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{♫Intro♫}
More than 90% of the plants on Earth are angiosperms,
flowering plants whose seeds are enclosed inside fruit.
And they’re everywhere --
but exactly how and when these plants
came to be so ubiquitous
is one of the most stubborn questions in science.
Back in the 1800s, Charles Darwin himself called it
an “abominable mystery”.
And despite nearly a century and a half
ofworking on it, the origins of flowering plants
are still a subject of debate.
The basic question is simple:
When did flowering plants evolve?
Our two best sources of data are
the fossil record and DNA evidence.
And they… provide two totally different
answers.
See, the oldest definite flowering plants
in the fossil record are from the
Early Cretaceous Period, around 130 million years ago.
But scientists can also use genetic information
to calculate a date in what’s called
a molecular clock estimate.
To do so, they compare genetic diversity
among living plants, and then use
the estimated rates at which
those plants’ genomes change
to calculate how long it would take them to
diverge from a common ancestor.
And this technique reproducibly comes up with
dates that are much earlier than the Early Cretaceous.
One particularly large study from 2019
used DNA from more than 2300 living plant species
to estimate an origin in the Late Triassic Period,
around 209 million years ago.
So, that’s over 70 million years of difference
between the fossil and DNA estimates
-- a gap that basically skips over
the entire Jurassic Period!
Now it’s not unusual for fossils
and DNA to disagree.
It’s happened before in estimating
the origins of birds and mammals.
But over time, those disagreements diminished
as our information got better…
but that hasn’t happened for flowering plants.
So… why not?
Who is wrong here?
Well, on one hand, it could be a fault in our fossils.
The fossil record is notoriously incomplete.
The chances of any particular plant being preserved
can depend on its size, shape,
and ecology, and not all plants fossilize equally.
And the odds are pretty low that
the oldest fossilized angiosperm also happens to be
the first ever angiosperm.
So maybe there are a bunch of early angiosperms
that we just haven’t found,
or were never preserved at all.
This is the explanation favored
by the authors of that 2019 genetic study.
They even proposed a name for this missing history:
the Jurassic angiosperm gap.
But here’s the thing …
some paleontologists are pretty sure
there isn’t a gap at all.
A separate 2019 study did a major review
of the oldest known flowering plant fossils,
specifically pollen.
Pollen is as great for paleontologists
as it is bad for people with seasonal allergies.
Those little grains are durable, and they get everywhere.
The study confirmed that angiosperm pollen
goes back to the Early Cretaceous.
Now, there are fossil sites even older than
that
where we can find pollen from other kinds
of plants, including close relatives of angiosperms.
But those sites don’t have angiosperm pollen.
We usually hear that absence of evidence
is not evidence of absence, but in this case,
the study authors say, the lack of pollen
seems to indicate that angiosperms
really just weren’t there.
And there’s more: when flowering plant pollen
does finally appear in the fossil record,
it does it in the order we’d expect.
We know from genetic studies which varieties evolved
in what order, and that’s the order
we see their pollen appear.
That really makes it look like the early evolution
of angiosperms happened in the Early Cretaceous.
It could still be an illusion of fossil preservation,
but it’s also possible that our
genetic estimates could be wrong.
See, despite the name, a molecular “clock”
isn’t a date written in a piece of DNA.
It’s a complex mathematical model,
and what that model puts out depends on
the data you put in.
That starting data can include relationships
between plants, the rates those plants evolve,
and the age of the fossils of angiosperms
and their relatives.
If we’re wrong about any of that,
it’s going to affect the date a molecular clock
analysis spits out.
In fact, studies testing these models have
found
that wrong inputs often lead to too-early dates.
If that’s happening here, then
the supposed “angiosperm gap” might be a result of
our own misunderstandings of plants.
But so far, genetic studies consistently place
angiosperm origins well before the earliest fossils.
It might be that the true answer is
somewhere in between.
Molecular clock estimates usually aren’t
a single date,
but a range of possible answers,
and many of these estimated ranges
have younger ends
in the Middle or Late Jurassic Period.
And even the fossil pollen study noted
that, even with the fossil evidence at hand,
slightly earlier angiosperms could have existed
in the later Jurassic before becoming
common enough to show up in the fossil record
and evolve the forms we recognize.
Ideally, fossils and DNA will work together
to arrive at the right estimate.
More fossils of flowering plants and their
relatives
will help us better understand the timing
of key evolutionary steps.
And more careful consideration of the data
on plant evolution will help us narrow down
those molecular clock estimates.
Resolving this puzzle isn’t just important
for knowing where our favorite flowers come from.
It’ll also help us understand past environmental changes
and the history of animals like insects
who evolved alongside flowering plants.
In the end, there will be a right answer.
But there’s bound to be some more data tug-of-war
before we find it.
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{♫Outro♫}
