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Eons: How Plants Became Carnivores

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      Sometime in the Eocene epoch, more than 35 million years ago, in a warm coastal forest near the Baltic Sea, the resin of a conifer tree dripped onto the narrow pointed leaves of a plant below. Over time that resin hardened into amber, trapping bits of the plant inside. These tiny leaf fragments, just a half a centimeter in length, belong to the same plant family as the modern genus Roridula. Today, those plants are found only in a section of southwestern South Africa called the Cape Floristic Region. But their family was clearly much more widespread during the Eocene epoch. And they're carnivorous. These plants actually trap prey. That makes these tiny bits of leaves encased in amber the best fossil evidence we have of carnivorous plants, but that doesn't mean they were the first carnivorous plants, because carnivory has actually evolved independently at least nine times in plants, and in plants that aren't all that closely related to each other. So it looks like something keeps driving plants to this seemingly extreme lifestyle. But what? How and why does botanical carnivory keep evolving? Well, it turns out that when any of the basic things that most plants need-- sunlight, water, and nutrients--aren't there, some plants can adapt in unexpected ways to make sure they thrive.

      (Describer) Title: Eons.

      If you find carnivorous plants strange and fascinating, you're not alone. Charles Darwin published an entire book about them in 1875, after spending a decade or so trying to figure out exactly how they worked, but it would take another 100 plus years before scientists would propose the definition of what counts as a carnivorous plant that's often used today. There are essentially two things that a plant has to do to be considered carnivorous. First, it has to have the ability to take in nutrients from dead prey on its surfaces or trapped inside it. That prey is usually insects, though sometimes it includes small vertebrates, like the northern pitcher plants that have been observed consuming salamanders. And by definition, doing this has to give the plant an advantage in growing or reproducing. It's not enough for the plant just to have defenses that can kill an animal that's trying to snack on it. It also has to get those animals' nutrients. Second, the plant needs to have at least one adaptation that actively lures in, catches, or digests its prey. Doing at least one of these things and absorbing the nutrients for your benefit makes you a carnivorous plant. But because this is nature, there are always exceptions to these rules, like the living relatives of that fossil plant preserved in amber do trap arthropods, but their sticky secretions can't digest them. Instead, the trapped prey attracts insects in the genus Pameridea, which don't get stuck to the plant. The insects then eat the trapped arthropods and poop on the plant, which in turn absorbs the nitrogen from their poop. So these plants get a mutualist to do the work of digestion for them, but they still benefit from the death of their prey. So some botanists count them among the carnivorous species. And there are actually a lot of plants alive today that meet the criteria for carnivory-- from about 580 to more than 800 species, depending on what definition of carnivory you're using. Carnivorous plants are found on every continent except for Antarctica, and they appear to have evolved between 95 million and 1.9 million years ago, based on molecular clock methods. Now this might seem like a really wide range of dates, but remember, there wasn't just one origin of carnivorous plants. For example, a key genetic change in the evolution of carnivory took place in a common ancestor of Venus flytraps and sundews that lived about 60 million years ago. Meanwhile, the pitcher plants of North and South America seem to have originated around 48 million years ago, and the youngest botanical carnivores appeared to be two species of bromeliad native to parts of northern South America that evolved around 1.9 million years ago. That means botanical carnivory is an example of convergent evolution, when organisms that aren't closely related develop similar adaptations independently in response to similar environmental pressures. Now over millions of years and across hundreds of species, plants have developed five different types of traps, most of them many separate times, and traps can be passive-- if prey just falls into them and can't escape-- or active if the plant actually moves to catch its prey. pitfall traps are the standard passive traps used by things like pitcher plants and the bromeliads. Prey lands on the plant slippery surface and slides down into a pool of digestive juices. Then there are flypaper traps, which are just what they sound like: prey become stuck in a sticky substance that is produced by the plant's leaves. These traps can be passive or active. For example, sundews have moving sticky tentacles that react to contact with prey. There are also snap traps, which are active, using rapid modified leaf movements like those of a Venus flytrap to snag prey, and bladder-suction traps are found exclusively in plants called bladder warts. They create little negative pressure vacuums inside their traps, which when triggered by prey, pop open and suck the victim inside before snapping closed. Finally, there are eel-traps or lobster-pot traps, passive traps that force prey to move towards the plant's digestive organ by having little inward pointing hairs that keep prey from moving backward out of the trap. And what's even cooler is that all of these unrelated plants have not only developed the same kinds of traps, but it looks like they've also developed the same molecular mechanisms for digesting their prey. For example, the lineages of three different kinds of pitcher plants split more than 100 million years ago, probably well before any of them became carnivorous. And they each produced their own proteins that were originally used to defend the plant from attackers like fungi. But over time, all of those proteins became repurposed into digestive enzymes. Basically, their function remained essentially the same, but changes came about as to where and how they were being used. Fungi support their cell walls with a starchy polymer called chitin. And chitin is also the basis for arthropod exoskeletons. So proteins that were first used to fight fungal parasites eventually became chitinase, the enzyme in the digestive fluid of the pitcher plants that breaks down those crunchy exoskeletons. All three of these lineages have also evolved to use purple acid phosphatase, another enzyme to absorb phosphate from their victims. Okay, so how botanical carnivory keeps popping up seems pretty well understood. But there's still the question of why. Well, it goes back to that idea of convergent evolution. All these different carnivorous plants are responding to similar environmental pressures. Across the globe, they're generally found in open sunny places that have moist but nutrient poor acidic soils. Many of them live in bogs or fens, but a plant has to get nitrogen and phosphorus somehow. And in these kinds of habitats, botanical carnivory represents an evolutionary trade off, one that comes with both costs and benefits. See, a carnivorous plant has two types of leaves, regular ones that photosynthesize and ones that have been modified into their particular kind of trap. This means they have fewer photosynthesizing leaves than a regular non-carnivorous plant. So they have to live in places with lots of sunlight to try to maximize their ability to photosynthesize, and they have to make up the difference. Carnivory can only evolve in situations where it benefits the plant more than investing in regular leaves, like in places where the soil is lacking nitrogen and phosphorus. And carnivorous plants will even stop being carnivorous, at least temporarily, if they're placed in nutrient rich soil or if they don't get enough water or light. As for what plant was the first to evolve this strange adaptation? Well, we don't really know. Carnivorous plants are pretty rare, and they're only found in certain kinds of habitats, so they're just less likely to fossilize than other types of plants that are more widespread. And the oldest reported fossils of carnivorous plants, one from the Early Cretaceous period of China and the other from the late Cretaceous of the Czech Republic, are controversial. Beyond that, the only other halfway decent evidence we have of ancient carnivorous plants are pollen grains from the Paleocene epoch of India, and one fossil seed from the Eocene epoch of Australia that was destroyed in a freak lab accident after being photographed. But because neither of these are fossils of the actual leaves themselves, we can't be 100% certain whether the plants they came from actually were carnivorous, or whether they were just related to modern plants that are carnivorous today. So the jury's still out on these early carnivorous plants, leaving us with just the leaf fragments of that Roridula relative preserved in amber as the oldest undisputed evidence. It's kind of ironic if you think about it. A plant that used sticky goo to trap its prey got stuck forever in the sticky secretions of another plant. Ultimately, the how and why of carnivorous plants teaches us one of the fundamental facts of evolution, that distantly related organisms can wind up finding similar solutions to the same problem: how to survive.

      (Describer) A Venus Flytrap closes on a fly. Titles: In this episode, we refer to fossils and art from the homelands of the Kaurna and Peramangk peoples. Kallie Moore: Content Consultant, Host. PBS. Accessibility provided by the US Department of Education.

      Accessibility provided by the U.S. Department of Education.

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      How and why does botanical carnivory keep evolving? It turns out that when any of the basic things that most plants need aren’t there, some plants can adapt in unexpected ways to make sure they thrive. Part of the "Eons" series.

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      Runtime: 9 minutes 47 seconds

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