Around 6,000 years ago, in the Sahel region of Africa,
a lone female mosquito buzzed through the lush, green savannah.
She couldn’t know it, but the planet itself was about to change
in ways that would see her descendants evolve to live very different lives.
A sudden ecological shift would force them to go from living in
forests and feeding on a range of animals to specializing on just one single species: us.
These tiny insects would eventually evolve to become one of the most
dangerous animals in the world… and it would be partially our fault.
But what drove this species of mosquito straight into our arms? What changed
around them and ultimately changed them into the scourge that we know today?
Well, it looks like it began with a shift in our planet’s
orbital cycles and the birth of the Sahara desert.
Today, the subspecies of mosquito known as Aedes aegypti aegypti is
among the most despised creatures in the world, and for pretty good reason, too.
They have a strong innate preference for humans. They’re drawn in by our
scent and females take up to 95% of their blood meals just from people.
This makes them a major vector for a bunch of viral diseases, including Yellow Fever,
Zika, Dengue, and Chikungunya – but not malaria, that’s a different mosquito.
And they live in warmer parts of every continent except Antarctica,
which makes them a source of global misery for literally billions of people.
But they weren't always this way.
For most of their evolutionary history, Aedes aegypti was pretty harmless,
much less widespread, and not especially interested in us.
And we know this because forest-dwelling populations still exist in parts of Africa today where they
still live as generalists, feeding on a range of animals that they share the environment with.
They don't prefer us humans over those other animal hosts, and they lay their eggs in natural
pools of stagnant water, often in tree holes, where the larvae live out their aquatic stages.
In contrast, their specialist cousins are primarily attracted to our scent,
and their larvae have even adapted to hatch and mature in our stores of clean
water. This has helped them follow us around and exploit our settlements.
So how did this happen, how did we get here?
Well in 2020, a team of researchers published a study that revealed some
initial clues. They collected Aedes aegypti from 27 sites in Africa,
including both generalist and specialist populations.
And they found that the human-specialist populations that were most attracted to
people lived in urban communities of the West African Sahel region,
which suggested that this was where specialization on humans first emerged.
The reason why the Sahel may have been where they first made the switch, is because it’s
a pretty dry place. Natural pools of water are hard to come by for around 9 months out of the year.
And in 2023, these researchers published a new study tracing
the history of the mosquitos’ switch to humans by sequencing their DNA.
They compared the DNA of the generalist and the specialist mosquitos they had collected,
and used the steady accumulation of mutations between the two over time
as a molecular clock to try and figure out when they diverged.
Now, to set a molecular clock, you need to figure out how fast
mutations naturally accumulate in a species.
And to do that, you can turn to events in the history of a species that occurred at known dates,
which can be used to calibrate your analysis.
And for these mosquitos, we know that they spread from Africa to the Americas starting
about 500 years ago… because that’s when the transatlantic trade in enslaved people started.
The mosquitos stowed away on ships heading out of West Africa,
feeding on the crew and captives, and reproducing in stored barrels of water.
So by comparing patterns of mutation accumulation between the populations
of both continents since that known period of migration,
the researchers could get a more precise estimate of the species overall mutation rate.
And by using that to calibrate their clock, they found that the original
split between generalist and specialist populations occurred around 5000 years ago.
This date not only told us when specialization began, but it also hinted at why…
Because this was right around the end of the African Humid Period, which was a key
moment of environmental upheaval for the region.
See, between around 5,000 and 15,000 years ago,
what’s now the sandy, dry Sahara desert was totally unrecognizable.
It was a wet, green, savannah that stretched the entire length of the continent and into
Arabia. We’ve talked about it before here on Eons in our episode When the Sahara Was Green.
The African Humid Period was caused by cyclical changes in the planet’s tilt,
orbit, and wobble that increased the amount of solar energy that hit the planet,
drastically altering the climate during this time.
One of the major effects of these planetary-scale
changes was a strengthening of the African monsoon, which also moved further north.
This dumped massive amounts of water over the Sahara,
forming huge lakes and even a river system, and allowed a lush savannah ecosystem to flourish.
But around 5000 years ago, the earth’s orbit shifted once more.
These climatic changes were reversed, ending the African Humid Period over
just a few centuries and birthing the Sahara Desert that we know today.
So the fact that the mosquito DNA pointed to this exact period of rapid climate change as
the catalyst for human specialization potentially makes a lot of sense.
After all, the sudden loss of the savannah ecosystem,
with its many water sources and abundance of animal hosts, would have been a major
challenge for the ancestral, forest dwelling, generalist mosquitos.
So in the western Sahel region, with its especially long and arid dry season,
they latched on to people to cope with the sudden shift.
And there was also another major change that happened around 5000 years ago,
one that probably helped enable the transition.
People living in the Sahel region domesticated the pearl millet plant,
which we can tell from genetic and archeological evidence.
This was one of the oldest plants to be domesticated in Africa,
and early farmers often stored large amounts of water in clay pots to use in the growing process.
These clay pots were perfect replacement for tree holes
and rock pools that the mosquitos in the Sahel were suddenly lacking.
So these two key events right around the same time and place - a shift in climate and
a new form of human agriculture - triggered the onset of human-specialization in the mosquitos…
Simultaneously bringing together all the required elements for their new,
horrible ecological niche and making the whole thing partially our fault.
And the evolution of this love-hate relationship didn’t stop there…
In fact, it continues to change today, right now, as you watch this.
See, over the last 20 to 40 years, as many African cities have gotten bigger
and more densely populated, some have found that their previously
generalist mosquito populations have shifted to biting people.
And the researchers’ genetic analysis from 2023 suggested that this is due to an influx
of specialists that are introducing their adaptations into the generalist gene pool.
So as urbanization continues and cities become more densely populated,
these human-specialist genes are spreading in response,
meaning more mosquito populations may become increasingly specialized in the years to come.
Plus, as our activities continue to warm the planet,
the range of Aedes aegypti seems to be expanding, too.
So while their infatuation with us may have started with a natural shift in
climate thanks to a change in our planet’s orbital cycles, the relationship is now
being shaped by us humans and the way that we are radically changing the environment.
It’s just one more example of a recurring theme in natural history:
everything is connected. And things as big as the orbit of the planet
itself can affect things as small as the lifestyle of a mosquito.
Before you go I wanted to take a second to tell you about the
latest series from our friends over at Crash Course… Botany.
If you’re interested in plants - and you know we are - the course explores
in great detail how plants evolved, as well as how they function today,
and just how vital they are to life on Earth.
It’s got all the good stuff you’d expect from Crash Course,
and is hosted by everyone’s favorite plant lover and forager (including mine),
Alexis Nikole Nelson. We’ll put a link below. Thanks for watching!
If this episode made you wanna know the story behind malaria,
check out our episode “How Our Deadliest Parasite Turned To The Dark Side”.
And thanks to this month’s Eontologists, who we can’t resist!
Annie & Eric Higgins, Juan M, Melanie Lam Carnevale,
Raphael Haase, Jake Hart, and John Davison Ng.
Become an Eonite at patreon.com/eons and you
can get fun perks like submitting a joke for me to read! Here’s one from Joe H.
What did the horse say when it fell over? Help! I've fallen and I can't giddy up!
[laughs] Ok...
Wow... "HeIp! I've fallen and I can't giddy up!"
Very cute.
Now if only that had been at the end of, like, a horse episode...
And thanks as always for joining me in the Adam Lowe studio. Subscribe
at youtube.com/eons for more fabulous fossils.
By comparing patterns of mutation accumulations between the population
of both continents... since that - [loses words]
[claps hands while saying each word] Mutation accumulation between the populations...
[School House Rock style] Mutation accumulation, between the populations, of the po - [laughs]
[again] Mutation accumulation, between the populations, since the last migration...
It keeps going, y'all.