Isaac Asimov (1984) characterized the fallacy of the creationist understanding of entropy: “In kindergarten terms, the second law of thermodynamics says that all spontaneous change is in the direction of increasing disorder—that is, in a ‘downhill’ direction. There can be no spontaneous buildup of the complex from the simple, therefore, because that would be moving ‘uphill’.” Asimov reasons, “An argument based on kindergarten terms is only suitable for kindergartens.” In this section, we will apply an understanding of entropy beyond the kindergarten level.
The second law of thermodynamics clearly does not prohibit the building of complexity from simplicity, hence the existence of complex structures like termite mounds and toaster ovens. The physical world is filled with countless examples of spontaneous order emanating from a less ordered state, such as gases (e.g., water vapor in clouds) condensing into a more ordered liquid state (rain) and liquids freezing into an even more highly ordered solid crystalline state (e.g., ice crystals). Perhaps most dramatic and commonplace biological example of spontaneous order derived from a less ordered state is the development of a single cell, the zygote, into a complex multicellular (billions of cells), adult human possessing dozens of specialized organs, tissue classes, and terminally differentiated cell types. Clearly, snowflake synthesis and embryogenesis do not violate any physical laws, so what’s going on?
In a nutshell, the synthesis of order exacts an energetic price: The cost of converting a relatively disordered water droplet into a more ordered snowflake is the release of heat to the environment, and the cost of embryogenesis is the conversion of ordered nutrients into less ordered waste products and heat. In the end, the processes of snowflake synthesis and embryogenesis always contribute more net entropy to the system as a whole, consistent with the second law of thermodynamics. According to the creationist “kindergartener’s understanding of entropy” (Asimov 1984), neither snowflake synthesis nor animal development could possibly take place, let alone organismal evolution.
Having just discussed how individual organisms maintain consistently higher degrees of internal order compared with their surroundings, we now describe how the second law of thermodynamics is perfectly consistent with, indeed promotes, the progeny of some populations of organisms becoming incrementally more complex over evolutionary time.
A Gouldian Disclaimer
Natural selection produces organisms that are more adapted to their environments, but “more adapted” organisms are not necessarily more “complex” than their ancestors. Although natural selection has produced complex multicellular life from relatively simpler unicellular ancestors, we are in no way implying that complexity is the general evolutionary trend—which it clearly is not (see Gould 1997). For example, much of the unicellular Kingdoms Monera (bacteria) and Archaea (archaebacteria) (i.e., the vast majority of life on Earth) remain virtually unchanged over millennia, and similar (though far less dramatic) cases can be made for cockroaches and sharks, whose body forms have remained essentially unchanged throughout long stretches of animal evolutionary history. Furthermore, there are also examples of lineages that have become, arguably, less complex with evolution (e.g., loss of numerous organs and body parts in parasites, loss of eyes in deep sea and cave-dwelling fauna). Here, we are specifically addressing a thermodynamic paradigm that explains how evolutionary complexity can develop in the face of entropy, without suggesting that the development of complexity is inevitable. The anti-evolutionists’ caricature of evolution as inevitably increasing complexity as a whole is simply not the case, even if some adaptations may increase complexity.
Even though net entropy increases over time in a thermally isolated system, local regions of reduced entropy (e.g., complexity) can develop spontaneously in open subsystems as long as there is a greater decrease in entropy (decrease in complexity) in another interlocking part of the system. So long as entropy tends to increase in the entire system, the second law of thermodynamics is not violated. Evolution can occur locally within a system by moving thermodynamically “uphill” (building the complex from simpler precursors) in one subsystem (e.g., a population of organisms) as long as an interlocking part of the system (e.g., the Sun) moves thermodynamically “downhill” at a significantly faster rate and magnitude than evolution moves uphill.
Roger Penrose (1989) describes, “Contrary to a common impression, the earth does not gain [net] energy from the sun! What the earth does is to take energy in low-entropy form, and then spew it all back again into space, but in a high-entropy form. What the sun has done for us is to supply us with a huge source of low entropy. We (via the plant's cleverness), make use of this, ultimately extracting some tiny part of this low entropy and converting it into the remarkable and intricately organized structures that are ourselves.” These concepts can be challenging to visualize, and we present them in a simplified form in Fig. 2. The photons that emanate from the sun and arrive at Earth are highly directed (arrive from a narrow range of directions) and possess high energy (shortwave radiation). In contrast with incoming solar light, outgoing photons re-radiated from the Earth consist of low energy (longwave radiation) infrared light that is highly dispersed (photons are moving in many different directions). Because the total energy carried by the outgoing photons is the same as the incoming photons, there are many fewer photons traveling toward Earth than there are photons reradiating back into space. The Sun’s smaller number of highly directed, high-energy photons represent a state of much lower entropy compared with the greater number of highly dispersed, low energy photons reradiated to space.
The Earth’s primary producers (photosynthetic plants and bacteria) make use of this low entropy, thereby reducing their own entropy. Non-photosynthetic organisms reduce their entropy by eating these primary producers either directly or indirectly and using the oxygen released by photosynthesis for cellular respiration. Therefore, photosynthetic primary producers can be viewed as a rotating cog in the machinery of life, powered by the conversion of low entropy sunlight to higher entropy infrared light (Fig. 2). This rotating cog interlocks with virtually all of Earth’s organisms and powers the machinery of life. The powering of life by converting sunlight from low to high entropy is analogous to the powering of a city from a river whose water flow rotates hydroelectric turbines to generate electricity. As long as the river provides enough water flow to turn the turbines, the city will be able to use the resulting electricity to maintain itself and stay “alive.”
However, does the sun actually provide enough low entropy to not just simply maintain life’s status quo but to also drive the ‘uphill’ evolution of complex life? Or, using the river analogy, does the river flow provide enough hydroelectricity to not just simply maintain the city but to accommodate growth and development of the city (i.e., increased complexity in the forms of shopping malls, suburbs, water parks, etc.)? Using basic mathematics, physicist Daniel Styler (2008) has elegantly shown that the Earth is bathed in about one trillion times the amount of entropy flux required to support the evolution of complex life. Physicist Emory Bunn (2009) shows that the evolution of extant complex life is compatible with the second law of thermodynamics as long as the time required for life to evolve on Earth is at least ∼107 s or 116 days. Since life has had 4 billion years to evolve on Earth, the theory of evolution does not appear to be threatened by the second law of thermodynamics. Far from threatening evolution, as we will see, entropy actually functions as a thermodynamic driving force behind natural selection.