phenomenon took place. In this new island environment, natural selection began to favor mammoths with reduced body sizes, mostly because they needed less food. These smaller mammoths not only required less food, they also began eating different things. A 2015 analysis of scratches and pits found in their teeth showed that Pygmy mammoths ate a lot more twigs and leaves than Columbian mammoths did. This may be because they were better able to access the steep slopes near the peaks of the islands, where these foods grew. One study even compared things like the limb proportions and centers of gravity of both kinds of mammoths, and concluded that the pygmys could have climbed steeper inclines to reach parts of the islands that the Columbians couldn’t. Either way, the newer, smaller mammoths were able to take fuller advantage of the forests that were prevalent on the islands, and probably gave them an edge when sea levels rose and flooded the grasslands that the columbian mammoths preferred. Pygmy mammoths eventually grew so specialized to their unique, isolated environment, that by at least 80,000 years ago, they became what’s considered to be a new species. This is a classic example of Insular Dwarfism, the tendency for large organisms living in isolated environments to become smaller over evolutionary time. But there’s an interesting wrinkle in this tale: The two species -- Columbian mammoths and pygmy mammoths -- apparently coexisted on the island. Fossil evidence shows that Columbian Mammoths lived on Santarosae as recently as 15,000 years ago. But, based on the amounts of fossils from both animals, scientists estimate that pygmy mammoths may have outnumbered the Columbian mammoths by more than three to one. So, it’s possible that these big Columbian mammoths were from a population that -- for whatever reason -- never evolved a smaller body size. But others suggest that they might represent repeated waves of new visitors from the mainland. In which case, full-sized Columbian mammoths may have swum out to islands populated by what were essentially miniature versions of themselves, like a Pleistocene version of Gulliver’s Travels. Now, we understand pretty well what happened to the Channel Islands’ shrinking mammoths, because ... it happened more than once. For example, during the Late Pleistocene, Europe was home to an enormous elephant, Palaeoloxodon antiquus, also known as the straight tusked elephant. And at some point during an Ice Age cold snap, some of these elephants seem to have made their way to the island of Cyprus, where they too found themselves stranded when the sea levels rose again. By about 11,000 years ago, a very similar, but much smaller elephant was living on Cyprus, Palaeoloxodon cypriotes. While Palaeoloxodon antiquus probably tipped the scales at 10 metric tons, the new dwarf species weighed only 200 kilograms, a reduction in size of about 98 percent! And the same thing happened with a species of hippos that lived on Cyprus until about 9,000 years ago. This phenomenon is so common that it’s got its own Rule, proposed by biologist J. Bristol Foster in 1964. Foster’s Rule says that, in isolated environments, animals can acquire either smaller or larger body sizes, depending on the availability of resources like food. The rule was inspired by Foster’s study of modern island animals, like the pygmy raccoon on Cozumel in Mexico. Or the pygmy tree sloths that live off the coast of Panama. Both mammals are about half the size of their mainland counterparts. And Foster’s rule isn’t a set-in-stone scientific law. It’s more like a trend, to which there are plenty of exceptions. But in both the fossil record and modern ecosystems, we can find many examples of insular dwarfism, which enables organisms to handle the limited resources that come with living on an island. But, even miniaturization can't always save an animal from extinction. The Channel Island pygmy mammoths disappear from the fossil record shortly after a global cooling episode known as the Younger Dryas came to an end about 11,600 years ago. With the climate getting warmer and drier, the pygmy mammoths’ forests were replaced with coastal scrub and grassland. And while these animals had survived lots of warm periods before, this time, they also had to share their island with a new neighbor: us. The first humans appeared on the channel islands around 13,000 years ago. And while we don’t know for sure if people hunted the pygmies, there are butcher sites elsewhere in North America that show humans did hunt and eat mammoths. So, as the Channel Islands’ shrinking mammoths can attest, islands can offer safe refuge from predators, and harbor unique habitats that some animals can exploit. But when the climate changes, or when new predators arrive, there’s nowhere to run, and a mammoth’s paradise can become a trap. But did you notice was I said earlier? About Foster’s Rule? Even though mammoths may have shrunk after they arrived on islands, there are plenty of other ancient animals that became giants in island habitats. So join us again in a few weeks, when we’ll explore how an island grew massive waterbirds and huge hedgehogs, and how Foster’s Rule applies to them, too. Thanks to Google for supporting PBS Digital Studios. Their mobile app, Science Journal, lets you take notes and measure scientific phenomena such as light, sound, and motion using your phone, tablet, or Chromebook. You can find activity ideas and additional information on their website at g.co/ScienceJournal or check out the link in the description below. And extra big thanks to our current Eontologists, Jake Hart, Jon Ivy, John Davison Ng and STEVE! If you’d like to join them and our other patrons in supporting what we do here, then go to patreon.com/eons and make your pledge! Now, what do you want to learn about? Leave us a comment, and don’t forget to go to youtube.com/eons and subscribe.