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Why did life arise only once?

On a planet full of suitable environments, a single family of living things. Science's explanations and the idea of the “closing window.”

by L.R. · November 11, 2026 · 6 min read

In brief

All known living things descend from a single common ancestor, LUCA.
We do not know whether life arose only once, or several times with a single surviving line.
As early as 1871, Darwin noted that today a new beginning would be “devoured” by existing life.
The search for a “shadow biosphere,” meaning life of a different origin still present on Earth, has so far found nothing.

An enigma that goes largely unnoticed

People usually ask how life began. They ask how many times far less often. Yet the second question is just as strange.

The Earth of 4 billion years ago offered a great variety of environments: oceans, hot springs on the seabed, volcanic pools that dried out and filled up again, coastlines, clays. If life is a chemical phenomenon that gets going when the conditions are right, we would expect it to have started in several places, in slightly different ways. We would then have families of living things with different genetic codes, different molecules, perhaps with the opposite “handedness.”

Instead, wherever we look, we find a single family. Same code, same molecules, same choice of “left-handed” amino acids. All relatives.

Three possible explanations

1. It really did arise only once

Perhaps the origin of life is such an improbable event that it succeeded once, and that was all. This is the position of those who see abiogenesis as an extreme stroke of luck. The problem is that life appears on Earth very early, almost as soon as the planet allows it: an extremely rare event that happens so quickly is a little suspicious.

2. It arose several times, but only one line won

This is the hypothesis most widely held among biologists. There may have been several beginnings, but one line proved more efficient, took over the resources and drove the others to extinction. LUCA would then be the ancestor of the winners, not the only attempt.

The fact that microorganisms exchange genes directly may have helped: the best solutions, such as a particular genetic code, would have spread until they became universal.

3. Today there is no room for a new beginning

This idea is surprisingly old. In the same 1871 letter in which he imagined life beginning in a “warm little pond,” Darwin added that today such organic matter would be instantly devoured or absorbed, which would not have been the case before living things existed.

In other words: the life that already exists eats the molecules a new beginning would need. Every pond, every spring is colonized by microorganisms. A second beginning would not have time to build up its ingredients.

What if there were a “shadow” life?

In the early 2000s some researchers, among them Carol Cleland and Paul Davies, put forward a provocative idea: what if microorganisms of independent origin still existed on Earth, and we had never noticed them because our instruments look for life “as we know it”? They called it the shadow biosphere.

It is a testable hypothesis, and that is its merit. So far, however, every organism analyzed has turned out to belong to the one known family. In 2010 there was a stir over the announcement of a bacterium said to use arsenic in place of phosphorus in its DNA: later checks refuted the claim. It, too, was a relative of ours.

Where science stops

Explanations 2 and 3 are reasonable and probably partly true. But they leave one point hanging. If life arises easily, why do all the oldest traces we have point to a single family? And if it arises with difficulty, how did it appear so early?

The honest answer is that we do not know. We have no second example of life, on Earth or elsewhere, to compare ours with. Every line of reasoning rests on a single case.

From here on we are no longer talking about what science has established, but about a hypothesis: the one proposed in the book Lux Lucis. It should be read as such.

The closing window

Chapter 7 of Lux Lucis is in fact titled “The closing window,” and it takes up Darwin's insight and gives it a central role.

The book pictures life as something that “switches on” when three conditions meet: rich chemical ground, a stable source of light energy and molecular structures complex enough to act as an “antenna.” In this view life is not improbable: when the conditions are there, the switching on is almost inevitable, like ice forming below freezing.

But for that very reason, the first system to switch on changes the rules. It copies itself, spreads, and consumes the molecules built up over millions of years. The window in which another switching on could happen from scratch closes, and never opens again.

So the enigma is turned on its head: the single origin does not show that life is rare, but that, once it has started, it leaves no room for a second beginning. It is also one of the reasons the book holds that life may be common elsewhere in the universe, as we explain in the article on the Fermi paradox.

What remains open

The book's reading is compatible with what we observe, but it is not the only possible one, and it is not proven. A decisive test would come from the discovery of life elsewhere, for example on Mars, on the icy moons of Jupiter and Saturn or in the atmospheres of distant planets. If we found life of independent origin, we would finally know that ours is not a unique case.

Until then, the question remains one of the most fascinating in biology: why, on a planet full of possibilities, did life begin only once?

Go deeper in the book

This topic is developed in chapter 7 of Lux Lucis – Children of the Light.

Read the first chapter for free · English edition in preparation; available now in Italian and Spanish