Where Did Life Come From? (feat. PBS Space Time and Eons!) — Transcript

Full transcript

1. 0:26Hey guys, Joe here.
2. 0:28Some people would argue the most important year in the history of soup was 1962, when
3. 0:33Andy Warhol released his soup-er soupy pop art.
4. 0:37But I think soup’s best year came a decade earlier, in 1952, when a scientist named Stanley
5. 0:43Miller first cooked up primordial soup.
6. 0:46Miller’s experiment took some simple chemicals, like those found on early Earth, bubbled them
7. 0:50up through a tube, zapped them with electricity, and after a few days, floating in this soup,
8. 0:56he found amino acids–the building blocks of proteins, and one of the essential ingredients
9. 1:01for life.
10. 1:02This idea–that life’s origins could be found in a puddle of chemicals–is an old
11. 1:07one.
12. 1:08In the 1920s, two different scientists theorized about life arising from what they called a
13. 1:12“prebiotic soup”.
14. 1:14And this soupy speculation even goes back (unsurprisingly) to Charles Darwin, who in
15. 1:191871 wondered if life may have formed from chemicals “…in some warm little pond…”
16. 1:24What made Miller’s experiment so special was it gave us proof: regular non-life stuff
17. 1:29could become cool life stuff super-easily.
18. 1:32But… everything “living” we see today, even the most basic bacteria, is so complex,
19. 1:39built of such intricate machinery, it’s impossible to imagine they just popped into
20. 1:43existence out of some soup.
21. 1:45That’s because they didn’t.
22. 1:46We’re going to go on a journey in search of the origin of life, and along the way there
23. 1:51will be a few forks in the road, maybe a couple speedbumps, and we’re going to need help
24. 1:55from a couple friends.
25. 1:56We’ll come to see that Miller’s primordial soup isn’t exactly how this story began.
26. 2:02But the FIRST question we should ask isn’t how life started, it’s when.
27. 2:07Life on Earth couldn’t exist before Earth existed, and it formed around four and a half
28. 2:11billion years ago, at the dawn of the Hadean Era/Eon.
29. 2:14Soon after that, another planet collided with the young Earth, melted the entire crust,
30. 2:18and created the moon in the process.
31. 2:21After the crust cooled, there was even some liquid water… at least for a little while.
32. 2:25Because for the next couple hundred million years, Earth was showered with hundreds of
33. 2:29massive space rocks.
34. 2:31The oceans boiled away, the crust melted again, and Earth was basically no place for life…
35. 2:36until things settled down about 4 billion years ago, at the dawn of the Archean Eon.
36. 2:41This is the earliest possible time that life could have started on Earth, the beginning
37. 2:45of what we call the habitability boundary.
38. 2:48And fossil and chemical evidence tell us that early microbes existed by 3.7 billion years
39. 2:53ago, what’s known as the biosignature boundary.
40. 2:56At some moment in here, non-life became life: we call this abiogenesis.
41. 3:02Now, I don’t have a time machine.
42. 3:05As far as I know, no one does.
43. 3:06Therefore we can’t go back and find that exact moment.
44. 3:10But if we could, what would we look for?
45. 3:13This brings us to the next big question on this journey… what is life?
46. 3:18You’d think biology would have a good definition for life, the thing it studies.
47. 3:23But as a biologist I can tell you this is much harder than it sounds.
48. 3:26In one chapter of biologist JBS Haldane’s 1949 book What Is Life? he literally writes
49. 3:33“I am not going to answer this question.”
50. 3:36Life is a board game, a delicious breakfast cereal, and a highway?
51. 3:39According to the dictionary, it’s the time between birth and death.
52. 3:42But none of these definitions really help us.
53. 3:44I think we might be asking the wrong question, because life isn’t a thing that things have,
54. 3:51life is what living things do.
55. 3:53In school, many people learn a checklist for the characteristics a thing must have in order
56. 3:58to be “alive”: MRS GREN.
57. 4:01But this list came from looking at life as we know it today.
58. 4:03Life at the very beginning was probably much simpler.
59. 4:06A physicist, Erwin Schrödinger, looked at all these things that life does and saw something
60. 4:12only a physicist would see:
61. 4:14According to the second law of thermodynamics,  . But inside of living cells there’s a
62. 4:28huge amount of order and complexity.
63. 4:31In 1944, Schrödinger defined life as a struggle against entropy– the persistent resistance
64. 4:37of decay, the preservation of DISequilibrium.
65. 4:41Since then we're learned a lot more about entropy, and it may be that the rise of complexity
66. 4:46is as inevitable as its decay.
67. 4:49That sounds pretty good.
68. 4:51Life creates these little closed systems where it works to keep things nice and ordered.
69. 4:56But this definition still leaves out one important thing: Living things evolve.
70. 5:02Inside the very first living things must have been molecules–chains of atoms–that carried
71. 5:07information–instructions for building things or codes for doing stuff.
72. 5:11Those molecules must have copied and made more of themselves, some a little different
73. 5:15than the others.
74. 5:16And a few of those codes and instructions must have been better at doing whatever they
75. 5:20did, so they made even more of themselves.
76. 5:24What we’re describing is evolution by natural selection, Darwin’s famous idea, and for
77. 5:28life to move forward, it must have been there from the beginning.
78. 5:33Life is a product of evolution.
79. 5:35With all this in mind, maybe we’re finally able to come up with a better definition:
80. 5:41Life began the moment that molecules of information started to reproduce and evolve by natural
81. 5:46selection.
82. 5:47And now that we have a definition we can make some rules for what something
83. 5:53has to do to be “alive”.
84. 5:551.
85. 5:56A living thing must work to avoid decay and disorder
86. 5:592.
87. 6:00To do that, a living thing has to create a closed system, or be made of cells
88. 6:043.
89. 6:05They have some molecule that can carry information about how to build cell machinery
90. 6:094.
91. 6:10This information must evolve by natural selection Sounds pretty good, but rules are one thing.
92. 6:16The ultimate question is how would this actually happen?
93. 6:20Let’s take these rules one by one.
94. 6:22What would it require for these things to arise?
95. 6:25And–most importantly–how likely are each of these steps based on what we know from
96. 6:29good ‘ol real, actual, hard science?!
97. 6:32Today, no matter where we look on the tree of life, most cell machinery is made of protein–
98. 6:37chains of folded amino acids.
99. 6:39When modern cells make proteins, they copy genes from DNA into RNA and then use that
100. 6:44RNA as a blueprint for making the proteins.
101. 6:47We call this universal pathway the central dogma of biology,
102. 6:52because it sounds really cool, and because it’s something that all life shares.
103. 6:56But there’s a paradox hidden in here–a puzzle.
104. 6:59It’s a chicken and egg problem!
105. 7:01DNA needs proteins to make more of itself.
106. 7:05And cells need DNA and the instructions it holds to make proteins.
107. 7:08So which came first?
108. 7:10We can solve this paradox in a pretty simple way.
109. 7:13Just get rid of DNA and protein in the earliest days of life, and let RNA do everything.
110. 7:19RNA is the molecular cousin of DNA.
111. 7:22It contains the same four-letter alphabet code as DNA, only T is replaced by a similar
112. 7:27molecule, U.
113. 7:28And instead of two strings in a helix, RNA is usually found in just one string.
114. 7:33RNA is special, because in addition to carrying information in that 4-letter code, it can
115. 7:38fold up into interesting shapes and actually do stuff.
116. 7:41The same way that protein enzymes can do all kinds of chemical reactions, RNA enzymes–called
117. 7:48ribozymes–can work life’s machinery too.
118. 7:50It’s now thought that life began in an RNA world.
119. 7:54Before DNA became a more permanent form of storage, different RNA chains could have carried
120. 7:59information and been the machines for all of life’s important chemistry.
121. 8:04Unfortunately, the RNA-only world went extinct more than 3 billion years ago, but we can
122. 8:10make these RNA enzymes today.
123. 8:12Scientists have constructed ribozymes that can copy themselves, just like DNA gets copied.
124. 8:17And those copies occasionally have errors or changes, so RNA can evolve too.
125. 8:22If you need more proof you can find it right inside your cells.
126. 8:25The ribosome, the massive structure that stitches amino acids into protein, is mostly RNA.
127. 8:32We also find nucleotides, the single molecular units of RNA, inside a bunch of other molecules
128. 8:37our cells need for metabolism.
129. 8:39This all makes sense only if the earliest days of living chemistry were dominated by
130. 8:44RNA.
131. 8:45And it solves our chicken and egg problem.
132. 8:47The RNA world takes care of two of our four rules: A molecule that can carry information
133. 8:53(3), and that can evolve (4).
134. 8:54To find answers for the other two, we need to ask one more question: Where did life begin?
135. 9:00There’s been a lot of theories about where life came from, but they boil down to these:
136. 9:05Either life arose on Earth, or life arose somewhere else and was brought here.
137. 9:10It’s well-known that space is full of the chemical building blocks of life, from amino
138. 9:14acids to DNA and RNA letters...
139. 9:17...buried inside meteorites like this one that fell on Australia in 1969.
140. 9:22It shows the chemistry that makes biological molecules can happen pretty much anywhere.
141. 9:27But the idea that life was delivered to Earth on space rocks, which goes by the awesome
142. 9:31name panspermia… well there’s just no proof it ever happened, and it doesn’t really
143. 9:36explain the origin of life anyway.
144. 9:38It just moves it somewhere else.
145. 9:40Life probably started here.
146. 9:42No… zoom out a little.
147. 9:45We know early Earth had plenty of chemical ingredients, but the problem with that old
148. 9:49idea of primordial soup is that soup can’t do anything on its own–those chemicals can’t
149. 9:55react without outside energy.
150. 9:57We get a hint of where this primordial energy came from by looking (again) at our own cells.
151. 10:02Instead of lightning, or heat energy, our cells pile up a bunch of hydrogen ions (protons)
152. 10:07on one side of a wall, let ‘em flow downhill, and use this like a water wheel to push on
153. 10:12cellular machinery (and make things like ATP in the mitochondria)
154. 10:19We burn food to keep our hydrogen pump going, but the first life forms wouldn’t have been
155. 10:24able to do this, because tacos hadn’t been invented yet.
156. 10:28Instead, they would have needed some natural source, and they could have found it at the
157. 10:31bottom of the ocean.
158. 10:33Deep-sea hydrothermal vents are covered in microscopic little pockets, which could have
159. 10:38served as molds for the first cells.
160. 10:41Molecules with one oily water-hating end and one water-loving end have a neat habit of
161. 10:45forming bubbles and sheets all on their own
162. 10:48and there were plenty of these in the chemical soup near deep sea vents, ready to give rise
163. 10:52to the first cell membranes.
164. 10:54These vents also create natural streams of hydrogen ions near those little pockets in
165. 10:59the rock.
166. 11:00Imagine an early life form sitting there, wrapped in its little membrane bubble,
167. 11:04with a free source of energy flowing by, powering all the work it takes to create ordered life
168. 11:08and resist entropy.
169. 11:09But this would have been the absolute simplest form that life could take.
170. 11:11For this life form to become life that looks like what we know today, a lot more stuff
171. 11:14had to happen: it had to switch from storing its genetic information in RNA and started
172. 11:18using DNA.
173. 11:19Instead of using RNA and ribozymes to run all its cellular machinery, it had to start
174. 11:23stitching amino acids into proteins.
175. 11:25This opened up new possibilities for making and storing energy that let early life become
176. 11:28free-living and more complex.
177. 11:30One of these complex life forms is the ancestor of everything alive today, the last universal
178. 11:35common ancestor, or LUCA.
179. 11:37This is the end of our journey, searching for the origin of life on Earth.
180. 11:42A lot has happened since.
181. 11:43This story is based on things we’ve actually seen, not just on what’s possible.
182. 11:49We’ve figured out when life could have started.
183. 11:51We’ve come up with rules for what life is.
184. 11:54We’ve found clues inside our own cells that explain how the first life satisfied these
185. 12:00rules, and where that life might have started.
186. 12:03The only question we haven’t answered is why, but that’s not really a question for
187. 12:08science, is it?
188. 12:09There’s still quite a few gaps to fill in this story, and if you’re looking for a
189. 12:12nice, neat answer for how life started, you’re probably not going to find it.
190. 12:17Life is just a thing that happens.
191. 12:19It’s still happening today, and it will evolve and continue as long as there’s a
192. 12:24place it can happen.
193. 12:28Darwin didn’t know it when he wondered about that warm little pond, full of chemicals,
194. 12:32giving rise to life, but his theory of how things change and adapt turned out to be so
195. 12:37powerful it encompasses life not just in its endless forms, but also in its first ones.
196. 12:45Stay curious.
197. 12:47Wow.
198. 12:48That was a LOT.
199. 12:49This is probably the deepest story I’ve ever done on this channel, and it’s one
200. 12:53that involves some of the science I actually used to do, so this was a lot of fun for me.
201. 12:57I hope you enjoyed it too.
202. 12:59But this is only part of the story of how life began.
203. 13:02What happened before, to made Earth a place where life could happen?
204. 13:06And what happened after chemistry became biology, what life form lives at the bottom of our
205. 13:11tree of life?
206. 13:12For those answers, go check out these videos from our friends at PBS Space Time and Eons.