In the last part of my philosophy series, “Thinking about the ‘Metaphysics’ in Metaphysical Naturalism,” I discussed cosmology and the origins of our universe from a naturalist and atheistic perspective. In this next part of the series I will be focusing more particularly on the origin of life in our universe (at least on the only planet currently known to host life, i.e. Earth), and the evolution of lifeforms from simple states to the intelligent minds of human beings today. This journey will require discussing: 1) the abiogenesis of life from non-living matter, 2) the evolution of biological diversity from common descent, 3) and the mind-body physicalism of human minds in a naturalist universe.
The planet Earth has been around for about 4.6 billion years, when a cloud of interstellar gas — filled with particles of ice, dust, rock, and other particles — collapsed to a point of concentrated mass, causing rising heat and the formation of our Sun. Most of the matter in this collapsing nebula fell into our Sun, but other material formed into a planetary disc in orbit around the Sun, causing particles to collide and eventually planets to take shape from cumulative bombardments with solid objects. The third planet from our Sun, Earth, happened to be in the Habitable Zone, which is the region in a solar system where a planet is neither too close nor too far from its star to form liquid water. Earth eventually formed liquid oceans on its surface and also an atmosphere with just the right greenhouse balance for the planet to be neither too hot nor too cold for complex life. The first surviving fossils of life date to about 3.5 billion years ago (about a billion years into our planet’s history) and scientists estimate that life on Earth could have began anywhere from 3.9-3.5 billion years ago.
This sequence of events has left us with a vexing question: what started life on Earth so long ago and how did it get to the point of us humans beings here today?
Early creation stories — primarily those found in ancient religions and mythologies — often depicted the origin of human beings and life on Earth as being brought about through an act of supernatural, designed, and guided creation.
In Genesis 1, for example, the Jewish god Elohim creates the planet Earth through a process that lasts six days: the Earth itself is created on the 1st day, on the 2nd day the water on Earth is separated from the water in Heaven, land-based vegetation is created on the 3rd day, the Sun, Moon, and stars are created on the 4th day, sea life and birds are created on the 5th day, land animals on the 6th day, and finally human beings (made in God’s image) on the 6th day .
Other creation stories — such as those in Zoroastrianism, Hinduism, and Islam — contain the similar elements of creation by an intelligent being and an ordered process of creation with the intention of producing and sustaining life.
As I discuss in my article, “Defining the ‘Natural’ in Metaphysical Naturalism,” however, naturalism predicts a blind cosmos, without any intelligent being or willful process behind its formation. Naturalism thus entails a universe where life must have come about without any intelligent mind or intentional process behind it, one that is reducibly non-teleological, non-mental, and reducible to purely mechanical causes. As will be shown in the discussion below, based on all of the evidence and major theories of modern biology and anthropology, we have every reason to expect that the origins of life, humankind, and intelligent beings in our universe came about solely through natural processes, without the external influence of any supernatural agencies.
If the life on Earth did not come about through a willful and guided process, then it must have come about due purely to natural chance. But how could a planet like Earth, with all of the correct life-supporting features, come about without any design? The answer is that our universe is so massive and contains so many planets that planets like Earth, which just happen to have all of the correct life-supporting features, are bound to emerge. In fact, astronomers estimate that there may be as many as 17 billion Earth-sized planets in our galaxy the Milky Way alone (to say nothing of the whole universe, which may have as many as 500 billion galaxies). In terms of solar systems, astronomers now estimate that as many as 20% of stars like our Sun probably have Earth-sized planets in their Habitable Zone. With so many planets like Earth elsewhere in our universe, it is not at all surprising that our planet took shape with all of the correct life-supporting features, purely due to natural chance, without any guiding plan behind it. In fact, NASA scientists now estimate that we will probably discover another planet like Earth within the next 20 years.
Life on Earth is carbon-based. The reason why is that carbon is capable of forming far more complex matter arrangements than other elements. As astronomer David Darling explains:
“Carbon is the key elemental building block for all known terrestrial life. It’s commonly assumed in astrobiology that it will also provide the basis for most life elsewhere in the universe. The reason for this is not only carbon’s ability to form a vast range of large, complicated molecules with itself and other elements, especially hydrogen, oxygen, and nitrogen, but also its unique facility for maintaining the right balance of stability and flexibility in molecular transformations that underlie the dynamic complexity of life. In aqueous systems at temperatures common on Earth, carbon is so superior to any other atom as a polymeric unit, that it has come to be the basis for the structure of biomolecules essential for all basic metabolic processes.”
It may be possible that life could form based on other elements, with silicon-based life probably being the most plausible alternative. However, it is difficult to know, since all of the life that we have observed thus far in the universe is carbon-based. Nevertheless, we have only observed a small part of our universe, and astronomers like Carl Sagan have cautioned against “carbon chauvinism” in assuming that no other form of life is possible. Time will tell as we learn more about life in our universe.
An important aspect of life being carbon-based, however, is that carbon also exists in non-living matter. Imagine a universe where an intelligent God wished to create both life and non-living matter. Such a God could use two completely different forms of matter — one for life and one for non-life — where it would be clear that non-living matter could never reach a state in which it would become living matter (likewise, even if God used the same basic elements to create life and non-life, such a deity could manipulate these elements into different arrangements of living and non-living of matter, where it would be clear that the non-living arrangements could never reach the complexity of living arrangements). In such a universe, abiogenesis would be impossible and we might suspect that some form of design or creation was necessary to tune life in such a unique way . The opposite, however, is the case. Our life is composed of an element that also commonly appears in non-living matter arrangements. Our matter arrangements just happen to be more complex, producing multicellular lifeforms. All that is required is a natural process for how this complexity might emerge, then non-living matter could become living matter (and eventually us humans) through a purely natural process, without any special tuning or design. Two steps are necessary for this to occur: 1) abiogenesis, in order to get life started in the first place, and 2) evolution from common descent, in order for life to evolve into more diverse and complex forms.
Currently, scientists have not fully determined how life on Earth first began, so that any scientific account of life’s origins must belong to the category of “scientific theory” rather than “scientific fact.” Abiogenesis is the theoretical process of life emerging from non-living matter, such as simple organic compounds. For abiogenesis to explain life on earth, however, such simple compounds must have gone through a process to reach far more complex states, such as the coding found in human DNA. To describe this process, I will be summarizing the article, “The Origin of Life,” by scientist Albrecht Moritz, which summarizes a plausible scenario of natural abiogenesis.
1. Synthesis of Organic Molecules:
As Moritz explains, the first step is “the the availability of organic molecules as building blocks,” which are “essential to the spontaneous origin of life.” For life to have emerged through abiogenesis on Earth, these organic molecules, necessary for producing amino acids, needed to be present on Earth. Moritz explains that, if the early Earth had a reducing atmosphere (the evidence for which he provides in the article), then organic compounds may have been common in the Earth’s prebioitic ocean :
“In a reducing atmosphere … the concentration of organic compounds in the prebiotic ocean may have been relatively high (De Duve and Miller 1991, and references therein). Furthermore, locally a ‘prebiotic soup’ might have been greatly concentrated by such simple processes as, for example, evaporation in puddles or shallow lakes, possibly with long-term wet/dry cycles. It should be kept in mind for evaluating all chemical scenarios that, due to its nature, the origin of life must have been a very local event; this is also important for the issue of the origin of homochirality of amino acids and sugars…”
Moritz also explains that amino acids could emerge even without a reducing atmosphere:
“Yet even if the early Earth’s atmosphere would have been neutral rather than reducing, new data suggest that efficient amino acid synthesis would have been possible also under these circumstances. The group of Jeffrey Bada demonstrated that, contrary to previous reports, significant amounts of amino acids are produced from neutral gas mixtures under suitable conditions (Cleaves et al. 2008).”
Likewise, the Earth’s atmosphere may not wholly be relevant, if life arose in deep-sea hydrothermal vents:
“Of course, if life arose in deep-sea hydrothermal vents (see below), the composition of Earth’s early atmosphere would become largely irrelevant. To a certain extent, this also holds true for organic building blocks delivered to the earth by interplanetary dust particles and on carbonaceous meteorites.”
Either way, based on all current scientific assumptions and knowledge, there is no reason to suspect that our Earth would not have had the necessary organic compounds in its early stages that would be necessary for the abiogenesis of life.
2. An RNA World First?
“It is now widely agreed that at the origin of life there was not the current DNA/(RNA)/protein system for gene information on one hand and catalysis, regulation, and structural function on the other. It would beg the question, what came first, protein or DNA? Protein catalysis without gene information, which allows it to be maintained and propagated, is not sufficient in the long term, and DNA gene information without catalysis, necessary for the function of life, would be useless as well.
Instead, it is assumed that RNA acted as a precursor of both protein and DNA, in the sense that it can serve both as catalyst (like protein enzymes) and as carrier of genetic information. Even in the modern cell ribozymes (catalytic RNAs) still play a vital, albeit limited, role. In the ribosome, the synthesis of the peptide chains of proteins from RNA code is accomplished by ribozymes. They also catalyze splicing of RNA.”
Without going into too much biochemistry (which can be read in the article), Moritz explains how, while finding a process whereby organic compounds could form early RNA is difficult and problematic, current scientific evidence and research suggests that life could likely have plausibly formed from a spontaneous transition of organic compounds to RNA (or through another precursor between organic compounds and RNA):
“The hypothesis that a so-called RNA World was involved in the early evolutionary stages of life is now an almost universally held view (Joyce 2002, Orgel 2004, The RNA World 2006). Could this RNA World have stood at the ultimate origin of life? This is currently still an open question. The RNA system may be too complex to have arisen without synthesis by a genetic precursor or prior enzyme-less metabolism … Yet while there are still substantial problems, there are now good leads for simple, spontaneous processes on the early Earth for both the synthesis of nucleotides and their concatenation to oligonucleotides.”
With early RNA in place, self-replication would be the next step needed for the evolution and increasing complexity of life.
3. Gradual Build-Up of Complexity:
Moritz next explains how early early self-replicating structures could have gradually led to the more complex coding found in life today:
“Let us assume the plausible scenario that either RNA was directly synthesized, see above, so that out of a large pool of random RNAs a self-replicating RNA molecule could arise, or that such synthesis was accomplished by a precursor genetic/catalytic system (possibly on the surface of minerals, cf. Orgel 2004). Since fatty acids could have been available in the environment (Hanczyc et al. 2003, Orgel 2004), a primitive fatty acid membrane could have surrounded the first self-replicating RNA molecules (due to their molecular properties, fatty acids can form vesicles spontaneously); this would not have allowed passage of the RNA polymers so that they would have stayed together, but would have let the much smaller nucleotides through, fed in from spontaneous prebiotic synthesis or from a precursor genetic/catalytic system. Such a membrane would have had different characteristics of semi-permeability than modern lipid membranes, where a lot of molecule transfer is regulated through protein channels.
The group of Jack Szostak has performed extensive and plausible studies that these fatty acid vesicles as containers for RNA would have allowed growth and replication merely by physico-chemical mechanisms, until a more sophisticated membrane machinery, steered by the cell itself and more resembling what is found in current organisms, would have taken their place (Hanczyc et al. 2003, Chen et al. 2004, Hanczyc and Szostak 2004, Zhu and Szostak 2009).”
Once the more complex cellular structures for self-replication had been reached, the process of replication along with occasional mutations could allow for the natural selection that drives biodiversity today. As Moritz explains:
“In summary, based on available data a spontaneous origin of life as simple ‘cells’ containing a single genetic polymer, upon which natural selection could act, is feasible. A gradual evolutionary transition from these to common cellular complexity would have been possible.”
Moritz discusses much more in his article (which I highly recommend reading), including the importance of minerals to the origin of life, ‘metabolism-first’ versus ‘gene-first’ scenarios of the origin of life, and the origin of the homochirality of amino acids and sugars. Mortiz summarizes the outlook for the future scientific research of abiogenesis as follows:
“Even though recent, exciting research has provided plausible scenarios for the origin of life and has answered many questions, it is clear that a lot of research remains to be done, since much of the origin-of-life scenarios is still hypothesis. Experimental models are needed that are both realistic and of some appreciable complexity. Were it possible, for example, to show that a primitive RNA organism could be built in the laboratory (Szostak et al. 2001), it would be a significant step forward. For this, see Carl Zimmer’s article; there also the hope is expressed that evolution of such an organism might be observable on the lab bench.”
Future scientific discoveries will reveal more, but, as things stand now, it is completely plausible that non-life transitioned to life on Earth purely due to natural processes, and abiogenesis is the dominant theory among professional biologists today.
A final note is that abiogenesis did not have to even occur on Earth! Due to the debris that are shot into space when one planet is hit by an extraterrestrial collision, it is completely possible that life began on another planet, was ejected into space by an impact event, and was then carried through debris orbiting the Sun into an eventual collision course with Earth, thus seeding this planet with life. New scientific evidence even suggests that life in our solar system may have even originally began on Mars. Another recent study suggest that two of the building blocks of life on Earth — DNA components and amino acid components — could possibly have arrived on our planet via space dust. Since life can be carried in inter-planetary debris, it is perfectly plausible that life began on another world than our own and was later brought to Earth, opening up even more plausible scenarios for natural abiogenesis.
Unguided Evolution from Common Descent:
Once life had begun on Earth, modern biology is very clear that evolution from common descent is how we reached the state of biological diversity seen on Earth today. There is no serious debate about this in the biological community, and thus evolution from common descent may be termed a “scientific fact.”
What is evolution and how does it work? The University of California, Berkley provides an “Evolution 101” overview online that will be helpful for explaining the concept:
“Biological evolution, simply put, is descent with modification. This definition encompasses small-scale evolution (changes in gene frequency in a population from one generation to the next) and large-scale evolution (the descent of different species from a common ancestor over many generations). Evolution helps us to understand the history of life.”
Evolution is not just about change, however, but also is the unifying theory that explains the diversity and early origin of all life on Earth:
“Biological evolution is not simply a matter of change over time. Lots of things change over time: trees lose their leaves, mountain ranges rise and erode, but they aren’t examples of biological evolution because they don’t involve descent through genetic inheritance. The central idea of biological evolution is that all life on Earth shares a common ancestor, just as you and your cousins share a common grandmother. Through the process of descent with modification, the common ancestor of life on Earth gave rise to the fantastic diversity that we see documented in the fossil record and around us today. Evolution means that we’re all distant cousins: humans and oak trees, hummingbirds and whales.”
Technically evolution is a process that can occur with any form of descent and modification (including the evolution even of ideas with Memetic Evolution); however, as discussed above, biological evolution on Earth occurs only in carbon-based organisms through reproduction and the mutation of genetic coding.
The major processes that drive biological evolution on Earth are as follows:
- Descent and the genetic differences that are heritable and passed on to the next generation;
- Mutation, migration (gene flow), genetic drift, and natural selection as mechanisms of change;
- The importance of genetic variation;
- The random nature of genetic drift and the effects of a reduction in genetic variation;
- How variation, differential reproduction, and heredity result in evolution by natural selection; and
- How different species can affect each other’s evolution through coevolution.
Add time to the mechanisms described above, and speciation will naturally occur across generations, leading to the biological diversity that we see today, along with the origin of the human species.
The evolutionary history that led to the origin of the humans species is as follows:
- Simple cells: 3.6 billion years
- Complex cells: 2 billion years
- Multicellular life: 1 billion years
- Simple animals: 600 million years
- Fish and proto-amphibians: 500 million years
- Land plants: 475 million years
- Amphibians: 360 million years
- Reptiles: 300 million years
- Mammals: 200 million years
- Primates: 60 million years
- Hominids (great apes): 20 million years
- The genus Homo: 2.5 million years
- Anatomically modern humans: 200,000 years
As can be clearly shown, human beings were not fashioned in the image of any divine creator, but emerged solely due to the natural forces of mutation and natural selection driving the speciation of our species from previous species. Not only that, but human beings are not even the only intelligent hominid to even exist! Other intelligent apes, such as homo neanderthalensis accompanied us in our evolution. Such a progression demonstrates that there is nothing particularly special about human biology. We are the same as other animals.
Likewise, we civilized, city-dwelling humans hardly make up the bulk (let alone even a substantial fraction) of our species’ history. After anatomically modern humans emerged, there were still many more stages of our advancement into civilization:
- Anatomically modern humans emerge: 200,000 years
- Behavioral modernity: 50,000 years
- Humans inhabit nearly all ice-free parts of the globe: 12,000 years
- Agriculture begins: 10,000 years ago
- Earliest known monotheistic religion: 1351 or 1353 BCE (Reign of Akhenaten in Ancient Egypt)
As can be seen, for 75% of our existence we humans were not even behaviorally modern! We have only been living in permanent agricultural settlements for about 5% our species’ existence, and monotheism has only existed for a tiny fraction of 2% of our species’ history. If a monotheistic God exists, our species has left no trace of any concrete theology or knowledge of him/her/it for a whopping 98% of our existence! Instead, the numbers above show how we humans reached our conditions today through nothing but gradual changes. Most of our existence has been wild.
I described evolution as “unguided” under the heading of this subsection. The reason why evolution can be described as “unguided” is because it is driven by accidental mutations that occur during descent. While natural selection cannot be said to be “random,” it is certainly not guided, as natural selection occurs in large part due to random fluctuations in a species’ environment. The result is that evolution is a “blind” force of nature, in accordance with the definition of naturalism that I provided earlier in this series.
Some apologists, such as Alvin Plantinga, have advanced the pseudo-scientific idea of “guided evolution.” There is no scientific description of what this would be, but the underlying idea is that life is not purely driven by natural causes, but is also influenced by outside supernatural intervention. Bear in mind that evolution naturally produces all sorts of nasty things: contagious diseases, predators capable of ripping people apart, and harmful mutations that damage the host. Unguided evolution does this without any design, so that there can be no blame when unfortunate natural consequences occur. But it makes no sense at all to believe that a benevolent deity would intervene to create horrible diseases, such as Smallpox, for example.
Plantinga (Where the Conflict Really Lies: Science, Religion and Naturalism, pg. 39) claims:
“It is perfectly possible that the process of natural selection has been guided and superintended by God, and that it could not have produced our world without that guidance.”
But, given how natural selection produces all sorts of terrible diseases and problems, how can Plantinga reconcile these natural evils with his God’s alleged omnibenevolence? Here is Plantinga’s answer (pg. 59):
“Satan and his minions … may have been permitted a role in the evolution of life on Earth, steering it in the direction of predation, waste, and pain…”
I will leave it to the reader to decide whether it is more probable that diseases like Smallpox exist because of unguided natural mutations that sometimes have terrible consequences, or whether “Satan and his minions” are at work guiding evolution towards evil consequences. Regardless, according to actual biological science, evolution is a chance event, without any such malicious interference.
So, if life on Earth began when non-living matter arrangements took shape into living matter arrangements, and if it is due solely to physical mutations in the process of replication that evolution and biodiversity exist, then what does that say about us humans?
One consequence of the naturalist scenario described above, where non-living material transitioned into living material and eventually reached the physical state of us humans, is Mind-Body Physicalism . Physicalism is the view that everything in reality is physical, or is supervenient upon the physical. Mind-body physicalism entails that we humans, including our complex minds, thoughts, and memories are all driven by physical processes in the brain.
Mind-body physicalism is the dominant view in professional philosophy. 54.4% of professional philosophers with a PhD are physicalists (28.9% are non-physicalists and 16.7% are “other”). Moreover, out of philosophers who specialize in Philosophy of Cognitive Science, a whopping 74.6% are physicalists (18.9% are “other” and only a sparse 6.6% are non-physicalists), showing how belief in mind-body physicalism increases with professional expertise in the field.
The reason why physicalism is so dominant in professional philosophy is due in part to the overwhelming evidence that has been amassed in the study of neuroscience, along with the coherence of physicalism as a theory predicting all of the features of the mind that we observe today. As naturalist philosopher Richard Carrier (Sense & Goodness Without God, pp. 156-157) explains:
“The evidence seems clear: our mind, hence our very existence, depends entirely on the brain. As a mechanism, the brain must be kept healthy and active, so it can remain a system of coherent perception and thought, and we can remain ‘conscious’ and experience life itself. But stop the brain from functioning, and we can experience nothing. Our ‘consciousness’ ceases to exist.”
“The positive evidence for mind-brain physicalism also presents an almost insurmountable challenge for opponents. For example, scientists have confirmed that we only perceive things after our brains do, not the other way around. When we see a face, our brains already show activity in the area that recognizes faces a fraction of a second before we are aware of seeing a face at all. Likewise, when we make a decision, say to move our arm, we know our brain has already sent the signal to move the arm (and thus has already decided to move it) a fraction of a second before we become aware of making such a decision. This is very hard to explain unless physicalism is true. For only then would perception be a process occurring in a physical organ, one that takes time (a fraction of a second) to complete itself, and only then would self-awareness itself be such a process of perception.”
“Consider a different problem. There are many people who suffer from a condition called synesthesia, where their brain fails to physically separate sensory processing, so they ‘see’ sounds or ‘hear’ colors, and so on. If this is what happens when the brain’s wires get crossed, how can a disembodied soul experience distinct sensations? After all, the soul has no physical wires to keep such sensations separate, and clearly can’t keep them separate in synesthetes. Yet every out-of-body experience has been reported without synesthesia.”
“I could list a dozen other similar problems that opponents of mind-brain physicalism have a hard time answering, but that neurophysiologists can answer easily, often with hard evidence to back them up.”
But, if our minds are purely physical objects, moving with the matter and energy around them like everything else in nature, how could we ever have freewill? Wouldn’t we just be meat zombies with no internal awareness, changing as the result of physical processes around us?
In the next part of this series, I will be discussing Freewill and the relation of determinism with agency. Although people commonly think that determinism (entailed by both the B-Theory of time discussed earlier in this series and the discussion of Mind-Body Physicalism in this article) excludes the possibility of freewill existing, most professional philosophers do not see it that way. In fact, 55.7% of professional philosophers with a PhD hold to the view known as Compatibilism, which maintains that freewill can still exist in a determined naturalist universe (16.7% are Incompatibilists, 12.9% lean towards “no freewill,” and 14.7% are “other”).
I will likewise be discussing how philosophical concepts like Ethical Realism and Abstract Objects are also compatible with mind-body physicalism and naturalism, in addition to discussing Epistemology and warranted naturalist belief. Stay tuned to read more!
 It should be conceded that not all Christians literally believe the Genesis creation account. Nevertheless, Christian scriptures are clear that God directly created humankind in his own image, very different from a scenario involving natural abiogenesis and human beings evolving from previous animals.
 It should also be conceded that abiogenesis could hypothetically occur in a theistic universe, where a deity had simply created the universe and its laws, and then allowed life to emerge through physical processes within the universe. Such a creation, however, would be very different from the revealed theism described by most world religions, such as Christianity, which specifies in its sacred scriptures (Gen. 1:26) that God (not some natural process) directly created humankind in his own image. Certainly, a universe without abiogenesis would provide much better evidence for the existence of God than one in which the deity had almost concealed its involvement by allowing life to emerge under gradual processes, with the the appearance of no design whatsoever (see the problem of divine hiddenness). Regardless, in a naturalist universe, abiogenesis would be essential for the emergence of life, since such life would have to be reducible to gradual natural processes, excluding life with irreducible complexity or any signs of intelligent design. As this article argues, life reducible to abiogenesis is almost certainly the case. I would further argue that atheistic naturalism is a far better explanation for abiogenesis than a divinely hidden theism, although I concede that abiogenesis is conceivable in a theistic universe.
 As the Encyclopedia of Astrobiology (v. I, pp. 1062-1063) explains, “A mildly reducing atmosphere is an atmosphere containing small but significant amounts of reduced gases, for example, H2, CH4, NH3, H2S, or CO, and not containing a counterbalancing excess of oxidized species such as O2. Many believe that the prebiotic Earth’s atmosphere may have been mildly reducing due to mantle outgassing prior to the advent of biological oxygenic photosynthesis. In general, reducing atmospheres appear to be favorable for the abiotic synthesis of organic compounds.”
 That said, I do not think that a minimal definition of naturalism necessarily entails physicalism, as I explain in my article, “Defining the ‘Natural’ in Metaphysical Naturalism.” However, since I am a physicalist myself, I will be defending the view of mind-body physicalism in this metaphysics series.