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