Were There Alien Civilizations in Our Past?
When we think about alien civilizations, we tend to look into the vast space and far away planets. But we might be giving too little attention to another vast dimension: time. Could it be that over the last hundreds of millions of years, other civilizations existed on Earth? Technological species that rose and died out? And that they or their artefacts are buried beneath our feet?
Life on Earth has existed for four billion years, mostly as single-celled organisms. Some 540 million years ago, the Cambrian explosion announced the age of animals. A huge time window for our aliens. Where could we look for them, and what could we hope to find? Sadly, we only have one civilization to look at: ourselves.
Anatomically modern humans emerged about 300,000 years ago and probably lived in small groups of hunters, slowly spreading around the world. Progress was slow, and their lifestyle didn’t change much. About 10,000 years ago, the agricultural revolution changed our lifestyle forever, enabling massive population growth and technological progress. Rather than just being animals with culture and tools, we changed the face of the planet, cleared forests, and constructed cities at breathtaking speeds and scales. Until about 300 years ago, when we became an industrial species, and our numbers grew, along with our impact on the planet. We could say that our 300,000-year-old history has three phases. We were hunters for 97%, farmers for 2.9%, and industrialists for 0.1% of our history. And yet, we are super new on Earth.
If there were aliens before us, and they too went through these three phases, what would remain of them? How would we learn about them by looking at ourselves?
Hunter Aliens
We actually know that hunter aliens existed in the last few million years: our ancestors like Homo Erectus and cousins like Neanderthals, and probably many more that we haven’t found yet. They left fragments of their bodies, of weapons and tools, and even art. Considering for how long they existed and how little remains of them, it is easy to think there have been others. Intelligent creatures like us that could communicate and use tools and fire. Creatures that had cultures and art. But they could have vanished without anyone knowing, and without leaving any traces. Their artefacts 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? Well, per hundreds of years of Earth’s history, we only get a handful of good fossils. So, we might easily miss fossils of such people. But, even if we had any, we wouldn’t be able to identify them as hunters.
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 of stronger metals. Farming allowed them to specialize and develop tech from writing to navigation, architecture and government. Over thousands of years, city-states became empires and kingdoms, some existing for millennia before they fell. Many of the monuments and buildings they constructed are still around, and traces of them will exist for a few thousand years. And, because of the greater number of humans that existed during our farm and empire period, we can expect way more fossils and artefacts to be detectable, maybe for a few million years before they vanish. So, we can say with confidence that no alien civilization existed in the last few million years, because otherwise, we would have found something they left.
This still leaves hundreds of millions of years, back to the emergence of complex life, where we wouldn’t be able to tell. If there were alien pre-industrial empires and societies on the level of the Romans or the 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.
Industrial Aliens
What about industrial civilizations, like humans today? Imagine humans died out suddenly, by something like a pandemic, or cosmic rays, or something like that. What would remain? Our impact on the planet is much, much 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 the structures of our ancestors, skyscrapers and streets will crumble into nothing in a few millennia. But, because there were so many humans everywhere, for a few million years after our sudden extinction, there would be hints of our existence. The by-products of our industrial lifestyle might actually give us away for hundreds of millions of years.
We use massive amounts of artificial fertilizer which redirects the Earth’s flow of nitrogen, that is being deposited in the soil. Mining metals and rare Earth elements will leave long term scars and deplete natural resources. We saturate our oceans with plastic that finds its 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. And, of course, in our short industrial history, we have changed the amount of carbon dioxide in the environment. We may already have left a mark in the geological record.
So far, we’ve found no trace of an industrial alien civilization. No layers of weird chemicals or displaced elements. No radioactive layers indicate that advanced civilizations once waged nuclear war. We do see mass extinctions and shifts in the fossil record, but no evidence that they didn’t occur naturally. And 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 for that long. But if they become sustainable, their impact on the geological record may be tiny. If past industrial civilizations were sustainable before they died out, we have little to no chance of knowing about them.
In any case, over hundreds of millions of years, these signatures may become very subtle. They may get overlooked and interpreted as natural. Even if an industrialized alien society existed 200 million years and lasted 100,000 years, it still may be easy to miss it on the geological record.
In any case, all of this is speculation. In the end, we must 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 any 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 vast space. Maybe we’re alone in the universe and always were. Maybe we’ll find traces of others eventually, we don’t know. But there’s one important point: The continuation of our civilization is not guaranteed, and if we’re not careful, we may disappear forever. Let us hope that, in a few million years, there won’t be another civilization studying our layer in the fossil record.