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← Origins of Life: Astrobiology & General Theories for Life - Abstract and General Models for Life

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Showing Revision 6 created 11/21/2019 by Alejandro Ibáñez-Costa.

  1. so one of the critical questions of
  2. astrobiology is how do we model life and
  3. we can think about abstract or general
  4. models for life. The reason that we're
  5. after those is because we want to not
  6. only explain life on earth but we want
  7. to be able to explain life on other
  8. worlds and so we need to come up with as
  9. general a principle as possible. So far a
  10. lot of astrobiology has focused on the
  11. idea of trying to define life, so we have
  12. come up with a lot of different
  13. definitions for life
  14. from numerous different perspectives and
  15. this word cloud is just showing some
    examples, words,
  16. definitions for life and there are so
    many different definitions for life that
  17. has really become quite a muddle as to
  18. how we should be thinking about it in a
    more rigorous way.
  19. And so one of the things that when you
    think about as
  20. astrobiologists is really what's the
  21. value of a definition versus a theory or
  22. model and a lot of the traditional
  23. literature and origins of life has been
  24. focused on this idea of defining life so
  25. that we can actually be able to identify
  26. life on other worlds but the definitions
  27. for life are really rather ad-hoc and in
  28. some sense they're they're derived from
  29. observations of life on Earth so for
  30. example we know that life on Earth
  31. evolved so we might have an evolutionary
  32. definition of life or we know that life
  33. on Earth is cellular so we might assume
  34. that all life requires cells but what we
  35. ultimately really need to be aiming for
  36. in the field of astrobiology is to build
  37. better models and theories which might
    be more general and allow us to move
  38. beyond definitions of life that are
  39. anthropocentric to our own life but
  40. actually become predictive theories for
  41. how life might look on other worlds the
  42. challenge that we face with anything
  43. trying to get beyond an anthropocentric
  44. or human-centered or earth-centered
  45. viewpoint is that we only have a single
  46. example of life on Earth so despite all
  47. the diversity of life forms that we see
  48. trees cats people bacteria in your gut
  49. all of that life is related by a common
    ancestry
  50. and the way that astrobiologists talk
    about that is to talk about
  51. something called the last Universal
    common ancestor
  52. so if we look at the tree of life
    as shown here and we trace
  53. the evolution of all the life-forms that
  54. exist today backwards in time they all
  55. converge on what's called this last
  56. Universal common ancestor which is a
  57. population of cells that lived on the
    primitive earth that we think that all
  58. modern life descended from and the
  59. properties that that life-form would
  60. have had would have had to have DNA and
  61. a translation machinery proteins and
  62. cellular architecture much like a modern
  63. cell so it was actually a very advanced
    life-form it doesn't take us all the way
  64. back to the origins of life on earth but
  65. the fact that all life shares is kind of
  66. common biochemical architecture is
  67. actually really limiting because it
  68. means that we only have one example of
  69. life to go on and extrapolating any kind
    of general principles from one example
  70. is actually rather hard so what people
  71. have done traditionally in the origin of
    life field is to try to come up with
  72. models that are based on sort of the core
  73. components of that architecture of life
  74. that we know today and two of those core
  75. components that have been dominant
  76. models for origins of life are what are
  77. called genetics first and metabolism
  78. first so we know that cells metabolize
  79. they need to acquire food from their
  80. environment or you or I need to acquire
  81. food in order to survive in order to
  82. reproduce so metabolism is obviously a
  83. critical component and then in the
  84. genetics first view we also know that
  85. life requires genetic information in
  86. order to be able to reproduce and to
  87. evolve over many generations and so if
  88. you split these two kinds of core
  89. components of biology and to just what
  90. that essential thing is we have this
  91. sort of genetics idea where people have
  92. proposed that the first living entities
  93. might have just been molecules like RNA
  94. that could copy themselves and here on
  95. the slide is shown sort of an
  96. abstract model that kind of process
  97. where you might just talk about the the
  98. binary digits in a sequence if it was an
  99. RNA molecule it would be the sequence of
  100. ribonucleotides in the actual RNA so
  101. that actual basis and you can actually
  102. talk about reproducing that information
  103. via copying and then the reason that the
  104. RNA world
  105. has been so popular as an idea is that
  106. RNA also has a catalytic function
  107. associated with it whereas in modern
  108. organisms we have DNA and protein and
  109. DNA controls genetic heredity and
  110. proteins are primarily the catalysts
  111. that actually execute reactions in the
  112. cell, RNA can do both those functions so
  113. genetics first has emerged of this idea
  114. that you can model quite simply with
  115. these kind of models where you talk
  116. about copying and heredity and this idea
  117. of evolution through this kind of
  118. process as the core thing that emerged
  119. first as a first living entity and the
  120. metabolism first perspective there's an
  121. alternative view that the first kinds of
  122. living entities were not individual
  123. molecules that could replicate but sets
  124. of molecules that were reacting together
  125. and could collectively reproduce due to
  126. the organizational patterns of their
  127. reactions, and that idea is called
  128. autocatalytic set theory and there's an
  129. example shown here about a catalytic
  130. sets, using that same kind of
  131. representation of representing molecules
  132. as binary strings, just strings of zeros
  133. and ones which is a way of modeling
  134. these kinds of processes in artificial
  135. chemistries. And so in this metabolism-
  136. first view the first kinds of living
  137. systems would have been these organized
  138. patterns of chemical reactions. And so
  139. both of these perspectives allow one to
  140. model certain attributes of living
  141. systems. But it's really nice if you
  142. actually put them side-by-side and look
  143. at something like the binary polymer
  144. representation of them because you start
  145. to see that both of them are different
  146. ways of propagating information in
  147. chemical systems, and a theme starts to
  148. emerge about what kinds of theories
  149. might unify different approaches to
  150. origins of life. And so this gets back to
  151. the idea that what we need to start
  152. doing to move forward in origins of life
  153. whereas traditionally we've had these
  154. models like genetics first, metabolism
  155. first, and there's other models like
  156. compartment first, where
    we're talking about
  157. mineral surfaces and all kinds of
  158. things and that we need to really start
    thinking about what are the theories for
  159. life, and how do we actually develop more
  160. predictive models that are more generic
  161. too different chemistries and allow us
    to actually go to the lab and predict
  162. under what circumstances we should start
    getting things that look more lifelike.
  163. And a nice example of the need for
  164. theories and thinking about origins of
  165. life was given by Carol Cleland and
  166. Chris Chyba and a paper that they wrote on
  167. defining life where they talked about
  168. trying to define water and how difficult
  169. it was to actually define water before
  170. we had a molecular theory for water so
  171. you might describe water as a clear
    liquid, you might describe it based on
  172. the fact that it's liquid at a certain
  173. range of temperatures that it you know
  174. doesn't have a strong odor there's a lot
    of different ways that you could
  175. describe what water that might lead to a
  176. definition of water but none of them are
    really exclusive to water because
  177. there's other clear liquids that you
  178. might describe there's other things that
    are also a liquid at room temperature
  179. and so the way that we really precisely
  180. define what water is is actually to have
  181. an atomic theory that describes
  182. molecules and their interactions and we
  183. can precisely define water now as h2o
  184. and so their thought was that what we do
  185. now is sort of phemenologically define
  186. life, we have a lot of heuristics or a
    lot of ideas about what we think life
  187. might be but ultimately what we need is
  188. a theory and that our definition should
  189. derive from the theory not the other way
    around.
  190. And one way I like to think about
  191. that is actually to think about like the
    emergent properties of life
  192. so water for example,
    one of the defining properties
  193. that we think water has is that it's wet
  194. but wetness of water is an emergent
  195. property it requires many many many
  196. millions of water molecules potentially
  197. to be wet although there's
    actually an active debate
  198. about how many water molecules
  199. if it's a few hundred a few
  200. thousand and people have been working to
  201. develop models to quantify when water
    gets wet
  202. likewise if we're thinking about
  203. emergent properties of life evolution is
  204. often considered to be a defining
  205. property of life but evolution exists at
  206. the level of population so it requires
    many interacting individuals
  207. in order to be an evolutionary
  208. system and so in some sense evolution
  209. that we use as a defining property of
    life is also an emergent property of
  210. life and so one of the things that we
  211. really need to challenge ourselves with
  212. is to try to find the underlying theory
  213. that explains that emergent property in
    the same way that we have an atomic
  214. theory for water that explains some of
    its emergent properties
  215. and so one of
    the ways that we might think about that
  216. is actually to think about life as an
  217. information processing system so this is
  218. kind of a newer proposal about trying to
  219. unify different properties of life that
  220. a lot of people have been very
    enthusiastic about in the field
  221. and a lot of people are working on from
  222. different perspectives but if we go back
  223. to thinking about that genetics versus
    metabolism picture
  224. and we had the binary polymer model.
  225. Both of those were kind of
  226. a representation of an informational
  227. system that was capable of reproducing
  228. itself they were just very different
  229. architectures for that kind of thing and
  230. one might think of genetics first as a
    digital type of information processing
  231. and metabolism first as an analogue type
  232. of information processing and so so
  233. there is this idea in the biological
  234. community and also emerging in
  235. astrobiology about information possibly
  236. being a unifying principle for how we
  237. should think about life across all
  238. scales and that may be organisms are
  239. really organized by flows of information
  240. so one way to think about the origin of
  241. life potentially as kind of a new
  242. perspective is to think about it as a
  243. transition and how information is stored
  244. propagated and used and this might be
  245. sufficiently general to be able to
  246. predict properties of alien chemistries
  247. that can also process information in a
  248. similar way potentially to Earth's
  249. biology but might allow different
  250. chemistries than that biochemical
  251. architecture that we have on earth as
    characteristic
  252. of the last Universal common ancestor
  253. so if we could
  254. understand how information works in
  255. biochemical networks on earth we could
  256. potentially understand what other
  257. possible chemistries could enable those
  258. same kinds of emergent properties on
  259. other worlds and maybe predict alien
  260. chemistries