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  • 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.

  • Orsomething.

  • 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 thisplant, 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 metazoansanimals 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.

In 1946, a geologist named Reginald Sprigg was working in the Ediacara Hills of southern

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The Other Explosion You Should Know About

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    joey joey に公開 2021 年 05 月 01 日
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