Advocates of Intelligent Design (ID) creationism like to play Stump the Scientist by asking evolutionary biologists, "what would disprove evolution?" Several answers have been given1, but I want to add another: "spontaneous generation," meaning the creation of organisms without parents. The question of how to disprove evolution draws its power from the idea of falsifiability, which was formalized by the philosopher Karl Popper as the test to separate science from non-science2. This question also raises the more general issue of whether biologists are open to non-evolutionary explanations for the diversity of life.

My response demands a mechanism that could supplant evolution as an explanation for the patterns of biodiversity that we observe in the world. This is not the most elegant way to evaluate evolution, especially because we have no way to pursue research into spontaneous generation. My focus on how an alternative theory could undermine evolutionary theory should not be interpreted to mean that  the only way to discredit evolutionary theory is to find a better explanation. There are several facts about the world that are necessary for evolutionary theory to be meaningful; if these facts were not true, then evolutionary theory would be discarded and we would have no good scientific explanation for the diversity of life. However, these facts have been investigated so thoroughly over the past 150 years that their description is best covered in textbooks, not in an essay3.

Consideration of spontaneous generation as an alternative theory is valuable for how it informs the current debate surrounding evolution. It demonstrates how a new observation could cause researchers to discard evolutionary theory, thereby illustrating the scientific process, and consequently showing how evolutionary theory is well founded whereas ID is nothing more than wild speculation. As a bonus, it provides some historical context for the development of evolutionary theory, since many scientists believed in the reality of spontaneous generation, both before and after the 1859 publication of Charles Darwin's On The Origin of Species4.

This device was central to his experiments of 1860-1862, demonstrating that broth remains sterile even when air is permitted to enter the flask, but dust is excluded. These experiments largely ended the debate on spontaneous generation, although heat-resistant spores continued to confuse the issue until 1876. Figure: Pasteur's swan-neck flask: this device was central to his experiments of 1860-1862, demonstrating that broth remains sterile even when air is permitted to enter the flask, but dust is excluded. These experiments largely ended the debate on spontaneous generation, although heat-resistant spores continued to confuse the issue until 1876.

To evaluate how spontaneous generation could discredit evolutionary theory, we first need to define "evolution" and "spontaneous generation". Evolution is a vast and complicated field of study containing many individual hypotheses -- after all, it encompasses the entire diversity of life on Earth over the course of billions of years. The most inclusive hypothesis that I can think of is the assertion that all life on Earth is descended from an ancient population that lacked most of the diversity that we see in modern life. I have high confidence in this hypothesis, which I'll call the Universal Ancestor Hypothesis. As for spontaneous generation, it could take any number of forms, including a conjurer who causes life to appear by speaking a few words, a magical cave in a tropical jungle with new life pouring out of it, or a laboratory concoction that suddenly teems with life. It could not rely on a complicated modern technology; otherwise, it could not explain anything that existed before that technology was developed.

The rejection of the Universal Ancestor Hypothesis would not mean that "evolution" had been rejected, though its explanatory power would be substantially diminished. For instance, if the Universal Ancestor Hypothesis were found to be false, it might be replaced with hypotheses such as, "all plants descend from a single ancestral population, while all animals descend from a separate ancestral population," as Darwin himself suggested in On the Origins of Species5. Only by rejecting all genetic relationships between species would evolution be completely rejected as a historical theory, though mutation and selection would still have a role in describing contemporary population dynamics.

The discovery of spontaneous generation would only undermine the Universal Ancestor Hypothesis if it provided a better explanation for the patterns of biodiversity that exist in both contemporary populations and the fossil record. Researchers would first have to show that spontaneous generation occurs on a regular basis, both so that it can be validated as true spontaneous generation, and also so that it could have made substantial contributions to biodiversity. The implications of this discovery for evolutionary theory would depend on how similar these spontaneously generated organisms are to known life.

We may find that they are nothing like known life, in which case spontaneous generation would have no evolutionary implications, except perhaps in the distant past. At the other extreme, we may find that all types of known life can be spontaneously generated, leading us to conclude that existing populations were actually established by spontaneous generation, rather than evolution. For instance, if we find a spontaneously generated mouse that is fully interfertile with known mice, then we would probably conclude that mice had not evolved from non-mouse ancestors. If we observe a wide variety spontaneously generated organisms from several branches of the tree of life, we might conclude that the tree does not represent any sort of historical process. However, there would still be room for explanations that involve both spontaneous generation and evolution. If we find mice but not rats, we may suspect that rats had evolved from mice. If we find only microbes, we may develop a theory where "lower" organisms are constantly generated spontaneously, but "higher" organisms have evolved from the lower organisms that populated the earth eons ago. This theory was actually somewhat influential during the 19th century, when it looked like spontaneous generation only involved microbes. Among its many supporters was Jean-Baptiste Lamarck, who was known for developing a theory of evolution that lacked  natural selection.

The point of the above research program would be to describe a variety of processes that can create organisms so that historical models can be built around them. Without this basic research, we could not compare a historical model based on spontaneous generation to the standard evolutionary model based on genetics and ecology. Unless a historical model is constrained by independent knowledge of how life exists, it can explain anything.

Legitimate evolutionary theory is always limited to events that are plausible given our existing knowledge of life, and historical inferences often give rise to novel hypotheses about what life is capable of. For instance, if an analysis of DNA sequence data leads to the inference that an antibiotic resistance gene has been transferred between two species of bacteria, then we can test whether this transfer of resistance can be observed under laboratory conditions6. This type of experiment has been common since Darwin tested whether various seeds could germinate after exposure to seawater, to test his evolutionary hypothesis that plants had spread from the continents to oceanic islands.

The scientific process described above illustrates the fundamental problem with Intelligent Design theory. Having never witnessed anything approaching this "intelligent design"-- not even any non-human bioengineering or breeding -- we cannot say how a history of ID would be apparent in today's world. ID theory pretends to explain patterns of biodiversity, but it makes no concrete reference to the process that supposedly generated that biodiversity. From one perspective, we can say that this makes ID implausible (i.e.the chance that ID happened is infinitesimal). From a different perspective we can say that this makes ID unfalsifiable because any possible observation of the modern world would be consistent with ID -- we would just have to assume that the designer was the type that would create the world as we see it. This infinite malleability is what Popper identified as the fundamental flaw of the Marxist school of historical analysis2, and likewise this is what makes ID pseudoscience. Of course, advocates of ID claim that it is testable7, but in the absence of any independent knowledge of what ID looks like, a responsible scientist cannot infer that it has occurred in the past.

The historical debates over spontaneous generation also inform the philosophical issues surrounding the general creationist movement, which has latched onto ID as though it could prove the existence of a god, despite its secular raiment8. Both Darwin's evolutionary theory and the spontaneous generation theories of his contemporaries were viewed as threats to religion. Spontaneous generation was thought to occur due to the intrinsic properties of matter, self-organizing to create living cells, so it too contradicted the traditional Christian creation story. One lesson from this is that evolution is not the only materialistic theory that could explain the diversity of life, so the rejection of evolution does not implicitly support creationism. Indeed, if we are willing to make historical inferences that are unconstrained by our knowledge of how life behaves today, then we could surely think up countless explanations that are just as plausible as creationism.

The flip side to this is that religion does not depend on creationism. While many people who were raised on creationism had trouble incorporating evolution into their personal theology, plenty of religious leaders have recognized that the core of their theology does not depend on what we can discover about the material world8. If a person can't find God in philosophy or their day-to-day experiences, then a half-baked historical inference such as ID isn't going to change that.


[1]Wikipedia has several examples of answersto the question of what would disprove evolution.


[3]Others have compiled catalogs of the evidence for evolution.

Two requirements for evolution are so important that I will address them briefly here: life must be capable of changing between generations, and life must have existed for sufficient time to allow large amounts of diversity to accumulate. The issue of age has been settled by geologists, astronomers, and physicists, while the mutability of life has been the focus of cutting-edge biological research for the past century. In microbes, beneficial mutations were known to occur on a regular basis by the 1920's, and over the next few decades these mutations were shown to occur before exposure to the environment that selected for the new traits, as posited by Darwin (PDFs are available for two classic papers:Luria and Delbrück (1943) and Lederberg and Lederberg (1952)).

The dependence of evolutionary theory on these facts puts to lie the claim that evolutionary theory only survives because there in no better explanation for the patterns of biodiversity in the world. There's no excuse for these critics to claim that scientists are incapable of distinguishing between a good explanation and a poor explanation, because anyone familiar with evolutionary theory would know that scientists readily recognize their ignorance regarding the origin of life on earth, despite numerous clues as to how it may have happened.

[4]Sparks of Life:Darwinism and the Victorian Debates over Spontaneous Generation by James E. Strick.

Reviewof Sparksof Life by Gregory Radick

[5]"I believe that animals have descended from at most only four or five progenitors, and plants from an equal or lesser number" --Charles Darwin, On the Origin of Species, Recapitulation and Conclusions.

[6]For instance: Interspecies recombination between the penA genes of Neisseria meningitidis and commensal Neisseria species during the emergence of penicillin resistance in N. meningitidis: natural events and laboratory Bowler LD, Zhang QY, Riou JY, Spratt BG. in the Journal of Bacteriology.

[7]Confusions On Evolution, Creationism,And Falsifiability


  • Some religious groups see evolution fundamentally as a religious
    or philosophical
    issue, and are happy to find any pseudoscince that might
    strengthen their rhetorical position.

  • Many people adhere to "theistic evolution," the viewpoint that God created the world through evolutionary processes. Polls have indicated that the vast majority of  "evolutionists" in the USA are theistic evolutionists. Examples of this view include the

    • Catholic theologians have admitted space for evolution in their theology by emphasizing the role of God as the "primary cause" behind the "secondary causes" of natural law, which is the same distinction that Darwin made.

    • A self-described creationist chemist offers scientific criticisms of ID theory, and a theological criticisms of the tendency to latch onto it.
Image credit: Pasteur's swan neck flask