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The natural world today has been changed pretty
radically by humans. Stand in the center of
New York City or at the edge of a giant corn
field and the human signature is obvious.
But when you think about the past, it’s
easy to imagine a world where the landscape
was pristine and untouched.
Where people lived in perfect harmony with
their surroundings, and the ecosystem was
just the backdrop where they lived their lives.
But, in many cases, that’s not how it was.
Many of the existing ecosystems people think
of as pristine — things like jungles, forests,
and savannahs — were, in fact, shaped by
the humans of the past.
Granted, these people weren’t changing the
planet on a global scale like we are now.
But by understanding the ways that past societies
did affect their ecosystems, we can uncover
histories that have been forgotten, lost,
or in some cases purposefully buried.
Now, throughout the natural world, there are
countless examples of ecosystems obviously
shaped by humans from the past. There are
ecosystems shaped by irrigation, crop cultivation,
or the introduction of new species — to
name a few.
But it’s not always obvious that you’re
looking at a disturbed environment, because
nature takes a lot of different shapes and
forms.
Like, ever since they were colonized by the
British, the rainforests of the Solomon Islands
east of Papua New Guinea have often been thought
of as a pristine ecosystem.
And flying over them, you might see an unbroken
blanket of trees.
But after hearing oral histories about people
who lived there pre-colonization, researchers
looked a little closer and spotted sections
of the woods where the tree species weren’t
distributed like you’d expect in an untouched
rainforest.
There seemed to be patches where the forest
filled in a former clearing. These patches
were, essentially, abandoned settlements.
In general, strange distributions of species 
like this one can be a strong clue that we’re
not looking at pristine nature.
The signs may be subtle, but for those familiar
with the life cycle and history of plants
and animals, an out-of-place grove of trees
might stand out
like a rhino in the middle of Times Square.
In fact, there’s a similar type of clue
right here in the U.S.
Honey locust trees naturally grow in rocky
and dry upland areas of the eastern United
States, but they can also sometimes be found
near rivers and floodplains throughout the
region, which might seem unusual.
But today, scientists think these lowland
populations may actually descend from trees
planted by the Cherokee,
who grew them for food and medicine.
Overall, odd distributions of species
may be signs that people were clearing forests
for wood or grazing, encouraging the growth
of useful plants or clearing out harmful ones,
or even introducing whole new species.
But there are other telltale signs of altered
ecosystems, too.
For instance, aerial or satellite photography
can reveal unnatural features of the environment
that are impossible to see from down below,
like human-made mounds, lines, or other feats
of engineering.
And a close-up look can reveal subtler clues,
like changes in the soil. Things like unusual
levels of nitrogen, silt, or charcoal might
be signs of past humans fertilizing the soil,
irrigating crops, or setting fires.
Finally, there’s another kind of evidence.
History.
This can include records like government documents
or photographs, but it can also include oral histories.
Traditionally, scientists have been wary of
using oral histories as evidence. Since, by
nature, they’re so subjective, they’re
often seen as unreliable.
But oral histories often contain specialized
traditional knowledge and can offer insight
on how people interacted with their environment.
This means they can be both a really useful
starting point for finding human-altered environments
as well as a lens for interpreting them — especially
in places with suppressed or hidden histories.
Overall, each of these types of information
can help us figure out how seemingly pristine
ecosystems might actually be the echoes of
things like agricultural practices or woodland management.
And by understanding how past societies interacted
with their environments, we can build a more
nuanced picture of what their lives might
have been like.
For example, let’s go to a Bolivian region
called the Llanos de Moxos.
Here, in the southwest Amazon basin, the rainforest
opens up into a large, flat, grassy savannah.
It’s threaded by multiple rivers, so the
savannah often floods.
But the landscape is dotted with little forested
hills that are just tall enough to avoid the
yearly floods, letting the trees grow tall
and lush.
These are called forest islands, and many
are roosting sites for bird species, like
the endangered blue-throated macaw.
Decades ago, it was thought that this area
was essentially untouched and unable to sustain
a large human population.
But since then, a number of discoveries have
changed how we view the region.
The scientific debate around the level of
human influence here and throughout the Amazon
region isn’t settled — but scientists
have realized that the floodplain contains
remnants of significant human activity, including
works of engineering like canals, causeways, and raised fields.
In fact, the forest islands themselves may
largely be the product of human engineering.
Some may be natural, but at least some of
these forest islands seem to actually be the
remnants of ancient levees constructed sometime
between 1,500 and 1,000 years ago.
And to draw this conclusion, scientists used
just about every tool in the toolbox.
For instance, they used mapping techniques,
including satellite mapping, to create diagrams
of these earthworks.
Some of the mounds were simple, but others
were arranged into complex, intentionally-created
shapes or connected by causeways.
Meanwhile, through archaeological excavations,
they found pottery, shells, and human remains in the mounds.
They even checked out old plant material and
found evidence of domesticated gourds, maize,
and the root vegetable manioc.
This suggests that not only was this area
populated, but the people there may have already
been domesticating plants — even though
full-blown agricultural societies probably
didn’t show up until much later.
Scientists are still arguing over these results
and what they could mean. But at least some
of the evidence suggests that, far from being
a pristine ecosystem, the unique environment
at Llanos de Moxos is actually an echo of
the agricultural specialization that happened
there more than a millennium ago.
And even here in the U.S., clues in our ecosystems
can tell us something about the past.
For example, take the forests along the Eastern
United States. Accounts from Europeans described
these forests as open, kind of like giant,
wooded parks, with very little undergrowth.
And some people have assumed that this was
how the forests naturally grew. That, if only
people would stop messing with them, the landscape
would return to that stable, natural state.
Except, that open, park-like structure is
not how those forests naturally grow.
A natural forest would have a whole tangle
of smaller saplings and underbrush growing
up in the shadow of those larger trees.
The only reason they wouldn’t look that
way is if frequent, small fires were burning
through the region.
In that case, the underbrush would regularly
burn away, while large trees could survive.
And even today, you don’t see as many fire-sensitive
trees, like maples, growing in certain woods.
Instead, they’re dominated by tougher oaks
and pines, which supports the idea that these
regions experienced frequent fires.
The thing is, regular, controlled burns like
that aren’t the work of something natural,
like lightning. They’re the work of people.
Aside from these environmental clues, several
lines of evidence, including historical records,
archaeological evidence, and soil samples
point to the fact that this environment was
shaped by human hands.
The groups living on these lands — many
of which are still around today — had and
still have detailed traditional knowledge
about how fire affects the ecosystem.
And setting small, frequent fires could have
had a number of benefits. For instance, it
could have made hunting easier, decreased
the risk of giant, catastrophic fires, fertilized
the soil, and encouraged the growth of useful
plant species.
In fact, we now recognize that people all
throughout North America
used fire to shape their environments.
So, environments around the world hold clues
about the people who shaped them, even long
in the past. And there’s a lot to learn
by investigating them.
For one, research like this can confirm oral
histories, lending weight to narratives that
may have been ignored.
And in some cases, like on the Solomon Islands
or in North America, this evidence can help
us better understand the true population size
pre-colonization.
It can also help us rediscover or better understand
how pre-colonial populations shaped their
environments in a way that supported them.
But perhaps most importantly, it can help
us put together pieces of history that are
missing from the usual historical narratives.
In some cases, we’ve forgotten things simply
because time and human memory will naturally
bury certain pieces of history.
But, sometimes the reason we don’t recognize
ecosystems as affected by human hands is because
much of history has been written by colonizers.
We sometimes think of science as being separate
from humanity and all its messiness, but situations
like this are reminders that that isn’t
true.
Many times, colonizers have misunderstood,
lied about, or purposefully set out to stop
indigenous practices and destroy indigenous
culture, knowledge, and histories.
Plus, any observations that were made in good
faith during colonization were often made
during times of extreme population collapse
and social disruption.
So, in places that have been colonized, we
often can’t rely on the prevailing history
that’s been passed down to paint an accurate
picture
of how indigenous people lived before colonial times.
We mention this not to give a history lecture,
but to acknowledge that this does affect huge
amounts of science — especially because,
even today, indigenous people have often been
ignored or barred from participating in and
shaping science and history.
But at times, the land itself has held on
to pieces of history that have been lost or rewritten.
As a whole, these studies tell us that wherever
people have gone, we have affected and shaped
the world around us — whether that’s
in our homes or our ecosystems.
This is true today and has been true for a
long, long time, even if it’s not always obvious.
But by studying the human echoes in ecosystems
around the world, we can better understand
our pasts and how our decisions today will
shape the future.
Thank you for watching this episode of SciShow!
And thank you to all our patrons, whose support
and curiosity about the world makes these
videos possible. We couldn’t do it without you!
If you’d like to learn more about how to
support SciShow, you can do that over at patreon.com/SciShow.
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