字幕表 動画を再生する 英語字幕をプリント 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.