Alien life on Earth

March 15, 2022

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When we search for alien life, we tend to look into the vastness of space for planets that have conditions similar to our planet. But instead of looking in all three dimensions in space, there is another dimension one can look into, when searching for aliens, namely, time. Our planet, Earth, is estimated to be 4.5 billion years old and life on Earth has been around for approximately 4 billion years. Is there a possibility that in that vastness of time, other intelligent species and civilizations have existed and died out?

Intelligent life on Earth

So far, we only know one intelligent species to have ever roamed the Earth and that is us, Humans. Therefore, it makes sense to look at our own history, to understand what we have to look for when searching for alien life on Earth [1].

For the majority of time, since the beginning of life on Earth, there have only existed single-celled organisms. Due to the simplicity of these organisms, it is safe to say that until this moment there was no possibility for the existence of intelligent life. This all changed 540 million years ago during the Cambrian explosion. This was a period of 13 to 25 million years, during which practically all major animal phyla started to occur [2]. It is safe to say that until this moment there was no possibility for the existence of intelligent life, due to the simplicity of the living organisms. Since then a wide variety of multicellular animals have roamed the Earth continuously [3].

The anatomically modern human as we know it has been around for 300,000 years. For most of its time, it has lived in small groups as hunter-gatherers. During this time technological progress was very slow. This changed around 10,000 years ago during the agricultural revolution. During this revolution, the human lifestyle transitioned from traveling hunter-gatherers to a sedentary lifestyle involving agricultural activities. This provided a more stable source of food, enabling population growth and technological progress [4]. This was also the first time, that we as a species changed the surface of the Earth by cutting forests and constructing cities [5]. The next great technological revolution happened only 300 years ago, the industrial revolution. Just like with the last revolution the population grew exponentially, as did our impact on the planet [6].  

Alien life on Earth

We can divide our human history based on technological progress and its impact on Earth, in the following three categories:

– Hunter-gatherers from 300,000 till 10,000 years ago (97% of human history);

– Agriculturalists from 10,000 till 300 years ago (2.9% of human history);

– Industrialists since 300 years ago (0.1% of human history).

If we make the assumption that all alien life on Earth must go through these three phases, we now know what to look for in the past 540 million years, when looking for intelligent life. This may, however, be harder than it sounds. The oldest surface on Earth is the Negev Desert. This desert is 1.8 million years old, everything older is either buried in the ground, covered in a thick layer of ice, in or beneath the ocean, or crushed to dust [7]. This makes it hard to look for marks left by any life before that time. For example, the Anthropocene, in which we live now, will only be a couple of centimeters thick in a few million years [8].  

Hunter-gather aliens

There are multiple species that we know of that lived as hunter-gatherers in the last 7 million years. Some of them lived at the same time as when the modern human arose and most of them have existed longer than we humans currently do [9]. We know of their existence due to the remains they left in the form of weapons, bones, and art [10]. If any alien civilization that ever lived on Earth did not surpass the technological level of hunter-gatherers, then at worst they would leave no traces after just a few thousand years [11].

Agricultural aliens

Agricultural aliens would leave much more remains. The stable source of food causes populational growth which leads to kingdoms and empires, with buildings of sturdier materials, being able to last thousands of years [12]. The pyramids in Egypt, for example, are expected to remain for 100,000 years and the population growth will lead to much more remains like bones and tools, which will be detectible for a few million years. This, however, still means that any agricultural civilization that has lived more than ten million years ago, 98% of the time since the Cambrian explosion, would still be undetectable today [13].

Industrial aliens

When looking for the remains that industrial civilizations would leave behind, we can think of a scenario where humans would die out instantly and look at what we leave behind. It are the byproducts of our industrial lifestyle that will remain detectible the longest, one example is plastic. Some plastics will sink to the ocean floor where they may persist for hundreds of millions of years [14]. CO2, since the start of the industrial revolution we have dramatically increased the proportion of CO2 in the atmosphere. So much, that we might have already left a mark in the geological record, leaving a layer with an exceptional high amount of CO2 in Earth’s crust [15]. So far we have not found any layers of displaced elements or any other signs of industrial civilizations [16].

Sustainable aliens

There is a scientific consensus that we have to become a sustainable society to reduce the impact that we as humans have on our environment. We still have a long way to go, but in the end there are two possible outcomes, we succeed or we fail. If we fail, we will eventually stress our ecosystem enough to cause our own extinction. Hence, such civilizations will die out quickly, limiting our marks on the planet. If we succeed, then our society is sustainable, meaning that our impact on the Earth is also very limited and thus the marks we leave on the planet are very limited [17].

Time vs space

When looking for aliens in time versus in space we see a lot of similarities. In both we see that there are places were the conditions for intelligent life is met and the vastness of both time and space makes you think that we cannot be the only intelligent species out there. But, till now, neither in time nor in space, have we found intelligent alien life.

[1] Wright, J. T. Prior indigenous technological species. International Journal of Astrobiology, 2018.

[2] S. A. Benner, When did Life Likely Emerge on Earth in an RNA-First Process?, 2020

[3] Britannica, Cambrian Explosion, 2019

[4] Peter Bellwood and Marc Oxenham, The Expansions of Farming Societies and the Role of the Neolithic Demographic Transition, 2008

[5] Christopher E. Doughty, Preindustrial Human Impacts on Global and Regional Environment, 2013.

[6] Ellis et al., Anthropogenic transformation of the biomes, 1700 to 2000

[7] Matmon A. et al. Desert pavement–coated surfaces in extreme deserts present the longest-lived landforms on Earth, 2009.

[8] Schmidt, G., & Frank, A. The Silurian hypothesis: Would it be possible to detect an industrial civilization in the geological record?, 2019.

[9] Smithsonian Institution, Human Evolution Interactive Timeline, 2021

[10] Krause et al.,The complete mitochondrial DNA genome of an unknown hominin from southern Siberia, 2010

[11] Durvasula and Sankararaman. Recovering signals of ghost archaic introgression in African populations. 2020.

[12] Chapter 6. Agrarian Societies, 2021

[13] Wright, J. T. Prior indigenous technological species. International Journal of Astrobiology, 2018.

[14] Anthony L. Andrady, Persistence of Plastic Litter in the Oceans, 2015.

[15] Schmidt, G., & Frank, A. The Silurian hypothesis: Would it be possible to detect an industrial civilization in the geological record?, 2019.

[16] Haqq-Misra J. & Kopparapu R. K. On the likelihood of non-terrestrial artifacts in the Solar System, 2012.

[17] Haqq-Misra and Baum, The Sustainability Solution To The Fermi Paradox, 2009.

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