Just what does the Catholic Church think of the theory of evolution? To be honest, it depends on which theory of evolution we mean. More than that, it depends on what we believe about the explanatory power of the theory. There are some who seem to treat the “theory of evolution” more like an ideology than like a scientific theory. So we need to be clear on what we mean by “evolution”.
There is an amazing variety of living creatures on this planet. These creatures, however, can typically be classified into broad groups which share common characteristics. Plants are different from animals, for example. Among animals, vertebrates are different from invertebrates. Among vertebrates, mammals are different from birds. The family of mammals can itself be broken down into its own subdivisions (the cat-like mammals being different from the ape-like mammals, and so on). Eventually we get to a special level known as a species, and this is where the fun begins.
The levels of biological subdivision above that of the species are usually known as a genus, a term that is related to English words like “generic” and “general”. A genus is a category of species that possess certain common traits, such as having a backbone (vertebrates) or having feathers (birds). The word “species”, on the other hand, is related to English words like “special” and “specific”. This word also describes a biological subdivision, but a biological subdivision with a special feature for creatures that reproduce sexually: its members are able to interbreed and produce non-sterile offspring. The capacity to reproduce is what makes individual animals members of the same species. A chicken is a vertebrate; so is a monkey. But a chicken and a monkey, despite both being vertebrates, cannot reproduce together. They may be part of of the same genus, but they are of two different species.
As an aside, I should point out that there are types animals that are so closely related that they can produce offspring, despite being members of two different species. A donkey and a horse, for example, can mate and produce the animal we know as a “mule” (or in rarer cases, produce a “hinny”). A tiger and a lion can also mate, believe it or not, and their offspring are known as “ligers” and “tigons”. A key feature of these inter-species hybrids, however, is that the offspring are either sterile or can only mate with a member of a parent species. As such, they do not consitute a new species of their own.
I should also point out that biologists also identify groups below the level of species, known as a sub-species. Again, these are animals which share particular characteristics, and so are grouped accordingly; however, they are still able to inter-breed freely within the larger species, and so cannot be considered separate species of their own. One of the most visible examples of this principle can actually be found within our own species, Homo sapiens. People from sub-Saharan Africa tend to have dark brown skin; people from northern Europe often have blue eyes or blonde hair; and people from eastern Asia tend to have upper eyelids without a visible crease. These distinctions mean nothing, however, when it comes to the capacity to reproduce, such that we humans really are part of one overall species.
Where did all these different species come from? The question is particularly compelling when we examine the fossil record, where we discover the remains of countless creatures that simply don’t exist anymore. One of the most common fossil types is the trilobite, and yet they disappear from the fossil record at a distant point in Earth’s past; it would seem that trilobites have been extinct for at least 250 million years. If species after species can be shown to have gone extinct, why are any species alive now at all, and with such amazing diversity? While trilobites disappear after a certain point, however, what is really interesting is how evidence of certain types of creatures cannot be found before a certain point. One of the best documented cases of this is that of the modern horse.
Evidence for the existence of modern horses can only be found starting from about 1 million years ago. However, prior to that point there is evidence of another creature, called “pliohippus”, whose remains closely resemble that of a modern horse while not being exactly the same (it is significantly smaller with differently-shaped bones). Remains of pliohippus, however, can only be traced back to approximately 10 million years ago. Prior to pliohippus, we discover the remains of a similar-but-different creature called “merychippus”. Merychippus cannot be found prior to 30 million years ago, but “mesohippus” (another similar creature) can, which in turn cannot be found prior to 40 million years ago. The earliest horse-like creature we can find in the fossil record is “esohippus”, which was approximately the size of a dog and had toes. If we were to put a modern horse and an esohippus beside each other we might have trouble believing that they were related creatures at all, but that is without the evidence of the intervening years. It is like putting a picture of a person at age 1 and another picture of the same person at age 100 next to each other — the two look very different, and yet there may very well be a strangely compelling similarity between them at the same time. If we were to “fill in the blanks” with other pictures from intervening years, though, the similarity would be even more striking, until a true continuity could be found. We would realise that all the photos were actually of the same person!
While individual human beings certainly do change (dare we say evolve?) during their lifetimes, however, the case of the modern horse is an example of what “evolution” really means: that a species taken as a whole can change over time.
Human beings have long recognized that children inherit certain characteristics from their parents, and not only among our own kind: after all, we have been breeding dogs for centuries, selecting parent dogs with particular traits so that those traits might appear in a concentrated form in the offspring. We have typically bred dogs for their utility in certain environments: huskies, for example, are excellent cold-weather animals, while Newfoundland dogs have webbed feet and an amazing capacity to swim. These dog breeds are another example of sub-species, in that while the different breeds might be quite different from each other they can still mutually reproduce. But what would happen if specialized breeding eventually meant that each breed became so different from the other that they couldn’t make babies with each other anymore? They would then be considered two different species, and an “evolutionary leap” called speciation would be considered to have taken place.
Speciation is difficult to observe in practice, because it is assumed to be a gradual process that takes place over many generations — generations which, taken together, add up to a longer period of time than the lifetime of the observer! Despite this, we do know that the human breeding of animals has indeed resulted in speciation. Domestic sheep are the result of centuries of the breeding of what were originally wild sheep, including the mouflon. But while the mouflon still exist as an independent species, domestic sheep and mouflon can no longer produce viable offspring together.
So what is evolution? In short, it is the theory that speciation does not only occur as the result of human intervention and breeding programs, but is also a naturally-occurring phenomenon. A particular species may give rise to sub-species, which in turn eventually become their own separate species, and so on, eventually contributing to the amazing diversity of life on this planet as we know it.
This theory of evolution has amazing explanatory power, but it raises a serious question: how does speciation occur in nature? It took centuries for domestic sheep to become a distinct species from the mouflon, and that was with direct human intervention. Given the time scales involved, therefore, we should not be surprised to discover that natural speciation has never been directly observed.
Going beyond the “how” is the “why”: why should speciation occur in the natural world? After all, if particular species are already well-enough adapted to their environments to be able to maintain relatively stable populations, why change?
Enter Charles Darwin, whose book The Origin of Species is considered the ground-breaking book in this area. Darwin not only proposed that new species arise from old, but also offered an explanation of the “how” and the “why”. In short, while each species is in harmony with its environment, that harmony is itself unstable. A major change in climate, for example, can have far greater deleterious effects on one species than on another, causing a major shift in population balance. Beyond this, a change to one of the species that is part of this “balance of nature” can itself upset the current balance. We know that predatory behaviour exists, with nature being “red in tooth and claw”; there is even competition for simpler resources like sunlight, where the tallest trees have the advantage. The potential hostility that exists in nature means that only the fittest creatures survive to pass on their successful traits to their offspring. Why, therefore, do new species arise and achieve a significant level of population? It is because the unstable and changing circumstances of the balance of nature require that creatures be able to adapt to those circumstances, and those adaptations would be heightened by the “survival of the fittest” mechanism which would tend to reduce the availability of mating partners to those creatures that did, in fact, survive.
While the “survival of the fittest” mechanism does tend to explain how one species might replace another, it does not however explain how a new species arises from another. After all, why should we observe new species as a result of a change in climate? Why not just simply a redistribution of the species that currently exist? A second “how” mechanism is therefore required, called mutation. In essence, mutation proposes that changes frequently occur in a species from one generation to the next. Many, perhaps even most of these mutations are actually defects, i.e. they make survival more difficult, and so they are eliminated by the survival of the fittest mechanism. Others may be relatively innocuous, but some confer a marked survival advantage, an advantage that is often most apparent when the balance of nature undergoes a shock (like a climate change). Again, the survival of the fittest mechanism would select for these traits, and the species as a whole would eventually be transformed as the new characteristic spread through the population. It is not really that a new species arises and kills off the old, but that the new species *is* the old one, only improved. Think of the primitive horses mentioned earlier: if this theory of mutation is correct, the modern horse did not “replace” the esohippus, the modern horse *IS* the esohippus after millenia of gradual changes, changes including the development of longer legs and hooves.
The mutation argument alone is not sufficient, however, to explain the rise of new species, because it can only explain the evolution of a single species within its own population. For mutation to result in speciation, therefore, something else is required, such as geographic isolation, or the instinct to only mate with members of the same species.
Think about it. Suppose an animal is born with the ability to better camouflage itself against predators. That animal could then pass this trait on to its offspring, who will tend to survive better and therefore pass that trait on to more offspring, and so on, such that the trait on that one individual becomes a trait of the species. Suppose this same original species also sees the rise of an individual that has an improved ability to catch food. It too could then pass this trait on to its offspring, who will tend to survive better and therefore pass that trait on to more offspring, and so on. One could say that two new sub-species have arisen within the original species thanks to these mutations, but one *cannot* say that speciation has taken place, as the two groups can still interbreed. Indeed, given the “survival of the fittest” mechanism, the best individuals would be those that possess both mutations, such that the two branches would tend to merge over time back into a single species branch (albeit one that has “evolved”). For mutation to result in speciation, therefore, either single mutations must be sufficiently “large” as to result in incompatible breeding populations right away, or there has to be a way for two populations of the same species to remain distinct long enough for the many small mutations to add up to a breeding incompatibility.
The theory of “large” mutations has not tended to be favoured, simply because it would require that several individuals arise simultaneously with the same mutation; otherwise, the new individual would be (in effect) a species with a population of 1, and would go extinct as soon as that one individual died (as the breeding incompatibility would mean it could never pass on its traits to fertile offspring). Typical evolutionary theory proposes that mutations occur randomly, and so mutations that result in speciation (no matter how successful for the individual) simply do not happen. On the other hand, if it were possible to propose an evolutionary theory that allows for simultaneous “large” mutations, it would go a long way to explaining speciation.
For “small” mutations to result in speciation, therefore, some other mechanism is required. Geographical isolation is a commonly proposed explanation, the idea being that two populations that are sufficiently isolated will develop independent sets of minor mutations, the cumulative effect of which will be populations so distinct that they can no longer interbreed (i.e. they will have become separate species). Again, while speciation has never been observed in nature, we do nevertheless see a tantalizing hint of the speciation mechanism in the Larus gulls that live around the North pole. These birds have a very broad geographic range, and gulls that live on one extreme of the range cannot breed any longer with gulls that live on the opposite extreme. These populations are different species to each other.
Another mechanism to explain how populations become isolated does not depend on geography but on the instinctual mechanisms for selecting mating partners. The basic idea is that “like attracts like”. How do cats know to only mate with other cats, and not with dogs? On some level they grasp their own “catness”, and they are drawn to mate only with other cats. The same effect may occur when a mutation happens: mutants may be drawn to mate only with similar mutants, while original members of the species might tend to reject mating with mutants in favour of their own kind. At first glance it might appear that this approach would require the appearance of several mutants at once, and therefore this theory would suffer the same problems as the “large mutation” approach already seen, but this is not necessarily the case. A new mutant would be at a disadvantage, of course, but because its mutation is not “large” it can still breed with the dominant population. It the mutation truly did confer a major survival advantage, such that this advantage meant that it could “force” its traits onto the population at large, eventually a small mutant population would develop, and the “like attracts like” mechanism could take place within those different populations. New mutations would accumulate within those populations, until they speciated. They two groups may not be separated by distance, but they nevertheless do not mix until eventually they cannot. This “like attracts like” mechanism has been observed experiments with multiple generations of fruit flies, where the descendants of these multiple generations show they prefer to associate with (and mate with) fruit flies that share similar characteristics.
The problem with both of these arguments is that while individual populations may be out of direct contact with each other, they are rarely without some sort of mediated contact. For the gulls, the “extreme end” populations may be species to each other, but they are considered sub-species with relation to the population in the middle of the range, as both populations can breed with the population found in the middle. The case of the Larus gulls certainly appears to be an example of sub-species become species in their own right; on the other hand, it is also possible that the reverse is occurring, and that the two previously incompatible branches will actually collapse into a single branch thanks to the presence of this “intermediate population”. In other words, while evolution as described could result in the rise of divergent species, it could also result in the merging of previously incompatible species thanks to new “bridge” species emerging (i.e. evolution could also be “convergent”). Nevertheless, if dependent on random mutations, evolution is likely a largely divergent process, as more specific mutations would be needed for the creation of reproductive compatibility than reproductive incompatibility.
Despite the incomplete nature of these explanations, and the practical impossibility of directly observing a speciation event, it is still possible to demonstrate the likelihood of evolutionary speciation in an indirect way. If new multiple new species truly can arise from a single parent species, these new species can be said to have a common ancestor. If the existence of a common ancestor can be demonstrated, then the fact of speciation can be affirmed even if it cannot be entirely explained. Much of the sifting through the fossil record has been an attempt to find such common ancestors in the distant past. Unfortunately, because resemblance does not necessarily demonstrate descent, this is essentially impossible to prove. For example, one would expect that a common ancestor of birds and reptiles would share features of both, but the discovery of a fossil that resembles a bird in some ways and a reptile in others does not necessarily mean that this fossil is that common ancestor. After all, perhaps they resemble each other because each independently evolved to be well adapted to a similar environment (such as the resemblence between dolphins and ichthyosaurs). Simply put, the fossil record cannot provide direct evidence of common ancestors.
The recent discovery of the importance of DNA in living organisms, on the other hand, has changed this situation. “DNA” is the short form for deoxyribonucleic acid, a chemical common to all living organisms that contains the “blueprints” for how that organism is to be structured (mostly by coding for the development of the proteins used in the various chemical processes of the body). The various amino acids found in DNA act as letters of the genetic alphabet, and their collective set is called the genome. The slightest change in the sequence of amino acids in a gene can have a major outcome: the origin of the deadly disease cystic fibrosis, for example, can be traced to changes in a single gene.
Because DNA patterns are transmitted from one generation to the next, common DNA patterns can be used to identify members of the same family — or even the same species. What research has also discovered, however, is that there are often large portions of DNA that are common across species boundaries. These portions are apparently inactive, so their presence cannot be explained by the same kind of independent-but-convergent process of natural selection that produced dolphins and ichthyosaurs. Instead, it is far more likely that these portions of DNA are simply the “fossil remains” of DNA that was once part of the genome of an ancestor and that is still passed down. But because these portions of DNA can be common across certain species boundaries (depending on the organisms under discussion), their presence points to the existence of a common ancestor. It is not direct proof, but it certainly is compelling evidence, such that the study of these similarities and differences in DNA patterns is provoking the revision of our current classification of species. Before, we used to classify the relatedness of species based on similar external characteristics; now this data is being completely reviewed, using the internal evidence of DNA.
So what is evolution? It is a scientific theory that states that the characteristics of a given species can change over the course of generations, and that these changes can result in reproductive incompatibility such that a new species can be said to have resulted. While the process of speciation has never been directly observed in the natural world, certain DNA evidence certainly points to the existence of common ancestors for reproductively divergent species. Evolution, therefore, is more than a mere hypothesis: there is serious evidence that it has actually occurred. How evolution works, however, is still open for discussion. The most common explanations involve minor random mutations accumulating over time, with defects being weeded out by a “survival of the fittest” mechanism, but as we do not know enough of how a genome “works” there are still plenty of questions to answer.
Where does the Catholic Church stand on all of this? Pope John Paul II made international headlines in 1996 when, in an October 22 speech to the Pontifical Academy of Sciences, he spoke of evolution as “more than an hypothesis”. Some were shocked, while others applauded, with both groups believing that such a statement represented a change in attitude of the Catholic Church on the question. Those aware of the issues, however, simply shrugged, as the Catholic Church has never had a real difficulty with evolution as such. Pope Pius XII, for example, wrote an encyclical letter in 1950 entitled Humani Generis, in which he expressly authorized research into evolution:
The Teaching Authority of the Church does not forbid that, in conformity with the present state of human sciences and sacred theology, research and discussions, on the part of men experienced in both fields, take place with regard to the doctrine of evolution, in as far as it inquires into the origin of the human body as coming from pre-existent and living matter—for the Catholic faith obliges us to hold that souls are immediately created by God. (HG, no. 36)
At the time Humani Generis was written, evolution was little more than a theoretical framework — a compelling one, no doubt, but lacking evidence. By the time Pope John Paul II spoke 46 years later, however, evidence (particularly DNA evidence) was now present. His statement, therefore, that evolution was more than a mere hypothesis was simply recognizing the current scientific state of affairs.
The theory of evolution has, however, tended to get hijacked. Because its explanatory structure is very compelling, evolutionary explanations have also arisen regarding the structure of societies and cultures, and even the structure of human ideas (such as the theory of memes). These evolution-like paradigms often become actual ideologies, with recommendations for how human behaviour should adapt (social Darwinism being one of the first examples). Some even propose theological conclusions from the evolutionary paradigm, implying that evolution is a closed system that needs no intervention from God (a position that completely ignores the philosophical distinction between primary and secondary causes, I might add). The Catholic Church, therefore, has tended to be cautious when it comes to voicing support for evolution — not because it is suspicious of the physical theory, but because the word “evolution” is also often associated with a package of ideas and conclusions that fall well outside the bounds of the merely physical theory.
As for myself, I am firmly in agreement with Pope John Paul II. The biological theory of evolution, which modestly states that speciation has occurred in the natural world, certainly seems to possess sufficient evidence to go beyond the stage of a mere hypothesis, although much more work remains to be done. But I also agree with the modesty of John Paul II (and Pius XII before him), when they caution against potentially misapplying the explanatory structure of the theory of evolution, particularly when that explanatory structure is still in need of fine-tuning regarding how speciation actually occurs. I accept evolution, but that does not mean I accept the package of secondary ideas that have become associated with what is otherwise a merely biological theory.
So why can’t we use this explanatory structure in other ways? Because doing so rests on the hidden assumption that man is merely a biological entity. The explanatory structure of the evolutionary paradigm is compelling thanks to its elegant simplicity, and so I suppose people can hardly be blamed for using it in the development of other theories, ideologies, and even theologies (or anti-theologies, as the case may be). But what if human beings also have spiritual souls? The evolutionary paradigm is not built to account for possibilities such as this, and therefore the application of that paradigm, as though it were complete in itself, to realities that it might not be able to explain fully could have monstrous consequences: the full reality of human nature would not be respected, and the resulting ideological recommendations for action would actually be dehumanizing and destructive to happiness.
It should be no surprise, therefore, that Pope Pius XII mentioned the concept of the spiritual soul as a boundary between evolution as a biological theory and evolution as an ideological package. And it should equally be of no surprise that part 2 of this series on human origins will be on the human soul.