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hello there and welcome back to video
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number five of evolutionary milestones
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everything until this point that we've
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looked at has been uh a member of this
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group the prokaryote so archaea or
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bacteria these are generally
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single-celled and they the cells have a
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structure that i described in video
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number four that kind of um cell with a
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single circular chromosome sitting in
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the cytoplasm uh there's actually a
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third branch however to the tree of life
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those are organisms which have more
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complex cells they're called the
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eukaryotes so without further ado let's
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learn about eukaryotes and
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the
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over the course of this video we'll
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learn about both eukaryotes and some
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elements of their life cycles
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so this group the eukaryotes includes
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fungi plants and animals but also a
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whole host of single-celled
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organisms
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including the
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zoa and for example the algae
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the cells of eukaryotes tend to be
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larger than those of prokaryotes they're
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between 10 and 100 microns in size
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they possess organelles these are
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membrane brown structures within the
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cytoplasm of the cell you can see some
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examples on this slide here in both
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animal and plant cells
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eukaryotic cells generally have
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mitochondria which we'll learn about in
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a bit and things like for example plant
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cells have
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chloroplasts in them which allow them to
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photosynthesize
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the dna in eukaryotes is held within a
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nucleus in linear chromosomes
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now it's generally accepted that the
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archaea and the eukaryotes are more
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closely related to each other than they
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are to bacteria
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okay so that's a statement of
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relationships there
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how they have their origins however is a
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really interesting and different tale
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we call the origin of the eukaryotes
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eukaryogenesis
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so essentially in its uh most
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basic form this is the origin of the
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nucleus and protein synthesis uh
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associated with that structure and the
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origins of this
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system are very much a matter of active
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research there have actually been
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massive strides uh made since 2015 in
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understanding some elements of the
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origin of eukaryotes but i should
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highlight that there is still a great
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deal of uncertainty around
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we do know
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that this process resulted from a
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symbiotic association between an
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archaean nowadays we even know which
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particular subgroup of the archaea
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the host belong to which is a member of
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a thing called the asgard clade and a
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bacterium the origin of the nucleus
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still has big error bars so i'm going to
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skip over that first lecture because i'm
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afraid there just isn't time
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but the far clearer picture exists for
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origins of some of the organelles those
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membrane bound structures which you find
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within eukaryotic cells
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and these structures
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occurred through endosymbiosis
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this is the long-term mutually
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beneficial collaboration between
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prokaryotes
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so you can see an example of a eukaryote
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cell on the left here with a load of
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these different organelles within it and
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the dna held within the nucleus let's
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zoom in on just one of those organelles
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the mitochondrion
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so mitochondria are an example of an
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organelle as you're breathing when
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you're watching this video within all of
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your cells your mitochondria are the
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things that are processing oxygen for
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you and allowing that oxygen to be
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converted into energy to keep you alive
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the origin of mitochondria is actually
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fairly well understood nowadays this
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occurred when an oxygen
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loving bacterium we even know what group
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of bacteria this belief two started a
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mutually beneficial
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relationship with an anaerobic archaean
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host cell
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the cell protected the bacterium
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and the bacterium respired for the cell
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at some point since this initial origin
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event mitochondria lose their cell wall
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they transfer some but not all of their
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genetic material to their host
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so mitochondria still have some of their
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own dna indeed in programs like a csi
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when people are talking about dna and
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artists of crime this is sometimes the
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mitochondrial dna they're talking about
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and they
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provide
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energy efficient aerobic respiration
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that has allowed the eukaryotes to
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colonize new ecological niches so
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through this beneficial relationship
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both partners
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benefit and it's become a permanent
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thing it's found as i say in the vast
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majority of eukaryotes and there's a
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question about whether those eukaryotes
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that lack mitochondria
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lack them primarily i.e they never had
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them or whether they liked them
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secondarily i either actually just lost
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them again
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the origin
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of eukaryotes had probably happened
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somewhere by 1 500 million years ago
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exactly when as with many of these
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things remains a matter of
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debate most of the examples of early
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eukaryotes that sample around this time
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close to their origins
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are taken from the
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1492 million year old roper group of
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northern australia so these are some
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really really old fossils
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in the top left is a fossil called
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tepania this is posited as a eukaryote
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because it's quite large in size it has
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preservable cell walls that have been
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recovered in this case through
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dissolving the host rock with acid and
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complex processes so these are the
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things sticking out of those cell walls
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as well as possible budding structures
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which suggest possibly
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eukaryotes like life cycles
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people have posted that this
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organism was a member of the fungi
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panels c
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and e are examples of things that we
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think are probably prokaryote so i
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probably progress i meant of course
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probably eukaryotes from the rope
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formation but they're problematic we
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just don't know what they are there's no
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particular reason to think that
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everything that was live
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at this time all of the eukaryotes are
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related to two living groups today and
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so these could be examples of groups
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that have gone extinct and in the bottom
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right is grypania this is a fossil
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that's known from china it's about 1500
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million years old and it's amongst the
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first macroscopic so visible with the
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naked eye
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fossils is unclear what it actually was
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what it may have been more closely
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related to but generally we agree that
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this was a eukaryote
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so
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when can we really be sure that we have
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eukaryotes that are similar to living
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groups on earth well this happens by
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about 1.2
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billion years ago this is the earliest
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really clear evidence for eukaryotes
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that are similar to living groups it's a
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fossil called
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bangiomorphopubessense it's a
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filamentous microfossil
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of about 1.2 billion years old
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it's solidified so preserved within
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silicon dioxide
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in carbonates from arctic canada
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and we can say based on its morphology
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and its reproductive biology and its
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life cycle
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that it is probably a form of red algae
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a group that's still around today
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if we're looking at the fossil record um
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there are various fossil deposits after
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this point and those allow us to say
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that at about 800 million years ago
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the fossil record suggests that there
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was an increase in the diversity of
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eukaryotes
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a key feature in many eukaryotic cycles
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is sexual reproduction
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this is especially true of macroscopic
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organisms so these are animals plants
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fungi for example
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now
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you may never have thought about it but
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actually sexual reproduction is kind of
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weird it has a two-fold cost to the
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organisms that use the system
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only half of the individuals can bear
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young
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and it requires the males to find the
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females when they do find each other
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they only pass on half of their genetic
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material
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an example of the real
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um
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uh consequences
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of this two-fold cost is the anglerfish
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i've put an image of one of these
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creatures here they live in the deep
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dark ocean if you ever seen the movie
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finding nemo they feature for a short
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while in that movie
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and these have a really interesting
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life cycle because a male needs to find
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a female
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relatively quickly in the deep dark
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ocean and they're all the boys and the
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girls are quite far
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um spread apart
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when the male does manage to find a
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female the male and the female fuse this
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is the girl and that tiny thing there is
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the boy
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the male becomes dependent on the female
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host for survival it receives nutrients
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by a shared circulatory system and
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provides sperm to the female in return
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so this is a really good example of a
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life cycle that's had to adapt to the
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costs of sexual reproduction within the
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deep ocean environment
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despite these costs sexual reproduction
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is still really common in animals and
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plants and fungi and such
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complex multicellular organisms
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why may this be the case there were a
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number of theories outlining
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the um selective pressure towards sexual
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reproduction and i'm going to introduce
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just one as part of this course
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please do feel free to ask me about some
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of the others in our zoom session if
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you're interested in this
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we still don't know for sure it's
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probably a mix of all of these factors
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but we know
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that sex outcompetes asexuality when
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there is a factor that kills a large
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proportion of a population
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one that is sensitive to genetic
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variation and one that changes rapidly
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between generations
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all of those things are true of
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parasites and of pathogens
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these are nasty things that co-evolve
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with their hosts and they change rapidly
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so i'm sure we're all acutely aware of
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the problems that pathogens cause
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because covert 19 is one such pathogen
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and as i've mentioned previously it has
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rna as an informational molecule so it
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evolves fairly quickly
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sexual reproduction increases variation
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within population
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within populations and it speeds up the
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dispersion of novels novel traits
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possibly ones that for example could
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confer immunity to a disease and so for
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this reason we think that sexual
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reproduction
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may result
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from
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pathogens and
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parasites
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and this is an idea that's called the
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red queen hypothesis the origin of sex
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and this is taken from alice in
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wonderland where she meets the red queen
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who says that my dear we must
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run as fast as we can just to stay in
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place and if you wish to go anywhere you
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must run twice as far as that more
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generally speaking the red queen
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hypothesis stresses inter-organism
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interactions and evolution as opposed to
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environmental forcing
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so i think that's really really
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interesting but i can't talk about it
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any longer otherwise this video will get
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far too long
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it's likely that sexual reproduction
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this system dates back to the origin of
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the eukaryotes as a group
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once more though the earliest
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incontrovertible evidence we have for
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sexual reproduction within eukaryotes
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and the fossil record goes back to
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banjo-morpho pubescence which have has a
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cellular structure suggestive of a
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sexual life cycle
