[MUSIC PLAYING] Our universe is prone to increasing disorder and chaos.

So how did it generate the extreme complexity we see in life?

Actually, the laws of physics themselves may demand it.

[MUSIC PLAYING] How did life begin?

We can seek the answer in the chemistry of the early Earth or in the biology of the first cell.

In fact, our friends at PBS "Eons" and "It's OK to be Smart" will do just that in companion videos to this one.

But we all know that chemistry and biology are just applied physics.

So can we approach the question of the origin and the very nature of life from the point of view of physics?

We're sure going to try.

To understand life, we need to understand entropy.

The universe tends toward disorder, decay, and equilibrium.

A hot cup of coffee will tend towards the same temperature as the room, and the hot, dense of our universe must expand.

Stars always burn out.

Black holes evaporate.

The particles that make up any system all have some degree of random motion.

That random motion tends to drive the system towards the most common arrangement of particles.

Such a random disordered, unspecial arrangement is a high entropy state.

Interesting arrangements, like thermal energy being concentrated in your cup of coffee or all the matter in the observable universe being crunched into an infinitely dense point are low entropy.

They're highly specific configurations that almost never happen by chance.

So entropy is sort of a measure of the boringness of a system, the commonness of the arrangement of particles.

The second law of thermodynamics tells us that a closed system will only increase in entropy.

The universe will only get more boring.

But there's one type of system that seems to resist the second law of thermodynamics and maintain low entropy.

That system is life.

Life has a very low internal entropy because its structure is extremely specific and non-random.

The molecular machinery of even a single cell defies belief.

You know what?

I'd prefer to let a real biologist convince you.

Hey, Joe, tell them how it is.

Cells are complex.

OK. That's a massive understatement.

Inside just a single one of your cells, you have six billion base pairs of DNA, storing hundreds of megabytes of data.

Intricate molecular machinery made of RNA and protein unpacks, transcribes, cuts and splices, and processes that data to build and control an entire factory of protein molecular machines, which in turn, power the entire biological process that is you.

Easy, right?

Not only is life stunningly complex, but that complexity increases over extremely long time scales, in fact, over eons.

Right, Blake?

When we look at the fossil record, we see the evidence of evolution carved in stone.

When we trace the development of fossils over the nearly four billion years of life on Earth, we see clear as day the study trend toward greater complexity, from the first single-celled organisms to simple ocean invertebrates to an explosion of complex animal life and finally, to us.

Naively, this presentation and increase in order appears to contradict the second law of thermodynamics-- entropy appears to either stay constant or decrease.

The Earth's biosphere, at least, becomes less boring over time.

But let me be clear, there is no violation of the second law.

The second law tells us that closed systems must increase in entropy.

So a system's unable to exchange energy with the outside environment.

But living organisms and indeed the Earth's biosphere are not closed.

Both receive energy from outside.

Ultimately, that source of energy is the sun.

Its light warms the atmosphere in the oceans and it powers photosynthesis at the bottom of the food chain, driving a complex chain of nutrient synthesis that ends with whatever you had for dinner last night.

On the other hand, the system of the Earth plus the sun is increasing in entropy.

Life acts to reduce its own internal entropy by increasing the entropy of its surroundings.

This was first pointed out by Ludwig Boltzmann, who described life as a struggle for entropy, well, more accurately, against entropy or for negative entropy.

Erwin Schrodinger, in his 1944 book, "What is Life," describes life as a process feeding on negative entropy.

Life absorbs order and it ejects disorder into its surroundings.

The type of order that life feeds on can be thought of as free energy.

By free energy, I mean the special out-of-equilibrium energy sources like a cup of coffee or the sun.

Another way to say this is that life feeds on energy gradients.

When two systems with very different energy densities come into contact, energy must flow.

Life feeds on that flow.

In fact, the importance of energy gradients to life can help us understand the actual origin of life and its precursors.

The origin of life on Earth isn't known.

We think it started with a self-replicating molecule similar to RNA.

The companion episode, over on "It's OK to Be Smart," will get into the nitty-gritty of that.

Following that synthesis, evolution took hold, and the first protocell and then first true living cell pulled itself together.

PBS "Eons" will cover that part as they explore LUCA, the last universal common ancestor.

But where on Earth did this all happen?

There are a few hypotheses.

Perhaps it was in tidal pools or around deep sea hydrothermal vents or even on the undersurface of Earth's ice caps.

These environments share a critical property.

They sit at persistent energy gradients.

The water of tidal pools is both cooled by the earth and the ocean and warmed by the sun.

Around deep sea vents, the searing gases from Earth's hot interior meet the frigid water of the ocean depths.

Beneath the thick ice caps, there's the transition between the solid and liquid phases of water.

These are places struggling to return to equilibrium.

These systems are doing their best to obey the second law of thermodynamics by redistributing their energy as evenly and randomly as they can.

Heat energy flows from hot to cold, seeking a uniform temperature, but energy is also dispersed into every form it can take consistent with the laws of physics.

Some of that energy gets distributed into chemical bonds as simple molecules form via every chemical reaction that's possible given the available raw materials.

As those molecules form, new channels open up for distributing energy into the chemical bonds of increasingly complex molecules.

Normally, this local rise in complexity would all cease when the system reaches thermal equilibrium, energy is perfectly evenly distributed and new molecules break apart exactly as often as they're formed.

But when our energy source is flowing into a much larger reservoir, why, the ocean, for example, then equilibrium is never reached.

Complexity can increase indefinitely as a byproduct of the system striving to redistribute the endless gradient in energy.

And at some point, natural selection takes over.

Molecules self-catalyze.

They help drive the very reactions that create more of the same.

Molecules better at that process become more abundant, and at some point, they become true self-replicators and eventually, they become life.

But even life and self-replication might be a very natural part of the same thermodynamic drive to dissipate energy.

I mean think about it.

Living things are incredible heat dissipation entropy-maximizing machines.

The most random possible form for energy is thermal radiation, and the lower the energy of its component photons, the higher the entropy.

A plant absorbs the concentrated ultraviolet light from the sun and reprocesses it into a much higher entropy infrared heat glow.

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

Life is great at dissipating energy, and more generally, it may be that self-replicating systems are the best possible energy dissipators of all.

This is a new idea proposed by MIT biophysicist Jeremy England, who puts the thermodynamics of life on more solid theoretical grounds.

He's demonstrated mathematically that self-replicating molecules and simple single-cell life are extremely good at shedding heat in the act of reproduction.

Self-replication randomizes the environment, even if each new replicator is highly ordered.

And it's not just life that does this.

Consider a perfectly streamlined or laminar flow of some fluid.

This organized flow is disrupted by introducing turbulence.

The laminar flow has a lower entropy than the turbulent flow because there are fewer ways to rearrange the particles in the former while preserving its global properties.

But watch the transition from laminar to turbulent.

While the global structure is disrupted, substructure develops.

Waves and vortices have their own complex and regular structures, but they ultimately serve to dissipate the flow.

Any given eddy taken separately has a lower internal entropy than its chaotic surroundings, but the source of that local incidence of low entropy is the streamline flow that it formed in.

And those turbulent eddies ultimately serve to increase the entropy of the greater flow.

So given a much larger source of order, the global process of dissipation of that order results in eddies of low entropy.

Life appears to be just such an eddy.

In the case of life, the original source of extreme low entropy is the Big Bang itself.

In the process of redistributing energy into the most random possible state, little eddies of order, like galaxies, stars, planets, and life naturally arise.

These blips in order are actually serving the second law helping the universe disperse its early extreme low entropy state.

So I guess that makes you a little eddy of order, a momentary fluctuation of interesting but ultimately, in service of the spread of disorder and dullness, an agent in the inexorable trend to maximize the entropy of space-time.