 |
| How Did We Get Here |
Stephen's discovery may have pinpointed the time and place for the origin of life, but it tells us nothing about how life actually started. To do that, scientists must re-create early earth in their labs and try to catch a glimpse of that first magical spark. Planet Earth, 4 1/2 billion years ago. If we stepped on its surface, molten lava would incinerate us immediately. One breath of its atmosphere would kill us. How could life have formed in this bubbling, poisonous hell? What you're making is simple molecules, what we call monomers. Life as we know it is made up of polymers, complex molecules that are made up of monomers.
That crucial step towards life may not come from a bubbling flask in a lab. Jen Blank is sure of that because she believes life needed something else to get started on earth and it came from the sky At 20,000 miles per hour. We know that in the early history of the solar system, comets were slamming into the planets, and maybe this would have been a vehicle for delivering prebiotic materials to the early earth. Comets mountain-sized lumps of ice and dust circling the sun. This time, we're coming in at a 15-degree angle from the horizontal. To see what happens to the amino acids, Jen needs to get her hands on some serious firepower. Here's where you might want to have a picture of a gun or something. And, really, at the end of the experiment, if all things go well, it looks much the same as when we first started. Now Jen breaks open the bullet capsule to retrieve its smashed contents and discover what happened to this cometary soup of organic chemicals. And here's an example with two different amino acids in it glycine, which is the simplest amino acid, and proline, which is another one. And here's the initial solution. You can see just two. Here's an analysis of the solution afterwards. We're at the tip of a tall branch of the tree of life. Down in the deepest roots are microbes whose bodies are just a single cell. But each microbe's biology is just like ours. No matter how different it looks, its identity resides in a strand of DNA. Every living organism we know belongs somewhere on this tree. But why did the tree of life grow in the first place? To solve that mystery, we have to find the seed. Scientists don't know much about this seed, but they are sure about one thing. Every living organism on earth shares one common feature a tough outer layer that separates it from the world outside. Every cell has a membrane. The first seeds of life must have had one, too. Well, we think we need this kind of primitive cell membrane to keep the genetic molecules trapped inside. You can't just have everything diffusing around.
You have to have this compartmentalized. Jack Szostak at Harvard Medical School is on a quest to solve one of life's biggest mysteries how the earliest life-forms walled themselves in, defined me from not me. This is how seeds blossom into flowers, how a caterpillar becomes a butterfly, and how a baby becomes an adult. Soap bubbles grow and divide with nothing more than a puff of air, so Jack slightly jiggles the vesicles and watches something incredible happen. The membrane will start to grow spontaneously. What we see in the microscope is that it grows in a very peculiar way. The whole initial vesicle turned into a long, flexible tube. These fatty-acid membranes are achieving one of evolution's most essential jobs self-replication. And they appear to do it automatically. To Jack, this is a sure sign he's getting close to understanding how the miracle of life began. At the University of Manchester in England, chemist John Sutherland is trying to discover how DNA came to be the key to discovering how we came to be. And you inevitably wonder how on earth did that structure arise? How on earth did chemistry produce it? RNA's structure looks simple, but looks can deceive. Each building block is actually made of two parts a sugar molecule and a nuclear base. Chemists found they could make the nuclear bases, and so, when they then realized they could actually make the sugars, they just thought, we must be able to join them together. And so they tried for many years, but the problem was, chemically, you just can't join them together. And incredibly, for the first time, these chemists achieve what none before them ever did they create two of the four basic building blocks of RNA. We don't know how a mix of nonliving chemicals turns itself into a living thing. We don't even know whether this is a very likely sequence of events or very unlikely sequence of events. But let's suppose it's very likely. Then shouldn't it have happened many times over right here on earth? Paul's term for possible homegrown alien life is the shadow biosphere. And he has a plan for how we might discover it.
We could look at places on earth where conditions are so extreme, so harsh, they're beyond the reach of life as we know it to see if there's some hardy alien type of microorganism living there. One of Paul's colleagues, Felisa Wolfe-Simon, is looking for a shadow biosphere by digging through the mud. So, the life that we might find in, let's say, this much mud we could have billions of different microbes that are as different as you and I are to a mosquito. In fact, we are more closely related to mosquitoes than they are to each other. That's how different these microbes are. Felisa works at the U.S. Geological Survey in Menlo Park, California, but it's NASA's astrobiology program that pays her to study mud. She did, indeed, find bugs that could survive these highly toxic doses of arsenic. I'm interested in how it dissolves, where you might find it. No matter how big a dose of arsenic she gave them, even many times more than the sky-high levels in Mono Lake, the microbes just kept on growing. Maybe they use similar proteins to ours, but maybe they use different amino acids than we do. If arsenic atoms are somehow replacing phosphorus atoms in these microbes, then these bugs do not fit on our tree of life. They may not look any different from life as we know it, but these bugs could be the descendants of an entirely separate genesis. So if we found something that even did something a little different, it could mean that here on Earth, there was not just one tree of life, but there could be multiple trees of life. Humanity has spent an eternity thinking about the loneliness of being us. It would provide for us an example of something else, some other form of life that was also successful. If we found those microorganisms, then bingo.
We could say life on Earth has happened at least twice. Two out of two on one earth-like planet surely means that the universe is teeming with life. It would be inconceivable life had happened twice on one earth-like planet and not at all on all the other earth-like planets. If Felisa's bugs are the offspring of a second genesis here on Earth, then life could be a cosmic norm. We would not be alone in the universe. But that leads us to an even more intriguing possibility life on Earth may not be from Earth at all. The scientific quest to discover the origin of life has revealed something totally unexpected There might have been more than one genesis. Our planet might harbor not one, but two or more trees of life, each growing from a separate seed. Where did these seeds come from? That question is forcing us to reassess who we really are because the answer could be out of this world. Planetary scientist Ben Weiss has pieces of another world in his lab. They are rocks that have traveled from Mars to Earth, and he thinks microscopic Martians may have hitched a ride on some of them. About a ton of Martian rocks lands on earth every year, and over the history of the solar system, billions of tons of materials have been transferred. So it's possible that we, in fact, are Martians. 4 billion years ago, when Earth was being pounded by meteorites and comets, so was Mars. So, what you see here is a slice of the meteorite. In fact, on the very outside of it over here on the left, there's a little melted zone. Perhaps the next space probe we send to Mars will stumble across them, and we'll be able to study our long-lost ancestors. But it's also possible the first living things on Earth are still here, lurking in the shadow biosphere. You and I are the latest chapter of a story that's been unfolding for billions of years. How that story begins is still unknown. Did comets seed the Earth with the raw ingredients of life? Was Mars our original birthplace before we jumped to a new planet? Or are we the Earth's second or third incarnation of life? Aliens might be living among us. We might all be Martians. In the end, the stuff of science fiction might lead us to a cosmic truth and answer that eternal question How did we get here?
0 comments:
Post a Comment