In search of life

Our Earth was formed around 4.5 billion years ago, and fossil evidence of life can be found in rocks that are over 3.5 billion years old. Since then, life has evolved and adapted, but it has never disappeared again. We find life everywhere on Earth. In every cubic centimetre of air, in the deserts, in the depths of the oceans and even in the Earth’s crust.

Plants make up by far the largest proportion of biomass, followed by bacteria. Animals and humans, meanwhile, add up to only a negligible fraction of total biomass. When it comes to single-celled organisms – bacteria and archaea – much of this largest share of the biomass is as yet unknown to us. In other words, even here on Earth, there is still plenty of life to discover.

Life has had a significant role in shaping our home planet. Not only the creatures themselves but also the oxygen and methane they produce have permanently changed the Earth’s crust, the oceans and the atmosphere. Without plants and algae, there would be no oxygen in the atmosphere. And without oxygen, there would be no animals or humans. All spheres of the Earth, and all living beings to be found there, are connected to each other by a gigantic chemical-physical network.

Today’s diversity stems from a single ancestor

As incredibly diverse as the life forms on Earth may seem – bacteria, archaea, plants, animals – they all have one fundamental thing in common: all, absolutely all, known life on Earth can be traced back to a single common origin. This doesn’t mean that life originated only once on the early Earth. But if it did happen several times, only one instance has endured.

What we don’t know, however, is exactly where, how and when the transition from inanimate matter to life occurred. From a biological point of view, “LUCA”, our last universal common ancestor, was already a very complex system that contained all the main characteristics of a modern cell, such as metabolism, cell walls and genetic information.

When trying to investigate life and its origins scientifically, we’re faced with an exciting challenge. Although we can assign attributes and characteristics to life as we know it and describe what “counts” as life, we don’t have an exact scientific definition of what life really is.

Where do we come from?

And so it is that we’re still unable to answer one of humankind’s most fundamental questions: Where do we come from? The aim is to understand which chemical building blocks were present on the early Earth and how these first building blocks reacted chemically with each other to produce ever more complex molecules and new functions. And how they ultimately provided the elementary components from which the first cell-like entities could form.

We want to understand these processes scientifically and be able to explain them. It’s important to determine precisely what the external conditions for these chemical reactions were as they prevailed on the early Earth at that time. This concerns, for example, the composition and density of the early atmosphere and the presence and chemical properties of water. Tackling these complex and multilayered issues calls for coordinated interdisciplinary approaches, which is why leading universities around the world have set up research centres for this purpose. With its Centre for Origin and Prevalence of Life, founded in 2022, ETH Zurich is playing a prominent role in this.

Are we alone in the universe?

Another fundamental question about life concerns life beyond the Earth: Are we alone in the universe? At the moment, Earth is the only place in the cosmos that we know is home to life. Empirical evidence of life on other planets or moons within our solar system or beyond has yet to be found. Such a discovery would be a sensation from a scientific point of view, and it would certainly have implications for other disciplines such as philosophy, sociology, religion and ethics. It would suggest that the emergence of life may be more universal rather than being linked to specific, unique initial conditions.

Enter the exoplanets. This is what we call planets that orbit not our sun but other stars. More than 5,500 such exoplanets have been discovered in the past 30 years, and more are being discovered almost every week. Most of them are located within just 3,000 light years of us, and many are even in the immediate vicinity of our sun; indeed, there are at least two planets circling Proxima Centauri, the sun’s nearest stellar neighbour.

Statistically speaking, every star ought to have planets, and many of these have masses, sizes and orbits similar to our Earth. We still know very little about these other worlds, and we can only guess at the conditions prevailing there. Finding proof of an “Earth 2.0” requires a new generation of ground- and space-based telescopes – for example, ETH Zurich’s LIFE mission, which will characterise Earth-like exoplanets in detail. In this case, detecting life will be a question of analysing the atmospheres of the exoplanets: because life as we know it leaves its traces everywhere, including in the atmosphere.

Future missions such as LIFE will be able to study the atmospheric composition of dozens of Earth-like exoplanets to detect these biological signatures. But there’s just one problem: we can only search for what we know and understand. Logic dictates that it will be difficult to detect traces of unknown life. This is another reason why it’s important for us to continue the search for as yet unknown or new forms of life here on Earth. The better we understand life on Earth, the greater our scope for discovery beyond the Earth.

Admiration grows with knowledge

Life on Earth, in all its fantastic diversity, unimaginable complexity and prodigious resilience, deserves our respectful admiration. And we should treat it accordingly.

For me, getting closer to the origin and nature of life and investigating its possible extent beyond the Earth is one of the greatest and most exciting challenges of modern science.  Perhaps there’s a habitable exoplanet in our immediate astronomical neighbourhood. Perhaps the emergence of life is a cosmic imperative. Perhaps life is even more fascinating than we can imagine. Let’s find out.