字幕列表 影片播放 列印英文字幕 Hey guys, Joe here. Some people would argue the most important year in the history of soup was 1962, when Andy Warhol released his soup-er soupy pop art. But I think soup’s best year came a decade earlier, in 1952, when a scientist named Stanley Miller first cooked up primordial soup. Miller’s experiment took some simple chemicals, like those found on early Earth, bubbled them up through a tube, zapped them with electricity, and after a few days, floating in this soup, he found amino acids–the building blocks of proteins, and one of the essential ingredients for life. This idea–that life’s origins could be found in a puddle of chemicals–is an old one. In the 1920s, two different scientists theorized about life arising from what they called a “prebiotic soup”. And this soupy speculation even goes back (unsurprisingly) to Charles Darwin, who in 1871 wondered if life may have formed from chemicals “…in some warm little pond…” What made Miller’s experiment so special was it gave us proof: regular non-life stuff could become cool life stuff super-easily. But… everything “living” we see today, even the most basic bacteria, is so complex, built of such intricate machinery, it’s impossible to imagine they just popped into existence out of some soup. That’s because they didn’t. We’re going to go on a journey in search of the origin of life, and along the way there will be a few forks in the road, maybe a couple speedbumps, and we’re going to need help from a couple friends. We’ll come to see that Miller’s primordial soup isn’t exactly how this story began. But the FIRST question we should ask isn’t how life started, it’s when. Life on Earth couldn’t exist before Earth existed, and it formed around four and a half billion years ago, at the dawn of the Hadean Era/Eon. Soon after that, another planet collided with the young Earth, melted the entire crust, and created the moon in the process. After the crust cooled, there was even some liquid water… at least for a little while. Because for the next couple hundred million years, Earth was showered with hundreds of massive space rocks. The oceans boiled away, the crust melted again, and Earth was basically no place for life… until things settled down about 4 billion years ago, at the dawn of the Archean Eon. This is the earliest possible time that life could have started on Earth, the beginning of what we call the habitability boundary. And fossil and chemical evidence tell us that early microbes existed by 3.7 billion years ago, what’s known as the biosignature boundary. At some moment in here, non-life became life: we call this abiogenesis. Now, I don’t have a time machine. As far as I know, no one does. Therefore we can’t go back and find that exact moment. But if we could, what would we look for? This brings us to the next big question on this journey… what is life? You’d think biology would have a good definition for life, the thing it studies. But as a biologist I can tell you this is much harder than it sounds. In one chapter of biologist JBS Haldane’s 1949 book What Is Life? he literally writes “I am not going to answer this question.” Life is a board game, a delicious breakfast cereal, and a highway? According to the dictionary, it’s the time between birth and death. But none of these definitions really help us. I think we might be asking the wrong question, because life isn’t a thing that things have, life is what living things do. In school, many people learn a checklist for the characteristics a thing must have in order to be “alive”: MRS GREN. But this list came from looking at life as we know it today. Life at the very beginning was probably much simpler. A physicist, Erwin Schrödinger, looked at all these things that life does and saw something only a physicist would see: According to the second law of thermodynamics, . But inside of living cells there’s a huge amount of order and complexity. In 1944, Schrödinger defined life as a struggle against entropy– the persistent resistance of decay, the preservation of DISequilibrium. Since then we're learned a lot more about entropy, and it may be that the rise of complexity is as inevitable as its decay. That sounds pretty good. Life creates these little closed systems where it works to keep things nice and ordered. But this definition still leaves out one important thing: Living things evolve. Inside the very first living things must have been molecules–chains of atoms–that carried information–instructions for building things or codes for doing stuff. Those molecules must have copied and made more of themselves, some a little different than the others. And a few of those codes and instructions must have been better at doing whatever they did, so they made even more of themselves. What we’re describing is evolution by natural selection, Darwin’s famous idea, and for life to move forward, it must have been there from the beginning. Life is a product of evolution. With all this in mind, maybe we’re finally able to come up with a better definition: Life began the moment that molecules of information started to reproduce and evolve by natural selection. And now that we have a definition we can make some rules for what something has to do to be “alive”. 1. A living thing must work to avoid decay and disorder 2. To do that, a living thing has to create a closed system, or be made of cells 3. They have some molecule that can carry information about how to build cell machinery 4. This information must evolve by natural selection Sounds pretty good, but rules are one thing. The ultimate question is how would this actually happen? Let’s take these rules one by one. What would it require for these things to arise? And–most importantly–how likely are each of these steps based on what we know from good ‘ol real, actual, hard science?! Today, no matter where we look on the tree of life, most cell machinery is made of protein– chains of folded amino acids. When modern cells make proteins, they copy genes from DNA into RNA and then use that RNA as a blueprint for making the proteins. We call this universal pathway the central dogma of biology, because it sounds really cool, and because it’s something that all life shares. But there’s a paradox hidden in here–a puzzle. It’s a chicken and egg problem! DNA needs proteins to make more of itself. And cells need DNA and the instructions it holds to make proteins. So which came first? We can solve this paradox in a pretty simple way. Just get rid of DNA and protein in the earliest days of life, and let RNA do everything. RNA is the molecular cousin of DNA. It contains the same four-letter alphabet code as DNA, only T is replaced by a similar molecule, U. And instead of two strings in a helix, RNA is usually found in just one string. RNA is special, because in addition to carrying information in that 4-letter code, it can fold up into interesting shapes and actually do stuff. The same way that protein enzymes can do all kinds of chemical reactions, RNA enzymes–called ribozymes–can work life’s machinery too. It’s now thought that life began in an RNA world. Before DNA became a more permanent form of storage, different RNA chains could have carried information and been the machines for all of life’s important chemistry. Unfortunately, the RNA-only world went extinct more than 3 billion years ago, but we can make these RNA enzymes today. Scientists have constructed ribozymes that can copy themselves, just like DNA gets copied. And those copies occasionally have errors or changes, so RNA can evolve too. If you need more proof you can find it right inside your cells. The ribosome, the massive structure that stitches amino acids into protein, is mostly RNA. We also find nucleotides, the single molecular units of RNA, inside a bunch of other molecules our cells need for metabolism. This all makes sense only if the earliest days of living chemistry were dominated by RNA. And it solves our chicken and egg problem. The RNA world takes care of two of our four rules: A molecule that can carry information (3), and that can evolve (4). To find answers for the other two, we need to ask one more question: Where did life begin? There’s been a lot of theories about where life came from, but they boil down to these: Either life arose on Earth, or life arose somewhere else and was brought here. It’s well-known that space is full of the chemical building blocks of life, from amino acids to DNA and RNA letters... ...buried inside meteorites like this one that fell on Australia in 1969. It shows the chemistry that makes biological molecules can happen pretty much anywhere. But the idea that life was delivered to Earth on space rocks, which goes by the awesome name panspermia… well there’s just no proof it ever happened, and it doesn’t really explain the origin of life anyway. It just moves it somewhere else. Life probably started here. No… zoom out a little. We know early Earth had plenty of chemical ingredients, but the problem with that old idea of primordial soup is that soup can’t do anything on its own–those chemicals can’t react without outside energy. We get a hint of where this primordial energy came from by looking (again) at our own cells. Instead of lightning, or heat energy, our cells pile up a bunch of hydrogen ions (protons) on one side of a wall, let ‘em flow downhill, and use this like a water wheel to push on cellular machinery (and make things like ATP in the mitochondria) We burn food to keep our hydrogen pump going, but the first life forms wouldn’t have been able to do this, because tacos hadn’t been invented yet. Instead, they would have needed some natural source, and they could have found it at the bottom of the ocean. Deep-sea hydrothermal vents are covered in microscopic little pockets, which could have served as molds for the first cells. Molecules with one oily water-hating end and one water-loving end have a neat habit of forming bubbles and sheets all on their own and there were plenty of these in the chemical soup near deep sea vents, ready to give rise to the first cell membranes. These vents also create natural streams of hydrogen ions near those little pockets in the rock. Imagine an early life form sitting there, wrapped in its little membrane bubble, with a free source of energy flowing by, powering all the work it takes to create ordered life and resist entropy. But this would have been the absolute simplest form that life could take. For this life form to become life that looks like what we know today, a lot more stuff had to happen: it had to switch from storing its genetic information in RNA and started using DNA. Instead of using RNA and ribozymes to run all its cellular machinery, it had to start stitching amino acids into proteins. This opened up new possibilities for making and storing energy that let early life become free-living and more complex. One of these complex life forms is the ancestor of everything alive today, the last universal common ancestor, or LUCA. This is the end of our journey, searching for the origin of life on Earth. A lot has happened since. This story is based on things we’ve actually seen, not just on what’s possible. We’ve figured out when life could have started. We’ve come up with rules for what life is. We’ve found clues inside our own cells that explain how the first life satisfied these rules, and where that life might have started. The only question we haven’t answered is why, but that’s not really a question for science, is it? There’s still quite a few gaps to fill in this story, and if you’re looking for a nice, neat answer for how life started, you’re probably not going to find it. Life is just a thing that happens. It’s still happening today, and it will evolve and continue as long as there’s a place it can happen. Darwin didn’t know it when he wondered about that warm little pond, full of chemicals, giving rise to life, but his theory of how things change and adapt turned out to be so powerful it encompasses life not just in its endless forms, but also in its first ones. Stay curious. Wow. That was a LOT. This is probably the deepest story I’ve ever done on this channel, and it’s one that involves some of the science I actually used to do, so this was a lot of fun for me. I hope you enjoyed it too. But this is only part of the story of how life began. What happened before, to made Earth a place where life could happen? And what happened after chemistry became biology, what life form lives at the bottom of our tree of life? For those answers, go check out these videos from our friends at PBS Space Time and Eons.