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hello everybody and welcome to
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evolutionary Milestones over the course
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of this series of videos as I said in my
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introduction just above we're going to
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be looking at the origins of life and a
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series of major steps in the evolution
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of Life documenting roughly its first
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3,000 million years of evolution or so
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the first few videos are going to be
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focusing on an event called a biogenesis
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a biogenesis
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is another word the the study of the
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origin of life and these first few
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videos are going to cover how when um
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and possible processes by which a
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biogenesis may have occurred so let's
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Jump Right In in order to understand the
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appearance of Life first we have to
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Define what life is one definition which
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I've taken from the reference shown at
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the bottom here is is that a living
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thing should have a
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metabolism that is a coordinated system
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of chemical reactions contributing to
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its maintenance A system that Imports
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energy to maintain order and it should
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also have hereditary replication that is
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a system of copying in which the new
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structure resembles the old I quite like
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that definition I think it covers most
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of the major bases but there are
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actually if you look around and do some
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reading you'll find out there are a
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number of different definitions and this
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um kind of idea of what life is is
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actually something that can be quite
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tricky to pin down and I would say for
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our purposes all of the Des definitions
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of life that are useful for us to
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understand a biogenesis for for the next
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few videos are that living things have
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three things in common they are things
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that are able to maintain M eles um they
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can replicate so they are able to copy
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themselves and they do so imperfectly
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imperfect replication or um in the case
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of life on Earth mutations are kind of
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the the the raw variation upon which
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Evolution
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acts I've put some things in a quiz
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below this video and when you're done
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watching this video do have a go at
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figuring out whether those are dead or
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alive by taking that quiz and see how
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you get on
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and as I suppose a slight aside when
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we're thinking about what life is one
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thing I would really like you to take
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away from this series of videos is that
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even what we may call the simplest life
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forms on Earth today are really very
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very complex no matter what organism it
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is that you're looking at today if it's
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still alive we are seeing the result of
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3,800 million years or so of evolution I
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think a really good example of this is
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how DNA is converted to proteins across
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the Tree of Life in a process that's
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called protein
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synthesis this process is shown for a
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cre an organism that has a uh nucleus
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here on this slide and you can see that
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we start off with DNA but in order to
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make um go from DNA to proteins which
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will actually act within cells there are
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many different steps we have to transcri
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Des cribe the DNA to RNA um that has to
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go through the nuclear membrane in this
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case because this organism has a nucleus
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that then um is acted upon by ribosomes
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which eventually make a protein so this
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is a mind-bendingly complex the process
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that's made up of a series of individual
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and its themselves complex steps and we
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think that generally this must have
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evolved in a peace Mill fashion to think
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anything else really doesn't make sense
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and at life's Origins we may expect that
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everything living organisms were much
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simpler so I would identify that that
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fact that things were simpler and we're
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now liing when we study living organisms
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at the results of such a long period of
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evolution is coupled with the fact that
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actually we lack chemical and fossil
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evidence of what when early life was
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around that is because we don't actually
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have sedimentary rocks in which we'll
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find fossils that record the very early
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Earth maybe have a think about why that
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may be the case but both of those facts
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mean that we have to deal with a lot of
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uncertainty when we're studying life's
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Origins and I think I will have
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succeeded spectacularly if I am able to
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finish the series of videos and you're
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able to kind of get a feel for where the
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uncertainty lies within this particular
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area so when we're talking about the
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origin of life we have to place this
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within the framework of the history of
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Earth um if you read old textbooks there
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have there was an idea um back many back
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in the 70s and ' 80s that was quite
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popular called panspermia which suggests
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that maybe life had originated elsewhere
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um that's no longer
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held or no longer holds I should say a
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broad consensus and we think that
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probably a biogenesis occurred on Earth
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but actually that's very hard to prove
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either way but if we assume that life
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started on Earth um then we have to
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consider Earth's history from its very
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Origins so this timeline is there to
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allow us to do just that so on this
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timeline F kind of orientation I've put
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um
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Homo sapiens here on the far right in
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fact every Emperor and King every uh POA
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everyone that's ever lived that was a
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human fits basically into one 100,000th
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of the right hand pixel of this um bar
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here so H Homo sapiens history is quite
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short on this time
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scale dinosaurs which are shown by this
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black bar here were around for quite a
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long quite a bit longer but even so they
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and indeed the trilobites which are
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represented by this um black bar here
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which lived for really quite a long time
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somewhere in the region of 180 to 200
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million years they kind of they pale
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into the book end of this long time
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scale this time scale starts at
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4,560 million years ago and we think
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that around this time a accary phase of
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Earth History built up there there were
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during the accretionary phase high
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temperatures that precluded um liquid
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water being present on Earth um water
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would have been there in the form of
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steam and that would that temperature
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would have incinerated organic compounds
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it was probably followed by Cooling in a
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period where water and simple organic
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compounds could
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accumulate oxygen oxygen Isotopes which
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are taken um from 4.4 billion year old
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detrital zircons ziron is a very tough
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mineral that lasts for a very long time
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um and we can study the ice tapes of
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oxygen in that mineral and that suggests
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that there was um liquid water present
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from about 100 to 200 million years
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after the beginning of that accre phase
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so that's actually very very early in
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Earth history in fact this paper here
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which um reported these oxygen isotop
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results suggested that from about 4.4 to
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4 billion years ago extensive liquid
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water oceans existed for long periods on
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the surface of the Earth those would
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have been cool enough to allow the
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survival of organic compounds although
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there is significant debate about
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exactly what temperature those oceans
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may have been the Earth was cooling very
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rapidly so oceans may have been a lot
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hotter back then Moon cratering so if we
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look at the craters and the moon
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suggests that possibly this period from
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4.4 to 4 billion years ago was
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relatively impact free there weren't
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lots and lots of um asteroids and comets
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and meteorites hitting the Earth during
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this time and it's in that quiescent so
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this kind of quite um cold and quiet
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period I say cold it may have been a lot
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warmer but at least we had liquid o it
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was in probably this interval that the
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key steps in the origin of life might
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have occurred that is a statement of my
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opinion rather than a broad
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consensus however the other thing that
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we need to consider in this timeline
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when we're looking at the origin of life
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is that at 3.9 billion years ago there
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was an event called the late heavy
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bombardment that we um have lots of
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evidence from from Moon cratering shown
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here in the middle and here's a um a
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computer generated reconstruction of
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what the moon may have looked like in
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the Earth at that
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time and that is a period which we think
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represents a spike in Impact rates so
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you can see that represented um on this
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graph here where we have impact rate on
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the y-axis time on the x-axis and this
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lhb is the late heavy bombardment that's
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labeled here so around 3.9 billion years
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ago there was a spike in the number of
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things that were hitting the Earth and
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this is currently thought to have lasted
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between 20 and 200 million years most
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recent estimates tend to think it's
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around the lower limit so around 20
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million years it was previously thought
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and older textbooks will tell you that
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this U um killed all life and basically
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sterilized the the Earth so a biogenesis
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must have occurred after the late heavy
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bombardment the current thinking um
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often based around
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uh kind of computer modeling studies um
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is that life need not necessarily
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postate the late heavy bombardment
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computer models um such as those in this
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Source here suggest that there is no
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plausible situation in which the Habit
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habitable zone on Earth would have been
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fully sterilized by this
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process so where does that fact leave us
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well we know we have this late heavy
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bombardment here somewhere between 3.9
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billion years um and and a bit younger
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than that but we also know we have this
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nice quiet period that was conducive to
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life shortly after the origins of Earth
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in that confusion phase in which a
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biogenesis could have occurred and so uh
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the balance of probabilities suggests to
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me personally that it wouldn't be
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ridiculous to look for the origin of
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life and abiogenesis to have occurred on
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Earth in that quiescent period and then
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to have survived through the late heavy
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bombardment and then after that point
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Evolution continued along this timeline
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here so that's it for video number one
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in the next videos we're going to look
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at some uh possible processes by which a
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biogenesis may have occurred so I'll see
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you in those shortly thank you for
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watching
