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  • 420 million years ago, a giant rose along the river banks of what would become North

  • America.

  • It feasted on the dead, growing slowly into the largest living thing on land.

  • This towering colossus wasn’t animal, plant, or mineral.

  • Instead, it belonged to an unlikely group of pioneers that ultimately made life on land

  • possible -- the fungi.

  • If youre anywhere on land right now -- like, not on a fishing boat or a floating oil rig

  • -- then all of the life that you see around you is there, because of fungi...

  • ... fungi like the small, unassuming little decomposer known as Tortotubus protuberans.

  • It’s one of the earliest known fungi in the fossil record, dating back 440 million

  • years, to the early Silurian Period.

  • Tortotubus grew near coastlines and rivers on the supercontinents of Gondwana and Laurentia

  • -- in regions that would eventually become New York, Saudi Arabia, Sweden, and Scotland.

  • But back then, life in these places was anything but lush.

  • On land, signs of life were sparse -- there were bacteria, algae, early plants like liverworts,

  • and possibly some of the first terrestrial arthropods.

  • But if it weren’t for fungi, things probably wouldve stayed that way.

  • Because, even though they weren’t very big, early fungi like Tortotubus had a superpower

  • -- the ability to break down almost anything, using digestive enzymes.

  • Fungi eat by releasing enzymes that break down organic matter.

  • This allows them to take the nutrients they need from the material, while helping the

  • rest to decompose.

  • But! These enzymes can be so powerful that they can eat into solid rock.

  • Given enough time, and some help from erosion, many fungi can weaken rocky earth and eventually

  • form fertile soil that plants can sink their roots into.

  • So big, complex life on land -- like vascular plants -- probably wouldn’t have been possible

  • if Tortotubus and its fungal predecessors hadn’t prepared the land first.

  • Once plant life started to spread and diversify on land in the Devonian Period, fungi were

  • there to help again.

  • Some scientists think that the first land plants used symbiotic fungi in their roots

  • to help them gather and transport nutrients.

  • In fact, most plants still do this today.

  • If you pull a plant out of the soil, youll probably see a lot of dirt sticking to little

  • white hair-things on the roots.

  • Those hair-things are actually bundles of nutrient-absorbing tendrils -- called hyphae

  • -- from fungi living symbiotically with the plant.

  • And these fungal structures are incredibly important for making arable soil that plants

  • can grow in.

  • They help stabilize soil, retain moisture, and hang on to nitrogen.

  • So without fungi, soil would just be ... non-nutritious dirt.

  • But ya know what’s weird?

  • Despite their starring role in making life on land possible, ancient fungi haven’t

  • been studied very much by scientists.

  • That’s probably why, back in the 1800s, when paleontologists discovered a fossil that

  • was eight times taller than the tallest plants of its time, no one expected it to be a fungus.

  • Geologists first discovered this enormous fossil in 1843, during a coal survey in Gaspé

  • Bay, Canada.

  • It was more than 8 meters tall, shaped kind of like a tree trunk.

  • And it came from a layer of earth that dated back 420 million years, when the Silurian

  • Period gave way to the Devonian -- a good 20 million years after Tortotubus first appears

  • in the fossil record.

  • And for a long time, researchers were totally stumped by this specimen.

  • Stumped? Is that supposed to be some kind of tree trunk joke?

  • The fossil remained unstudied in a museum collection for years before a Canadian paleontologist

  • named John William Dawson had found more, new specimens in the 1850s, and tried to classify

  • them.

  • Dawson thought this thing might be a primitive conifer tree with some sort of fungus growing

  • on it.

  • So he named it Prototaxites, which meansfirst yew,” after the tree he thought it most

  • resembled.

  • Then, in 1872, botanist William Carruthers wrote a paper saying that Prototaxites couldn’t

  • possibly be a plant.

  • He said it was probably a giant mound of algae, possibly kelp.

  • And, to his credit, English botanist Arthur Harry Church also studied Prototaxites in

  • 1919, and he said it probably was a fungus.

  • But nobody really paid much attention to him at the time which is...too bad.

  • So, based on Carruthersfindings, Prototaxites was filed underprobably weird algae

  • for more than a century.

  • Until the 1990s, when Francis Hueber, a curator at the Smithsonian Museum of Natural History,

  • took yet another look.

  • And he focused on a feature of Prototaxites that had long puzzled scientists: these strange

  • rings.

  • They looked kind of like growth rings, like what you find in modern trees.

  • But the rings in Prototaxites were lopsided and not always concentric.

  • So Hueber examined them under the microscope and found long, tube-shaped cells that were

  • similar to the hyphae found in modern fungi.

  • He concluded that Prototaxites wasn’t a plant at all, but rather a member of Team

  • Fungus.

  • And this was the same conclusion of another study done in 2007, that looked at the chemical

  • makeup of the fossils.

  • Now, how can chemistry tell you whether something’s a fungus?

  • The answer -- as always! -- is carbon.

  • We talk about carbon a lot around here, because carbon is the stuff of life.

  • And different kinds of living things can be identified by how much carbon, and of what

  • kinds, a fossil contains.

  • The thing that researchers focus on here is the ratio of various isotopes of carbon, like

  • carbon-12 and carbon-13.

  • Plants have pretty consistent ratios of Carbon-12 to -13, because they all get their carbon

  • from the same place -- the CO2 in the air.

  • But things that eat instead of photosynthesize -- like animals and fungi -- pick up carbon

  • isotopes from their food, so their ratios can vary a lot.

  • And the 2007 study compared the isotopes in several fossils of Prototaxites from different

  • eras and found that those ratios changed radically over time, probably as its food sources changed.

  • This means Prototaxites was not a photosynthesizer, but an eater -- it got its food from other

  • living things.

  • Now, there’s still a chance that this enormous not-plant-thing was something else, like a

  • lichen.

  • Despite looking like weird plants, lichens are actually fungal hybrids that house their

  • own algae or photosynthetic bacteria.

  • So far, no one’s come up with a good way to prove whether Prototaxites was a pure fungus

  • or a lichen.

  • But, either way, it was a giant in its day.

  • And it marks the peak of fungi’s reign over life on land.

  • When they first appeared in the Early Devonian, the fungal spires of Prototaxites towered

  • over everything else that grew, burrowed, and crawled around them.

  • Like the humble Tortotubus, Prototaxites fed on dead stuff.

  • But unlike its tiny predecessor, the giant fungus sent out huge networks of hyphae in

  • all directions, sending food back to its central pillar.

  • And in turn, it may have been a source food, and shelter, for early invertebrates.

  • Scientists have even found littlebore-holesand tunnels that look like insect burrows

  • in some of its fossils.

  • All told, the heyday of the giant fungus spanned 70 million years -- a short time that saw

  • a lot of change.

  • When Prototaxites first arrived on the scene, 420 million years ago, vascular plants had

  • just begun to colonize the land.

  • But by the time they vanished 350 million years ago, the first trees started to tower

  • over the fungi that had paved the way for them.

  • No one’s sure why the giant fungus went extinct.

  • I for one am bummed that I’ll never get to see one.

  • Some speculate that it grew too slowly to recover from being chewed on by invertebrates

  • all the time

  • Others suggest that the rise of land plants brought too much competition for nutrients.

  • Either way, ever since the demise of Prototaxites, for hundreds of millions of years, fungi have

  • continued to thrive, but with a much lower profile.

  • Today theyre mostly found in the dark, close to the surface, and even underground.

  • But the world we live in was made possible by these fungal pioneers -- with their ability

  • to digest rock to create soil, and to derive life from death.

  • Thanks for putting the fun in fungi with me today!

  • Now, what do you want to know about the story of life on Earth?

  • Let us know in the comments.

  • And don’t forget to go to youtube.com/eons and subscribe!

  • But don’t stop exploring now!

  • Do yourself a favor and check out some of our sister channels from PBS Digital Studios.

420 million years ago, a giant rose along the river banks of what would become North

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当巨真菌统治时(When Giant Fungi Ruled)

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    joey joey 發佈於 2021 年 05 月 03 日
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