What is life? How does the simple arrangement of atoms transform into something as complex as a living being?

How do we differentiate the living from the non-living? Extremely complex animals, like us, human beings, are made of atoms. How complex must this arrangement of atoms be to transition from chemistry to biology?

For centuries, these questions have been debated by philosophers and naturalists, but with few mathematical and physical tools. Only in the 20th and 21st centuries did physicists begin to intensely analyze these questions and add others, equally interesting, such as: why does life exist? Can life be completely derived from the laws of physics? And, furthermore, is life an inevitable consequence of the laws of the Universe?

Increasingly, quantum biology and the physics of complex systems are bringing new discoveries that expand human understanding of these issues. In October 2023, a study was published in the journal Nature about the development of a new theory called Assembly Theory, which aims to explain and quantify Darwinian evolution based on the fundamental laws of physics.

This theory proposes a way to map the necessary steps to form a specific molecule or object, initially mapping the steps to form simple structures and later identifying how these simpler structures can arrange themselves to form more complex structures, until, after some “levels of complexity,” these arrangements may present characteristics that are generally only observed in living beings.

The definition of life is a thorny topic that has been discussed for centuries by philosophers and scientists. There is no single definition accepted by all, but there are some common characteristics that are generally associated with life. Even NASA scientists, the U.S. space agency, study the topic. After all, how would we recognize something alive that is not at all similar to the life we are accustomed to here on Earth? According to NASA, life is a self-sustaining chemical system subjected to Darwinian evolution.

But what is Darwinian evolution? Three points are fundamental for us to understand this concept:

1 – Random processes (such as mutations in genes) within a population of individuals of the same species generate hereditary differences in physical and behavioral characteristics.

2 – Selective pressure causes some hereditary characteristics to become more common in a population, as they may confer advantages and increase reproductive chances.

3 – Heredity ensures that certain characteristics are passed on to the next generations through some mechanism of chemical memory (such as the DNA molecule).

Although the Theory of Evolution is extremely important for understanding life on Earth, and for explaining how living beings evolved from unicellular individuals to conscious humans, it does not explain how atoms arrange themselves in such a way as to become living cells.

But physics can explain much of what we know in the Universe if we base ourselves only on fundamental concepts, such as the interactions between atoms and molecules mediated by forces and energy exchanges. However, none of the intellectual arsenal of physics can prove the phenomenon of life as a consequence of fundamental laws. 

Therefore, very complex objects found in large quantities indicate the phenomenon of life. Thus, according to Assembly Theory, life is the ability to store chemical memory (as in the DNA molecule), which ensures the ability to repeatedly produce complex objects that are statistically impossible to be found in a large number of copies simply by random processes.

What makes Assembly Theory really interesting is that it is based on basic principles, such as statistical analysis, and on measurable quantities experimentally, such as the number of identical copies of an object. For example, the Assembly Index can be measured through mass spectrometry.

Among the interesting predictions of the theory, we can highlight the measurement that an Assembly Index greater than 15 means that the studied object is alive or derived from something alive. For example, a cell, an animal, or a simple plant has this index greater than 15. A computer is obviously not alive, but it has a high Assembly Index and number of copies, as it is a product of life — after all, without life, there would be no computers.

Thus, Assembly Theory manages to unify living and non-living objects into a single theory. Life emerges in the theory as a property of the nature of assembling molecules.

Marcelo Sousa
IDOR/Bright/UFC

“I have always been fascinated by the explanations that physics provides about living organisms. If we stop to think, from the perspective of the laws of physics, life is very counterintuitive! The question that caught my attention the most was: “why does life exist?”. This article was the first I studied in which physics is used not only to describe living organisms but also to explain the fact that life simply exists.”

Matheus Araña
École Polytechnique de Paris

“Assembly Theory aims to quantify the complexity of a given system. Being well defined mathematically, “complexity” can be calculated and measured through the Assembly Index. In my opinion, this is a significant step for the physics of complex systems, which studies everything from biological systems to the formation of civilizations. However, before it can be considered a breakthrough, we need to see the theory in action. I eagerly await fascinating applications.”

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This scientific dissemination text was produced by Marcelo Sousa and Matheus Araña exclusively for Pioneer Science.

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Link to the article: https://www.nature.com/articles/s41586-023-06600-9

Link to the report: https://www.santafe.edu/news-center/news/new-assembly-theory-unifies-physics-and-biology-explain-evolution-and-complexity