Importance of Natural Resources

What Did the First Animal Look Like?

[ intro ] One of the central concepts in evolutionary
theory is common descent, meaning that all living organisms can be traced
back to a single common ancestor. In other words, all life on Earth is related. That means, in theory, you could trace your way from child to parent
to connect even the most distant of evolutionary cousins. And if you did, you would, at some point, come face to face with the
very first animal. But what that animal would look like is…
a good question. There are clues all around us. They’re hidden in layers of rock and in
our DNA. And scientists have been piecing together
these clues for centuries in the hopes of understanding exactly how
animals came to be. Sponges—those marine animals perhaps most
famous for their use as cleaning implements— don’t have a lot of traits that other animals
do so it seemed pretty logical that the animal
that came before all of us might have looked a lot like them. But as more and more evidence comes to light, the idea that sponges are our best link to
our ancestors has become somewhat shaky. And that’s led to a heated debate among
scientists. Some stand by the sponges. Others think mysteriously beautiful animals
called comb jellies give us a better picture of our past. Today, we’re going to unpack this fierce
academic debate. And all though we can’t say which side is
right, understanding the arguments for both does
help us envision what the first animal might have looked like. Starting way back in the 1800s, pioneers of evolutionary theory like Charles
Darwin and Alfred Russel Wallace began drawing evolutionary trees to map out
how animals related to one another. And building evolutionary trees is still an
important tool for understanding phylogeny – the evolutionary history and relatedness
of different kinds of organisms. Basically, each branch on these trees represents
evolutionary innovation— a new trait or set of traits that separates
the organisms on that branch from their closest relatives— while each stem represents a common ancestor. Figuring out this branching can be tricky. But, by paying close attention to the things
that differentiate species from one another, scientists can reconstruct how they think
organisms might have separated. And when there are a couple different branching
patterns that might fit what they observe, they tend to follow the principle of parsimony: basically, that the simplest explanation is
probably the best. One neat thing with such evolutionary or phylogenetic
trees is that you can use the information on different
branches to basically back-calculate what traits a common ancestor likely had. And since the first animal is the common ancestor
at the very base of the animal tree if we can construct that tree accurately,
we can infer what that animal looked like Like Darwin and Wallace before them scientists today look at physical traits or
morphology when mapping out phylogenetic trees but they also consider physiology, behavior,
and genetics, and use computer modeling techniques to fill in the gaps. And while this has the potential to make the
picture much more complete it can also make things a lot more complicated. You see, the first animal family tree was
built with that parsimony principle in mind and assumed that the simplest animals were
the oldest. We consider ourselves to be pretty complex,
and the fossil record confirmed that bony skeletons are a pretty new development, so it made sense that the phylum humans, hummingbirds
and angler fish belong to—Chordata— should be the furthest branch from the tree’s
roots. Other animals that share a lot of traits with
our branch —like having three main tissue layers and
a body that’s symmetrical when split down the middle— are the next branches down as you work your
way towards that first animal. Then, way further down, you reach the point
where jellies and their relatives in the phylum Cnidaria branched off. Cnidarian bodies are considered simpler than
other invertebrates because they consist of only two layers of
tissues with a kind of special goo sandwiched in between. And after them, there are really only two
phyla left: the Ctenophora, delicate gelatinous predators often called comb jellies, and the Porifera, filter-feeders more commonly
known as sponges. Sponges are considered the simplest animals
because they lack the complex structures found in most other phyla, like a digestive system or nerves. They don’t even have true tissues— they’ve just got some gooey stuff sandwiched
between two thin layers of cells, all of which is supported by tiny hardened
bits called spicules. Compared to sponges, comb jellies seem downright
complex. Not only are their bodies arranged in a symmetrical
way, they have muscles and nerves and the ability
to capture their food with the help of sticky tentacles. In fact, they look a lot like the swimming stinging jellies which can ruin an otherwise
delightful day at the beach. So it’s generally been assumed they were
somewhat close to their gelatinous kin. which leaves sponges as the first animal offshoot. If this tree is correct, we can predict the
ancestor of all animals probably looked quite… spongey. Like sponges, they would have had asymmetrical
bodies composed of specialized layers of cells rather than tissues And they would have probably stuck in one
place, lacking the ability to swim around like comb jellies. But . . . when scientists have tried to recreate
this classic evolutionary tree with genomic data, they keep getting a weird result. The genetics often say those comb jellies
are the lowest branch — at least, according to computer models of
how DNA has evolved. Some scientists argue this is an artifact
of genomic sequencing. They think that ctenophores evolved really
quickly— basically, their gene sequences changed more
rapidly over time than those of sponges, and that’s throwing off the evolutionary
models that are used to reconstruct the big family tree. And that might be the reason why they appear
to be closer to the common ancestor of all animals when you infer relatedness by examining
genomes. But other researchers disagree. Strongly. They say the math is sound and the placement
of comb jellies as the earliest offshoot isn’t an artifact. You might think the fossil record would sort
all this out. All you’d have to do is look back to see
which group of animals started appearing first. But while we think of the fossil record as
this neat, detailed log of life on Earth, it’s more like your organic chemistry homework
after the dog’s had a go at it. The pages are wrecked, there’s chunks missing,
and if we’re being totally honest, it wasn’t complete to begin with. That’s because soft, fleshy things don’t
fossilize well, so their fossil record is patchy at best and that’s before geologic processes wear
down or completely erase what might little have existed. In fact, fossil ctenophores are so hard to
find that the first was described in the 1980s. Still, some really great comb jelly fossils
have been found the oldest of which dates back to about 520
million years ago placing it in the early Cambrian—the period
of rapid animal diversification which began 541 million years ago. Because of their hardened spicules, sponges
have a fossilization advantage over ctenophores, so unsurprisingly, we have a lot more ancient
sponges to examine. But, as a March 2018 paper in the journal
Paleoworld points out there still aren’t any clear sponge fossils
from before the Cambrian. And something really interesting happens when
you look closely at the sponge fossils we do have from that early Cambrian period when
we know ctenophores were around. These first sponge fossils share a lot of
features with… ctenophores. As that 2018 review paper put it, their characteristics
seem to put them on a continuum with the earliest known comb jellies— which could actually support the idea that
comb jellies came first! There are slightly older fossils, dating back
to about 600 million years ago, which some claim are sponges. But they also have features which make them
very un-sponge-like, so some have argued they aren’t really sponges
at all. So in the end, the fossil record—at least
so far— doesn’t do a good job of clearing things
up. Which leaves us with looking at how these
animals are now. Comb jellies might look a lot like true jellies
and their relatives, at the cellular level, they’re vastly different. That’s because they have a nervous system
that’s unlike any other animal on Earth. For instance, they have completely different
neurotransmitters than the rest of us animals. Everything from jellies to us uses serotonin,
dopamine, and acetylcholine to communicate between neurons. But comb jellies use small proteins called
neuropeptides as chemical messengers. So, if ctenophores were the first animal lineage
to spit off then it’s not entirely clear what came before
them. Some think that the common ancestor lacked
any neural structure more like a sponge, and that comb jellies
just happened to develop their own nerves independently. But that would imply that nerves evolved twice. And this is a really hard argument for a lot
of scientists to swallow because of that whole principle of parsimony thing. Neurons and neural communication is incredibly
complex. And it does seem kind of far-fetched that
something that intricate would have evolved in ctenophores and then again in the ancestor of everyone
else. But, just because it’s unlikely, doesn’t
mean it’s impossible. When species that are really different from
each other develop similar traits it’s called convergent evolution. And it’s happened over the tree of life. Like, echolocation in bats and dolphins. Or, the wings of birds and insects. Heck! Our eyes are extremely similar in form and
function to the eyes of octopuses and squids, even though our last common ancestor occurred
over 500 million years ago. So if something as complex as an eye can develop
separately in different species, the independent evolution of nerves doesn’t
seem that far-fetched. Or, maybe they didn’t evolve more than once. It could be that the common ancestor to all
animals did have a simple nervous system like comb jellies which was then modified a lot
before the cnidarians split off. Though, that may suggest sponges lost this
trait. And that possibility raises an important point. Though it’s sometimes or even often implied
in evolutionary biology, the complexity of an organism doesn’t tell
you how old or how quote “evolved” it is. Some argue that the assumption of increasing
complexity causes us to overlook the fact that animals gain and lose traits through
natural selection all the time. Parasites are a clear example of this—as
they coevolve with their hosts, they often lose things that their hosts can do for them. For example, this kind of reductive evolution
can be seen in the genomes of Rickettsia— he pathogenic bacteria behind diseases like
typhus that have evolved to live inside other cells. They’ve straight up lost a little over a
quarter of their genome, and a lot of what’s left is scrambled nonsense. And we see it all the time when animals adapt
to new environments just look at pretty much anything that lives
in a cave. I mean, Mexican blind cavefish have completely
lost their eyes! The truth is, there’s no reason that animals
farther up in an evolutionary tree have to be more complex than those that are closer
to that common ancestor. We might try to blame the idea that evolution
equals complexity on things like the fossil record, since we see more complex animals in it than
simpler, gooier creatures. But really, this idea that evolution produces
increasingly complex things probably comes from the fact that we humans like to think
we’re pretty special, and we look for the traits of ours that set
us apart from our animal kin. The thing is, in the process of indulging
in such biases, we risk taking for granted how amazingly complex so-called simple creatures
like sponges and comb jellies actually are. I mean, comb jellies can generate light as
a defense mechanism. Chemical reactions in their tissues allow
them to shimmer magnificently in blues and greens. Can you do that? And sponges aren’t as simple as they might
seem. One sponge species found on the Great Barrier
Reef has over 40,000 genes in its genome— that’s about twice the number you have. Why does it need such a complex genome? We don’t know. And when you think about it, getting past
our own preconceived notions of how the world works is kind of the whole point of this debate. Whether you’re on team sponge or team comb
jelly, it’s the science that counts. At the end of the day, scientists care which
animal is closest to our common ancestor because they want to have a richer and more complete
understanding of our evolutionary past. It doesn’t really matter who is right or
wrong, what matters is that we aspire to get to the truth of how animals actually evolved
– even if the truth looks different than we
thought. Evolutionary biologists will continue to debate
how animals evolved for many years to come. Maybe someone someday will find some incontrovertible
evidence that settles things— or, maybe they won’t, because there just
isn’t any to be found. As long as they keep an open mind and continue
asking good questions, all of us ultimately benefit, because the
debate enhances our knowledge about the world we live in. Questioning the status quo and digging for
the truth about life on this planet is what biologists have always done. And I, for one, hope they keep it up! Thanks for watching this episode of SciShow! If you liked this deep dive into the origins
of animals, you might like our episode looking at the oldest fossils we’ve found and what
they tell us. And if you want to be among the first to watch
our newest episodes — about everything from fossils to physics
— you can go to and subscribe. [ outro ]

Reader Comments

  1. So the ICCP is working on its latest findings based off of CMiP 6 (the data set and modeling parameters) only they had to release 2 sets of parameters this time. one with more accurate modeling of our local star containing 3+gb of data or one containing aprox 80 mb of data(released 3 months later). which data set do you think they will use ….the old view or new??

  2. Interesting that you used Rickettsia as an example as it, or something very like it, may have been the origin of mitochondria. You know, the powerho……

  3. Here's my two cents, cone jelly's seem have a lot more in common in bacteria than sponges which is a big deal because the first animal didn't just appear, also it seems to me that sponges are a lot more niche which usually happens when a creature diverges off from something right?

  4. I propose we get a team together (probably including Nic Cage and Tom Hanks; they're into that sort of thing, I hear) and do a raid on the Vatican, steal their Chronovisor and find out for ourselves. Who's with me?

  5. what evidence do we have that we all evolved from one common ancestor? do we all have some of the same DNA? what i'm getting at is could unrelated single celled organisms convergently evolve animals? is it possible that there are more then one 1st animal? could animals with bilateral symmetry and animals with radial symmetry have evolved from 2 different "1st" animals? could early single celled organisms evolve independently from amino acids and the building blocks for life? could none living ingredients that created the 1st living cells have convergently combined to create life, thus having multiple starter animals that are not even connected to the same trees?

  6. I lean towards team jelly. It is easier to swallow that sponges lost their nervous system rather than jelly's developing their own individually.

  7. Haha what do u mean I thought god created all animals??!?!?!!!!!


  8. If we're choosing between sponges and comb jelly than it was definitely comb jelly. Everything is pointing to it and it makes more sense. Just because sponges are more simple doesn't mean it's the original. In fact, the very way in which it is simple could have come from something more complex and likely did.

  9. Long story short… it's the comb jellies. The last bit in this video was somewhat confused and mixed, but almost all of it is pointing towards comb jellies. Simple things can come from complex things and complex things can be downright redundant.

  10. I am of the opinion, that the "spongingly gelatinous comb jellysponge" was the first animal… call it a compromise

  11. You said that the reason simple lifeforms like ancient sea sponges "appear to be more genetically complex" is "unknown" but it isn't. Potatoes, trees, blades of grass, all these simple life forms are well understood to have extremely complex genes and the reason is because they have very simple body plans. As long as they can do basic things like photosynthesize and sense the ideal environment to grow in, they can have hundreds of broken copies of genes or other abnormalities and be fine. It doesn't impede them at all to duplicate their entire genome several times over. Humans, and other complex animals however, are comparatively very fragile and even one mistake in our genes can prove fatal. Our respiratory systems and nervous systems are so complex that we can't afford to have something go wrong with them and it results in relatively straight forward genetic blueprints for us, where as a potato will always be a potato no matter how messed up its genes get. It's not going to make bad tactical war decisions or fail its math test, it's a potato and it'll be fine. An antelope with slightly impaired legs won't outrun a cheetah though, and it will not be fine, so these errors never get to build up.

  12. Forget Team Edward and Team Jacob, Team Sponge vs Team Cnidaria is far more fascinating. Change my mind…

    …Though, admittedly, making a Twilight joke in 2019 feels about as outdated as the early Cambrian and probably falls as flat as a layer of stromatolites.

  13. Here’s a possible objection to the common descent theory:

    At some point, three to four billion years ago, life began on earth. There is one important way that this life wasn’t the same as all the other life we know. That is the fact that it wasn’t the result of some other life form’s reproduction.

    We know life comes from life and from that knowledge we posit that all life has a common ancestor — but we also must posit that, at least once, life came from non-life. There was, at least once, a living organism that had NO ancestor but, presumably, was itself the ancestor to all the life that has existed since.

    Even if you believe that life never did begin on earth and came to us extraterrestrially, you must concede that there was an occasion, sometime and somewhere in the universe, when life proceeded from non-life.

    It happened, and it happened because it’s possible to happen, and if it’s possible to happen once it’s possible that it could have happened twice, and if it happened twice the progeny of the organism that came to life the first time has NO common ancestor with the progeny of the organism that came to life the second time.

    What objection can you raise to this idea?

  14. I definitely think prioritizing complexity is focusing on the wrong thing, all natural selection means is that individuals with a certain set of traits had more success surviving and passing on their genes, but complexity and ability to pass on genes are not the same, something can be highly effective at surviving and passing on its genes without being very complex and complexity isn't going to be "promoted" unless there's sufficient environmental pressure making the more complex individuals be at an advantage while the less complex individuals are at a disadvantage anyway. Selective pressures are pretty important and a big part of explaining why we have things that serve no purpose anymore, we only lose useless things if the people with those now useless things are at enough of a disadvantage, if there's nothing to select out the useless trait we tend to just keep it, evolution isn't some intelligent force dedicated to making the most efficient being or anything. In a way survival of the fittest gives this incorrect impression, it's really probably closer to survival of the satisfactory, as long as you're good enough to pass on your genes, chances are any other flaws in your genetic make up will be irrelevant.

  15. Great content. And I have to congratulate the host for all his effort for improving the quality of his presentations. It is paying him back. Good job on this one.

  16. They haven't been looking for CENTURIES for what came before people… Most people still think people just magically happened from God or some magical power. Almost everyone thought Darwin was insane andor a heathen

  17. I loved this video you have a new subscriber. Im no evolutionary scientists like you guys and i know im talking about things probably after the "first animal" but going on yr principle of the simplest explanation is usually the best i thing the ultimate ancestor to vertebrates has got to be a worm like creature that Eventually turned into a simple eel like fish and from there other species evolved. I also have a lot of scepticism about the "Cambrian explosion" i personally believe life didnt suddenly become complex in the Cambrian i believe this is when animals became better for fossilization (developing hard parts etc) i believe there were ancestors to the Cambrian animals but they were more simple and therefore did not fossilize as well and this is why it looks to us in the fossil record why there was an "explosion" of life in the Cambrian. This point of view is supported by discoveries of life in the last few decades before the Cambrian i believe a new period has even been named, i know that the animals discovered before the Cambrian are completely unrelated to species alive today or even to Cambrian species (as i understand) but my point is there could of been soft bodied animals living along side these fractual animals but they just didn't fossilize well. I think this is far more likely than life went from single cell level to trilobites and other Cambrian fauna just like that, it makes more sense to me anyway but like ive said already im not an evolutionary scientist like you guys

  18. I think there has been some genetic transference from comb jellies or their ancestors to unrelated organisms. I also think that the different neurotransmitters is itself a more parsimonious argument for different organisms evolving the same kind of “structures”, than supposing the original structures from which nerves evolved—if there were any—were particularly unique to one lineage. I also have to suppose that gene-transference was probably more common in organisms with simpler—more archaic—cell membranes.

  19. Evolution is just the process of throwing s*it at the wall until it sticks.
    Anything is possible.
    Thanks for the brain food.

  20. why would mexican cave fish lose their eyes ? even if they weren't using them anymore in the dark, to lose them, they would have to be somekind of liability to select against as they reproduced over and over again, and how long would this be, how many generations ( species life spans ) would it take to lose them ( ? ) how long have they been in these caves ?

  21. 40,000 genes is like bloated software with linked libraries and pictures (possibly scrambled). The end result is still a sponge.

  22. adult sponges are very simple…Maybe what needs to be looked at is the more mobile juvenile sponges. those can swim and even have something of a brain.
    Maybe that would help determine if sponges are more or less complex by comparison.

  23. Hey Stefan, what if neither sponges, nor ctenophores are the progenitor of all living animals, but they share a close common ancestor that was? Imagine an animal with none of the special characteristics of sponges, nor ctenophores. No nervous system, nor ability to move, nor stinging cells, nor even a single mouth like ctenophores & no spicules that sponges…….. gee, doesn't that sound like Tribrachidium which was a filter feeder on the ocean bottom of the ocean. There is evidence Dickinsonia may have moved a little, but there is no paleontological evidence Tribrachidium could move at all.

  24. I like that when you’re about 6 minutes in they expect you to understand the topic and start talking all exciting like Sir I still don’t know what a ctenophobia is.

  25. Maybe it's best to look at things that aren't animals to answer the question. Because all life has common ancestors right? Maybe at one point, animals and probably fungi where the same thing that breathed oxygen and released carbon dioxide.

  26. Another amazingly well done video! What stands out to me on most of the SciShow videos that I've seen is that the material is not "dumbed down" – accurate and current scientific terms and names are used (even though they are probably over the heads of most lay persons), such as phylum names like Cnidaria, Ctenophora, and all of the subtleties of how these species are different, and how one establishes those differences. Great work!

  27. It would most likely be (and look) like bacteria or bacteria is the first thing to exist for life (and other single cells maybe)

  28. Why no mention of dickinsonia? There is debate over whether it's plant, animal, or fungus but it IS the oldest contender for animal life that we know of (the fact that the plant/fungus/animal debate exists at all honestly makes it seem like a good place to look to me).

  29. 134 people came here for a definitive answer. 134 people don't understand how science actually works.

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