In 1946, a geologist named Reginald Sprigg
was working in the Ediacara Hills of southern Australia when he discovered fossils of what
looked kind of like jellyfish. Or … something. He wrote up a paper that described his discovery,
but soon he ran into a problem. Not many people took his claims seriously. That’s because Sprigg’s little jelly-looking-things
were found in a stratum of rock that was more than 550 million years old. But at the time, the oldest known fossils
of large, complex organisms were only about 540 million years old, dating from that huge
profusion of life known as the Cambrian Explosion. It would take decades for scientists to fully
recognize the importance of Sprigg’s discovery. And it turned out, his weird little jelly-things
were only the first in a whole slew of exciting and strange discoveries that, together, would
lead experts to realize there was another explosion of life before the Cambrian Explosion. Though less famous than the one in the Cambrian,
this sudden burst of diversity marked the dawn of truly complex life on our planet. It’s come to be known as the Avalon Explosion. Now, evidence of big complex organisms that
pre-dated the Cambrian had been found in isolated fossils, since the 1800s. But Sprigg's strange, soft-bodied fossils
marked the first time that a whole community of these things had been found. And that suggested that multicellular life
was not only present before the Cambrian Explosion, but that it was much more elaborate and diverse
than anyone thought. Still, most experts needed more evidence before
they’d start rethinking their long-held ideas about the history of life. Then, in 1957, another weird fossil was discovered
accidentally, by a kid playing in the Charnwood Forest of England. No one had ever looked for fossils there before,
because the rocks were up to 600 million years old -- way too old for signs of complex life,
or so it was thought at the time. The frond-like fossil was named Charnia. It was several centimeters long and had a
clearly organized structure that marked it as a living thing. And Charnia convinced geologists to start
looking for fossils in rocks they had previously ignored. Then, in the 1960s, researchers made a huge
find at a place called Mistaken Point on the Avalon Peninsula in Canada. There, in rock dated between 560 and 579 million
years old, there were not just a few fossils of big, complex organisms -- there were thousands
of them. OK but, what were all of these Precambrian
life forms, exactly? Well, some of the earliest, and simplest,
macro-organisms from this time are known generally as rangeomorphs. They all generally resemble Charnia, very
thin and shaped like the frond of a plant. But rangeomorphs were definitely not plants
-- they lived too deep in the ocean to be able to photosynthesize. Instead, they held fast to the seafloor and
probably absorbed nutrients in the water. But they weren’t really animals either. Because, for one thing, they grew in a way
that no living animal does. Rangeomorphs grew by branching fractally -- meaning,
they repeated a single basic pattern over and over again as they developed. So, if you look at a rangeomorph closely,
you’d see that each branch is actually just a smaller version of the whole. Nothing alive today grows like this. And other Precambrian organisms were equally
strange, in their own unique and charming ways. For example, the vast majority of animals
today -- including myself for the most part Except my nose is crooked -- exhibit bilateral symmetry. We have two symmetrical sides. But some Precambrian organisms -- like Tribrachidium--
had three symmetrical sections that spiraled out from the center of its body. They had trilateral symmetry! And again, there’s nothing alive today that
grows like this — plant, animal, or otherwise. So, it’s unclear where many of these early
organisms fit in our current understanding of the tree of life. Some scientists think that they may have been
some type of stem-animals, an ancient lineage that we modern animals share a common ancestor
with. Other studies have proposed that they’re
more ancestral to algae, or fungi, or protists like kelp. It’s even been suggested that these organisms
represent an entire, separate kingdom of life that’s now extinct. But not all Precambrian life was so alien-looking. Some organisms, especially those that appeared
later, had traits that made them seem distinctly animal-like. They were likely the first metazoans — animals
with differentiated body plans and specialized cells. Take, for instance, Haootia. Not only was this bizarre creature probably
one of the first cnidarians -- the group that includes jellies and sea anemones -- it also
contains the earliest evidence of muscle-like cells. But unlike me, it didn’t have to bother
with leg day. It doesn't have legs If you have to explain it it's not funny Then there were the creatures that showed
the first evidence of bilateral symmetry, and motion. One of these was Kimberella, a squishy mollusk-like
creature that left tell-tale marks in the sediment. These tracks seem to suggest that it dragged
some proboscis-like appendage behind it as it fed. And there was Spriggina -- named for Reginald
Sprigg. This long, segmented creature had a crescent-shaped
head with evidence of rudimentary sensory organs. And it’s thought by some to be ancestral
to the beloved trilobites. All of these organisms, taken together, revealed
to scientists an entire chapter of the history of life that, until the 20th century, they
didn’t know existed. But it wasn’t until 2008 that we had a name
for it. That’s when a team of paleontologists at
Virginia Tech surveyed all of the research that had gone into these Precambrian fossils,
and determined that they represented a specific, evolutionary event. They named it the Avalon Explosion, after
the Canadian site that had preserved so much from that time. But, there’s still the question of how this
profusion of life happened in the first place. How did life make the jump from microbial
mats and simple sponges to things with muscles and sensory organs? Well, about 635 million years ago, a long
period of intense cold that had enveloped the world, known as the Cryogenian Period,
came to an end. As glaciers retreated, huge amounts of nutrient-rich
water from the melting ice flooded the oceans, causing mass blooms of oxygen-producing cyanobacteria. And as the oxygen content of the oceans slowly
rose, conditions became more hospitable to life. Then, the thinking goes, about 575 million
years ago, life reached a tipping point, and the Avalon Explosion began. Now, geologists have come to understand this
whole episode as its own distinct period of geologic history, too. So in 2004, the official keepers of the Geologic
Time Scale -- a group known as the International Union of Geological Sciences -- added a new
period to the GTS for the first time in over a century. They named it the Ediacaran Period, after
the Australian Hills where Reginald Sprigg found those game-changing Precambrian fossils. The end of this period is marked by the disappearance
of most Ediacaran life from the fossil record, 542 million years ago. It’s not clear what happened to them. But it probably was a combination of things,
like a drop in oxygen levels in the oceans, along with the appearance of Cambrian predators. Ediacarans might also have just stopped showing
up in the fossil record, because conditions became less favorable for their preservation. We still don’t fully understand how Ediacaran
life relates to the organisms we know today. But we’ve come a long way since Sprigg found
his fossils of squishy little things in 1946. Our ideas about natural history, and our definitions
of geologic time, are always changing. Because, you never know when a funny shape
on a rock, where no one thought to look, might completely upend what you thought you knew
about the history of life on earth. Thanks for joining me again! And as always, I want to know what you want
to learn about! So leave me a note in the comments below! And be sure to go to youtube.com/eons and
subscribe. Now, if you’re interested in all the weird
forms that life can take, then you should really check out Deep Look, a channel that
presents a close-up look at the natural world in Ultra HD. It’ll help you look at the world in a whole
new way.
"Fossils found around the world suggest that multi-cellular life was not only present before the Cambrian Explosion, it was much more elaborate and diverse than anyone thought. This is the story of the sudden burst of diversity that marked the dawn of truly complex life on our planet."