1 00:00:02,200 --> 00:00:05,067 We finished off the last video in the Hadean Eon. 2 00:00:05,067 --> 00:00:10,190 It was named for Hades or the ancient greek Underworld 3 00:00:10,190 --> 00:00:13,733 Hades is also the name of the guy who ran the ancient greek underworld 4 00:00:13,733 --> 00:00:17,415 Zeus's older brother and it was an appropriate name, 5 00:00:17,431 --> 00:00:21,200 although the idea, the ancient greek notion of the underworld 6 00:00:21,200 --> 00:00:24,533 isn't exactly the modern notion of hell, but it was a 7 00:00:24,533 --> 00:00:28,354 hellish environment, you had all this lava flowing around 8 00:00:28,354 --> 00:00:32,954 and things impacting the earth from space. As far as 9 00:00:32,954 --> 00:00:36,933 we can tell right now it was completely inhospitable 10 00:00:36,933 --> 00:00:39,667 to life. And to make matters worse, even though the 11 00:00:39,667 --> 00:00:42,533 earth started to cool down a little bit, maybe the crust 12 00:00:42,533 --> 00:00:45,267 became a little bit more solid, maybe the collisions 13 00:00:45,267 --> 00:00:48,533 started to happen less and less as we started to go 14 00:00:48,533 --> 00:00:51,600 a few hundred million years fast forward after 15 00:00:51,600 --> 00:00:54,600 Theia ran into the early earth and formed the moon 16 00:00:54,600 --> 00:00:57,533 There is something like the "Late heavy bombardment" 17 00:00:57,533 --> 00:01:01,046 and right now the consensus is that life - whatever we 18 00:01:01,046 --> 00:01:05,133 are descended from - would have had to come about after 19 00:01:05,133 --> 00:01:07,867 the late heavier bombardment, because this was a time when 20 00:01:07,867 --> 00:01:10,067 so many things from outer space were hitting earth, 21 00:01:10,067 --> 00:01:12,467 it was so violent, that it might have killed off any 22 00:01:12,467 --> 00:01:14,800 kind of primitive, self replicating organisms or molecules 23 00:01:14,800 --> 00:01:17,533 that might have existed before it. And I won't go into 24 00:01:17,533 --> 00:01:20,467 the physics of the late heavy bombardment. 25 00:01:20,467 --> 00:01:23,333 What we believe of what happened, because Uranus and 26 00:01:23,333 --> 00:01:26,333 Neptune, so this is the sun right here, That is the sun. 27 00:01:26,333 --> 00:01:29,467 This is the asteroid belt. That's outside the orbit of the 28 00:01:29,467 --> 00:01:37,533 inner rocky planets. But Urans and Neptune, 29 00:01:37,533 --> 00:01:45,067 their Orbits moved outward. What that caused is 30 00:01:45,067 --> 00:01:50,933 gravitationally it caused a lot of the asteroids of the 31 00:01:50,933 --> 00:01:53,400 asteroid belt to move inward and started impacting 32 00:01:53,400 --> 00:02:02,067 the inner planets, and earth was one the inner planets. 33 00:02:02,067 --> 00:02:05,667 And also impacted the moon. It also obvious on the moon 34 00:02:05,667 --> 00:02:08,933 The moon doesn't have an atmosphere to smooth over the 35 00:02:08,933 --> 00:02:12,200 impacts. The consensus is that only after the late heavy 36 00:02:12,200 --> 00:02:15,267 bombardment was Earth kindof ready for life. 37 00:02:15,267 --> 00:02:19,333 We believe that the first life formed 3.824 billion 38 00:02:19,333 --> 00:02:26,600 years ago. Remember, G for Giga, 4 billions years ago. 39 00:02:26,600 --> 00:02:30,067 When we're talking about life in this period, we're not 40 00:02:30,067 --> 00:02:33,067 talking about squirrels or panda bears. We're talking about 41 00:02:33,067 --> 00:02:37,800 extremely simple life forms. Prokaryotes. 42 00:02:37,800 --> 00:02:44,000 Let me give you a little primer on that. 43 00:02:44,000 --> 00:02:49,067 We're talking about Prokaryotes. I'll compare them 44 00:02:49,067 --> 00:02:53,667 to Eukaryotes. 45 00:02:53,667 --> 00:02:59,000 Prokaryotes are for the most part unicellular organisms 46 00:02:59,000 --> 00:03:04,533 that have no nuclei. They also don't have any membrane 47 00:03:04,533 --> 00:03:09,067 bound organelles, or what we would call parts of the cells 48 00:03:09,067 --> 00:03:12,256 that would perform specific functions, like Mitochondria 49 00:03:12,256 --> 00:03:14,569 So their DNA is just floating around. 50 00:03:14,569 --> 00:03:16,990 Let me draw this character's DNA. 51 00:03:16,990 --> 00:03:19,995 So it's just floating around ,just like that. 52 00:03:19,995 --> 00:03:23,308 Prokaryote literrally means : "Before kernel" or 53 00:03:23,462 --> 00:03:25,621 "Before Nucleus" 54 00:03:25,621 --> 00:03:30,425 Eukaryotes DO have a nucleus with all their DNA. 55 00:03:30,441 --> 00:03:34,236 So this is their nuclear membrane with the nuclear 56 00:03:34,482 --> 00:03:37,292 DNA floating inside it 57 00:03:37,292 --> 00:03:42,369 It also has other membrane bound organelles, Mitochondria 58 00:03:42,369 --> 00:03:45,821 is kindof the most famous of them 59 00:03:48,267 --> 00:03:52,200 Mitochondria, we think is basically one prokaryote 60 00:03:52,200 --> 00:03:54,933 crawling into the membrane of another and becoming stuck 61 00:03:54,933 --> 00:04:00,554 and they both becoming a symbiotic organism with each other 62 00:04:00,554 --> 00:04:08,236 So at this time we're still talking Prokaryotes at this stage. 63 00:04:08,313 --> 00:04:13,267 This shows our current understanding of where things 64 00:04:13,267 --> 00:04:18,041 branched off from, at this point of the tree 65 00:04:18,041 --> 00:04:23,067 is some common ancestor to prokaryotes and eukaryotes 66 00:04:23,067 --> 00:04:25,718 prokaryotes right over here 67 00:04:25,718 --> 00:04:28,262 the bacteria and archeae 68 00:04:28,277 --> 00:04:31,138 and here is the eucaryotes 69 00:04:31,138 --> 00:04:36,467 and this first living thing we think might have been 70 00:04:36,467 --> 00:04:41,338 some self replicating molecules, maybe a little bit more 71 00:04:41,369 --> 00:04:45,646 organized membrane, DNA and RNA became 72 00:04:45,646 --> 00:04:52,738 the method of passing information from one generation to the next 73 00:04:52,738 --> 00:04:56,641 It is still an open question what that first life is 74 00:04:56,641 --> 00:04:59,662 or how you even define that first life 75 00:04:59,862 --> 00:05:04,677 Based on the makeup of DNA molecules of the current 76 00:05:04,692 --> 00:05:09,662 organisms, this is how the think the tree of life came about 77 00:05:09,662 --> 00:05:15,333 We have one common ancestor, then the Archeae and Prokaryotes 78 00:05:15,333 --> 00:05:20,467 have a common Ancestor, it's different from the Bacteria 79 00:05:20,467 --> 00:05:30,800 this is an example of Bacteria, Ecoli 80 00:05:30,800 --> 00:05:35,600 We think that the earliest lifeforms we anaerobes 81 00:05:35,600 --> 00:05:40,400 these did not need oxygen and for the most part 82 00:05:40,400 --> 00:05:43,200 found oxygen poisonous and the earliest 83 00:05:43,200 --> 00:05:47,000 lifeforms probably also did not perform photosynthesis 84 00:05:47,000 --> 00:05:49,467 They might have gotten their energy from other sources 85 00:05:49,467 --> 00:05:52,133 Chemically probably in that extremely volatile 86 00:05:52,133 --> 00:05:56,067 environment that they were in 87 00:05:56,067 --> 00:06:01,800 Lets forward to a major event, these are big time scales 88 00:06:01,800 --> 00:06:04,400 we are talking about here 89 00:06:04,400 --> 00:06:07,067 This is a big timescale, we're talking about 800 million 90 00:06:07,067 --> 00:06:12,733 years, humans and chimpanzees have ony 91 00:06:12,733 --> 00:06:19,867 diverged 5 million years ago 92 00:06:19,867 --> 00:06:25,600 From ancient greece to now we're talking about only 93 00:06:25,600 --> 00:06:33,733 2,500 years ! You multiply that by 1,000 you get 94 00:06:33,733 --> 00:06:39,200 2,5 million years. 95 00:06:39,200 --> 00:06:43,733 And this is 800 million years ! 96 00:06:43,733 --> 00:06:47,867 These are extremely huge periods of time. 97 00:06:47,867 --> 00:06:50,733 That's why we call them Eons. 98 00:06:50,733 --> 00:06:55,067 The dividing line between the Hadean Eon and the 99 00:06:55,067 --> 00:06:58,733 Archean Eon is a bit fuzzy. 100 00:06:58,733 --> 00:07:01,867 Most people place it at about 3.8 Billion years ago. 101 00:07:01,867 --> 00:07:09,133 It's about the earliest rocks we can observe. 102 00:07:09,133 --> 00:07:14,067 We have rocks from about that time. 103 00:07:14,067 --> 00:07:21,067 So there are 2 things, Rocks have survived and that is 104 00:07:21,067 --> 00:07:28,000 also when we think life has started. So now we are in the 105 00:07:28,000 --> 00:07:30,467 the Archean Eon. 106 00:07:30,467 --> 00:07:38,400 But there is still none to little oxygen in the Environment 107 00:07:38,400 --> 00:07:41,200 If you went to the earth at this time it would have looked 108 00:07:41,200 --> 00:07:44,467 something like this, a reddish sky. You would have had 109 00:07:44,467 --> 00:07:48,467 nitrogen and methane and carbondioxide in the atmosphere 110 00:07:48,467 --> 00:07:52,933 There would still have been a lot of volcanic activity 111 00:07:52,933 --> 00:07:56,600 This right here, these are pictures of stromatolites 112 00:07:56,600 --> 00:08:04,733 These are formed from Bacteria that are bringing in 113 00:08:04,733 --> 00:08:07,267 sediment particles, and over time these things get 114 00:08:07,267 --> 00:08:09,400 built up. 115 00:08:09,400 --> 00:08:13,667 The most important even in the Archean Eon in my opinion 116 00:08:13,667 --> 00:08:20,933 , we believe this happened about 3.5 billion years ago 117 00:08:20,933 --> 00:08:26,400 Prokaryotes, or especially bacteria evolving to use the 118 00:08:26,400 --> 00:08:29,400 energy from the sun 119 00:08:29,400 --> 00:08:31,933 To actually do photosynthesis 120 00:08:31,933 --> 00:08:38,954 The side effect from this was the production of oxygen 121 00:08:38,954 --> 00:08:48,123 And at first this oxygen didn't really accumulate in the atmosphere 122 00:08:48,138 --> 00:08:59,133 because there was a lot of dissolved iron in the oceans 123 00:09:02,267 --> 00:09:06,267 All of the life for the next billion years all occured 124 00:09:06,267 --> 00:09:09,467 in the oceans. There was no ozone layer. 125 00:09:09,467 --> 00:09:13,467 The land was being irradiated, a completely inhospitable 126 00:09:13,467 --> 00:09:18,600 environment for life. 127 00:09:18,600 --> 00:09:22,267 The first oxygen that got produced, instead of being 128 00:09:22,267 --> 00:09:26,800 released into the atmosphere instead bonded with the 129 00:09:26,800 --> 00:09:33,267 iron that was dissolved in the ocean 130 00:09:33,267 --> 00:09:38,467 Once a lot of that iron was oxidized later on and the 131 00:09:38,467 --> 00:09:44,400 oxygen did get released into the atmosphere it had 132 00:09:44,400 --> 00:09:47,600 a cataclysmic effect. 133 00:09:47,600 --> 00:09:53,000 Especially on the anaerobic life that existed until then. 134 00:09:53,000 --> 00:09:58,867 It was neccessary though for us to happen later on. 135 00:10:03,829 --> 00:10:16,771 Still a few billions years though before things start 136 00:10:16,771 --> 00:10:21,486 flopping around on the land. See you at the next video.