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 to a decade earlier, in 1952, when a scientist named Stanley Miller first cooked up the 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 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 HadeanEra/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.
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 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 as the water wheel to push on cellular machinery (and make things like ATPin 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.
0 comments:
Post a Comment