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we have many videos on Khan Academy on things like evolution natural selection we think we have a fairly solid understanding of how life can evolve to give us the variety the diversity that we and the complexity that we've seen around us but it still leaves unanswered a very fundamental question and this might be the biggest question known to us and that is the origins of life how did life first emerge at least on earth and that even starts to lead to other questions about is there life outside of this planet and what could it be like and so let's start with what we actually know and I'm going to start with a timeline so let's go 1 billion years ago let's go 2 billion years ago 3 billion years ago 4 billion years ago so this is now and once again we're talking 1 billion years ago you'll sometimes see the abbreviation BY a billion years ago which is a it's it's an unfathomable amount of time going going into the past but we know that Earth along with the rest of the solar system was formed around 4.6 4.6 billion years ago so that's when earth was formed and right at 4.6 or even you know you wait a casual 100 million years after that 4.5 billion years ago we believed that earth wasn't very suitable for even very simple life to form and that's because the solar system was a crazy place you had collisions of all scales happening all of the time the moon itself was formed from the collision of of two planet-sized objects one would kind of call it the proto-earth and and another planet-sized object and they collided and then they started to spin around and one part became the moon they became it was tightly tightly linked with the earth but you could amend that's not an environment where it would be easy for life to form and even once the moon was formed you have a heavy bombardment of things continue in the solar system the solar system was a messy place it took a long time for the stability that we now observe out there and so that continued we believed until about 3.9 billion years ago which is the earliest that we currently think that earth might have been suitable for life before that there might have been pockets where the Mubarak stop stops and maybe some type of primitive life might have form but then it would have gone away with the heavy bombardment but who knows maybe they could have survived that somehow but that's that's the current mainstream belief the other thing we know is that we see fossil evidence for life 3.5 billion years ago and these are stromatolites the fossil evidence microorganisms they form these structures that actually are continued to be formed today these types of these types of structures continue to be formed today and although it might not feel like microorganisms are complex life when you think about what has to happen within a microorganism they're actually incredibly complex especially if you compare them to very simple non living organisms so we our current belief is well some place in some place in this region life must have arisen on earth but that still doesn't even if we were able to answer that question though it was exactly 3.7 billion years ago was the first time that some RNA decided to we're not decided or ended up getting in the right conformation so it could replicate itself in some way even if we even if we know that date it still leaves unanswered maybe the more interesting question which is the how the how is really at least to me more important arresting than the when and to the how question there's a couple of layers on it the first is well let's just start with the most simple molecules that we would have expected to found to find on early Earth here are some examples of it right over here this is h2o or more commonly known as water right over here is co2 more commonly known as carbon that's a little hard to see let me do a lighter a lighter color so we have we have carbon dioxide right over here here we have molecular nitrogen you have some ammonia you have some phosphate these are and and many other of the the elements that we see on on earth today they would they might have been available in that early Earth but how how do they form at least even the next step up which is the slightly more complex or actually good bit more complex organic molecules and when people talk about organic molecules they might be talking about things like this these are amino acids these are the building blocks of proteins amino acids you see over here nucleotides these are the building blocks of RNA DNA other things and so the first question is and these aren't the only simple organic molecules you could think about sugars and all sorts of other things but the question is is it realistic can how do we at least understand how we can go from these very simple molecules up here to these more complex often called organic molecules and the simple answer is we now have a lot of evidence that this is doable that you can go from these things to these things abiotic lis and without the presence of without the presence of life you'll hear that word abiotic a lot think about it antibiotic you're killing life you're killing bacteria abiotic that is without life and the points of evidence that we now have are we believe and we've seen evidence that there are there's amino acids and and organic molecules related to them on comets meteorites on other planets that it can they formed spontaneously in space once again without the presence of life there we've even been able to form amino acids and other molecules like this from these more simple elements in the laboratory the most famous experiment there is the Miller and Urey experiment this was in the 1950s where they were able to show with with some energy they provided a spark you could imagine that in the early early Earth it could have been from lightning and they tried to set up a mix of gases that they believed was similar to the mix of the atmospheric mix in the early Earth which didn't have much oxygen in the atmosphere then we needed life to actually started to produce some of that oxygen and even though today we think that they probably didn't have the mix of gases right they were they didn't do something significant they were able to show that with that with that mix of gases at least they thought were in that atmosphere and some energy being added to that that they were able to form some of these organic molecules so we should feel pretty good that at least this first step is doable now the next question is as well these organic molecules by themselves that's not life in fact these aren't even the most complex molecules that are we believe essential for life proteins are where things start to get really interesting and a protein of protein our proteins are one of the places a protein might have thousands of amino acids thousands of amino amino acids things like DNA and RNA also we believe essential for life or at least life as we know it could be made up of tens of millions of nucleotides for one DNA molecule so for example this is this is just a small part of a DNA molecule but you can already see much much more complex than what we what we see over here and there too we have evidence that you can go from the amino acids to the proteins or you can go from the nucleotides to the DNA without the presence of life that these things can happen spontaneously if you have the right context the right energy available some people believe or or it's been observed that if you have the right surfaces that these molecules can be organized in the right way to form these more complex things now I know what you're thinking alright proteins are really cool DNA RNA is really cool but then how does that become life what what point would we start carrying well that was a that was a proto life-form and this is where we really get into the area of the unknown because we don't know and there's a couple of hypotheses out there one of them is called the RNA world hypothesis I'll write that down RNA world hypothesis and this is the idea that the first prototype was self-replicating RNA molecules and the reason why people tend to focus in on RNA a little bit more than DNA is that even in cells today RNA doesn't just store information it can actually play a role as a catalyst when you think about things like tRNA and you think about ribosomal RNA and so maybe some of that first proto life was RNA that information that replicated itself and catalyzed the replication of itself maybe it somehow got organized into membrane-bound structures so it could separate so you had environments that were separated from the outside world but the simple answer is we don't know another mainstream hypothesis is the metabolism first hypothesis metabolism metabolism first and this is the idea that a lot of a lot of basic pathways that you might study and say a biochemistry book that these were kind of first just happening in in well all of this could have been happening in this primordial soup where you had these organic molecules and in the right conditions may be around heat vents and whatever else but the metabolism first is some of these mechanisms that we now study in biochemistry these might have happened outside of a cell or outside of life and they just kept creating more and more complexity but at some point these things started happening in in in kind of self-organizing membrane-bound structures maybe there's some kind of combination of the two the simple answer is we just don't know but there's some fascinating clues even if we observe current biology and in fact even if we see the commonalities of things that happen central dogma of biology if we if we see how proteins which proteins which which structures are common to all life as we know it it might give us clues or hints at what some of that very earliest life or proto life was actually like
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