We think we’re the first advanced earthlings—but how do we really know?

How do we really know there weren’t previous industrial
civilizations on Earth that rose and fell long before human
beings appeared? That’s the question posed in a scientific
thought experiment by University of Rochester
astrophysicist Adam Frank. Credit: University of Rochester
illustration/Michael Osadciw

Imagine if, many millions of years ago, dinosaurs drove cars
through cities of mile-high buildings. A preposterous idea,
right? Over the course of tens of millions of years, however,
all of the direct evidence of a civilization—its artifacts
and remains—gets ground to dust. How do we really know, then,
that there weren’t previous industrial civilizations on Earth
that rose and fell long before human beings appeared?


It’s a compelling thought experiment, and one that Adam Frank,
a professor of physics and astronomy at the University of
Rochester, and Gavin Schmidt, the director of the NASA Goddard
Institute for Space Studies, take up in a paper published in
the International Journal of Astrobiology.

“Gavin and I have not seen any evidence of another industrial
civilization,” Frank explains. But by looking at the deep past
in the right way, a new set of questions about civilizations
and the planet appear: What geological footprints do
civilizations leave? Is it possible to detect an industrial
civilization in the geological record once it disappears from
the face of its host planet? “These questions make us think
about the future and the past in a much different way,
including how any planetary-scale civilization might rise and
fall.”

In what they deem the “Silurian Hypothesis,” Frank and Schmidt
define a civilization by its energy use. Human beings are just
entering a new geological era that many researchers refer to as
the Anthropocene, the period in which human activity strongly
influences the climate and environment. In the Anthropocene,
fossil fuels have become central to the geological footprint
humans will leave behind on Earth. By looking at the
Anthropocene’s imprint, Schmidt and Frank examine what kinds of
clues future scientists might detect to determine that human
beings existed. In doing so, they also lay out evidence of what
might be left behind if industrial civilizations like ours
existed millions of years in the past.

Human beings began burning fossil fuels more than 300 years
ago, marking the beginnings of industrialization. The
researchers note that the emission of fossil fuels into the
atmosphere has already changed the carbon cycle in a way that
is recorded in carbon isotope records. Other ways human beings
might leave behind a geological footprint include:

  • Global warming, from the release of carbon dioxide and
    perturbations to the nitrogen cycle from fertilizers
  • Agriculture, through greatly increased erosion and
    sedimentation rates
  • Plastics, synthetic pollutants, and even things such as
    steroids, which will be geochemically detectable for millions,
    and perhaps even billions, of years
  • Nuclear war, if it happened, which would leave behind
    unusual radioactive isotopes

“As an industrial civilization, we’re driving changes in the
isotopic abundances because we’re burning carbon,” Frank says.
“But burning fossil fuels may actually shut us down as a
civilization. What imprints would this or other kinds of
industrial activity from a long dead civilization leave over
tens of millions of years?”

The questions raised by Frank and Schmidt are part of a broader
effort to address climate change from an astrobiological
perspective, and a new way of thinking about life and
civilizations across the universe. Looking at the rise and fall
of civilizations in terms of their planetary impacts can also
affect how researchers approach future explorations of other
planets.

“We know early Mars and, perhaps, early Venus were more
habitable than they are now, and conceivably we will one day
drill through the geological sediments there, too,” Schmidt
says. “This helps us think about what we should be looking
for.”

Schmidt points to an irony, however: if a civilization is able
to find a more sustainable way to produce energy without
harming its host planet, it will leave behind less evidence
that it was there.

“You want to have a nice, large-scale civilization that does
wonderful things but that doesn’t push the planet into domains
that are dangerous for itself, the civilization,” Frank says.
“We need to figure out a way of producing and using energy that
doesn’t put us at risk.”

That said, the earth will be just fine, Frank says. It’s more a
question of whether humans will be.

Can we create a version of that doesn’t push the earth into a
domain that’s dangerous for us as a species?

“The point is not to ‘save the earth,'” says Frank. “No matter
what we do to the planet, we’re just creating niches for the
next cycle of evolution. But, if we continue on this trajectory
of using and ignoring the climate
change it drives, we human beings may not be part of Earth’s
ongoing evolution.”

Explore further:

Earth as hybrid planet: New classification places Anthropocene
era in astrobiological context

More information: Gavin A. Schmidt et al. The Silurian
hypothesis: would it be possible to detect an industrial
civilization in the geological record?, International
Journal of Astrobiology
(2018). DOI: 10.1017/S1473550418000095

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