Perhaps the most common objection made to naturalistic evolution is that some of the steps in the development of life as we know it seem far too implausible to have occurred as the product of natural forces alone. Curators of this view advocate something called “intelligent design,” which is, in their words, “[t]he theory…that certain features of the universe and of living things are best explained by an intelligent cause, not an undirected process such as natural selection.”
Note that intelligent design is not the same as young Earth creationism. While young Earth creationism implies intelligent design, the reverse is not necessarily true. It is possible for believers of intelligent design to accept established scientific facts about the age of the Earth and the billions-year timescale over which life incrementally developed on it. What this ID perspective claims about evolution is specifically that some of its developments are too difficult to have arisen naturally, so they suggest a role for divine intervention to have ordained such “irreducibly complex” biological structures.
Whereas young Earth creationism does offer directly falsifiable (and false) claims about Earth’s geological and biological history, old Earth ID advocates make the not-so-falsifiable claim that such seemingly implausible steps in life’s evolution only came to be because they were facilitated at the behest of an intelligent designer.
That leaves us with the following two theories: (1) evolution aided by divine intervention to get beyond certain difficult junctures; (2) evolution as wholly the product of undirected natural selection, even where ostensibly implausible. So which is it? Since this question is presently unamenable to direct experimentation, I propose making use of some Bayesian inference.
For Bayesian inference, we need to take prior probabilities and revise them one way or another based on additional evidence. As far as evolution is concerned, the key to understanding how such a process that spawns complex and varied life forms is possible via an undirected natural selection is appreciating the brute enormity of the time afforded—we’re talking over four billion years that the Earth has existed for life to develop upon it. That would appear to be plenty of time for the perceivably improbable to occur naturally (in incremental steps) sooner or later.
So it is not difficult to understand how natural selection is the scientifically favored and comprehensive theory for what has driven the process of evolution. (If science has already demonstrated it to explain so many facets, why not the rest? Occam’s razor.) And so it does deserve a high prior probability on this reasoning alone.
But that point being made, it is clear that some in our society are still not convinced. So what I would like to show is that bringing to bear evidence from modern astrobiology—typically unincorporated into the evolution pedagogy—makes the case for undirected, naturalistic evolution of life even stronger than it already is.
First, consider Fermi’s Paradox. In 1950, Italian physicist Enrico Fermi pondered why, amidst the very many stars in the nearby universe which could potentially allow advanced civilization to develop, we had not seen evidence of extraterrestrial civilizations (and still have not, by 2015), nor had we been visited by extraterrestrials (dubious UFO reports notwithstanding). Hence Fermi’s famous question, “Where is everybody?”
Efforts to substantiate the answer (for which gathering empirical evidence is difficult) have been formalized into models like the Drake Equation, and more recently the Great Filter hypothesis. The idea is that one or more conditions for or successive steps in the progressive development of life—past and future—are unlikely enough as to reduce the incidence of advanced civilizations from the vast number of observable stars down to some figure that is orders of magnitude closer to zero. Considering how this galaxy could have been colonized many times over by now, and SETI has yet to ascertain any ET radio signals (the Great Silence), this suggests there are (barring decidedly inconspicuous aliens) significant filters standing athwart the development of life to a sufficiently advanced stage.
So what could be filtering against the proliferation of advanced life? To be sure, there are many candidates, and certainly multiple contributing factors. But one prominent explanation is precisely that certain innovative steps in the evolution of life are prohibitive—due to their natural, undirected origins! Even most planets with the basic requisite conditions may not be lucky enough to have a few of their molecular structures collide in just the right way in order to have obtained a novel innovation that will significantly enhance the development of life upon it. But some Earths out there would—if only by an aberration of random processes on a microscopic level.
In short, the seeming unlikelihood of the natural formation of certain life developments, along with the Great Silence—are complements to one another, and would serve to corroborate purely naturalistic evolution against the need for intelligent design arguments. Why, if there were intelligent fine-tuning for life in this universe, would we not expect to see more signs of it elsewhere? Would it not be more commonplace for divine intervention to have cleared the otherwise multitudinous challenges to it? Indeed, this lacking evidence of extraterrestrials pulls our Bayesian probabilities further in the direction of natural evolution from where they were before.
(In case anyone mistakenly thinks it cuts the other way, let me reiterate: The intuition is that the chance of naturally evolving complex biology is quite low. The fact that complex biology did arise suggests its occurrence is disproportionate to this implausibility, which is what persuades some to seek a special explanation like ID. However, the absence of ET in a surveying of the sky suggests advanced life’s propensity to occur is closer to this low probability associated with natural, non-ID evolution. So Bayesian reasoning uprates natural evolution after observing the lack of ET.)
Oh, but there’s more! Not only does Fermi’s Paradox afford the possibility that the undirected, and thus unlikely quality of evolutionary advancements accounts for the rarity of civilization, but statistical inference from the history of life’s evolution on Earth actually hints toward this explanation. The fact that key biological developments (e.g. the formation of a eukaryotic cell from prokaryotes), and ultimately civilization itself, did not occur for a significant while into Earth’s habitable timeframe suggests they may very well be low-probability events rather than inevitabilities, even on a habitable planet like Earth.
Understanding the novelty of this point requires some knowledge of statistical probability distributions. In the abstract, we can model the occurrence of the “first event” of a critical life advancement by an exponential distribution. To the uninitiated, exponential distributions look like this:
At any given instant in time, the event in question has the same constant probability of occurring. However, since our concern is with the “first” occurrence, any subsequent occurrence will become irrelevant. For this reason, the distribution of probability exhibits a frontloaded, decaying functional form.
Although exponential probability distributions are infinite, the planet’s habitable timeframe is not. So a truncated exponential distribution is what we would need to consider. For events that are more likely, this truncated distribution would be more frontloaded; but for events that are less likely, this distribution would be much flatter, approaching the shape of a uniform distribution.
Moreover, if we are considering that there are more than one such unlikely, probabilistic life developments requisite for civilization, we would want to model two or three or more iterations of these exponential distributions, each successive one within the timeframe remaining after the fruition of the preceding development. And when we combine them (forming a negative binomial distribution of sorts), we get a final probability distribution (for when civilization arises) that is backloaded near the end of the timeframe insofar as some component distributions are sufficiently near-uniform, i.e. represent difficult biological innovations that could take, on average, upwards of tens of billions of years to randomly occur on the planet.
And this is more or less what our own civilization’s timing in the Earth’s habitable timeframe suggests. Earth’s habitability is believed to have begun around 3.9 billion years ago, give or take; it will become unable to sustain complex life as soon as 0.8 billion years in the future as the Sun increases in luminosity, heating up the Earth and throttling the carbon cycle. Standardized on a scale of 0 to 1, our existence as a civilization is around 0.83 in the window, which lends credence to a backloaded distribution, seemingly indicative of there having been one or more low-probability steps in the development of advanced life, which were completed in great measure out of sheer luck.
Note that what I outline here is a basic model for conceptual elucidation. In a more thorough scientific analysis, I would advise additional considerations: (1) the effects of time-correlated variables like solar luminosity, oxygen level, and temperature in biasing the fecundity of life across the period; (2) climate variation rather than biological random walk may account for some of the stochastic nature of life’s progression; (3) “easy steps” (not unlikely, but possibly prolonged as a continuum of imminent steps) should be factored into a more robust model, in which stochastic steps would be mapped on the conjoined intermittent time for statistical inference.
As any good statistician would acknowledge, the limitations of this sample are such that we cannot know what the large-sample parameters actually are based on this information alone. But it would be careless to think our observation is not useful at all! In fact, it is very useful in persuading for or against our priors; here, the distribution of apparent “hard steps” and the fruition of civilization relative to the timeframe of Earth’s habitable history persuades us to further update our Bayesian probabilities in favor of advanced life being a low-probability, chance development, even on a habitable planet—i.e. the result of a purely natural evolution.
And this obviates the need for any “God of the gaps” intelligent design theory to explain why sophisticated life arose through intuitively difficult steps, given that it actually did arise with a timing in history and a rarity in the stars suggestive of being a function of low-probability, random variables. It is thus not without irony that ID proponents would point to the null evidence of extraterrestrial civilization, when actually this rarity of civilization corroborates evolution as an undirected process which can only surpass the odds to reach advanced life in few and far between places throughout the universe.
Those who have a firm intellectual grasp on natural selection have long been able to appreciate the sheer magnitude of evolutionary timescales which would allow intuitively unlikely biological innovations to eventually occur. However, observer selection bias amidst time-limited planetary habitability means that even our evolutionary timescale is more likely than not to be an underestimate of how long it would truly take, on average, for such developments to naturally arise on an indefinitely habitable Earth-like planet. Many advocates of natural evolution, including astronomer Carl Sagan, have seemed to regard the evolution of life on Earth as an inevitability of sorts. They have failed to realize the clues to just how unlikely it probably was.
Not only can difficult steps in evolution help us make sense out of the Great Silence and the spaced, filled-out timing of life’s base developments relative to the habitable window, but it also helps to explain why so many people still find natural evolution to be implausible in certain aspects. Intuitively, the unguided, natural formation of complex biological structures seems hard, but that’s only because it is hard—whether that means 5 billion or 500 billion expected years for intelligent life to evolve. Yet it is only because we beat the low odds that we are here at all to observe our fortuitous history. The sheer vastness, inhospitability, and superfluity of space and time with respect to our civilization’s locale loudly testifies that we are a random aberration of complexity amidst an entropic universe.
There are many things more inevitable than advanced life, and one is this—that advanced life in this universe will want to ascribe to higher intelligence the deliberate creation of such complex, biological structures, given the extraordinary unlikelihood of its natural, chance arrival as intuitively grasped by intelligent beings still limited in their perceptions of the truly immense depth of space and time and the rare possibilities it affords on sparse occasion throughout.
Indeed, intelligent design explanations of life’s origins would be more persuasive in a world so very limited in space and time. But if we have arisen by sheer luck against daunting odds, an old, vast, and ostensibly uncivilized universe is exactly what we would expect to observe in our surroundings. Given the abundance of space and time, and the null evidence of any other advanced life within the current limits of our detection, we are left with no good reason to conclude anything else.