[00:00.00]Listen to part of a lecture in a paleontology class.
[00:05.05]Professor: So, the best evidence we have suggests that earth is around 4.5 billion years old, right? [00:12.43]So what we'd like to know as paleontologist studying the origin of life is how long after that did life begin on earth? Yes, Keith?
[00:22.89]Male Student: Umm… I think the book mentions of fossil, the oldest fossil ever found from like 3.5 billion years ago？
[00:31.68]Professor: Good, that's right. That's the oldest fossil ever found that was left by what was clearly a living organism. [00:38.36]Plus there's some older rocks that show traces of chemicals that are highly suggestive of life. [00:43.84]But let's ignore that for now. [00:45.69]So it's been claimed that as much as 1 billion years passed between earth formation and the origin of life on this planet. [00:53.11]But we have to consider, does this fossil evidence tell us conclusively when life began?
[00:59.74]Male Student: Well, it can't really be conclusive. Can it? [01:03.36]Cause like, just because we haven't found any older fossils, doesn't mean there weren't any right. [01:08.65]I maybe we just haven't looked hard enough.
[01:11.55]Professor: Well, you're on the right track. [01:13.63]The evidence we have certainly isn't conclusive, but you can't really say we haven't. [01:19.51]Well, let me get back to that in just a second. [01:22.25]First, think about this. Is it reasonable to expect it to take 1 billion years for life to evolve here?
[01:29.92]Female Student: Well, that depends on lots of other factors? Doesn't it? [01:33.42]Like, how long did it take for earth's temperature to cool down enough to support life? [01:38.19]For other conditions to be right?
[01:40.74]Professor: Ok. A fair question. [01:42.58]But if I may rephrase it slightly, what you really wanna know is how long was it before water could exist in liquid form? [01:51.74]Let's just say that by the time earth's atmosphere had stabilized, and this was not long after the creation of the moon, say, 4.4 billion years ago. [02:02.00]By this time, there were already substantial oceans covering the earth. [02:06.56]So, we're still talking a 900-million-year gap before that first fossil.
[02:12.51]Female Student: Well, I guess when you put it that way and it does seem like rather a long time.
[02:17.48]Professor: Ok, I thought you might think. so, but to get back to the point I was making before, we're not gonna resolve this question just by searching the world for older fossil records. Why? [02:28.97]Well, before that oldest fossil we have from 3.5 billion years ago before that, any kind of fossil record really had no chance of surviving. [02:40.47]The process is going on within the earth during that time, the convection of molten rock within earth's mantle, rising to the surface cooling and then sinking again, the volcanic activity, earthquakes. [02:52.94]All the things that led to the creation of plate tectonics, in fact, were so intense than any rocks pre dating that first fossil would have been crushed and very likely melted. [03:03.55]So earth has, in effect, erased its earliest history. [03:07.77]There really are no early samples of earth left on earth.
[03:11.93]Female Student: So, you mean, that's it. There's no way we can tell when life first appeared on earth.
[03:18.15]Professor: Well, there might be, let's think about this a minute. [03:22.97]If no earlier rocks on earth could have survived with their fossil records intact. [03:28.52]Is there anywhere else we could look?
[03:31.12]Female Student: What? You mean like an outer space or something?
[03:35.62]Male Student: No, wait, I think I see where she's going with this. [03:39.94]Remember a few years ago when they found those meteorites and like Antarctica or somewhere. [03:45.04]And it turned out they actually came from Mars.
[03:47.71]Female Student: Yes?
[03:48.36]Male Student: So maybe the reverse could have happened, right? [03:50.93]If rocks from Mars could end up on earth, why couldn't there be earth rocks on Mars or even better on the moon?
[03:59.19]Professor: Exactly. In fact, it's almost a certainty. [04:03.39]We know that there were lots of meters in our region of the solar system during this time period. [04:09.07]And quite a few of them hit the earth with a big enough impact, tons of earth rocks would have been tossed up into space. [04:16.84]So it's really not much of a leap to theorize that a good number of them might have ended up on the moon. [04:22.74]In fact, researchers have calculated that there are probably thousands of tons of earth rocks on the moon just waiting to be found. [04:30.40]And if we could find, say, some 4.2, 4.3 billion year old earth rock there, it happened to contain evidence of life. [04:40.33]Well, that would tell us something amazing, wouldn't it? [04:43.17]Cause that would mean that life on earth began quite quickly as soon as it possibly could have, which in turn would suggest that life is easily generated. [04:52.77]And so it can probably be found all over the universe.
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