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
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. 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 hundred-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 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 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 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
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, 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 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 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 bladderworts. 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 toward 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 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. And they each produced their own 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. 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 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. 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 kinds of plants that are more widespread. And the oldest reported fossils of carnivorous
plants - one from the Early Cretaceous Period of China and another 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’re just related
to modern plants that are carnivorous today. So, the jury is 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 secretion 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. Hey everyone! Feeling a little stuck inside? Then
you should check out Overview, a brand new show on PBS Digital Studios’ science and nature channel
Terra. Overview combines mesmerizing drone footage with in-depth science storytelling to reveal all
the things shaping our planet from the 10,000 foot view (literally). So go over to Terra, subscribe
and make sure to tell them Eons sent you! We’d also like to say thanks to our friends over
at Deep Look who provided us with some spectacular up-close footage of sundews. Be sure to check
out their episode, “Cape Sundews Trap Bugs in A Sticky Situation” to learn more about these
carnivorous plants - link is in the description. Big thanks to this month’s Eontologists
for sticking with us: Lucan Curtis-Mahoney, Sean Dennis, Jake Hart, Jon Davison Ng, Patrick
Seifert, and Steve! Become an Eonite by supporting us at patreon.com/eons. And remember - Eonites
get perks like submitting a joke for us to read! Like this one from James H What did the paleontologist name his son? Cam Brian! Get it? Cam..cam...Cambrian? It's good. Thanks James! And as always thank you for joining
me in the Konstantin Haase studio. If you like what we do here, go to
youtube.com/eons and subscribe!
I love that channel
I've been wondering about their evolution. Thanks!