Did another advanced species exist on Earth before humans?

Our Milky Way galaxy contains tens of billions of potentially habitable planets, but we have no idea whether we're alone. For now Earth is the only world known to harbor life, and among all the living things on our planet we assume Homo sapiens is the only species ever to have developed advanced technology.

But maybe that's assuming too much.

In a mind-bending new paper entitled "The Silurian Hypothesis" — a reference to an ancient race of brainy reptiles featured in the British science fiction show "Doctor Who" — scientists at NASA's Goddard Institute for Space Studies and the University of Rochester take a critical look at the scientific evidence that ours is the only advanced civilization ever to have existed on our planet.

"Do we really know we were the first technological species on Earth?" asks Adam Frank, a professor of physics and astronomy at Rochester and a co-author of the paper. "We've had an industrial society for only about 300 years, but there's been complex life on land for nearly 400 million years."

If humans went extinct today, Frank says, any future civilization that might arise on Earth millions of years hence might find it hard to recognize traces of human civilization. By the same token, if some earlier civilization existed on Earth millions of years ago, we might have trouble finding evidence of it.

See more on our universe:

15 PHOTOS
Astonishing space moments of 2017
See Gallery
Astonishing space moments of 2017
REFILE - CORRECTING HOW PHOTO WAS TAKEN A composite image of 21 separate photographs taken with a single fixed camera shows the solar eclipse as it creates the effect of a diamond ring at totality as seen from Clingmans Dome, which at 6,643 feet (2,025m) is the highest point in the Great Smoky Mountains National Park, Tennessee, U.S. August 21, 2017. Location coordinates for this image are 35�33'24" N, 83�29'46" W. REUTERS/Jonathan Ernst TPX IMAGES OF THE DAY
Enthusiasts Tanner Person (R) and Josh Blink, both from Vacaville, California, watch a total solar eclipse while standing atop Carroll Rim Trail at Painted Hills, a unit of the John Day Fossil Beds National Monument, near Mitchell, Oregon, U.S. August 21, 2017. Location coordinates for this image is near 44�39'117'' N 120�6'042'' W. REUTERS/Adrees Latif TPX IMAGES OF THE DAY
A jet plane flies by the total solar eclipse in Guernsey, Wyoming U.S. August 21, 2017. REUTERS/Rick Wilking TPX IMAGES OF THE DAY
A total solar eclipse occurs on August 21, 2017, at Mary's River Covered Bridge, in Chester, IL, USA. (Photo by Patrick Gorski/NurPhoto via Getty Images)
The total solar eclipse Monday August 21, 2017 in Madras, Oregon. Emotional sky-gazers stood transfixed across North America Monday as the Sun vanished behind the Moon in a rare total eclipse that swept the continent coast-to-coast for the first time in nearly a century. / AFP PHOTO / ROB KERR (Photo credit should read ROB KERR/AFP/Getty Images)
ROXBURY, NJ - AUGUST 21: The Moon is seen passing in front of the Sun during a Solar Eclipse on August 21, 2017 at Roxbury High School in Roxbury, NJ. A total solar eclipse swept across a narrow portion of the contiguous United States from Lincoln Beach, Oregon to Charleston, South Carolina. A partial solar eclipse was visible across the entire North American continent along with parts of South America, Africa, and Europe. (Photo by Rich Graessle/Icon Sportswire via Getty Images)
People watch the total solar eclipse in Guernsey, Wyoming U.S. August 21, 2017. REUTERS/Rick Wilking
Cheerleaders use solar viewing glasses before welcoming guests to the football stadium to watch the total solar eclipse at Southern Illinois University in Carbondale, Illinois, U.S., August 21, 2017. Location coordinates for this image are 37�42'25" N 89�13'10" W. REUTERS/Brian Snyder
Guests watch the sun re-emerge after a total eclipse at the football stadium at Southern Illinois University in Carbondale, Illinois, U.S., August 21, 2017. Location coordinates for this image are 37�42'25" N 89�13'10" W. REUTERS/Brian Snyder
One of the last looks at Saturn and its main rings as captured by Cassini. When the spacecraft arrived at Saturn in 2004, the planet's northern hemisphere, seen here at top, was in darkness in winter. Now at journey's end, the entire north pole is bathed in sunlight of summer. Images taken October 28, 2016 and released September 11, 2017. NASA/JPL-Caltech/Space Science Institute/Handout via REUTERS ATTENTION EDITORS - THIS IMAGE WAS PROVIDED BY A THIRD PARTY
Cassini team members embrace after the spacecraft was deliberately plunged into Saturn, at NASA's Jet Propulsion Laboratory in Pasadena, California, U.S., September 15, 2017. NASA/Joel Kowsky/Handout via REUTERS ATTENTION EDITORS - THIS IMAGE WAS PROVIDED BY A THIRD PARTY. MANDATORY CREDIT.??
The spacecraft Cassini is pictured above Saturn's northern hemisphere prior to making one of its Grand Finale dives in this NASA handout illustration obtained by Reuters August 29, 2017. NASA/Handout via REUTERS ATTENTION EDITORS - THIS IMAGE WAS PROVIDED BY A THIRD PARTY.
An illustration released by NASA on October 16, 2017 shows a hot, dense, expanding cloud of debris from two neutron stars before they collided. The image was released to mark the first time scientists detected light tied to a gravitational-wave event, from two merging neutron stars in the galaxy NGC 4993, located about 130 million light-years from Earth in the constellation Hydra. NASA/Handout via REUTERS ATTENTION EDITORS - THIS IMAGE WAS PROVIDED BY A THIRD PARTY
The collision of two black holes - a tremendously powerful event detected for the first time ever by the Laser Interferometer Gravitational-Wave Observatory, or LIGO - is seen in this still image from a computer simulation released in Washington February 11, 2016. Scientists have for the first time detected gravitational waves, ripples in space and time hypothesized by Albert Einstein a century ago, in a landmark discovery announced on Thursday that opens a new window for studying the cosmos. REUTERS/The SXS (Simulating eXtreme Spacetimes)/Handout via Reuters FOR EDITORIAL USE ONLY. NOT FOR SALE FOR MARKETING OR ADVERTISING CAMPAIGNS. THIS IMAGE HAS BEEN SUPPLIED BY A THIRD PARTY. IT IS DISTRIBUTED, EXACTLY AS RECEIVED BY REUTERS, AS A SERVICE TO CLIENTS TPX IMAGES OF THE DAY
WASHINGTON, Oct. 16, 2017 -- Jo van den Brand, spokesperson of the Virgo collaboration, speaks at a news conference about the update on the search for gravitational waves in Washington D.C., the United States, on Oct. 16, 2017. Scientists announced Monday that they have for the first time detected the ripples in space and time known as gravitational waves as well as light from a spectacular collision of two neutron stars. The detection of the gravitational wave signal, called GW170817, was made at 8:41 a.m. EDT (1241 GMT) on August 17 by twin detectors of the Laser Interferometer Gravitational-Wave Observatory (LIGO), located in Livingston, Louisiana, and Hanford, Washington. (Xinhua/Yin Bogu via Getty Images)
HIDE CAPTION
SHOW CAPTION
of
SEE ALL
BACK TO SLIDE

In search of lizard people

The discovery of physical artifacts would certainly be the most dramatic evidence of a Silurian-style civilization on Earth, but Frank doubts we'll ever find anything of the sort.

"Our cities cover less than one percent of the surface," he says. Any comparable cities from an earlier civilization would be easy for modern-day paleontologists to miss. And no one should count on finding a Jurassic iPhone; it wouldn't last millions of years, Gorilla Glass or no.

Finding fossilized bones is a slightly better bet, but if another advanced species walked the Earth millions of years ago — if they walked — it would be easy to overlook their fossilized skeletons — if they had skeletons. Modern humans have been around for just 100,000 years, a thin sliver of time within the vast and spotty fossil record.

For these reasons, Frank and Gavin Schmidt, a climatologist at Goddard and the paper's co-author, focus on the possibility of finding chemical relics of an ancient terrestrial civilization.

Using human technology as their guide, Schmidt and Frank suggest zeroing in on plastics and other long-lived synthetic molecules as well as radioactive fallout (in case factions of ancient lizard people waged atomic warfare). In our case, technological development has been accompanied by widespread extinctions and rapid environmental changes, so those are red flags as well.

After reviewing several suspiciously abrupt geologic events of the past 380 million years, the researchers conclude that none of them clearly fit a technological profile. Frank calls for more research, such as studying how modern industrial chemicals persist in ocean sediments and then seeing if we can find traces of similar chemicals in the geologic record.

He argues that a deeper understanding of the human environmental footprint will also have practical consequences, helping us recognize better ways to achieve a long-term balance with the planet so we don't end up as tomorrow's forgotten species.

Then again, he's also a curious guy who's interested in exploring more far-out ideas for finding Silurian-style signatures: "You could try looking on the moon," he says.

See more on our world:

6 PHOTOS
Archaeologists find ancient settlements in the Amazon
See Gallery
Archaeologists find ancient settlements in the Amazon

Parts of the Amazon thought uninhabited were actually home to up to a million people.

(Photo: University of Exeter)

HIDE CAPTION
SHOW CAPTION
of
SEE ALL
BACK TO SLIDE

Lunar archaeology

The moon is a favored target of Penn State University astronomer Jason Wright, one of a handful of other researchers now applying serious scientific thinking to the possibility of pre-human technological civilizations.

"Habitable planets like Earth are pretty good at destroying unmaintained things on their surfaces," Wright says. So he's been looking at the exotic possibility that such a civilization might have been a spacefaring one. If so, artifacts of their technology, or technosignatures, might be found elsewhere in the solar system.

Wright suggests looking for such artifacts not just on the lunar surface, but also on asteroids or buried on Mars — places where such objects could theoretically survive for hundreds of millions or even billions of years.

SpaceX's recent launch of a Tesla Roadster into space offers an insight into how such a search might go. Several astronomers pointed their telescopes at the car and showed that, even if you had no idea what you were looking at, you'd still quickly pick it out as one weird-looking asteroid.

Finding technosignatures in space is an extreme long shot, but Wright argues that the effort is worthwhile. "There are lots of other reasons to find peculiar structures on Mars and the moon, and to look for weird asteroids," he says. Such studies might reveal new details about the history and evolution of the solar system, for instance, or about resources that might be useful to future spacefarers.

If the efforts turn up a big black obelisk somewhere, so much the better.

FOLLOW NBC NEWS MACH ON TWITTER, FACEBOOK, AND INSTAGRAM.

Read Full Story