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When we think about alien civilizations we  tend to look into the vastness of space,  

to far away planets. But there is another  

incredibly vast dimension that we might  be giving too little thought to: time.

Could it be that over the last  hundreds of millions of years,  

other civilizations existed on earth?  Indigenous technological species that  

rose and died out? And that they or  their artifacts are buried beneath  

our feet? What does science have to say about  this and what are the implications for us?

Life on earth has existed for 4 billion  years, mostly as single celled organisms  

until some 540 million years ago the Cambrian  explosion heralded the age of animals. A huge  

time window for our indigenous aliens. Where would  we look for them and what could we hope to find?  

Sadly we have only one  civilization to look at: Ourselves.

Anatomically modern humans emerged around  300,000 years ago and probably lived in small  

groups of hunter gatherers, slowly spreading  around the world. Progress was slow and our  

lifestyle did not change much, although there  may have been local bursts of complexity.

Until about 10,000 years ago, when  the agricultural revolution changed  

our lifestyle forever, enabling massive  population growth and technological progress.  

Rather than just animals with culture and tools  that would have been invisible from space,  

we changed the face of the  planet, cleared forests and  

constructed cities and temples for our  gods at breathtaking speeds and scales.

Until about 300 years ago, we became  an industrial species and yet again  

our numbers grew exponentially  as did our impact on the planet.

We could say that our 300,000 year  long history has three phases:  

We were hunter gatherers for 97%, farmers for  2.9% and industrialists for 0.1% of our history.

And yet we are super new on earth.  

On geological timescales, even the sturdiest  things lose their durability. The oldest large  

scale surface is in the Negev Desert, a meager  1.8 million years old. Everything older has been  

crushed to dust or turned over and it is either  below the ground, or covered by ice or ocean.  

Our age, the Anthropocene will be a layer only  a few centimeters thick in a few million years.

If there were aliens before us and they  too went through these three phases,  

what would remain of them? What can we learn  about indegnous aliens by looking at ourselves?

Hunter & Gatherer Aliens

We actually know that in the last few million  years, there were hunter gatherers aliens.  

Our ancestors like Homo Erectus and cousins like  the Neanderthals or Denisovans and probably many  

more that we haven’t found yet or are lost to  time forever. They left fragments of their bodies,  

of weapons and tools and even art. Considering how  long they existed and how little remains of them,  

although they lived not that far from our present  it is easy to think there have been others.

Intelligent animals like us that could talk and  use tools and fire, that had culture and art. For  

the last two million years, most hominins lived as  hunter-gatherers – so if these aliens never moved  

up higher on the tech tree, hundreds of different  species and cultures could have existed without  

leaving any traces. Their artifacts lost to  biological and then geological processes.  

At worst they would leave absolutely nothing  to be found after thousands of years.

But what about fossils? We talked about how  unlikely the process of fossilization is in more  

detail in our dinosaur video, but in a nutshell,  per hundred thousand years of earth’s history  

we only get a handful of good fossils. So we might  easily just miss fossils of such people – but even  

if we had any, we would not necessarily be  able to identify them as hunter gatherers.

Agricultural and Empire Aliens

Looking at humans again, agricultural societies  left much more to be dug up and found,  

because they used more sophisticated tools made  from sturdier materials and had to feed millions,  

leaving many more artifacts. Farming  allowed them to specialize and develop tech  

from writing to navigation, architecture  and government. Over thousands of years city  

states became kingdoms and empires, some  existing for millennia before they fell.

Many of the buildings or monuments  they constructed are still around  

and traces of them will exist for a few thousand  more years. Some only as outlines in the ground,  

but still recognizable to future archaeologists.  The pyramids are piles of limestone so massive  

that they will probably be around for hundreds  of thousands of years. And because of the  

greater numbers of humans that lived during our  farm and empire period, we can expect way more  

fossils and artifacts to be detectable, maybe  for a few million years before they vanish.

So we can say with confidence that no such  indigenous alien civilization existed in the  

last few million years because otherwise we  would have found something that they left.

This still leaves a window of hundreds of millions  of years, back to the emergence of complex life,  

where we would not be able to tell. If there were  alien pre industrial societies and empires on the  

level of the ancient Romans or Chinese, they would  have likely disappeared without a trace. Their  

tools, even advanced metals, would have rotted  away. If they dug up canals, cleared forests and  

built cities, their traces would be very hard to  distinguish from nature. Maybe there once was an  

empire of cephalopods occupying impressive cities  in swamps and lakes, made from wood and stone.  

Their engineers flooding land to build wet cities,  their poets reciting poems in a language of  

colors. Maybe they never industrialized – maybe  their society was too stable or not inventive  

enough or they never got a chance. A single  event like an asteroid, an epidemic or an ice age  

could just have deleted their civilization,  ground up their temples as their soft bodies  

rotted away. Sadly we don’t have the tiniest  amount of evidence for any such civilization.

After a few million years at best, their  achievements would have dissolved into nothing.

Industrial Indigenous Aliens

What about industrial civilizations,  like humanity today? Imagine humans  

died out suddenly, through a pandemic or cosmic  rays or something like that. What would remain?

Our impact on the planet is orders of  magnitude greater than that of our ancestors.  

The fossil record will show a  great extinction of wildlife  

and an explosion of fossils from human associated  animals, like rats, cows, pigs and chickens.  

Like the structures of our ancestors, skyscrapers  and streets and harddrives will basically crumble  

into nothingness in a few millennia. But  because there were so many humans everywhere,  

for a few million years after our sudden end,  there would be clear hints of our existence.

The byproducts of our industrial lifestyle might  

actually give us away for some  hundreds of millions of years .

We use massive amounts of artificial fertilizer  which redirects earth’s flow of nitrogen that  

is being deposited in the soil. Mining metals  and rare Earth elements leaves long term scars  

and depletes natural resources. We saturate  our oceans with plastics that find their way  

to the ocean floor and may persist  for hundreds of millions of years.  

There are radioactive elements and their decay,  unnatural accumulations of elements that do not  

exist outside labs or weapons. And of course, in  our short industrial history we have changed the  

proportion of CO2 in the air by burning massive  amounts of fossil fuels, increasing the acidity  

of the oceans and so on. We may already  have left a mark in the geological record.

So far we have found no traces of  an industrial alien civilization.  

No layers of weird chemicals or displaced  elements. No radioactive layer to indicate  

that once great nations waged nuclear  war. We do see mass extinctions and  

massive shifts in the fossil records but no  evidence that they did not occur naturally.

And ironically we’ve hit upon an interesting  problem here: If industrial societies stress the  

ecosystem enough to cause their own extinction,  they won’t be around that long. But if they become  

sustainable their imprint on the geological record  may be tiny. If past industrial civilizations were  

sustainable before they died out, we may have  little to no chance of ever knowing about them.

In any case, over hundreds of millions  of years these signatures may become  

very subtle and get overlooked or interpreted as  natural. Even if an industrialized alien society  

existed 200 million years ago and lasted  for 100,000 years, 300 times longer than  

industrial humanity, it still might be  easy to miss it in the geological record.

In any case, all of this is speculation. In  the end we should not use our imagination to  

trick ourselves into thinking we know anything  about our blind spots. Just assuming a thing  

happened because we don’t have evidence  against it is a trap we should avoid.

So for now, if we look at the  vastness of time it seems as  

devoid of aliens as the vastness of space. Maybe  we are alone in this universe and always were.  

Maybe we’ll find traces of others eventually, we  don’t know. But there is one important takeaway:  

The continuation of our civilization is  not guaranteed and if we are not careful,  

we may disappear forever. Let’s  hope that in a few million years,  

there won’t be another civilization  studying our layer in the fossil record.

It’s frustrating to know there are so  many things in our past we can’t access.  

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