How to understand the physics behind life on earth?

Updated: Dec 19, 2020

In 1944, the physicist famous for killing cats in his thought experiments Erwin Schrodinger thought about the question 'what is life'? There was no such thing called biophysics or astrobiology back in his days. He naturally tried answering a few questions in biology from a physicist's point of view.

Schrodinger had views mainly about two aspects of life, one is genetics, and the other is thermodynamics. In his analysis of genetics, he estimated the number of atoms in a gene, then suggested that it encodes the genetic information in something resembling an aperiodic crystal-like structure.

Schrödinger also proposed that organisms can create ordered systems within themselves, by producing even higher disorder in the environment.

We will talk about order and disorder quite a lot in this post, let's save it for later and proceed.

What is life?

The answer to this fundamental question might vary depending on the person you ask. If you ask a philosopher, a biologist and a physicist, the same thing their answer might differ. The best way to go about this is to understand life through its properties. What are some properties of life that separates them from the inanimate world?

  • Self-replication:

Self-replication is one of the remarkable properties of life that makes it stand out. A table doesn't find a matching table, have sex and produce more tables, but a rat living under that table does! All life on earth is a consequence of replicating genes.

  • Living organisms are chemical systems:

It takes living organisms a plethora of biochemicals to carry out life functions whereas, a robot requires machinery, components and software to function.

  • Evolution:

Before including evolution into the equation, let's consider the fact that not all self-replicating systems are alive. For instance, take crystals, they are also self-replicating chemical systems, are they alive? No!

Evolution is a fundamental property of life that has adaptability etched into it. If the offsprings produced through genetic replication is not perfect, it offers the organism a choice through genetic mutations; those mutations might offer a better chance for survival or might not.

Therefore, we can conclude that life is a self-replicating chemical system capable of evolution.

Before getting into the physics of life and all that, let's understand the requirements for life.

What are the requirements for life?

  1. Life requires a rich source of elements and compounds that can support its energy, protective and reproductive requirements.

  2. Life needs a solvent in which the elements and compounds can mix and combine to form more complex organic biomolecules.

  3. A solid surface or a liquid substrate to live in and carry out functions of life because dense organic molecules cannot stay afloat forever.

  4. And importantly, life needs the energy to sustain itself!

Did we originate from stardust?

As we read in the article about the Big Bang, none of the heavier elements needed for life formed in the early universe. 3,78,000 years after the big bang, protogalaxies formed, these were the earliest seeds for all the galaxies, star clusters, planets and black holes we witness today.

The Big bang left us with a universe that had an abundance of the most basic gases, hydrogen and helium. These gases acted collided and clumped together, forming the first protostars. Because of the plethora of hydrogen and helium available, the first generation stars were exceedingly massive.

We learnt in the life cycle of stars that, the more massive stars have a shorter life.

Stars that had eight times the mass of our sun, the temperature rises enough to form heavier elements than just hydrogen and helium, Materials such as carbon, oxygen, magnesium and silica form in their cores. This fusion process continues till the core forms iron; hitting iron means the star has hit rock-bottom and further fusion cannot occur.

The image shows a supernova explosion
Supernova explosion

The energy produced at the core halts abruptly and the core shrinks, this sends a shock wave throughout the still fusing outer layers of the star, causing it to explode in a grand explosion called a supernova.

These events can outshine entire galaxies. It produces elements heavier than iron in stars and disperses through supernova explosions.

Supernovas can enrich stars and planetary systems with material that can lead to formation and sustenance of life.

Through the above process, our planet got its rich supply of various elements, compounds and metals.

How planets formed?

Now we have all the material needed for life, but where do they go? Planets are the splendid abodes for life. Let's examine how planets form around stars.

Protostars develop a flat disk-like cloud of dust and gas that rotate around them, called an accretion disk. Material floating in the disk eventually bombard and clump together forming millions of protoplanets. These protoplanets fuse to form planets.

The inner region of the accretion disk that is closer to the star receives more heat from it. Hence non-volatile elements that don't evaporate away easily are abundant here, and they formed the rocky planets such as Mercury, Venus, Earth and Mars. These planets are rich in iron, Aluminum and silica, making it more suitable for life.

The outer region of the accretion disk receives less heat from the star and gets an abundance of volatile material that stays in gaseous or liquid states. Hence formed the gas giants like Jupiter, Saturn, Uranus and Neptune.

How did life on earth originate?

Even though we don't yet have the exact mechanism of how life originated on earth, we sure have some plausible theories.

Scientists believe life formed near hydrothermal vents deep inside the ocean. The ancient earth without life on it wasn't pretty to picture; it was teeming with UV radiation and volcanic activity.

Hydrothermal vents are these fissures on earth's surface where molten lava meets seawater inside the earth's ocean. Seawater seeps into these vents and immediately pushed back outside because of the tremendous heat.

But the seawater that comes out is just not plain old seawater anymore; it's now a wealthy slurry of minerals and chemical compounds. All this make hydrothermal vents a strong candidate for facilitating the origin of life.

There is an alternate approach to this idea; we know it as panspermia. It suggests that organic matter formed outside earth and was bought here through asteroids or meteors.

We have checked most of the 'requirements for life' list; there's just one more left, that is energy. Energy part is simple to understand. But before getting to that part, we need to understand the entropy behind it.

What entropy has to do with life?

We learnt about entropy when we learnt about the arrow of time and how the big bang is the ultimate source of low entropy in our universe.

Entropy is the measure of disorder in a system or entropy is the universe's way of pushing towards its much-coveted state of equilibrium.

We also learnt that according to the second law of thermodynamics, the entropy in the universe always increases. But what about life?

Life on earth seems ordered. Since the formation of the first living cell to the evolution of human beings, the level of orderliness has relentlessly increased. Doesn't this violate the second law of thermodynamics? Let's take it slow from here.

Even though life has an extreme amount of order within it, it contributes to the universe's increasing entropy. To understand this, let's go back to the classic example of the broken egg.

Imagine you have an unbroken egg, the egg has low entropy, and high energy concentrated within it. Now you accidentally leave the egg somewhere near your puppy. Is the egg going to stay whole when you come back? No! You get back to a splattered, half-eaten egg on your carpet. The energy concentrated in the egg before has spread out throughout your carpet and has high entropy. There is no way you could put this energy back together, and it is a one-way process.

From this, we understand that energy never randomly concentrates in one place, unless there is a living organism behind it. Living organisms do this all the time; they have somehow figured out how to accumulate energy and even store it.

How life exchanges entropy for energy?

Living organisms act to reduce their internal entropy by borrowing energy from the sun. According to physics, living organisms are this high functioning entropy maximizing machines. Life is a process feeding on low entropy; for life on earth, the sun acts as a source of low entropy. This idea first came from Ludwig Boltzmann, who observed that life is a struggle for entropy; more accurately a struggle for lowering entropy.

The most arbitrary form for energy is thermal radiation. Plants absorb the concentrated energy from the sun (source of low entropy) and convert it into high entropy and heat.

Animals consume high-energy-density packets of matter called food and convert it to lower energy density waste and that same heat.

Therefore, all living forms, including the microbes, gather energy and give back heat and entropy; keeping the second law of thermodynamics alive.

The ultimate source for the low entropy for the universe is the big bang itself. The universe must increase its entropy to reach equilibrium, all systems, including our universe, prefer to attain the boring state equilibrium where nothing happens.

While the universe takes its grand steps of distributing energy and reaching equilibrium, these systems of extreme order like stars, galaxies, planets, moons, life naturally arises; we are a rare and precious part of it.


1. Astrobiology: A Very Short Introduction Book by David Catling

2. From Dying Stars to the Birth of Life: The New Science of Astrobiology and the search for life in the universe Book by Jerry L. Cranford

3. First Life: Discovering the Connections Between Stars, Cells, and How Life Began Book by David W. Deamer

4. Astrobiology: A Brief Introduction Book by Kevin W. Plaxco and Michael Gross

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