|MadSci Network: Evolution|
Hi Hakan, Since we do not know how life began on Earth, the panspermia hypothesis cannot be ruled out. This hypothesis is credible in the sense that it has not been disproven or eliminated as a possibility for the origin of life. I can tell you that origin of life studies are very difficult, since we can never know for certain whether any of the hypotheses hit the jackpot or not. Though I haven't read much of Francis Crick's writing on this topic, I have been to several conferences where the origin of life is a main subject. Some origin of life hypotheses include the primordial soup hypothesis, a more geochemical origin of life, and panspermia. 1. The primordial soup hypothesis involves an origin of life in the midst of an ocean, where organic molecules interact to form larger organic molecules such as amino acids, which theoretically might form proteins. (According to the RNA world notion, formation of nucleotides might be more convincing, since scientists have known for about two decades that RNA can have catalytic functions. For more information about this, there is a book out titled The RNA World (Tom Cech, editor) that has lots of fascinating information about RNA and implications for the origin of life.) 2. The second hypothesis involves interactions between aqueous substances and minerals. The minerals provide a scaffold and possibly contributions that would lead to the formation of complex molecules that might eventually become biological. Certainly, this method for the origin of life in more probable in the sense that molecular interaction at a surface is more likely to occur than interaction in an ocean. The molecules at a surface are localized in a more 2-dimensional environment, rather than floating around in a body of water. 3. Panspermia is just what you said it is: the seeding of life on Earth by extraterrestrial matter such a the Martian meteorites. When I first got into exobiology research, I read that regardless of whether there were organics on the Earth's surface early on, large impact events would effectively volatilize the organics. That means that Earth would be sterilized with regard to organics, and an extraterrestrial contribution such as comet debris would be required to get any sort of organic synthesis. There is a paper by Carl Sagan and Chris Chyba about this topic. In that sense, life on Earth was clearly seeded from extraterrestrial matter. If you've heard anything about the putative worm in the martian meteorite ALH84001, you might know that people are thinking of more obvious panspermia. Actual organisms rather than organic molecules. This is a hotly debated topic at the moment. It is possible for organics to exist within an object traveling from one location to Earth without being degraded. There is another option that involves an extraterrestrial geochemical origin of life. If life originated at the rock-water interface, the abundance of biologically relevant elements in cells should be similar to those in the rocks. This is not a necessary relationship, but it makes sense. If this is in fact the case, the statistics for how much phosphorus is in martian rocks agrees roughly with how much phosphorus is in a cell. This is not the case with rocks that are from the Earth. I do not know if this is the case for any other biologically relevant elements or not, though. This is obviously very complicated. Panspermia is just one of the many theories about the origin of life. Though you specifically asked about that hypothesis, I figured that explaining some of the other prominent ideas would be beneficial so you could get a feel for what ideas are out there and decide for yourself which makes the most sense. If you want more definitive literature citations, please write back and I'll look them up for you. Sarah Earley CU Boulder Admin Note: While panspermia gives a possible hypothesis for the origins of life on Earth, it still doesn't solve the pre-biotic origins of life, but simply moves it to another place, e.g. if life on Earth was seeded by meteorites from Mars, then how did life evolve on Mars?
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