In 1879, Charles Darwin wrote to his colleague
Joseph Dalton Hooker about an “abominable mystery"
in the fossil record.
He was perplexed at how rapidly major families
of flowering and fruit-bearing plants,
known as angiosperms, emerged during the Cretaceous
over the span of 10 just million years.
The oldest known fossils of angiosperms date
to around 135 million years ago,
about 10 million years before this explosion of diversity.
Now, researchers are using tools in molecular biology
and fossils to shed light on this fascinating history.
In 2019, researchers turned to chloroplasts,
the organelle in plant leaves that converts
sunlight into energy via photosynthesis.
Chloroplasts are a great tool for understanding
plant evolution, because it’s easy to extract
them and sequence their DNA.
Changes in that DNA from species to species
can reveal how closely different plants are
related to each other—
and when certain species and groups of species evolved.
Researchers sequenced the chloroplasts of
plants from angiosperm clades, groups of plants
with a common ancestor.
They used the oldest fossil of major clades
to calibrate their molecular analyses.
Next, researchers analyzed genetic markers
in living plants to understand when their
genomes might have diverged from their ancestors.
This led to a much earlier date for the origin
of angiosperms—more than 200 million years
ago in the Upper Triassic,
versus the oldest angiosperm fossils which date to
135 million years ago.
Molecular analysis also suggests an earlier
explosion of angiosperm diversity, in the
late Jurassic and early Lower Cretaceous.
Another clue to this mystery comes from the
flowering plant’s helper--pollinators,
including butterflies and bumblebees.
As with angiosperms, recent molecular evidence
has placed the origin of pollinating insects
further back than the fossil record.
The earliest fossil evidence for Lepidoptera,
for example, an order of insects which include
butterflies and moths,
was from 195 million years ago.
But molecular analyses suggest that they’ve
been around since 300 million years ago, predating
the new estimate of the origin of angiosperms
by tens of millions of years.
In the past, researchers thought that the
shift from wind pollination in non-flowering plants,
or gymnosperms, to animal pollination
in angiosperms, led to more diverse flowering plants.
But recent discoveries show that some extinct
gymnosperms were pollinated by insects,
complicating the story.
Other questions remain—including whether
certain characteristics in plants,
such as long floral tubes,
formed in response to longer pollinator tongues,
or the other way around.
The discovery of new fossils,
and studies that use similar methods and calibration,
could further illuminate exactly when
certain insects and plants emerged.
For now, the answer to Darwin’s mystery
well over a century later?
It’s still complicated.
