How Earth’s First, Unkillable Animals Saved the World


640 million years ago, Earth was a totally
different place. Conditions were so harsh that our world was
nearly uninhabitable. Because it was, above all, very, very cold. By some estimates, the temperature at the
equator could get as low as negative 43 degrees Celsius, and at the poles it got down to negative
130 degrees It was so cold that the entire planet was
probably covered in ice. Scientists call this chapter in our history
Snowball Earth. Yet despite the cold temperatures, life survived. In fact, some of the earliest animals we know
about show up during this time. And one group of those creatures in particular
was just super hardcore. Heroic, even! Because they were practically unkillable. They have survived every catastrophe and every
mass extinction event that nature has thrown at them. Which is why they still live among us today. These resilient animals are … the sponges. Yes! Sponges. They may look a bit like plants or funny-shaped
rocks, but they actually form one of the earliest branches of the animal family tree. And today they live all over the world, in
oceans and lakes, from the arctic circle to the equator, in shallow waters and down in
deep thermal vents. They can survive freezing temperatures of
negative 80 degrees Celsius for months; they can be left to sit on a shelf for years and
still survive; and some individual sponges have been known to live for 20,000 years
or more. And it’s a good thing that sponges are so
epic, because as some of the very first animals, they evolved in probably the worst period
possible for complex life. But the fact is, sponges not only survived
Snowball Earth, they may have actually shaped the future of our planet, making the world’s
oceans more liveable and paving the way for all animal life to come. By being the little, filter-feeding, water-cleaning
creatures that they are, sponges may have saved the world. The sponge you probably know best is the big
yellow thing you use in the bath, or maybe that one you watched goof around with a starfish
on TV. But sponges aren’t all soft, squishy, yellow
squares with pants. Actually none of them are in real life. They’re actually quite diverse, but they
do have a few things in common. First, they’re all multi-cellular organisms
that don’t have any organs. Second, they all have features called pores
and canals — open spaces in their bodies where water passes through. Sponges use tiny hair-like filaments to pump
water through those spaces so they can filter out food, like plankton. And third, though they seem soft, many sponges
have a skeleton. The basic unit of this skeleton is called
a spicule, which can be long and sharp or short and chunky, depending on whether the
sponge uses it for structure or defense. And while sponges might not look like animals,
they meet the most basic definition of the animal kingdom: They’re multi-cellular,
their cells are tightly joined together, and they produce collagen in a space outside of
their cells, instead of cellulose like plants do. Still, I’ll grant you that sponges are pretty
weird as animals go – probably because they branched off very early from the rest of the
animal family tree. In fact, they might’ve been the earliest
branch, though some scientists think ctenophores — also known as comb jellies — might’ve
come first. Either way, sponges are definitely old. Molecular clock analyses suggest they evolved
about 780 million years ago. And they’ve managed to survive for all this
time because sponges are basically unkillable. Well, not the sponge itself, but its offspring. Sponges can reproduce both sexually and asexually
– and when they take the asexual route, usually in harsh conditions, they produce
a tiny bundle of embryonic cells called a gemmule. And gemmules can survive just about anything. They can endure temperatures of negative 80
degrees Celsius and levels of radiation five times higher than what would kill a human. Some can dry out for months and still hatch
to form healthy sponges. And they can also survive 20 times Earth’s
gravity! I have no idea why anyone would test that
– maybe there’s a secret NASA plan to send sponges through a small black hole or
something. But like any animal, sponges still need to
eat. They feed mainly on plankton, like photosynthetic
cyanobacteria. And their food would have really struggled
to survive in an ocean covered in ice. So how did they – and other life – survive
Snowball Earth? Well, first, I don’t want to be that person,
but Snowball Earth really should be called Snowball Earths. Because the world actually froze over twice:
once between 716 million and 680 million years ago, and again about 650 million to 635 million
years ago. How the world froze over is still heavily
debated and really complicated, so we’ve dedicated a whole other episode to that. And how much of the world froze has also been
debated. The geological evidence suggests that glaciers
basically covered the earth, with open water being rare. If that’s true, though, then there’s a
problem: Most unicellular life at the time was photosynthetic — like cyanobacteria and
phytoplankton – and they need light to live. Which is why many biologists have proposed
that Snowball Earth was really more like Slushball Earth, with many areas of open ocean. That might account for the biological evidence
that life survived this cold period, but it doesn’t explain a lot of the geological
evidence. So it may be that life was more versatile
than we’ve given it credit for, and could have survived below the ice. Sponges are no stranger to cold water. There are sea sponges in Antarctica that typically
grow very slowly, but can also grow quickly and explode their populations when the environment
is just right. Plus, some recent research suggests that during
Snowball Earth, ice at the equator was maybe just a few meters or even centimeters thick,
which would have allowed light to pass through. And we do know that sponges were around during
Snowball Earth. But the fossil record of the time is really
limited. There are a few things that might be sponge
fossils, like 760 million year old blobs found in southern Africa, and some other round and wishbone-like fossils from South Australia that are 635 million years old. But these could be the fossils of colonial
unicellular organisms, bacterial mats, or something else entirely. So, the earliest solid evidence of sponges
comes from 640-million year old chemical traces found in rocks in Oman. These chemicals are the residues of sterols
— compounds that are produced by all plants, animals, and fungi — and this particular
kind has been linked to the class of sponges known as Demospongiae — a group that makes
up 90% of sponge species today, including both freshwater and saltwater species. So sponges were definitely around in Snowball
Earth, despite the cold. But ice wasn’t the only challenge back then. There was another problem: the oceans had
basically no oxygen. And it’s here where sponges may have totally
changed the game for our planet. The atmosphere contained oxygen since about
2.4 billion years ago. But during snowball earth, there was very
little oxygen in the oceans. And whatever oxygen that was available was
mostly being produced by photosynthetic cyanobacteria near the surface. But sponges lived on the seafloor, where oxygen
was extremely low. Fortunately, sponges are unkillable. Unlike most animals, they can live in really,
really low-oxygen environments. Like, in conditions with around 1 percent
of today’s oxygen. And their gemmules can survive environments
that don’t have any oxygen at all. But some researchers think that sponges actually
helped oxygenate the world’s oceans, by making room for bigger oxygen-producing organisms. In 2014, a team of researchers proposed that
the slow rise in ocean oxygen levels that we find in the geologic record was due directly
to sponges’ feeding habits. Sponges eat small plankton, like that abundant
cyanobacteria. And they also eat the nutrients that cyanobacteria
need to grow. So how did eating the oxygen-making bacteria
create more oxygen? By selectively eating the bacteria, researchers
think, sponges made room for other, bigger photosynthesizers to evolve. In fact, the fossil record shows that, at
the end of the first snowball earth episode about 660 million years ago cyanobacteria
lost their standing as the dominant photosynthesizer in the seas and were gradually replaced by
a new up-and-comer: algae. These new algae helped increase oxygen in
the oceans, because they lived a little deeper in the water column, and they were bigger,
so when they died they sank to the bottom. Before they came along, any oxygen that was
produced was being used up at the oceans’ surface. That’s because of scavenging bacteria, which
ate the dead cyanobacteria and consumed all the oxygen. But the new, bigger algae sank when they died,
starving those scavenging bacteria and allowing oxygen to build up in the water. Now, while all this was going on, big events
were taking place on land. New bursts of volcanic activity were releasing
massive amounts of CO2, causing temperatures to rise and oceans to warm up. And by the time the ice started to melt for
the last time 635 million years ago, the oceans were clearer and more oxygen-rich, thanks
to the sponges, which allowed animal life to take over. By about 580 million years ago, oxygen levels
in the oceans had risen enough for the big, beautiful animals of the Ediacaran Period
to completely take over. Molecular clocks estimate that many of our
earliest animal ancestors diverged during the Snowball Earth period, and diversified
immediately afterwards in the Ediacaran, like the last common ancestor of all vertebrates. And many scientists think none of this would
have happened if it hadn’t been for the filter-feeding of those early sponges. Thanks to this month’s Eontologists: Patrick
Seifert, Jake Hart, Jon Davison Ng, and Steve. If you’d like to join them and our other
patrons in supporting what we do here, then go to patreon.com/eons and make your pledge! Thanks for joining me today in the Konstantin
Haase studio, and if you want to learn more about when our planet first became oxygenated
then check out “That Time Oxygen Almost Killed Everything”.

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