More than 35 million years ago, in the Eocene Epoch, 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. Just half a centimeter in length, the tiny leaf fragments belong to the same plant family as the modern genus Roridula. Those plants are found only in a section of southwestern South Africa called the Cape Floristic Region. Their family was much more widespread during the Eocene Epoch. They’re carnivorous! It makes these tiny bits of leaves encased in amber the best fossil evidence.
Nepenthes rajah is the largest carnivorous plant species. Although this is the largest carnivorous plant, there are many more species found all over the world. They have evolved into many different forms developing their unique ways of catching prey. That plants are defined by their ability to make their food. How and why did these plants take such a different evolutionary pathway?
Carnivory has evolved at least nine times independently in plants and plants that aren’t closely related. So it looks like something keeps driving plants to this seemingly extreme lifestyle. How and why does botanical carnivory keep evolving? 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. Charles Darwin published an entire book about them in 1875, after spending a decade. It would take another hundred-plus year before scientists would propose the definition of what counts as a carnivorous plant that’s often used today.
How did plants become carnivorous?
There are many different types of carnivorous plants that have very different methods of trapping their prey. Many of them are not even related. As carnivory in plants has evolved separately on other occasions. Even pitcher plants are made up of multiple families of plants that have evolved their liquid-filled traps multiple times. Cephalotus follicularis is a pitcher plant found in Australia and is more closely related to the star fruit than any pitcher plant found in Southeast Asia.
Among all of the plants have different methods of catching their prey. The evolutionary process of developing their traps may have been different. But the reasons behind why they evolved the traps are the same. Although they are called carnivorous plants, this isn’t exactly correct because they aren’t getting any energy from their insects. They can still photosynthesize, just like any other plant. They digest animals for nutrients.
Carnivorous plants are found growing in water-clogged mossy or very acidic soil low in nutrients and have become carnivorous to supplement their poor diets. Growing in these nutrient desert environments comes with benefits as it is hard for other plants to grow in these areas. So the carnivorous plants have low competition and won’t get overgrown. Over time this has forced them to get their nutrients from other sources to survive and reap the benefits. Unfortunately, the wet boggy environments forced carnivorous plants to evolve.
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 observed consuming salamanders. By definition, doing this has to give the plant an advantage in growing or reproducing. It’s not enough for the plant to just 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 makes a carnivorous plant. 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, absorbing 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. Some botanists count them among the carnivorous species.
There are many plants alive today that meet the criteria for carnivory, from about 580 to more than 800 species. Carnivorous plants are found on every continent except Antarctica. And they appear to have evolved between 95 million and 1.9 million years ago, based on molecular clock methods. 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.
The youngest botanical carnivores appear 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. The 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 moves to catch its prey. Pitfall traps are the standard passive trap used by things like pitcher plants and bromeliads. Prey lands on the plant’s slippery surface and slides down into a pool of digestive juice. Then there are flypaper traps, which are just what they sound like prey becomes stuck in a sticky substance 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 active traps, using rapid modified leaf movements like a Venus flytrap to snag prey. And Bladder-suction traps are found exclusively in plants called bladderworts. They create little negative pressure vacuums inside their traps. When it is triggered by prey, pop open and suck the victim inside before snapping closed.
Passive traps force prey to move toward the plant’s digestive organ by having little inward-pointing hairs that keep prey from moving backward out of the trap. All of these unrelated plants have not only developed the same kinds of traps. But it looks like they’ve also evolved 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.
They each produced proteins that were originally used to defend the plants from attackers, like fungi. But over time, all of those proteins became repurposed into digestive enzymes. 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. It is 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.
How botanical carnivory keeps popping up seems pretty well understood. But there’s still the question of why? It goes back to the 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. In these kinds of habitats, botanical carnivory represents an evolutionary trade-off – one that comes with both costs and benefits.
- A carnivorous plant has two types of leaves: regular ones that photosynthesize and ones that have been modified into their particular kind of trap.
It means they have fewer photosynthesizing leaves than regular, non-carnivorous plants. So they have to live in places with lots of sunlight to maximize their ability to photosynthesize. And they have to make up the difference. Carnivory can only evolve when it benefits the plant more than investing in regular leaves, like in places where the soil lacks nitrogen and phosphorus.
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, 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 kinds of plants that are more widespread.
The oldest reported fossils of carnivorous plants:
- The Early Cretaceous Period of China.
- The Late Cretaceous of the Czech Republic.
Beyond that, the only other halfway-decent evidence of ancient carnivorous plants are pollen grains from the Paleocene Epoch of India. And one fossil seed from the Eocene Epoch of Australia was destroyed in a freak lab accident after being photographed.
Evolution of carnivorous trapping mechanism
All known carnivorous plants are angiosperms with flowers that are known to have evolved about 135 million years ago. It is known that none of the currently living species can be any older than this. Plus, it is known that nearly all of the traps on carnivorous plants were originally regular leaves that have been heavily modified to catch animals. All plants can absorb nutrients through their leaves, known as foliar feeding.
Carnivorous plants just build on this basic function. A standard hairy leaf can hold a small amount of water that small insects can sometimes drown inside of. As the insect rots away and breaks apart, the plant can absorb the nutrients using foliar feeding.
Proto-carnivorous plants rely on nutrients from sources other than soil, like carnivorous plants but lack many advanced features. One of the best examples of proto-carnivorous plants is the bromeliads which is the family that contains pineapples. Many species in this family catch water in their crown of leaves that sometimes have whole ecosystems of animals living inside them like frogs and insects.
The animals that live in bromeliads bring nutrients to the plants in their droppings and when they die. But there are at least two species of this group that are carnivorous and will break down any prey that falls inside them. It is known that they descended from the other members of the family, showing they would have gone through a proto-carnivorous stage before becoming completely carnivorous. And most carnivorous plants would likely have gone through a stage like this before becoming truly carnivorous.
When carnivorous plants became more specialized for capturing and killing, the features that helped meat-eating plants catch and digest their food were often co-opted from common features among plants. Some carnivorous plants use nectar and bright colors to trick pollinating insects into their traps. The cocktail of chemicals carnivorous plants uses to break down and digest their victims is related to those other flowering plants use to fend off pathogens.
For instance, many species of plants use an enzyme that breaks down chitin. This substance is found within the cell walls of the fungus. So they use this in defense against fungal infections. But Australian pitchers and other carnivorous plants have repurposed this enzyme to digest insect exoskeletons which are also chitin.
Another carnivorous plant that uses simple traps is the flypaper traps that use sticky leaves to catch their prey. These plants also would have had a very similar evolutionary process as pitcher plants. Just instead of their leaves becoming more curved, they got stickier. Some group members just have a pretty regular-looking leaf that has a sticky surface for catching insects. In contrast, others are quite different from other plants, like sun juice covered in sticky hairs.
Surprisingly DNA studies show that one of the most complicated and most famous carnivorous plant traps. The snap traps descended from these simpler sticky leaf trapped plants.
- Venus flytraps are the most famous snap trap. But there is another aquatic plant that catches its prey like this found in Europe known as the water wheel.
Venus flytraps are so highly modified. It is a lot more challenging to see how they came from a regular non-carnivorous plant. But a venus fly trap is analogous to the leaf of any other plant where the hinge of the trap is the vein of the leaf. And the sides have folded up to create the shutters. They have just been so heavily modified.
- Its leaves are divided into two lobes hinged along the midrib. Small trigger hairs on each lobe are susceptible to touch. When these hairs are bent, ion channels at the base of the hairs open. It generates an electrochemical signal, which then changes cells in the midrib and allows the lobes held under tension to snap shut. The leaves shut in two phases. At first, the closure is fast but only part of the way trapping larger prey. Over the next five to 12 days, the insect is digested, after which the flytrap will open again.
Flypaper carnivorous plants will also fold up when they have trapped their prey to stop them from escaping. They are just really slow. And it is known that this was the beginning process behind the evolution of the highly specialized snap traps. The advantage of a snap trap over a sticky surface is that the shutters protect the prey from being stolen by larger animals. Due to the increased leverage of a snap trap, they can catch larger animals which will give.
More nutrients, for instance, most flypaper traps catch small flying insects. Despite the name, venus flytraps more commonly trap large animals like spiders and millipedes. So the first snapping trap would have started with sticky leaves. To catch larger prey, they would have adapted larger and more concave leaves. That can close quickly to reduce the chances of the stronger and larger prey escaping.
As the snapping mechanism becomes faster and stronger, the stickiness of the leaf would be less relevant in the capture of its prey. The most complicated trapping mechanisms out of all the carnivorous plants are deployed by the Utricularia, better known as the bladderworts. These plants are found worldwide and in many different habitats but are often aquatic. Their small flowers are often quite eye-catching. Their bottom half-hidden under the water or the soil possesses one of the most deadly and complicated traps of any carnivorous plant.
They have bladder-like traps that suck up their prey to be digested. Some species can catch animals as large as tadpoles. The mechanism behind this trap is so complicated. It is puzzling to think how a structure like this could have evolved. But there is one member of this group of plants that may hold the answer Utricularia multi feeder found in Australia is a Utricularia. That has traps similar to any other member of the group. Only their traps don’t suck, and instead, it works more like a lobster trap where the prey will be funneled further and further into the trap but can’t get out once it’s in.
So carnivorous plants may seem complicated and different. But their elaborate traps are simply the product of repurposing, modifying, and warping simple functions possessed by nearly all plants. By doing this, they could survive and sometimes thrive where other plants could not previously grow.
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