Our fragile existence on Earth

“Humans are a second on Earth’s clock.” I recently began taking a course in Astrobiology and this sentence, said by my professor in our first lecture, has stuck with me since. Out of pure curiosity and fascination with space since childhood, I chose to study an extracurricular course in astrobiology alongside my normal archaeology bachelor’s. I have been enjoying this course a lot, and I think it offers an insightful context for human evolution and archaeology, opening a wormhole of existential curiosity for Earth’s past and future.

A short history of what we know about Space

If the universe is an ocean, we have only searched a teacup full of water, is a metaphor mentioned by Jill Tarter, a radio astronomer best known for her work on the Search for Extraterrestrial Intelligence (SETI). We know very little about space, yet with developing technology and great astronomers, astrobiologists and other interdisciplinary scientists, we get to take little sips of said space teacup.

The Giant Impact hypothesis suggests that during the Hadean Eon, Early Earth collided with a Mars-sized co-orbital protoplanet. Some of the ejected debris from the impact event later reaccreted to form the Moon.

At first Earth was covered by magma oceans: hundreds of kilometres deep of molten rock layers were melted by the energy released during this collision. The atmosphere was drastically different (composed 90% of carbon dioxide), and volcanism was an extremely dominant process, occurring intensely for a long time. This allowed for rapid chemical exchange and released volcanic gases which formed the early atmosphere – these conditions might have been key for developing life.

During this time, compounds that did not form into planets assembled instead into small bodies (e.g., asteroids, meteorites, and comets) and bombarded our planet, probably bringing water and organic compounds that might have brought water and organic compounds to Earth.

The first water oceans formed through two sources of water:

1. Water on Earth (Internal and External): as the Earth’s surface cooled below 100°C, the atmospheric water vapour condensed to form clouds. Prolonged periods of rainfall that lasted for centuries and thousands of years, filling the planet’s great hollow areas and forming the first oceans.
2. Delivery from Space: Water-rich asteroids and some comets, as well as the giant impactor forming the Moon, contributed significantly to Earth’s water supply.

The first continents formed 3.5-3 billion years ago:

• Vaalbara is considered one of the earliest hypothesised landmasses, around 3.1 billion years ago. Includes parts from the Kaapvaal Craton in South Africa and the Pilbara Craton in Western Australia.
• Ur is thought to have formed a supercontinent around 3 billion years ago, including parts of India, Australia, and Madagascar.

In these extreme conditions, the emergence of continental crust, the presence of fresh water and the start of the hydrological cycle likely facilitated environmental niches for life to develop around 4 billion years ago (age still highly debated).

Understanding how Life evolved on Earth

In the beginning, life was simple and unprotected, but it had to adapt quickly to an unstable and extreme environment, evolving:

1. A stable and semi-permeable membrane which encapsulates cell components
2. Genetic material which can be passed on in cell formation and which controls cellular behaviour and function
3. Energy generation via metabolic pathways which enables growth, self-maintenance, and reproduction

Recent studies and samples show that the building blocks for life (such as carbon) are common in the universe. An interesting example is the asteroid Bennu, which contains organic molecules, including amino acid precursors, that are important for the formation of proteins and life. These discoveries highlight that the essential ingredients for life are not unique to Earth but are widespread throughout space. Now, how might we detect signs of life elsewhere? By looking for biosignatures.

Biosignatures help us identify environments beyond Earth where life could potentially emerge. By studying how life began on our own planet, we can better understand what signs to look for elsewhere. In a way, we can reverse the process and imagine how Earth might have appeared to an outside observer in different points in time:

• Modern Earth: We have detailed data about our planet’s current atmospheric and environmental conditions. Using this as a reference, we can search for exoplanets with similar characteristics — though this represents a fairly narrow range of possibilities. To broaden the search, we can also look to Earth’s past and potential futures.
• Past Earth: Geological and environmental records allow us to reconstruct some of Earth’s earlier states.
• Future Earth (?): We can model different scenarios of what Earth might look like in the future — both sustainable and less optimistic outcomes. Each scenario would produce distinct biosignatures

With these models, we’ve broadened the scope of our search and can more effectively exclude environments where life is unlikely to emerge. If life does exist elsewhere in the universe, it’s reasonable to assume it may not be as widespread or complex as it is on modern Earth. Instead, it might exist in small quantities — perhaps as simple organic molecules or primitive life forms similar to LUCA. This is why many current astrobiological instruments are designed with high precision and resolution: to detect subtle chemical or biological traces and focus our efforts on the most promising hotspots for life.

A fragile existence

Our current scientific knowledge points to the idea that life is rare and Earth is unique. There is a specific mixture of conditions in order for life to exist, and even if it were to exist, life is very fragile and could be wiped out and forgotten in a few seconds compared to the universe’s timescale.

The survival of humans is the result of both sheer luck and rapid adaptation. Even here on Earth, life almost didn’t make it (several times!). At one point, the population of Homo sapiens may have been reduced to around 10,000 individuals.

When we look at distant exoplanets, we are also looking back in time. Any potential life forms we might detect could already have vanished — or perhaps they have not yet emerged. We may simply be observing these planets at the wrong moment in their histories, either before or long after any civilisations arose. If (intelligent) life is so unique, we should recognise Earth’s value and strive to protect our environment and us.

Open-ended questions

Some food for thought:

• How do we define life?
  • There is no consensus. Yet, NASA has a working definition: life is a self-sustaining chemical system capable of Darwinian evolution that uses energy and matter to maintain itself (metabolism), grows and reproduces and can evolve (change over generations through genetic variation and natural selection).
  • But this definition is not perfect, and this shows in borderline cases. Do we consider viruses, prions, artificial life or artificial intelligence life?
• Why does life form around water? or Why do we think that water is a key condition for life to form?
  • Astrobiologist David Grinspoon emphasises that life requires a liquid medium to enable the complex molecular interactions necessary for its emergence and persistence. On Earth, that medium is water — our universal solvent. While other liquids such as ammonia or methane could, in theory, support similar chemistry, water’s unique physical and chemical properties make it especially well-suited for sustaining life as we know it.
    • Water acts as a solvent, allowing organic molecules to move freely, interact, and form complex structures.
    • Solid state: molecules are locked in place and cannot interact dynamically.
    • Gas: would move too chaotically to form stable bonds.
  • Water is also abundant throughout the universe and has chemical characteristics that, so far, no other substance has been able to replicate in laboratory settings for similar complexity. However, we cannot rule out the possibility that elsewhere in space, other solvents — like ammonia or methane — might support alternative biochemistries.
• How long will humanity last? Will we leave a mark in the universe?

References

Featured image: NASA Cat’s Paw
Notes from my Astrobiology course
David Grinspoon answering questions for WIRED
David Grinspoon interview for NASA Astrobiology
Giant Impact Hypothesis