字幕列表 影片播放 列印英文字幕 Hi! I'm Emily from MinuteEarth. Or, as I've sometimes heard it pronounced, "my-nute Earth." Coming up, we've got four short stories about the science of size, why big things are big, small things are small, and as far as Mother Nature is concerned, size really does matter. First up, a quest for the biggest organism on the planet. Blue whales are the biggest animals ever to exist on Earth. They can weigh upwards of 150 tons, which is more than the largest dinosaurs. But the blue whale is not the biggest living thing. That title goes to... well, it depends on what you mean by "biggest." The tallest may be a California redwood nicknamed "Hyperion." At a towering 115 meters, this giant is taller than the Statue of Liberty. The most extensive organism is a very old humongous fungus that covers a whopping 2385 acres in a national forest in Oregon. At the base of trees, bunches of honey mushrooms appear. They are the fruiting bodies produced by the fungus, which otherwise lives out of sight. Imagine if apple trees grew underground and only the apples apples were visible to us. That's basically what the fungus does, except that it spreads its mycelia not just through the soil, but also through the roots and bark of trees in the forest, attacking them and stealing their nutrients, so it can continue spreading outwards. However, if we're talking about the good old heaviest organism ever found, that prize goes to a giant panda living high on a Utah plateau. Just kidding. It goes to a single quaking aspen named "Pando" that weighs over 6000 tonnes, as much as 40 blue whales. If you go to the Fish Lake National Forest, though, you won't see a giant tree trunk. You'll just see a forest of regular sized trees. But thanks to genetic testing, we've learned that this stand of aspen covering 106 acres of land is actually a single clonal organism that grew from a lone seed long ago. That single tree was able to spread so much because its roots send up shoots that grow into what look like individual trees. Since all 47,000 trees are part of the same organism, the forest behaves somewhat unusually. For example, the entire forest transitions simultaneously from winter to spring and uses its vast network of roots to distribute water and nutrients from trees with plenty to trees in need. Speaking of water, if you include water when weighing these giant organisms, then the humongous fungus might actually way more than Pando but foresters at least care only about the mass actually produced during growth, the dry mass. And since fungi are mostly water, Pando wins. Either way it's likely that some of the below ground connections whether roots or mycelia, have become severed over time, meaning these giants are probably comprised of smaller but still ginormous and genetically identical patches. And finally, because of the extensive testing required to confirm "biggest anything" claims, the fungus and Aspen can only profess to be the largest living organisms ever found. There may be even bigger monsters lurking right under our feet just waiting to be discovered. So, it's possible that the biggest organism hasn't been discovered yet, even if it isn't possible that that organism is an underground panda. The thing is, though, animals don't just wake up one day gigantic. Something weird has to happen to make them that way. Animals come in all different sizes but usually over evolutionary time each type of animal stays roughly the same size. Every once in a while though, something crazy happens that allows an animal to get truly gigantic. Take insects and other arthropods, which have tiny bodies, in part because they breathe by sponging up air through their exoskeletons, and the available oxygen can only diffuse so far before getting used up. If they had bigger bodies oxygen wouldn't reach far enough inside. But about 300 million years ago, Earth's atmospheric oxygen levels spiked. With more oxygen in the air, arthropods' bodies could grow way bigger, leading to mega-bugs like a dragonfly the size of an eagle, and a millipede the size of a two-person kayak. Dinosaurs on the other hand got pretty darn big without any outside help, but at some point they hit a limit due to the so-called "square-cube law": body strength is based on a cross sectional area of bones and muscles, but weight is based on volume, and just like doubling the height of a cube causes its cross-sectional area to get four times larger but its volume to get eight times larger, when an animal gets bigger, it does get stronger, but it gets WAY heavier. Fossil evidence suggests that dinos were nearing the size of which they could no longer lug around their own bodies, when they stumbled upon an evolutionary breakthrough: a system of air pockets and air sacs throughout their skeletons that allow them to get bigger without getting heavier, and have incredibly long but light necks, which granted them access to a huge bounty of leaves. Eventually though, a group of land animals got around the square-cube problem altogether by climbing back into the water, which buoyed their weight. And since they took their lungs with them into the water, they could breathe oxygen-rich air, rather than being stuck with oxygen-poor water, allowing these mammals to grow almost twice as big as the biggest fish. But these giant creatures just didn't have enough food to get any gianter than that. Then, a few million years ago, changing ocean currents brought tons of nutrients up from the depths, which fuel huge localized phytoplankton blooms, which in turn attracted enormous concentrations of scrumptious zooplankton. With this new "krillion"-calorie diet together with their air-breathing lungs and water-supported bodies, blue whales quickly tripled in size to become not just gigantic, but truly the largest animals to have ever lived. And that is certainly something to spout about. In the ocean life comes in all sizes. It turns out that we humans need to be eating more of the small stuff. Anyone who goes fishing probably has a story about the one that got away. "It was this big, don't cha know!" Yeah, that was a bummer, but it's actually quite important that big fish get away, both for fish and fishermen. For most of the species that we fish, commercial and recreational fishermen are only allowed to keep individuals above a minimum legal size. The idea behind these laws is to protect juveniles so they can grow big enough to reproduce at least once before becoming our dinner. In theory, that means there will always be enough fish for dinner tomorrow, and ensuring dinner for tomorrow is important enough that the English Parliament discussed protecting young―that is, small―fish as early as 1376, and today it's a common regulation for fisheries worldwide, except it doesn't really work. First, large individuals have the greatest number of successful offspring, both because bigger fish produce more eggs and because the eggs they produce also contain a more generous food supply for the baby fishies. So by removing the largest individuals of a given species, we severely decrease the population's ability to replenish itself. Second, if we only remove the largest fish, that means fish that are small for their age and thus smaller when they first reproduce, are more likely to live long enough to make babies, so individuals with small fish genes tend to stay in the water, reproduce and pass on their genes to new generations, while big fish and big fish genes become rarer and rarer. We're basically breeding smaller fish, unintentionally, and it's not a small change. Size-selective fishing has caused the body mass of large commercial fish to be cut in half over the last 40 years. Let me say that again. Heavily-fished fish are now half the weight they used to be. Six-year-old haddock, for example, weigh 40% of what they did in 1970 Imagine if full-grown men weighed 65 pounds! Clearly, size-selective fishing means fewer and smaller fish in the water, which suggests it's not the best way to keep our fish supply stocked for future human generations. And in fact, there's a new idea called "balanced harvesting" ready to save the day. Instead of reeling in all the largest individuals, fishermen would catch a smaller number of fish across a wider range of sizes, keeping the numbers and sizes of fish... well, balanced. However, old habits die hard, and the use of size limits is deeply ingrained in our collective fisheries management DNA, but sooner rather than later, we'll have to accept that it's good to let some of the big ones get away, for only they can change the course of fish-tory. Fish-tory! We can't be proud of all of our puns, but while we're talking tiny and water, let's talk about tiny water. How many water molecules does it take to make a drop? Somewhere inside of every raindrop is a tiny impurity―a touch of salt, a speck of soot, a grain of clay―that's absolutely crucial to the raindrop's existence. In fact, without these microscopic pieces of dirt there would be no rain because water vapor can't condense into droplets on its own, which is kind of weird because water molecules like each other. If they didn't, they wouldn't cling to each other like this, and in the air vaporized water molecules collide and stick together all the time, but they also break apart all the time, thanks to bond-breaking heat energy. Only when the air cools down past a certain point called the "dew point" does this breaking apart slow down enough for little clusters of water molecules to grow into droplets. But actually, that's only true if the cluster is big to start with. If it's too small its surface is so curved that the molecules on the outside have few neighbors to bond to which makes them easy to break off so the cluster as a whole has higher chances of losing molecules than gaining them, even below dew point, which means that up until a certain critical size, a cluster's chances of shrinking are better than its odds of growing. Unfortunately, that critical size is 150 million molecules, and while there are millions of 5-molecule clusters in a golf ball-sized volume of air at dew point, odds are that only one of those clusters will grow to a size of 10, and you'd need a golf ball of air 10 million miles across to find a single 50 molecule cluster, which basically means that clusters of water molecules never get to that 150 million mark on their own. Fortunately, they don't have to! They can start off at that critical size by condensing onto one of the gajillions of little pieces of dirt floating in our atmosphere and then grow and grow until they're a droplet in a rain cloud and ultimately it's these little pieces of dirt surrounded by water that make life possible on our big piece of dirt surrounded by water. And, from my spot on this big piece of dirt, thanks for watching!