If humans weren't the first intelligent species to evolve on this planet, would we know it?

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We humans have a natural desire to know if we’re alone in the cosmos. We search the stars for radio signals sent by other civilizations.

But what if we’re looking too far afield? What if the proof we seek is beneath our feet?

After all, complex life has existed for over 500 million years. How sure are we that humans are the first intelligent species to have evolved on this planet?

Trilobite cities and Cretaceous tool users

Could there have been a Cambrian civilization of intelligent trilobites, building undersea cities powered by hydrothermal vents? Or big-brained Cretaceous avian theropods capable of tool use and language? (Both of these capabilities arguably still exist in parrots and crows.)

It’s not as absurd as it sounds. On the geologic time scale, we’ve only inhabited this planet for an instant, and industrial civilization has only existed for a fraction of that instant. If humans went extinct tomorrow, the relics of our society wouldn’t last very long.

Within a few hundred years, rust, rot and the freeze-thaw cycle would cause buildings and bridges to collapse. Nature would move in and grow atop the rubble of our cities. Vines would sprout and weeds would take root in every crack, crumbling brick, asphalt and concrete. Anything organic would decay and become part of the soil, and durable materials would slowly weather away. Glass and ceramic would become sand; steel girders would become unusual ore deposits. Satellites would deorbit and burn up in the atmosphere. Shipwrecks on the ocean floor would slowly dissolve.

If humans went extinct tomorrow, the relics of our society wouldn’t last very long.

Within a few thousand years, it would be almost impossible to tell that human beings had ever inhabited this planet. The most enduring evidence of our presence might be Mt. Rushmore or the pyramids of Egypt, both of which could last for millions of years before they’re erased by erosion. Even deeply-buried repositories like Onkalo will eventually be consumed by the churn of plate tectonics.

But, in cosmic terms, those are insignificant intervals. The Earth has existed for 4.5 billion years, and the Sun has at least another 5 billion years to go before it becomes a red giant.

Intelligence across deep time

If another intelligent species were to evolve in the far future—say, 65 million years from now, as distant from us as we are from the dinosaurs—there’d be no tangible artifacts of our civilization remaining for them to find. By symmetry, if there had been an intelligent species that died out in the distant past, we might not know about it either.

This is called the Silurian hypothesis. It’s a nerdy reference to Doctor Who, which posits a race of intelligent reptiles that predate humans. Two scientists, Gavin Schmidt and Adam Frank, coined the term in a 2018 paper.

In the paper, they asked if an extinct industrial civilization might leave any traces in the geologic record that we’d be able to discover. Of course, one way to answer this is to ask what evidence we’re leaving for far-future scientists.

Barring the unlikely chance of some clearly artificial object being fossilized, Schmidt and Frank suggest that the best place to look would be in lake beds and ocean floors. Human activity will be embarrassingly detectable there for tens of millions of years to come.

Our burning of fossil fuels has released gigatons of carbon-12, the lighter isotope which organic life preferentially takes up over carbon-13. We’ve belched out enough of it to change the isotopic composition of the biosphere. All that carbon is also causing a sudden spike of global warming, and rising temperatures affect which isotopes of oxygen are taken up by marine life. Both of these isotopic shifts will leave a clear signal in deep core samples.

Other waste products of our civilization will also leave their mark on the geological record. Industrial agriculture and the resulting fertilizer runoff will lead to an excess of nitrogen in sea-bottom sediments. Mining and nuclear power programs may enrich those sediments with heavy metals and long-lived radioactive elements.

Last but not least, the plastic trash that clutters the ocean will eventually break down into microscopic particles, some of which sink to the sea floor and accumulate like snow. We don’t know how long they’ll persist, but some of them may well survive over the long term. The chemical traces of plastic, made of molecular bonds that don’t occur in nature, will be a clear indicator of intelligence (though, clearly, not enough intelligence to dispose of it properly).

The signature of the Anthropocene

This will be the geologic signature of the Anthropocene: a sudden, simultaneous shift in carbon, oxygen and nitrogen ratios, together with a spike in heavy metals, radioactive elements, and microplastics. The castoffs of our civilization will be detectable even when our era is nothing but a colorful band in the rock record. Even if far-future scientists never find a single human fossil, they’ll be able to tell that something unusual happened around our time.

We haven’t found anything like this in the fossil record, although our knowledge of Earth’s turbulent history is far from complete. Geologists have catalogued mass dieoffs, global catastrophes and wild climate swings, some of which happened for reasons that are still mysterious.

We may not even know about all human technological civilizations. A case in point is the Antikythera mechanism, found in a Greek shipwreck from around 200 BC. It’s a clockwork computer of astounding sophistication, capable of calculating the phase of the moon and the positions of the sun, the moon and the planets on any given date. It could also predict the dates of eclipses. Nothing like it would be built until the Renaissance, over a thousand years later. We have no idea who made it or how.

It’s humbling to realize that the most enduring legacy of humanity could be our fossilized pollution.

It’s humbling to realize that the most enduring legacy of humanity could be our fossilized pollution. Then again, perhaps that’s what we should expect. As the 2018 paper says:

…the longer a civilization lasts, the more sustainable its practices would need to have become in order to survive. The more sustainable a society (e.g. in energy generation, manufacturing or agriculture) the smaller the footprint on the rest of the planet. But the smaller the footprint, the less of a signal will be embedded in the geological record. Thus, the footprint of civilization might be self-limiting on a relatively short timescale.

As our technology becomes greener, our impact on the planet may become harder to detect. The most sustainable civilization may well be one that leaves no trace at all. If those hypothetical far-future scientists find our remnants in the fossil record, and then see them die away in the next layer… that could be a sign that we paid the price of our folly and went extinct. Or it could be a sign that we finally grew wise, that we departed for realms unknown and left the planet as a nature reserve. We might even find the trilobites waiting for us when we get there.

DAYLIGHT ATHEISM Adam Lee is an atheist author and speaker from New York City. His previously published books include "Daylight Atheism," "Meta: On God, the Big Questions, and the Just City," and most...