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The origin of animals

  1. Jun 12, 2016 #1
    Dear PF Forum,
    Lately I've been watching this channel
    It's a very good science channel

    And I've been wondering about this one thing.
    The origin of animal
    At first, "life" couldn't do photosynthesys (what ever that we call "life")
    And at 3.4 billions years later, there life could do photosynthesys.
    1. Did animals evolve from these cells?
    And after that there were two distinct groups. Bacteria and Archaea.
    2. Did bacteria can do photosynthesys? Some of them can do I think.
    And from archaea there are two distcint groups.
    Animal and Fungi
    So, at first there were some cells that can do photosynthesys on earth about three and a half billions years ago.
    And later, from these cells, some of them lose the ability to do photosynthesys, such as animal and fungi?
    Were plants the ancestor of animal (and fungi) ancestor's?
    Thanks for the enlightment.
  2. jcsd
  3. Jun 12, 2016 #2

    Simon Bridge

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    Just to be clear...
    1. things do not evolve into other things ... they pass on their general characteristics to their offspring via their genome.
    Since celled organisms can exchange genetic material without reproduction ... you can think of this as a vector for mutations.
    Single celled organisms reproduce by splitting in two - so which is the offspring? usually count both.
    2. Some bacteria can photosynthesize but they are not counted as plants.
    3. early single-celled organisms did not do evolution by natural selection as we understand it today... though that process will have started.
    4. archaea are a kingdom on their own - they are not animals and they are not early organisms: they are modern creatures who we expect share many characteristics with early life after it had built some sophistication but before multicellular organisms emerged. At this level we have modern forms: archaea, bacteria, and eukaryota ... the third one includes us.

    The earliest animal known would have been the Urmetazoan (600mya) with some form of choanoflagellates as a likely precurser type...

    The precurser for plants is iirc usualy considered to be when a (possibly biflagellate) protozoan ate a cyanobacteria and didn't digest it.
    This produces a eukaryote with a chloroplast. You get plants when these 1-celled creatures formed colonies.

    notice how this is not a clear "diverging from a common ancestor" pattern for plants and animals? It's just not that simple.
    However - protozoa are historically classified as single-celled organisms with animal like characteristics so, in the way you have been thinking of it, the animals came first.

    Whatever: there should be enough terms there to keep you reading for a while. When it comes to pre-cambrian evolution it gets messy.

    Have a read of:
  4. Jun 12, 2016 #3
    Yes, thanks @Simon Bridge, for your good answer. And today I read many sources beside your two links.
    The evolution tree is pretty complex. And I don't mean the evolution of the TREE, coconut, palm, etc.
    Perhaps what I want to know is this.
    Life began to do photosynthesis 500 millions years ago.
    And now there are plants, animals, etc.
    So, which one is true?
    A. There were animals, and later some of them can do photosynthesis and becomes plants.
    B. There were plants, and later some of them loose the ability to do photosynthesis and becomes animal.
    C. None of the above, the evolution doesn't look like that.

    Bacteria can do photosynthesis.
    D. Early life couldn't do photosynthesis and the evolve in two different branches, and bacteria and plants "learned" to do photosynthesis seperately.
    E. After early life "learned" to do photosynthesis then they evolved in two different ways. Bacteria, Archaea and Eukaryota.

    Yes, that was wrong :smile:
    Of course not. And the mutation is random, right. Random mutation selected by natural selection. The evolution is not directed (Laplace), but it's selected by natur (Darwin), right.
    Vector for mutations, ok.
    WILL have started. So the natural selection didn't start at the beginning of life, 4.4 gya? I'll think about it.

    Terms?? Heck, I didn't even know what "life" is. If you consider stromatolite rocks are living things
    Thanks. I've been reading them now.
  5. Jun 12, 2016 #4

    jim mcnamara

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    Um. I do not know where to start on this.
    First of some terms:
    autotroph https://en.wikipedia.org/wiki/Autotroph Note the word chemosynthesis
    Heterotroph - https://en.wikipedia.org/wiki/Heterotroph Note the concept of 'fixing carbon'

    Life on earth ran solely by chemosynthesis and heterotrophically (anaerobic respiration) using fixed carbon up until the advent of blue green algae - for about about 1.5 billion years. For early life oxygen was toxic. There are many organism like Clostridium tetanii - the tetanus "germ" - that cannot live in the presence of free oxygen like we have in our atmosphere today. They exist only as inanimate spores until they are introduced into a wound in a mammal, then they are are able to grow as heterotrophs.

    After that came the great oxygenation event caused by blue green algae cranking out oxygen:
    [/PLAIN] [Broken]
    This caused a massive change in existing life - subsequently organisms developed aerobic respiration which is more efficient than anaerobic respiration.

    Please read the above links and follow up by reading about aerobic/anaerobic respiration. Then, please come back and re-ask your questions from a little more enlightened perspective.

    PS: your timeline for photosynthesis needs a tuneup, too. The GOE page will help on that score.

    Last edited by a moderator: May 8, 2017
  6. Jun 12, 2016 #5
    the division will be useful for a student like me, many thanks
  7. Jun 13, 2016 #6

    Simon Bridge

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    1st up: what Jim said - he's probably closer to the subject than I am ;)

    Pylogenetic tree.


    That's three ways - but they are best thought of as classifications.

    The more common models for how plants came about is that some eukaryotes that could already form ad-hoc colonies ate cyanobacterium (a common ancestor to modern cyanobacteria), and the bacterium didn't die or destroy the host... in some combinations both benefit. Provided nothing else fatal happened, the ability to live like this would have selective pressure. Actually lots for some later forms since the oxygen produced was killing off the competition.

    Well they are life like coral is life, or like seashells are life.
    The "rock" itself is more like evidence of life.

    I was thinking that a timeline may help:
    ... the article puts green algae at 750mya, now consider that the cambrian explosion started about 600mya ... this is where the modern animal body plans appeared (along with many others that are no longer present).

    Another timeline for comparison.
  8. Jun 13, 2016 #7
    Thanks Jim for your answer.
    4.3 gya is a good start:smile:
    Autotroph can grow (and construct organic compound) from simple element.
    Heterotroph cannot growth/construct organic compound from simple element, Heterotroph constructs (now that's the wrong choice of word, why would heterothrop construct organic compound from another organic compound? Perhaps I should use "grow" to avoid confusion, although they do construct I think)
    So, heterotroph grows by using another organic compound (either from autotroph or from another heterotroph?)

    I think for autotroph to grow and construct organic element they need energy. So they use sunlight or other reduced chemical.
    There are photoautotroph and chemoautotroph.
    Chemoautotroph. I think to store energy, some chemical elements must be reduced? So, if an autotroph organism uses reduced chemical element for their source of energy, why didn't their body oxidized? I think even though they use reduced element (such H2S, H2SO4, CH4?) they still can be oxidized further for heterotroph to grow. Is it something (remotely) like that?

    uses light to grow from organic compound. I don't think this is photosynthesys. It's different, right. But, if this heterotroph "eat" some organic compound, hasn't the organic compound been already reduced? All this photoheterotroph needs to do is oxydizing it? Perhaps lack of oxygen?
    Further reading, I read that
    Doesn't photosynthesys produce ATP? Ok, I let it go for now.
    Chemoheterotroph uses other chemical for their source of energy.
    If there are only two distinction of this heterotroph, I think human is chemoheterotroph. Because, once I passed out during flag ceremony even under sunlight, because I haven't eaten in the morning. :smile:
    Okay.., Now I want to re-ask my question in different way. Btw, the timeline of photosynthesys? I don't know, I just watch the video, it says that life learned photosynteys 500 millions years AFTER life started emerging on earth, not when the earth was 500 millions years old.
    Okay.., my question.
    I think first "life" on earth was autotroph. Chemoautotroph I think,
    then Photoautotroph?
    Then came heterotroph?
    Perhaps chemoheterotroph, then photoheterotroph.
    If it's so. Then animal is plant that learned how to "eat" another plant? Then they loose the ability to photosyntheys?
    Now, before I press "post reply" your post comes up @Simon Bridge
    Thanks Jim.
  9. Jun 13, 2016 #8
    The timeline says 3.4 gya. If the video says that it takes 500 millions years for life to learn photosynthesys, it means that first of on earth emerged at 3.9 gya.
    But some says, that if the condition of earth is fine tuned for life, life should have comes up at least 100 million years after the earth formation that puts around 4.5 gya. Or life wouldn't come up at all
  10. Jun 13, 2016 #9

    Simon Bridge

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    3.4gya is for the first photosynthetic bacteria...
    Abiogenisis is thought to have occurred between 3.8 and 4.1gya ... so the 0.5gya for life to "learn photosynthesys" is about right for a conservative estimate, and considering the uncertainty involved. Scientists have to make statements based on evidence after all. That is what the evidence suggests so far ...

    Who says? Citation please.

    By that argument, then, the Earth is not fine tuned for life.

    But note:
    Conditions do not need to be ideal in order for something to happen.
    Don't forget the time involved for the Earth to stop getting bombarded, cool down, and for liquid water to appear ... the conditions for life to appear do not exist.

    It does look like life appeared almost as soon as conditions looked remotely possible...
    After that, the surface of the earth and life grew and changed together.

    You want to take care though: the "fine tuned" argument is a creationist bugbear. There are plenty of atheist refutations online for this.
  11. Jun 13, 2016 #10
    The "fine tuned", Brian Greene says that. But with respect to the universe not life on earth! THIS universe is fine tuned for life to emerge. I don't know the exact argument, but the gravity constant or the mass of this universe is not so big that the BIG CRUNCH doesn't appear too soon. That the gravity is not weaker either that SMBH cannot construct galaxy, etc...
    But about the "earth is fine tuned for life" I think is not the word. But...!
    I forgot where, I have read (or heard) that the condition for life to emerge if appropriate is easier then the evolution from single celled to multicellular, and from multicellular to become organism with tissue is more difficult. That's why I think that many lifeforms emerged in Cambrian period, 600 mya. Before that only simple life form.
    Perhaps if the earth is 4.6 gy, then life didn't immediately appear in 4.5 gya. Just merely 100 million years. But, I think after the late heavy bombardment, the accumulation of water from comets, perhaps it only takes 100 years for earth to produce living cell.

    Now you say that :smile:. I'm reading your links. Thanks.
  12. Jun 13, 2016 #11
  13. Jun 13, 2016 #12
    And I'd say that our earth is really fine tuned for life!
    Our sun is (of course) 1 solar mass. That it takes 8 billions years for our sun to become red giant.
    This 8 billions years is enough for earth to produce life. Is it barely enough? If the primates didn't evolve to ape in this time range would there be human? If North and South America didn't collide and block the Atlantic current, would Africa experience drought?
    If Africa didn't experience drought would the monkey climbed DOWN the tree and became ape? And if it didn't and had to start all over again, say 15 millions years ago, given 1 billion years, can highly intelligent lifeform appear on earth?
    And don't forget the moon that balance the earth 23.5 degree, so the season is just right. But I think this argument deserves its own thread.
  14. Jun 13, 2016 #13


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    There are three basic classifications of life on Earth: bacteria, archaea and eukaryotes. Bacteria and archaea are both types of single-celled organisms. Eukaryotes have a more complex cellular structure (for example, they contain mitochondria), and although there are single-celled eukaryotes, all forms of multicellular life (including plants and animals) are eukaryotes.

    Photosynthesis evolved in a class of bacteria called cyanobacteria. Eukaryotes evolved when a species of archaea ate and fused with an species of bacteria (an alphaproteobacterium, to be specific). This alphaproteobacterium eventually became the mitochondria inside of the eukaryotic cells though a process called endosymbiosis. Neither the archaea nor the alphaproteobacterium that fused to become the first eukaryote had the ability to perform photosynthesis, so the original eukaryote also lacked the ability to perform photosynthesis.

    From this original ancestor, the many different types of eukaryotes evolved. One branch would become animal and fungi. A separate branch would become plants and algae. Along the branch that would become plants, these cells ate and fused with another type of bacateria (this time a cyanobacterium), which would become the chloroplast though endosymbiosis. This process is how plants and algae got the ability to perform photosynthesis. Animals cannot perform photosynthesis because they evolved along a different branch that never acquired chloroplasts.
  15. Jun 13, 2016 #14

    jim mcnamara

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    I disagree. The earth was never 'fine tuned' for anything. Life started out in an unimaginably awful (to us) environment - An atmosphere that would be completely toxic to everything (almost) on earth now - most especially Eukaryotes. Life, over 3+ billion years, actually changed the atmosphere. Many sedimentary formations are made from deposits, e.g., coal and limestone, from the remains of past life. So life has had a huge impact on the earth. In changing the environment like that, the extant organisms had to change themselves as well in order to accommodate the new environment. Or go extinct.

    FWIW you see estimated oxygen levels over time from zero to as much as 25% (Pennsylvanian) with drops at the end of the Permian to maybe 10%.
    Humans would have a hard time breathing for eons before the atmosphere would even slightly tolerable. With periods of time when breathing would be

    The overall point being that humans and all living things are fine tuned to live right now. Fine tuned by evolution, if you insist on the idea of fine tuned.
    There was never any intent or planned direction to evolution or the changes life made on earth. Abiogenesis and subsequent life here is due to a process usually described by the term 'emergent'.

    There was an older (IMO somewhat telological) concept for this - Gaia Hypothesis.
    Still used to model Earth Systems

    To see what I mean try:
    Examples of emergent behavior us humans can easily see -- Langston's Ant and John Conway's Life -
    simple rules iterated over and over and over -> complex result


    If you understand the above try a search on Cellular Automata
  16. Jun 14, 2016 #15



    So there were no learning process for plants? They just ate cyanobacterium, and they has the ability to perform photosynthesis. And Eukaryotes actually a type of archaea that DID NOT evolve. But they just ate alphaproteobacterium. Wow. These are all very mind boggling.
    THIS IS what I want to know. Thanks.
    I've been wondering...
    Did animals (and fungi) evolved from the plants that loose the ability to perform photosynthesis?
    Did plants develop photosynthesis differently than bacteria considering, invertebrate developes eyes differently than vertebrae.
    So, that's the answer. Thanks again

    So the difference between eukaryote and other single celled is that eukaryote has mitochondria, (the cell energy) right?
  17. Jun 14, 2016 #16
    So we (life) are fine tuned for the earth.
    Zero? Hm...
    Which caused giant dragonfly?
    Yes, I agree, it's us who adapt the earth condition. But there are many planets out there, Keppler has shown lately. Which perhaps ten or twenty years ago, I thought planets are scarce. Still the planets in the "neighborhood", I mean exoplanet, none of them seems to have life, or intelligent life that is trying to communicate with us.
    Perhaps the earth is not fine tuned, but..
    It's in habitable zone.
    Its sun is just the right mass.
    Its mass is sufficient to contain atmosphere
    It has moon to balance the tilt, I don't know if it has any impact on life, because if the earth tilt is somewhat like Uranus, the life on earth would adapt to the conditions.
    Moon is in habitable zone, but has no life.
    Eropa has liquid, but so far no life found.

    Yes, old windows game. I played before. And that's right! I often thinking how simple rule can produce very complex consequences.
  18. Jun 14, 2016 #17

    Simon Bridge

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    You mean Brian Greene?
    ... yes, there exist people, even with quite impressive credentials, who like to emphasise apparent fine tuning ... best to take this with a grain of salt.
    The mainstream view is that life is messy. Since life is plastic to the environment, pretty much anywhere life appears will look ideally suited to the life that is there.
    You don't see the life that was not suited to that environment. In the bigger picture, this Earth has killed off the vast majority of all the types of life that it has ever carried. That's not really "fine tuning" is it?
    In fact, if the World were so fine tuned for life, there would be no evolution - which happens because variation in populations means that nobody precisely fits their environment (in this case "fitting the environment" would include the ability to modify it to suit) and some offspring will be less suited than their parents (or other offspring).

    Biologists are quite familiar with the messiness of Nature - some hobbyists have made a list:

    ... you were mistaken. Historically the people who have bet that what goes on close to us is actually quite common have won.
    If one of them was exactly like modern day Earth right now, and they were pointing their instruments right at the Earth, they would probably not see signs of life here either. Or what they did see may be ambiguous. The closest candidate is 13.8ly away... see how little we can tell about it from here.

    Viewing Earth as an exoplanet:

    Check the definition of "habitable zone";
    This is meaningless - the star just needs to go through a long period where it is fairly stable.
    The habitable exoplanet candidates include a decent range of masses for their Suns.

    So is Mars and Venus.... at the bottom end, the Moon, Ganymede, Europa, Callisto, Rhea, Dione, Enceladus, and Titania all have (extremely thin) atmospheres. Most of these are comprised of some mixture of oxygen, methane, nitrogen, carbon monoxide, or carbon monoxide.
    i.e. https://www.nasa.gov/mission_pages/LADEE/news/lunar-atmosphere.html

    Mars has about the same axial tilt as the earth - no large moon.
    Venus has a very small tilt.
    The tilt affects the seasons.

    Well, the Earth is inhabitable but there are places in it that do not have life either.
    Remember that for billions of years the earth did not have life either.

    ...notice that Europa is also not in the HZ, yet is a candidate habitable world.
    No evidence has been found because it is very difficult to look ... used to be no evidence for life in deep ocean trenches either.
    But no life anywhere else in the Solar system does not mean much - all anyone is saying is that life appears where it can.

    Looks like it used to be habitable... nothing conclusive though.

    It's a diverting game to play: how far can we tweak the earth's parameters and still have some sort of life.
    Fact is, there is not enough information.

    There are some intreguing ideas about how life can show up to thrive where it can though.

    But I think your original question has been answered.
  19. Jun 14, 2016 #18


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    Yes. Plants did not acquire the ability to photosynthesize independently. Instead, they acquired that capability from the cyanobacterium they fused with.

    There are many differences between eukaryotes and prokaryotes (the collective term for bacteria and archaea). In addition to the mitochondria, eukaryotes have a nucleus (a separate compartment to hold their genetic material), an endomembrane system (a network of intracellular membranes to facilitate transport and other processes), and membrane-bound organelles (specialized membrane-bound structures to perform specific functions). There are also a number of differences in how eukaryotes and prokaryotes process RNA and DNA at the molecular level.

    So, the ancestral archaeon had to evolve considerably to become a eukaryote. Some hypothesize that acquisition of mitochondria was the key first step along this process, while others argue that mitochondria came later and the endomembrane system was first to evolve (see my Insight article for a more detailed discussion).

    No. Although plants and animals share a common ancestor, that ancestor was not photosynthetic. Plants and animals are two separate branches of the eukaryote tree, and photosynthesis was acquired only in the branch leading to plants.

    No, photosynthesis did not evolve independently in plants. Because they acquired the capability from cyanobacteria, the basics of photosynthesis remain the same in plants and bacteria. There are some differences in how photosynthesis gets carried out between plants and bacteria, but the processes share a common evolutionary ancestor.

    In general, when one sees similar structures in different organisms, these structures can either be homologous or analogous. Homologous structures are structures that share a common ancestor. So, we say that photosynthesis in plants is homologous to photosynthesis in cyanobacteria. Homologous structures can also have different functions. For example, arms and forelimbs in land mammals evolved from the pectoral fins of fishes. These structures are homologous even though they serve different purposes and look quite different. (We can tell they are homologous, however, for a number of different lines to evidence, from looking at the morphology of the bones that make up the structures, to looking at how these structures develop, to comparing the genetics of the structures).

    Analogous structures are structures that share the same function but evolved independently. Vertebrate and invertebrate eyes are a good example of analogous structures. Although both are involved in sight, the structures look very different, develop very differently, and function very differently at an anatomical and molecular level. Multicellularity is another trait that has evolved independently a number of times throughout the eukaryotic tree (it is estimated to have evolved independently over 40 times). For example, plants evolved multicellularity independently from animals. The molecular structures and signaling pathways that enable multicellularity are very different between plants and animals.
  20. Jun 14, 2016 #19


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    The “fine tuned conditions” argument is unfortunately meaningless. The only requirement was that the conditions be sufficient to support the development of life. If they were not sufficient, it could not be asserted that they were somehow “fine tuned”, that is because it is life that is making the assertion.
    Self referential arguments are not useful.
  21. Jun 14, 2016 #20
    Yes, Simon!
    I just don't know how to take it. I don't know much science myself. But thanks for your advise.
    Yes. I think this is very logical!
    I read that only 1% of the species survive extinction (or less?)
    Ahh, yes.
    But that was before Kepler. And I think Kepler has been discovering one or two exoplanets on daily basis.
    But if we send them prime numbers...?
    Thanks for your effort Simon. And yes, @Ygggdrasil has fulfilled my curiousity. And this discussion has evolved some distance.
  22. Jun 14, 2016 #21
    Thank you very much!
    Thank you very much
  23. Jun 15, 2016 #22

    Simon Bridge

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    0% of species survive extinction ... if they survived, they would not be extinct ;)
    Very few species survived each extinction event ... but you also don't get to see all the forms that may have evolved from them had the event not taken place either.

    Exactly - in other words: planets are common. Our star has planets ... the way to bet is that this situation is common, vis: other stars have planets. Those who bet on planets being common won.
    ... sure: the seti project broadcast on the hydrogen wavelength hoping that distant radio astronomers, naturally watching hydrogen lines in their astronomy would notice the wierd modulation in our Sun's spectrum. OK so how long did we do that for? What are the odds that a distant astronomer would happen to be looking during that time? Why would anyone else be any different?
  24. Jun 15, 2016 #23
    I think I've made a wrong sentence. I mean, for EVERY species that we see today, there are about 1 hundreds that were extinct.
    A curious case, if I'm not mistaken, is the coral reef. They were extinct then emerged then extinct then emerged. I forgot, now is what period for those coral reef emergence. Jacques Cousteau wrote in "The pharaoh of the sea" - The Ocean World of Jacques Cousteau. Of course it was Darwin (?) who first noticed it.
    Yeah, we shouldn't put Earth as a special thing in this universe
    - The centre of the universe??? Of course the Earth is the centre of the universe
    - The only habitable planet? No
    - Sun the only star who has planets? No
    - Anything that can happen to Sol, will highly probable happen to other stars as wel.
    They would if we send them, three days simultaneously, prime numbers before we send the primary message like in Contact.
  25. Jun 15, 2016 #24

    jim mcnamara

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    This is a good read to give you a perspective on the history of the earth's atmosphere

    Note these points in the article:
    Up until 800 million years ago land based Eukaryotes, and life in general could not survive on land due to UV. Ozone from oxygen become abundant enough to protect land dwellers from UV. Unfortunately for a long period after 800 million the entire land surface Earth was in the grips of massive Ice Ages. Look up 'snowball earth' to see more. Also google for the name Cryogenian.

    The top chart in the link shows partial pressure (amount of oxygen) from 3.8gya to present. Note the big spike during Carboniferous times. And also note that photosynthesis had a loooong trek to creating an atmosphere with something comparable to modern oxygen levels - i.e., possibly breathable by us humans.

    The second colorful chart shows a life timeline - which is directly relevant to the base concepts you need to understand how we got here and when we got here.
    Animals included. i.e., this is the answer to your first question.

    Please do us all a favor and stop telling us facts as you think you know them - take the time to look up everything. Way too many questionable statements with no citations. I'm not going to spend the time, energy and space required to do that. It is your turn to get things right. The first time. You can do it.
  26. Jun 15, 2016 #25

    Simon Bridge

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    Coral reefs are not organisms, then are ecosystems. An ecosystem can die out and get repopulated. That is not an extinction. If all the coral polyps died out, that is the end of reef building.

    Sure, and if anyone is looking in the right direction during those three days with equipment sensitive enough... then maybe they'll spot it.
    3 days is not long.

    Look at the SETI project - how long was any part of the sky looked at? When was another look taken? What are the odds that a 3-day broadcast was missed?
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