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 Jan 25 '15 at 18:26
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    $\begingroup$ This sounds like a question a creationist might ask. $\endgroup$ – HDE 226868 Jan 25 '15 at 18:29
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    $\begingroup$ 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 Jan 25 '15 at 20:25
  • $\begingroup$ @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. $\endgroup$ – Selene Routley Jan 26 '15 at 6:02

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.

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  • $\begingroup$ It's somewhat disconcerting that this is my highest rated answer… $\endgroup$ – canadianer Jan 26 '15 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 Jan 27 '15 at 11:28
  • $\begingroup$ 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 Jun 4 '15 at 8:00

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.

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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.

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    $\begingroup$ Although I agree with the broad scope of your argument, however I requires some further referencing! but still a nice response! $\endgroup$ – Behzad Rowshanravan Jan 25 '15 at 22:10
  • $\begingroup$ Sometimes the reduction of entropy is balanced by reduction in enthalpy (for e.g. RNA folding). $\endgroup$ – WYSIWYG Jan 26 '15 at 7:35

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.

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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)

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    $\begingroup$ 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 Jan 26 '15 at 6:39

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