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Today, I want to start off with a question: Can a species direct its own evolution?
Normally, when we explore the forces that shape the evolution of species here on Eons,
we’re talking about external factors…
Things like changes in climate and resource availability or
pressures from competitors and predators.
These common environmental challenges influence how natural selection shapes all
living things to fit their ecological niches, from E.coli to elephants.
But what if a species actively modifies the way it interacts with its environment,
constructing its own niche and altering the selective pressures it faces?
Well, then it would be shaping its own evolutionary journey, at least in part.
And there’s one species that seems to have a habit of doing this over and over again: us.
Thanks to our ability to develop and share complex learned behaviors across
generations - a thing we sometimes call culture - we have become the ultimate niche builders.
Cultural changes deep in human history introduced new selective pressures that
we wouldn't have otherwise faced, by exposing us to novel situations of our own creation.
These affected our genes, our bodies,
and our evolutionary journeys just like ancient shifts in climate and ecology did.
And it would take studying the DNA of people living today to tell the stories
of some of these cultural changes – from the ones that transformed our
diets to the ones that altered our consciousness.
From a genetic perspective, evolution is simply
a change in the gene pool of a population across generations.
And if that change has been selected for by natural selection because it increases
survival and reproduction - a.k.a. fitness - then it can be considered adaptive evolution.
It’s a change that helped the organism adapt to its environmental conditions.
Now, we know that ancient cultural shifts altered the way that we interacted with our environment.
And this exposed us to new selective pressures created by
culture that drove the adaptive evolution of specific genetic changes in response.
One classic example of this is the fact that around one in three people
living today can digest lactose - the sugar in milk - as adults.
Humans usually lose this ability after childhood, like most other mammals.
The gene that codes for the enzyme that breaks down lactose essentially turns off as we grow up,
meaning that, until fairly recently, everyone was lactose intolerant as an adult by default.
But, in the Holocene Epoch, multiple groups of ancient people began domesticating animals
that they could milk in parts of Europe, Asia, the Middle East, and Africa.
And, as a result of that cultural shift towards
dairying, those populations independently evolved to keep that gene turned on into
adulthood – sometimes by entirely different mutations that achieved the same end result.
These mutations were selected for by natural selection because being
able to digest milk as an adult was a survival advantage for people back then.
And this new lactose tolerance, in turn, might have reinforced the culture of
dairying and lactose consumption in a kind of self-perpetuating cycle.
Now, this topic is actually even more complex and probably deserves a whole episode to itself.
But it’s often considered a textbook case of gene-culture co-evolution - the process by which
culture can change DNA, and vice versa.
And the deeper we’ve dug into our DNA over the last two decades,
the more examples of it we’ve found.
On our first chromosome, for example,
is a genetic feature that stands out compared to our closest primate relatives.
It’s a gene called AMY1, and while other primates have just a couple copies of it,
us Homo sapiens usually have many more - about 7 on average.
And we even have more copies than our closest extinct cousins - the
Neanderthals and Denisovans - who have just the two copies of our shared common ancestor.
So what drove the expansion of this gene
in us Homo sapiens after our lineage diverged from theirs?
Well, the gene codes for a protein called salivary amylase, which is an enzyme that
we produce in our spit that helps us break down starch into simple sugars.
And having more copies of the AMY1 gene means more salivary amylase,
which means more efficient digestion of starch.
So some researchers think that the duplication of this gene is the result
of ancient cultural shifts that changed our diet, much like dairying did with lactose.
This time though, the shift was towards eating more starchy foods.
Now, this may have happened as far back as the middle of the Pleistocene Epoch,
as ancient humans mastered and shared the
art of processing and cooking starchy foods in the wild, like tubers and roots.
Later, the development of agriculture in the Holocene was another cultural
shift that suddenly increased the amount of starch in our diets.
One study even found that people from populations with traditionally high starch diets have,
on average, more AMY1 copies than people from populations with low starch diets.
Now, diet is a pretty common driver of evolution, because feeding yourself is
a very important and constantly shifting challenge that all living things face.
So it’s no surprise that consuming more starch and more lactose thousands of
years ago left genetic legacies that can still be felt today.
But gene-culture co-evolution isn't only tied to exploiting new resources and opportunities.
In theory, a cultural shift that introduces a new danger can also
lead to the emergence and spread of genetic adaptations for coping with that danger.
And the form that culture-induced dangers take can sometimes be pretty surreal.
See, one recurring quirk of human cultures is their search for emotional,
spiritual, and religious experiences.
For tens of thousands of years, in many cultures around the world, people have practiced shamanic
rituals to trigger those experiences - and many communities still do today.
And it’s not uncommon for these kinds of rituals to feature the consumption
of different hallucinogenic plants and fungi with traditional significance to each community.
In the Americas, for example, Indigenous people ranging from the Southern U.S. to northern Chile
and Argentina have used hallucinogenic cacti in shamanic rituals for nearly 10,000 years.
These cacti naturally produce psychoactive alkaloid compounds that alter consciousness.
But consuming these alkaloids can have toxic effects in some
humans - especially pregnant people and children.
In August of 2022, a team of researchers from
Argentina published a study looking at whether exposure to this toxicity had,
over thousands of years, left any adaptive genetic footprint in people living today.
They focused on Native populations from the Central Andes that have a
long history of consuming hallucinogenic cacti.
But first, to figure out what potential genetic adaptations to look for in their DNA,
the researchers turned to the DNA of a certain
species of fruit fly that also exploits these cacti.
They figured that if they could experimentally
analyze how the flies genetically resist the toxic alkaloids found in the cacti,
then they could look for signs of selection on the corresponding genes in people.
After all, fruit flies and humans have a surprisingly high proportion of genes
and genetic pathways in common - it’s part of what makes them
such useful model organisms for the study of human genetics and disease.
So the researchers raised the flies on both hallucinogenic and non-hallucinogenic cacti
species and altered the alkaloid dosage and measured and compared
gene expression under the different conditions.
From that gene expression data, they were able to find a list of
candidate genes that seemed to be involved in dealing with the toxicity of alkaloids.
Some of those genes had a corresponding version in humans, and of those,
ten showed signs of positive selection in the DNA of Native Andean populations.
And most of these were exactly the kind of genes that you’d expect to be under
selection pressure after thousands of years of exposure to hallucinogenic alkaloids.
They were associated with metabolizing, detoxifying, and clearing chemical substances,
as well as protecting and maintaining the activity of the brain and the nervous system.
So while more studies are definitely needed, this may be a rare example of
a religious practice with ancient roots subtly influencing the evolution of the human genome.
Shamanism didn't just alter the consciousness of ancient people and their modern descendants,
it might have literally changed their DNA, too.
All of these examples, and the many more we’ll no doubt find in the years to come,
are a result of the same general evolutionary principle.
From a gene’s perspective, selective pressures that come from culture are
no different than the ones that come from geology, ecology, or climate.
And our fondness for inventing and transmitting complex learned behavior means that,
more than any other species in the history of life, our evolution has been shaped not only
by the environments we found ourselves in, but also the environments we created for ourselves.
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So besides dairy and starchy foods - some humans have another odd dietary preference.
Check out, “How Chilis Got Spicy (and Why We Love the Burn)” to learn more.
And thanks to this month’s transformative Eontologists!
Gale Brown, Juan M., Jacksy Weiss, Melanie Lam Carnevale,
Annie & Eric Higgins, John Davison Ng, Colton, Chase Archambault, and Jake Hart.
By becoming an Eonite at patreon.com/eons, you
can get fun perks like submitting a joke for us to read.
Here’s one from Matt Hepler.
What is a geobacter's favorite thing about a spa day?
The mud bath!
And as always thanks for joining me in the Adam Lowe studio.
Subscribe at youtube.com/eons for more prehistoric stories.
I did not know "dairying" was a word until this episode.
What a word, dairying.