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welcome back and thank you for sticking
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with me you're doing really well we're
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now on video number four of evolutionary
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milestones and we're kind of starting to
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look at the point where life really
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takes a hold on Earth so everything to
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this point has been in the Arcane Eon
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this is an eon which lasted from 4 000
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to oh well to about 2500 million years
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ago bear in mind that has nothing to do
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with archaea the organisms apart from
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etymologically and linguistically they
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share the same origin that's shown here
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are the ancient Greek
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um
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means ancient
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um so we have this RK on Ian
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um
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where everything we've been looking at
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so far occurred but we're moving on from
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that now to the next major period in
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life history
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so shortly after what I presented
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earlier as potentially being the first
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fossils uh things cause stromatolites
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start appearing these are structures
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that are associated with bacterial mats
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and there's an example of mistonia is
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shown in this image here they have this
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really distinctive kind of wavy
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structure of different layers that you
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can see when you split through a rock
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and if you look at a bedding surface so
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down onto the rock you'll often see
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these um kind of mounded dismounded
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appearance
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these structures are represented with
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bacterial mats which are probably
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um a result of some form of prokaryote
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trapping layers of calcium carbonate as
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they grow upwards possibly towards
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getting uh towards a light source to
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power a photo
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um synthetic reaction of some form a
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note that some recent papers have been
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published that suggest that these
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structures stromatolites appear very
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very early so some of them in recent
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years have been suggested to exist at
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3.7 billion years ago but actually my
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two cents is that it's very hard to
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prove unequivocally or indeed to
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um to even make a case that is more
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likely than not that the earliest
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stromatolites are biological on Origin
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there are a few different ways that this
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kind of structure could have built up
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that doesn't necessarily rely on
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bacteria to help make it
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what I would highlight though is that
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widespread stromatolites appear at about
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3.4 billion years ago and they're really
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really common by about 3 billion years
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ago and this suggests that we can make a
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case that's quite robust for widespread
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bacterial colonization of Earth from
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around 3000 million years ago
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now I've talked about these structures
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and obviously it would be nice for you
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to be able to see some so I've put some
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3D models of these structures on the
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site
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um just below this video please do take
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a look at those to become a bit more
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familiar with what these things look
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like and bear in mind they're really
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cool these are the earliest macroscopic
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evidence for life on Earth the first
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thing you can pick up in your hand and
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be like you know what that is the result
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of Life on this planet so I think it's
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really really cool
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stromatolite is a relatively rare today
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they're found in a place in Australia
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called Shark Bay and that's where this
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photo comes from these are some Modern
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stromatolites and today these are
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structures that only develop in
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environments that nothing else can
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survive
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such as high salinity Bays or lagoons
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for example
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that's because today things love to eat
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the kind of bacteria that could have
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created a stromatolite whereas on early
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Earth there weren't other organisms such
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as invertebrates grazing
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um early prokaryotes thus preventing the
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growth of stromatolites through grazing
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so that's why they were very widespread
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in the past but are quite rare today but
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we can study these examples that are
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around today to better understand them
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so another thing to consider when we're
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thinking about early Earth is whether
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there has always been free oxygen or O2
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in the Earth's atmosphere today we kind
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of take it for granted you and I are
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breathing it right now I sincerely hope
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we're breathing it right now otherwise
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that could be quite problematic
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um and it seems that it will be um the
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kind of thing that has been there
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forever but actually I can tell you that
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in answer to this question well there
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has always been free oxygen the answer
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is no
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free oxygen
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um lagged the appearance of Life by some
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amount of time the the earliest
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evolution of Life occurred in anoxic
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conditions
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so that forces us to then think about
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how oxygen could have started building
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up in that up on uh in Earth in Earth
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history
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so oxygen started building up at some
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point in the Arcane in Earth history and
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the buildup of oxygen in the atmosphere
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results in a thing called the goe or the
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great oxygenation event sometimes called
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great org station event or variations
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thereof
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g o e requires the evolution of
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photosynthesis of the type that's shown
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on this slide here or something related
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this is a reaction that takes carbon
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dioxide and water and uses energy from
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the Sun to convert these two into sugar
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complex sugar and free oxygen and that's
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the origin of free oxygen on Earth so in
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order for oxygen to start building up on
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Earth this kind of reaction must have
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evolved within some form of life
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is plausible that photosynthesis may
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have paralleled the development of the
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stromatolites that we just met in the
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early Earth
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but it may well be the early
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photosynthesizers use sunlight in a
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slightly different reaction to the one
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that I just showed you
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what I can tell you is that the goe
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began really around 2.5 billion years
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ago and it's marked in up the rock
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record by the appearance and the
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increase in red oxidized rocks though
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such as those that are shown on this
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slide here from Northwestern Australia
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these are an example of a thing called a
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banded iron formation that
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appears as soon as there is oxygen to
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start reaction reacting with your iron
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in the ocean so these are red oxidized
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rocks related to free oxygen and at the
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same time these things appear there are
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this is a global phenomenon there are
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large numbers of Benidorm formations
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that occur around 2.5 billion years ago
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because the iron in the ocean is is
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being outside and as a result we see The
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Disappearance of minerals like iron
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sulfide pyrite or fool's gold which are
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really easily oxidized
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so that marks this kind of really
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distinct start of free oxygen appearing
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that we can see in the Rock record
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we think cyanobacteria are the first
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organisms to have evolve photosynthesis
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that releases O2 and certainly we can
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say that things like plants come much
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later in geological history and spoilers
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they actually make use of cyanobacteria
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for this more on that later
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thus cyanobacteria or the ancestors
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thereof probably cause the goe
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um the exact timing of this is and the
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appearance of free oxygen is still
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debated but I would say that current
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evidence favors a pre-goe origin of
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photosynthesis as represented by these
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arrows on this diagram showing time on
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the x-axis against the amount of oxygen
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in the atmosphere on the y-axis you can
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see that from early in Earth history
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oxygen
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um
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concentrations in the Earth's atmosphere
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were very very low but we start getting
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evidence after 3 billion years ago in
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individual kind of rocks I'm scattered
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around the globe that there was a local
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production of oxygen starting to occur
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we think though that um there we've got
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this pre-growe origin and then lag
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before free accumulation sorry before
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the accumulation of free oxygen in the
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atmosphere and that lag after this point
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here would have occurred due to
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buffering reactions that used up early
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for oxygen such as the reduction of
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hydrogen carbon sulfur and iron that we
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see in the world record
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you can say though is that then there is
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this kind of burst shortly after 2.5
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billion years ago in which the um levels
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of oxygen in the atmosphere really
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increased quite significantly and quite
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quickly before a very long period of
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relatively little change in oxygen
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atmosphere sorry of the concentration of
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atmospheric oxygen
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um before another bump about half a
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billion years ago
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that jump in oxygen that I've just
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talked about marks the beginning of the
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proterozoic Eon and that's the Eon that
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spans from the appearance of oxygen at
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about 2.5 2.4 billion years ago to the
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appearance of animals at somewhere
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around 541 million years ago so that's
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the protozoic Eon
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and
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Russell deposits a single cell life are
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smattered throughout this time period
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the proteins I've got one example
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um on this slide
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of Apostles from this time period That's
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the 1 870 million year old gun Flint
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formation so these are fossils from a
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rock
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um what they were collected
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um
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in Lake Superior in Canada and I've
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included these in this video because
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they're a really good notable example of
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a protozoic ecosystem that has been
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reserved in the fossil record in fact we
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often use them as a benchmark for free
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cambian cellular preservation so these
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are fossils of individual cells of
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things that were alive around 1 800
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million years ago Isn't that cool I
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think that's really really cool
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in this deposit there are a high number
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of individual micro fossils we see this
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big diversity of a form that's reflected
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in all of the images that you can see
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here which I've provided a source for at
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the bottom of paper from the pnas
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um and these range from filamentous
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bacteria such as this one that's shown
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here so it's a long and like a filament
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two spherical ones such as the ones
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shown at the bottom here that are quite
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rounded in shape who star-shaped
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bacteria such as those shown at the
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bottom right here onto a range of
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structures that are really quite unlike
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anything that's live today and they are
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typically described in this euphemistic
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sense of being as of unknown Affinity
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this thing on the top right is called
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eosphera and we just don't know what it
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is but it does look like a slightly more
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kind of um a complicated cell structure
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than many of the other fossils that we
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get from this time period so that the
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gun Flint Chad shows um is an example of
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the the rich fossil record that we do
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get appearing at times
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um during this protozoic eon
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um so it's just that life is getting
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gradually more complicated through this
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long period of time in Earth history and
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in our next video we're going to look at
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um how um
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multicellular organisms might have
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evolved and some of the challenges that
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organisms face once they become
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multicellular as well so I'll see you
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there very shortly
