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”.