I was confronted by this question:

Biological evolution of life on Earth, from simple prokaryote-like cells to large, multicellar eukaryotic organisms,

  • A) has occurred in accordance with the laws of thermodynamics.
  • B) has caused an increase in the entropy of the planet.
  • C) has been made possible by expending Earth's energy resources.
  • D) has occurred in accordance with the laws of thermodynamics, by expending Earth's energy resources and causing an increase in the entropy of the planet.
  • E) violates the laws of thermodynamics because Earth is a closed system.

Answer: A

However, I do not understand. Biological evolution does cause the system (living organisms)'s entropy to decrease. So, by the second law of thermodynamics, the entropy of the universe (in this case Earth), must have overall increased.

I would answer D... Where am I going wrong?

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    $\begingroup$ Life (excluding chemosynthetic bacteria and such) doesn't expend Earth's energy resources, it uses energy from the sun. So entropy increase on Earth is much more than balanced by the decrease on the sun. (Or is it vice versa? I forget which way it goes...) $\endgroup$
    – jamesqf
    Commented Jan 25, 2015 at 18:26
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    $\begingroup$ This sounds like a question a creationist might ask. $\endgroup$
    – HDE 226868
    Commented Jan 25, 2015 at 18:29
  • 3
    $\begingroup$ The second law of thermodynamic states the entropy can only increases in a closed system. Life is definitely not a closed system. I think that this question would be a better fit for physics.SE $\endgroup$
    – Remi.b
    Commented Jan 25, 2015 at 20:25
  • 3
    $\begingroup$ @Remi.b I think this question is a good fit here as well (even though I'm a physicist): this is the application of physics to a primarily biological problem and I think looking at and becoming well versed in this kind of thing is pretty essential for biologists, particularly of they don't want to get wiped out by well funded creationists with very big megaphones. $\endgroup$ Commented Jan 26, 2015 at 6:02
  • $\begingroup$ This area is extremely complicated and has been the subject of historical scientific disagreement, as the Wikipedia entry shows. At the best of times MCQs are, in my opinion, the antithesis of education, but to use them in this context seem to me quite absurd. It suggests to me that the so-called educators of the poor poster were force feeding him some glib line, which they then expected him to regurgitate. I feel for him, but as this was six years ago, I hope he may have recovered from the experience by now. $\endgroup$
    – David
    Commented May 1, 2021 at 16:50

7 Answers 7


However, I do not understand. Biological evolution does cause the system (living organisms)'s entropy to decrease. So, by the second law of thermodynamics, the entropy of the universe (in this case Earth), must have overall increased.

The universe and the earth are not equatable. Earth is not an isolated system. Life causes entropy of the earth to decrease. This is offset by increased entropy of the sun, which is the primary source of energy for the earth. Overall, the entropy of the universe increases.

  • 2
    $\begingroup$ It's somewhat disconcerting that this is my highest rated answer… $\endgroup$
    – canadianer
    Commented Jan 26, 2015 at 19:31
  • $\begingroup$ Yes, for example entropy of the sun decreases, because of nuclear reactions. I suggest to improve this answer so that it wouldn't be overrated. $\endgroup$
    – Mithoron
    Commented Jan 27, 2015 at 11:28
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    $\begingroup$ What if we had no Sun and all the biological process got their energy from Earth's internal energy? There would be no evolution? Please help me understand. $\endgroup$
    – Probably
    Commented Jun 4, 2015 at 8:00
  • $\begingroup$ @anthony You should post your own answer. $\endgroup$
    – canadianer
    Commented Apr 30, 2021 at 18:43
  • $\begingroup$ Okay, added a quick and dirty one, could use some restructuring maybe. $\endgroup$
    – Anthony
    Commented Apr 30, 2021 at 19:15

I would say A or D are acceptable, but A is probably the better answer: firstly, the entropy of the planet system alone is probably not increasing (indeed, it's probably near to constant) and as noted elsewhere, you need to consider a closed system to apply the Second Law, so you need to think about everything that comes to and leaves the Earth. This is the key to understanding why life on Earth doesn't violate the second law: Earth absorbs and re-radiates roughly the same amount of light, but that light's state is radically changed as this happens, and this state change and the huge entropy increase it brings about overwhelmingly offsets the decrease in Earth's organisms' entropy as they build themselve whilst making use of that light.

Let's look at the radiation balance in more detail.

The thermodynamic entropy of a system is basically the system's conditional information content (also known evocatively as equivocation in information theoretic circles) conditioned on the given macroscopic properties of a system. It's the logarithm of the number of ways a system's internal microstates can be arranged consistent with the observed outward properties. A simple explanation of the logarithm is that when you add two systems together, the number of ways that they can be arranged multiplies (think of two car number plate letters: one letter yields 26 different number plates, two letters $26^2$ number plates, three letters $26^3$ number plates, three letters and two digits yields $26^3\times 10^2$ number plates and so forth). So the entropies add when the possibilities multiply.

The entropy of a system of identical particles is proportional to the number of particles in that system. Think of each particle as like a letter in an alphabet of its states and then think of long "number plates" of concatenated particle states.

So now: let's look at the input to the Earth: around about $1{\rm kg}$ of energy in the form of sunlight per second is available to Earth systems to do work. It comes as $\frac{c^2}{h\,\nu}$ photons per second, where $\lambda = \frac{c}{\nu}$ is of the order of 500nm so $\nu\approx600{\rm THz}$. That is, about $2\times 10^{35}$ photons per second.

The energy output of the Earth is basically infrared heat: it's the same amount of energy as comes in, but it is now composed of many more photons, because now $\nu$ in the photon number $\frac{c^2}{h\,\nu}$ is of the order of $30{\rm THz}$ (corresponding to $\lambda=10{\rm \mu\,m}$). The Earth therefore radiates roughly twenty times more photons than it absorbs: about $4\times 10^{36}$ photons per second. Each photon can encode the same amount of information, so the entropy increase is roughly twentyfold. Life systems on Earth use about one thousandth of the incident Solar energy: it has been estimated that photosynthesis fixes energy for use by life systems at the rate of about $100{\rm TW}=10^{14}{\rm J\,s^{-1}}$, compared with an input of $1{\rm kg\,s^{-1}}\approx 10^{17}{\rm J\,s^{-1}}$. So, no matter how complex life organisms get, this barely makes a dint on the massive entropy "production" of the total Earth energy cycle: the nett production of entropy by the whole Earth system is still strongly positive notwithstanding the presence and evolution of life, and therefore in keeping with the Second Law.

An interesting calculation has been done on how much Solar energy is needed to evolve life on Earth to the present day. This is presented in the paper:

Emory F. Bunn, "Evolution and the second law of thermodynamics", Accepted for Publication Amer. J Phys.

Bunn calculates that less than a year of sunlight would be enough to power the evolution of all life on Earth over the last four billion years and still be in keeping with the Second Law.

I should cite where I first saw this idea for explaining the life versus second law problem: I took this line of argument from:

Roger Penrose, "The Road To Reality: A Complete Guide to the Laws of the Universe", 2004 Chapter 27, "The Big Bang and its Thermodynamic Legacy"

but I have heard it used in several lay explanations of physics since.


My 2 pennies on this discussion

Entropy in general is the total number of states a system can occupy which is also called degrees of freedom (so is with the supply of heat, the molecules vibrate and can occupy more states). Any system tries to avoid confinement and maximize the number of states it can occupy. However if the system is in a stable state and any movement would cost it energy, then it would remain in the present state at the cost of reduced degree of freedom.

This relationship is described by this equation: ΔG = ΔH -TΔS
Where ΔG is change in the Gibbs free energy, ΔH is change in enthalpy, ΔS is change in entropy and T is the temperature.

Though the process of natural selection destroys the number of phenotypic states that life can occupy, thereby causing a decrease in entropy, there are other processes which lead to gain in entropy (for example population growth and migration). Note that the process of reproduction itself creates order from disordered molecules, but as the population expands and spreads entropy is again maximized. As already pointed out by bobc, catabolic processes which fuel life processes increase entropy but anabolic processes do the opposite.

So my opinion is that the process of Natural Selection reduces entropy, so is the formaton of the eukaryotic cell. A free living bacteria has more freedom than a cell-locked mitochondria. However there are other energetic benefits that balance this reduction of entropy.

For earth I would guess that the overall entropy has reduced because the earth has cooled down since its birth. Since heat is removed entropy has reduced.

There is no violation of the second law in any of these cases. It is a universal law.


The correct answer is almost D: Life on Earth has occurred in accordance with the laws of thermodynamics by consuming Earth's free energy (which is continually being replenished) and causing an increase in the entropy of the universe. Life maintains it's own low entropy by increasing the entropy of it's environment. The Earth radiates energy of lower entropy than it receives (see citation 3), thus increasing the entropy of the universe. This happens by re-radiating the short wavelength radiation received as longer wavelength radiation. Life contributes to the process of converting the former to the latter. The outgoing radiation is in the form of more but lower energy photons than the incoming higher energy photos from the Sun. Keep in mind that the energy into the Earth is approximately equal to the energy out of it.

Useful resources for looking into the subject more:

  1. Ulanowicz, R.E. and Hannon, B.M., 1987. Life and the production of entropy. Proceedings of the Royal society of London. Series B. Biological sciences, 232(1267), pp.181-192.

  2. Schneider, E.D. and Kay, J.J., 1994. Life as a manifestation of the second law of thermodynamics. Mathematical and computer modelling, 19(6-8), pp.25-48.

  3. Kleidon, A., 2010. Life, hierarchy, and the thermodynamic machinery of planet Earth. Physics of life reviews, 7(4), pp.424-460. – Anthony 1 hour ago

  • 1
    $\begingroup$ The answer is A because D implies a fallacy that Earth is a closed system. $\endgroup$
    – Bryan Krause
    Commented May 1, 2021 at 16:42
  • $\begingroup$ Read carefully and notice that I didn't propose D and specifically point how it is not a closed system. The closed system is the universe. $\endgroup$
    – Anthony
    Commented May 2, 2021 at 11:20
  • $\begingroup$ Yes but some of the other answers were much more clear on that. $\endgroup$
    – Bryan Krause
    Commented May 2, 2021 at 14:01
  • $\begingroup$ Well I guess time will tell us where voters' reading comprehension is at. $\endgroup$
    – Anthony
    Commented May 3, 2021 at 9:33
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    $\begingroup$ Excellent reference list - at least, from a physicist's point of view! $\endgroup$
    – Roger V.
    Commented Jun 11, 2021 at 7:44

The Second Law is one of the most misunderstood and misapplied laws of thermodynamics. It has a scope largely confined to physics, nothing more should be read into it. Simply put, it states that "Heat cannot spontaneously flow from a colder location to a hotter location". (Wikipedia) There is nothing about life that violates that, or creates "negative entropy".

When you get into the details, you need to consider closed vs systems, and equilibrium vs non-equilibrium systems.

In fact, life contributes to positive entropy, like everything else. Energy at high potential is transformed to low potential, at the same time creating useful work and waste heat. Note that "useful work" can include storing energy in a form useful for future use.

  • 1
    $\begingroup$ Although I agree with the broad scope of your argument, however I requires some further referencing! but still a nice response! $\endgroup$ Commented Jan 25, 2015 at 22:10
  • $\begingroup$ Sometimes the reduction of entropy is balanced by reduction in enthalpy (for e.g. RNA folding). $\endgroup$
    Commented Jan 26, 2015 at 7:35

It is A for one simple reason the ultimate source of energy that drives evolution may be earth or the sun.

First Additionally it is arguable if a multicellular organism has less entropy than ancestral prokaryotes since there were far far more iterations of prokaryotes.

Evolution has no effect on individual organism's entropy. Evolution can only occur in a system that is consuming energy. To put it a different way if I build one car and then build a second more complex one, building the second car has no effect on the entropy of the first. So you can only be looking at the process itself, in which case the traits of the organisms are irrelevant.

Consider sorting in geology, particles can be sorted into organized patterns, but only if there is enough energy to move them in the first place. Sorting in theory might decrease entropy, but it can only occur if there is an overwhelming increase in energy to drive it. for living things ,depending on the organism, that energy/entropy may be coming from the earth or the sun, in the latter case the entropy of the planet may not be effected depending on how you define it.

That said that is a horribly written question, because sunlight could be considered part of earths energy resources, and it is written in a way to confuse different usages of entropy.


Evolution increases complexity (entropy) e.g., developing a new citrate path way in E. coli

It does this process by of expending energy. i.e., via lifeforms's metabolism

So to answer your question:

A,B,C are all correct (or part of a correct answer)

However D is the most correct answer (It is an all of the above type answer)

E is a none of the above answer (and most incorrect)

  • 3
    $\begingroup$ There is no way of breaking the second law of thermodynamics. And you "don't get away with it" by "using the the 0th and 1st". The second law is only valid for closed (energetically) systems, which the earth is not. So it is not correct to use this law here. $\endgroup$
    – Chris
    Commented Jan 26, 2015 at 6:39

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