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There’s a lot we still need to learn about the early history of humans.
Both ourselves, Homo sapiens, and other members of our genus.
Like, we know that even though we’re the only ones left,
once upon a time we coexisted with other human species.
And there’s one place on Earth that may have taught us more about that than any other
single site.
At least according to a study out this week in the journal Nature,
which tells us more about who lived there and when.
Denisova Cave is located in Russia, specifically in southern Siberia.
And it’s an interesting place, because we’ve found a variety of human fossils there.
That includes Neanderthals, our well-known early human cousins.
But there’s also another distinct human group first uncovered here in 2008
and named after the site: Denisovans.
In fact, one fossil found here is thought to be both:
the child of one Neanderthal and one Denisovan parent.
Researchers think this cave has been a haven for people
for hundreds of thousands of years.
But there aren’t a huge number of human remains in the cave,
so we haven’t been able to examine enough fossils to really nail down the timeline.
For example, we don’t know whether Neanderthals or Denisovans got there first,
what the world was like for them at the time,
or which groups were associated with archaeological evidence like ornaments and bone tools.
But a new analysis has shed some light on who was living in the cave over the millennia.
Previously, scientists have only been able to sequence and examine
DNA from eight fossils found in the cave.
But those bones aren’t the only source of DNA lying around.
The key is something called environmental or e-DNA, found in the cave’s soil.
Because people don’t just leave tools and campfires behind.
They also leave behind little flecks of skin and stuff containing precious DNA.
And apparently, those stuck around long enough in the cave to be recovered
and sequenced all these years later!
Building off a smaller pilot study,
the team took more than seven hundred sediment samples
and were able to extract human mitochondrial DNA from 175 of them.
Mitochondrial DNA is the relatively short segment of DNA that lives within our mitochondria,
separate from our main genome.
It’s often used to track evolution because it’s typically passed down only via egg cells
and doesn’t change too much over time.
A handful of these DNA samples were good enough to construct nearly full mitochondrial sequences,
though the rest were too degraded by time
For those, scientists compared what they had to known genomes from Neanderthals,
Denisovans, or modern humans to try and suss out their lineage.
They were also able to pull additional DNA from the soil,
to see what animals may have accompanied people through the years.
Together, the samples covered a stretch of time from about
two hundred fifty thousand to twenty thousand years ago.
Putting everything together,
the researchers were able to propose a timeline of who visited the cave when.
This may not be exactly perfect, as things like burrowing animals
may have mixed up some soil layers and muddled our ability to tell when they were laid down.
But overall, the evidence appears to suggest that Denisovans were
the earliest people to occupy the cave, appearing to be the major group
from around 250,000 to around 170,000 years ago.
Then, around that time, Neanderthals appear,
and while they share the space for a while,
they seem to eventually take over as the main group to live around Denisova Cave.
This may have coincided with changes in the outside world
as the world went from a warm period to an ice age,
the animals in and around the cave seem to have changed.
Eventually, around 45,000 years ago,
modern humans seem to appear, once again with a bit of overlap.
There’s evidence that each of these groups may have run into the other ones.
In one chamber, the researchers say it’s possible that all three groups
may have been present and using the cave at some point.
Placing what we’ve learned here in wider context,
we can see hints of larger possible stories,
such as how the Denisovans may have moved eastward into Asia
following animal migrations along the foothills of the Himalayas.
The overall picture is of a kind of crossroads
a place that many different people visited over time as the world changed around them.
Which is a remarkable thing to learn from plain ol’ dirt.
You have to wonder what other stories this cave will have to tell.
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For example, Denisova Cave is only the beginning when it comes to
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