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hello and welcome to video six of
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evolutionary milestones in which we're
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going to look at multicellularity which
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of course
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begs the question what is
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multicellularity
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i want you to have a think about this
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for maybe 20 seconds
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and see if you can come in up with an
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internally consistent
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definition of what a multi-cellular
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organism
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is
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so how do you find that was it easy
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do you think you've got it covered uh at
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first
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uh inspection this may seem like an easy
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easy thing to say
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so multicellular organisms are organisms
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that consist of more than one cell
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all good right but if we think about it
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a bit more
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and especially if we look at the
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organisms that are alive
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today it isn't actually so clear-cut
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and the next video um which i'm going to
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cut in straight after this
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shows us an example of why defining
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multicellularity
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is so difficult
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at the research station on heron island
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on the great barrier reef
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scientists are working to understand how
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it was that multicelled organisms began
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to colonize the earth
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to find the answer marine biologist
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professor bernard degnan
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is studying sponges
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the things that connect sponges to the
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rest of the animal kingdom
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we can find at the level of the cell and
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the gene
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when we look at its genes it's clearly
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an animal
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we look for the things that bind all
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animals together
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so what does a human share not only with
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the chimpanzee or for that matter a
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tiger but what it shares with the sponge
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if we can find any common threads we're
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getting really to the heart of the
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amount of multicellularity in the animal
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kingdom
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so that's the key
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[Music]
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a classic experiment gives us some
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insight
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first a sponge is cut into small pieces
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then it's pushed through a sieve at the
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end of a syringe
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this breaks the animal down into its
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individual cells
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this may seem a brutal thing to do to a
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living organism
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but to a sponge this is of no
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consequence
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in response it does something quite
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astonishing
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the cells begin to move
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and then they form clumps
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[Music]
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soon the clumps form bigger clumps
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until three weeks later a miniature
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sponge has formed
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[Music]
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sponges have this amazing capacity to
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regenerate themselves
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and what we can do is actually rebuild
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the sponge from the cell level up
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[Music]
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so i hope you found that really
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interesting example i certainly did
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of why it is so hard for us to define
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multicellular creatures
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um things organisms tend to do
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whatever benefits their survival and
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they don't really care about
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our definitions of whether something is
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the same multi or single cellular
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and this makes defining features such as
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this
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really kind of challenging what we can
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say with some
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certainty however is that for the
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majority of
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multicellular organisms
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this has probably a lot arrived through
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the symbiosis
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of organisms of the same species to form
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a colony so symbiosis being when
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organisms enter into mutually beneficial
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relationships
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this allows different individuals in the
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colony to have
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specialized roles and it's so common
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today that the boundary between colonial
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organisms
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and a multi-cellular entity is a diffuse
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one
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and i think a really interesting example
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of this is the manor war
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this creature is actually a nidarian
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closely related
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to for example jellyfish that's shown on
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the left here
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now this is actually a colonial organism
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made up of
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a large number of smaller individuals
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called zoids
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but these zooids are so specialized
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that they are no longer able to survive
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outside of this colony
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so for example the stinger cells
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and the float cells can't survive
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outside
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of a man or wall colony that's really
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interesting because the closest living
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relatives
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of the man of war are all individual
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organisms that don't form
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one of these colonies so this is a
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really nice example of that
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specialization
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of roles within an individual
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experimental work has demonstrated that
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predation provides
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selective pressure that promotes
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multicellularity
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at least in green algae so this could
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have been one of the selective pressures
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towards multicellularity within many of
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the groups that display this habit
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in our zoom session i can give you some
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mind-blowing examples
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of the complex life cycles that some
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eukaryotes have
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that actually straddle single-celled and
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multicellular lifestyles
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they're really really cool so the first
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convincing evidence we have of an
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organism that's made from multiple
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cells in the fossil record dates back to
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1 700 million years ago
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in the form of a fossil called
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kingshania
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however the first reliable evidence of
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cellular differentiation
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i.e this process of dividing labor
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within an
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organism which kind of is a hallmark of
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complex multicellularity
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is once more 1.2 billion years ago with
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the red algae
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ban geomorphopubescence this has a
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holdfast at the base for example
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which is different to the rest of the
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organism so that's fairly convincing
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as a form of multicellularity but these
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are all relatively small
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creatures if we're talking about
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macroscopic multicellular
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organisms that are widespread in the
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fossil record
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these only really appear where i've
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marked on this timeline here
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at the beginning of the ediacaran period
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so these
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organisms the ediacaran fauna appear
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as 635 million years ago and kind of
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disappear fairly abruptly
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at 542 million years ago
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i know that rob has covered these in the
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first year
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so some of you will be familiar with
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them already but if you're not
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just be rest assured that the um
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identity
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of these organisms i.e their affinities
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what they're related
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to is for the most part a wide open
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question and it's the
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uh subject of some really exciting uh
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and fast moving
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research right now that changes year on
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year
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so these organisms uh the ediacaran
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fauna
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not necessarily animals they come in
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multiple different
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forms you can see an example here on the
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far left
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of a frondaze form called the chania
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mesoni
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found in the uk in leicestershire in
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fact
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and these fronds were previously
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interpreted as
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sea pens those on the cnidarians so
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related to jellyfish
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due to the fact that they had a gross
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similarity of form you can see an
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example
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of a c pen in the middle here however
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ediacaran fondaze organisms have what we
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call
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fractal growth they exhibit a repeating
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pattern
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that displays at every scale in this
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case branching
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and alternate arrangements of tubules
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and fronds
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which are quite unlike those
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structures that we see in c pens so
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nowadays
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this superficial similarity
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is not widely held to be particularly
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accurate
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as a sign of a close relationship
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between those two organisms
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so in this case we're not 100 sure
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what these organisms are
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there are a number of radial forms
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uh which haven't been interpreted as
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jellyfish so nidarians again
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but based on my reading at least i'm not
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100 convinced by these
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arguments some of these round structures
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that you can see
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examples of in this slide could be the
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front bases
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i the um the hold fasts for those funds
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that i just showed you
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or they could be a microbial structure
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some may be sedimentary structures
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tri radial tetraradial and pentaradial
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discs you can see some of those on the
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right here
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whilst biogenic are very difficult to
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place we've just got no idea really
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what these creatures may be
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now most animals the creatures that um
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i was going to say rule most stressful
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ecosystems but that's incredibly
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arrogant of me and not true at all
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um animals are heterotrophs and thus
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tend to eat plants so plants
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rule uh most terrestrial ecosystems at
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least but nevertheless
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um animals which are very common today
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are generally bilaterally symmetrical
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if you draw a line down the middle of
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you or me say
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both sides will match and there are
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another
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there are a number of what appear to be
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bilaterally symmetrical ediacaran
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fossils are these animals we're not
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really sure
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some have symmetry which is similar to
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those fundays organisms so whilst they
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look superficially
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bilaterally symmetrical if you look in
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detail
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they're not really and they have no
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visible mouth
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uh or through gut which is what we see
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in
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all animals that are bilaterally
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symmetrical so it's unlikely
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those organisms that lack these features
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are um true bilaterally bilaterally
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symmetrical animals
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and indeed the evolutionary
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relationships for all of these creatures
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remain argued the truth is probably not
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that simple
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there is now a consensus that some
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ediacaran
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organisms represent animals based on
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developmental
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and chemical as well as morphological
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evidence
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so some of the creatures that i've shown
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you were fairly convinced
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belong somewhere either within
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the animals or are more closely related
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to animals than they are to any other
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creatures
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and there's lots of exciting work in
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recent years that has
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looked at the reproduction of these
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creatures their developmental biology
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their relationships and a lot more i'm
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acutely aware
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when doing this video that i skipped a
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lot of really active and exciting
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research
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in a very vibrant research film fitter
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so field and this is largely by
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necessity i just don't have time
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to cover everything that i would like to
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within this video
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so to make up for this i provided you
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with a bonus video
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at the bottom of this page which is a
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talk by my
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colleague dr frankie dunn which provides
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some up-to-date insights into the nature
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of these ediacaran creatures i would
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strongly encourage you if you have the
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the time and the energy to watch that
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video because it's a fantastic
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talk
