Around 55 million years ago, above a  warm tropical sea in what’s now Denmark,   an ancient butterfly flapped  its wings for the final time.

As it died, it tumbled into the water below,  sinking to the bottom and becoming buried.

In 2016, this tiny insect was identified  as the oldest butterfly fossil ever found.

And for a long time, this time  period - the Eocene epoch - was   assumed to have been when butterflies originated.

Not just because that’s when  their earliest fossils date to,   but also because that's when bats first appeared.

The idea that bats basically invented  butterflies might seem like a weird logical   leap, but here's what some scientists  were thinking around the year 2000.

The first complete bat fossils we have also  come from a little over 50 million years ago.

And it seems like echolocation  evolved pretty early on,   which would have made them skilled  predators of nocturnal insects…like moths.

And, butterflies are basically just day-moths.

So the idea was that the evolution of bats drove  one group of moths to abandon the night entirely,   becoming active during the day to  escape, giving rise to butterflies.

Or did they?

It turns out, instead of having  bats to thank for the existence of these   beautiful insects, the groups we should  actually be thanking are bees and beans.

Butterflies and moths make up one of the  most diverse orders of insects on the planet,   called Lepidoptera, which means ‘scaly wings.’ There are at least 160,000 species  of lepidopterans described so far,   with 75-to-85% of them being nocturnal moths.

For context, that means  that about one in ten known,   non-microbial species on Earth is a Lepidopteran.

They’re thought to have diverged from  their sister group, the caddisflies,   as far back as the Late Carboniferous  period, around 300 million years ago.

But the oldest fossil traces of them  only date back around 200 million years,   to the boundary between the  Triassic and Jurassic Periods.

Those traces are these microscopic moth scales,   reported in 2018 from sediment  from a site in Germany.

Which means that the fossil record  of lepidoptera is full of gaps.

And if you’ve ever held one in your  hand, you know how delicate they are,   so their relative lack of a fossil  record makes a lot of sense.

For example, that Eocene formation in Denmark  has produced around 25,000 insect fossils,   but only one has been identified as a butterfly.

So despite butterflies being incredibly diverse,  generally unproblematic, and distributed basically   everywhere on Earth, until recently we  could only really guess at their origin.

And it’s weird to me, personally, that  a creature that’s so familiar to most   of us would be such a mystery  to evolutionary biologists.

Like, were they really just moths that  were driven out of the night by bats?

Or did they have a deeper, more complex origin  story than their fossil record alone suggested?

In 2019, researchers turned to modern  evidence to try to solve the mystery,   comparing genetic data collected from 186 species.

They used it to build a big family tree  of moths and butterflies, with a set   of rare fossils giving them specific  dates for when certain groups evolved.

Their analysis allowed them to  narrow down the timeframe of   when butterflies actually split off  from their nocturnal moth ancestor.

And the fossil-dated family tree that  they created found that this split and   their switch to daytime activity took  place around 100 million years ago   in the mid-Cretaceous Period, long  before bats even first took flight.

And rather than the key trigger for the  evolution of butterflies from moths being   avoiding predators, the researchers argued it  was more likely tied to new food opportunities.

Which is my favorite kind of opportunity... You see, angiosperms - aka flowering plants - were   in the middle of a dramatic rise to  global dominance in the Cretaceous.

And they had already forged an evolutionary  partnership with other insects that are diurnal,   or active during the day, by this time,  like bees, which helped to pollinate them.

Flowering plants had become brightly colored  to attract the bees and developed easy-access   nectar to encourage them to visit, often blooming  only in the daytime when bees are most active.

And many ancient moth lineages were already  using a tube-like proboscis to suck up nectar,   sap, and other liquid foods when they could.

But their nocturnal lifestyle meant that they were  missing out on those prime daytime feeding hours.

So it would make sense that the first butterflies   in the mid Cretaceous became diurnal  to follow in the footsteps of bees.

Little tiny footsteps.

Now they could take advantage of the daytime  nectar resources and attractive, bright signals   that flowering plants had initially developed  for a totally different group of pollinators.

So they didn't abandon the night  to avoid new nocturnal dangers.

Instead, they embraced the daytime  for the new ecological opportunities.

So that gave us some clues  about the when and the why   of butterfly evolution, but what about the where?

After all, they're basically everywhere  today - even on remote and isolated islands,   but they clearly had a single origin  somewhere before their global dispersal.

Well that’s a more complex question to untangle,  and it requires an even more complex data set… One that covers not just how they are related,   but also how their distribution and lifestyle  patterns reflect those relationships, too.

But if you know any evolutionary biologists,  and I do, you know that they like a challenge… So in 2023, researchers published the most massive  and comprehensive analysis yet to tackle the   question of where exactly butterflies came from  and how they spread to every corner of the planet.

They collected DNA from almost  2300 species from 90 countries,   both from museum collections and the  ones that they collected in the field.

And they combined this with  information on species distribution,   habitat preferences, and plant relationships.

Much of this information was pulled  together from books and field guides   from around the world, translated  from many different languages.

Like, if this was a movie, this is  the moment where we’d get a montage   of the researchers blowing dust off  of old books and opening drawers in   the backs of museums and chasing  butterflies around with a net.

Like Indiana Jones, but with  butterflies, is what I'm thinking.

I would watch the hell out of that movie.

They analyzed all this data,  calibrated with key fossils,   and they modeled the rise and spread  of butterflies far back in deep time.

And it looks like North or  Central America was where   the first butterflies adapted to life in  the sun, around 100 million years ago.

And because modern butterfly lineages  often have very specific host-plants   that they feed on as caterpillars,  the researchers were able to work   backwards using a technique called  ancestral state reconstruction.

This allowed them to figure out which plants  the first butterflies were associated with.

And it turned out, in their earliest days, they  fed on just one group in particular - bean plants!

Their relationship with the legume family  was the original jumping off point for them.

And they were probably a pretty  good host family to start off with,   because they’re abundant in all sorts  of ecosystems and don’t usually have   defensive chemicals that keep  insects from feeding on them.

Eventually, the earliest butterflies added  other angiosperm hosts to their roster.

They diversified, coevolved,   and dispersed alongside them into the  nearly 20,000 species we know today.

And by the late Eocene, butterflies  had made it to every continent,   even passing through Antarctica at one  point, though they don’t live there today.

The story of butterfly origins reminds us that  ecological relationships in deep time aren’t   always obvious and that new ways to answer  old scientific questions are always evolving.

In this case, it meant rethinking our  initial, simple hypothesis about bats   in favor of a more complex understanding of the  place of butterflies within ancient ecosystems.

And it turns out, we owe the existence of  these familiar insects to American beans,   Cretaceous bees, and a series of  great migrations that brought this   group of colorful, day-flying moths  to almost every corner of the world.