“In three words I can sum up everything I’ve learned about life: it goes on.” -Robert Frost
Well, here we are, at the end of the week, and you know what that means: another Ask Ethan column! This week, I’ve been asked about the same thing by many different people, so I’ll credit it to Patrick E. and Matt T., who want to know:
I’m curious to hear your take on this: the 2nd Law of Thermodynamics applied to the origin of life.
For those of you who haven’t heard, this is what they’re talking about.
We’ve long known that the Universe didn’t start out with anything resembling life. Forget about complex, multicellular, highly differentiated creatures like mammals, angiosperms and insects; immediately after the Big Bang, the Universe didn’t even have atoms in it!
But out of that incredibly hot, dense soup of primordial matter and radiation came everything. And for many of the steps along the way, we completely know how.
The earliest quarks-and-gluons, part of the primordial plasma of the Universe, became tightly bound as the Universe expanded and cooled, decaying into the protons and neutrons that we’re more familiar with in short order. There are good reasons for this: energetically, it’s more favorable to have quarks and gluons bound together in those configurations — protons and neutrons — than to have them in any other configuration.
When there’s plenty of available, free energy, there’s nothing compelling you to be in a more stable state, but as the Universe expands and cools, down the ramp you go, until you reach the bottom. And that same process continues.
Those protons and neutrons — once the Universe cools to the point where photons don’t immediately blast apart the things they combine into — combine to form heavier, more stable atomic nuclei. If there were enough activation energy, they’d form even heavier, more stable elements at this time!
As it is, helium-4 is pretty much the practical limit at this time, but as the Universe continues to expand and cool, there becomes another way for the particles in the Universe to become more tightly bound: the atomic nuclei and electrons can bind together, forming neutral atoms for the first time.
Of course, they can only do this once the Universe is cool enough; if you have too much energy, the neutral atoms will be blasted apart back into ions and electrons, but if the ambient radiation/temperature/particle energy is below the binding energy of electrons to nuclei, you’ll get neutral atoms.
And you notice a theme here: some processes happen spontaneously, where once the energy of your surroundings drops below a certain threshold, things just wind up bound in lower energy states, and some processes require a certain amount of “activation energy” to climb over a threshold, resulting in a more stable state overall. Let’s go a little further.
After gravitation binds the Universe together into clumps — again an example of a spontaneous “binding” — the most overdense regions accumulate enough matter at high enough temperatures and densities that nuclear fusion ignites, producing heavier and heavier elements. This, again, is an example of requiring an “activation energy,” but in the end, the products you wind up with are more tightly bound (overall, once everything is averaged together) than the reactants you started with.
And after the heaviest of these stars go supernova, the interstellar medium becomes heavily enriched with all the atoms in the periodic table, and they bind together in a variety of ways to create molecules: simple ones at first, but eventually some incredibly complicated ones as well.
What I want you to keep in mind is that, at every step along the way, we’ve gone into a more energetically favorable configuration, and there were only two general ways we went there: either we spontaneously “rolled down an energy hill” to get there, or we “rolled up an energy hill and then down into a lower valley” to get there.
That’s it. In all cases, the energy reactions looked like one of the following curves, to some extent.
So, then, what about that big step? We can go from subatomic particles to atomic constituents, from protons, neutrons and electrons to atoms, from atoms to all the elements of the periodic table, from all the elements to a huge diversity of molecules, but at some point, we have to take that leap. You know the one I’m talking about: where non-life turns into life.
It’s pretty well accepted that either life originated very early on in Earth’s history, or perhaps even before that, in the cosmos somewhere prior to that. Once we have life, Darwinian evolution — mutation, limited resources and natural selection — can definitely get us from those first, self-reproducing organisms to the full breadth and diversity of life we see on the planet today.
But it’s not a fundamental explanation; it doesn’t tell us the mechanism behind how or why these changes occur, or what the physical process is that powers them.
It’s hard, in many ways, to draw the line between life and non-life. Those of you who grew up around the same time I did might be surprised at how well the old Sesame Street definition still holds up today.
You don’t need life for that; crystals are certainly not alive, but from rock to ice, crystalline structures self-reproduce.
Quite interestingly, so can sphere clusters on a molecular level.
And perhaps even more surprisingly, so can vortices, a simple example of fluid flow!
None of these things are alive, though, in the way we think a virus or bacterium is alive. But Jeremy England, a physicist at MIT, thinks that the same physical principles — very similar to what’s led us along every known step of the journey thus far – might be responsible for all of these things.
(The complete — and eminently readable — paper is here.)
It’s all based on energetic favorability, something a physicist is likely to simply call thermodynamics. The basic idea is that if you take a bunch of “whatever” — atoms, molecules, etc. — and you apply a low, variable amount of energy to them over time, they’re going to wind up shaking down to a more tightly bound state. This is the same way a gentle-but-inconsistent shaking of a jar of mixed nuts will eventually leave the largest nuts at the top and the smallest nuts down at the bottom: it’s a more energetically stable configuration.
Here’s the thing: what England says is definitely physically true. It’s also almost certainly not the entire story of life. For two examples, evolution doesn’t always (or arguably, even often) find the most energetically favorable solution to a problem.
We’ve never evolved wheels, even though that would be hugely more efficient for a variety of applications. Yet no animal has ever evolved it.
And all the plants in all the history of the entire world have always relied — as far as we know — on the same two molecules for gathering and storing energy from the Sun: Chlorophyll A and Chlorophyll B. These are not the only molecules capable of storing solar energy or even of permitting photosynthesis, as there are bacteria that use other molecules to achieve it. The solar spectrum even peaks in the green/yellow, while Chlorophyll A and B both absorb blue and red light, but reflect green/yellow!
But just because that one phenomenon doesn’t govern everything doesn’t mean it isn’t the phenomenon responsibly for the origin of life; it might be. But it’s very speculative at this point. As Harvard professor Eugene Shakhnovich says,
“Jeremy’s ideas are interesting and potentially promising, but at this point are extremely speculative, especially as applied to life phenomena.”
There are plenty of applications of thermodynamics — and of an increased entropy of the environment — to biology and to life processes, but does life emerge, causally, on account of entropy and thermodynamic phenomena?
It’s possible, but the jury is not only still out, they likely won’t be back for a long time. This is an incredibly difficult hypothesis to test, and we very likely won’t believe we have the answer until we’ve achieved the holy grail of abiogenesis: creating life from non-life for ourselves! So we still don’t know why or how life exists, but it’s conceivable and reasonable that thermodynamic processes may hold the answer to that. It’s also very cool that people have been not only researching this for decades, but that the research is leading us down all sorts of interesting paths, some of which might have something to do with the origin and evolution of living (and almost living) things here on Earth!
And that will wrap up another Ask Ethan for this week. Have a question or suggestion for what you’d like to see us tackle next? Ask away! And if you haven’t been over to the new Starts With A Bang at Medium this week, you’ve missed out on nine new fantastic articles this week alone; better catch up!