How do we even know that we're the first advanced civilization to call the planet Earth home?

It’s no secret that humans are the Earth’s most dominant species. And, without proof of life anywhere else in the universe, we can probably consider ourselves the most dominant species in space too.

But what if there were others just like us millions of years ago? How do we even know that we’re the first advanced civilization to call the planet Earth home? And, if there have been others, what kind of traces have they left for us to find?

We know that complex life has existed on Earth for 400 million years. That’s a heck of a long time, so it’s definitely possible that advanced civilizations have come and gone in all that time before us. But how could we look that far back? Ancient ruins and artefacts only take us back a few thousand years, geologic records only about 2 million, and while fossils go back further, the fraction of life that actually gets fossilized is so small that it could miss an entire civilization.

So, if we can’t rely on direct evidence, then where else could we find traces of a potentially extinct civilization?
And what can we learn to make sure we avoid the same fate?

Here are some numbers to make your brain hurt for a second. The universe is about 13.8 billion years old. The Earth is 4.5 billion years old. And humans have only developed our industrial civilization in the last 300 years.

With all that in mind, it seems like there was ample opportunity for another advanced civilization to emerge, flourish, and die, all before we ever showed up.

But finding any proof of a past civilized species seems like it could be harder than even starting the civilization itself.

Physical artefacts would be the most concrete evidence of past intelligent life, but it’s not very likely that we will ever find any. Over time, even our tallest buildings will crumble, and our strongest materials will disintegrate. Our cities cover less than one percent of the Earth’s surface, so any comparable cities from past civilizations would be easy for palaeontologists to miss.

Even if we knew where to look, it’s unlikely that these artefacts would last any longer than a few million years. Sure, there’s always fossilization, but it only provides a limited record of the past. Due to variables like when they lived and where, an industrial civilization that lasted 100,000 years – that’s 333 times longer than we’ve made it so far – might leave no fossil trace at all.

So maybe we’re better off looking for more indirect evidence. The best way to find that would be to look at what traces our own civilization would leave behind if it collapsed in its current state.

One place we’re leaving a clear trail is in the sediment at the bottom of our bodies of water. One look at this stuff, and you would immediately see the wacky chemical balances that would imply some sort of outside influence. For instance, the nitrogen fertilizers that we use to grow food are running off into our bodies of water, and producing low oxygen “dead zones” that would be visible in sedimentary layers.

On top of that, we’ll be leaving long-lasting synthetic molecules from radioactive fallout, steroids, and all the plastic we dump in the ocean. So yeah, future species are going to have a great impression of us.

In case we’re not embarrassed enough, our technological advances are rapidly changing the environment, which has brought on widespread extinctions that would be visible in fossil records.

Ironically, perhaps the most promising marker of an advanced civilization could be one of the very things that brought on its downfall. When we burn fossil fuels, we are releasing carbon back into the air. Fossil fuels ultimately derive from decayed plants and animals, which contain a variety of carbon that has a different atomic mass than most carbon in our atmosphere. When this kind is released it changes the molecular make up of the atmosphere, and leaves a clear signal to future scientists.

In a 2018 study, scientists Adam Frank and Gavin Schmidt hypothesized that civilizations could even have fossil fuel driven life cycles. When fossil fuel use leads to climate change, the oxygen levels decrease in the ocean and help create a breeding ground for new fuels like oil and coal. In this way, a civilization and its demise might sow the seed for new civilizations in the future.

In our search for intelligent neighbors from the past, we may never actually find any evidence, but we can learn a lot about ourselves. Analyzing our own long-term ecological footprint can have practical benefits. It helps us recognize where we can do better, to achieve a balance with our planet so that we don’t become the forgotten civilization of tomorrow.

While it doesn’t seem likely that intelligent life was here before us, that doesn’t mean there wasn’t any. For all we know, they were so advanced they were able to clean up their mess as they went. The only verifiable way to check if someone was here before us may be to rev up a time machine to go see for yourself.

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