Framing the debate on human origins: misconceptions and burden of proof

 0. Why

In this post, I’ll be attempting to clarify an important misconception about evolutionary reasoning. In the process, we’ll hopefully get a clear idea of what the shape of the human origins debate should be – who has what burden of proof, and why. One can consider this as a framing/prequel to my last article on how to explain the evidence for evolution. This post doesn’t really add anything substantive to the old article, just sets up the appropriate framing for it.

As a helpful teaching tool, I’ll be using this YouTube video from Stated Clearly on the evidence for evolution. The video is an extremely well-made presentation of perhaps the best argument for human-chimp common descent – the presence of endogenous retroviral elements in our genomes. I suggest the reader watch the video first before reading the rest of this article. It should be kept in mind, however, that the comments of the article apply not just to the specific claims in the video – but really, virtually all of common descent reasoning.

So here’s what we’ll be doing in this post. First I’ll talk in detail about what the video gets wrong – specifically, two gaps in their argument. The next section, however, will comprise the video’s redemption arc: I’ll argue that these gaps in logic, if framed correctly, are trivial and can be plugged in easily. This section basically represents the “steelmanning” of common descent. In the last section, we’ll discuss our approach to defeat this stronger argument for common descent, using an analogy with another contentious topic in philosophy of religion.

1. Gaps in common descent reasoning
Or, What Stated Clearly stated somewhat ambiguously

In brief, the video presents an effective example of what I referred to as the problem of gratuitous similarities. Endogenous retroviral sequences are elements in the genome that are shared between humans and chimps. Not only are their sequences similar, but these elements are found in the exact same spots in the human and chimp genomes. Plus, not many of them have been proven to be functional. In fact, since these elements are essentially carcasses of viruses that once infected the host, it’s reasonable to assume prima facie that at least some of them are exactly what it says on the tag – broken, non-functional remains of viruses, not functional genetic elements. But even if they do turn out to be functional, it’s difficult to argue that these exact sequences in the exact genomic locations were necessary for functionality of this sort. Meaning, this similarity between humans and chimps is gratuitous – it cannot be explained by appealing to their shared utility alone. This is also called non-adaptive similarities in the literature: the similarity in itself has no adaptive benefit.

After laying out this data – and let me just go ahead and say the science here is perfectly solid - the video moves in for its central argument. It’s worth quoting this section in full (forgive the redundancy):

If humans and chimps share a common ancestor and if at least some of the infections we find in our genome occurred before the chimp-human split, we should find the same virus genes in the exact same locations in both human and chimp genomes. In contrast, if humans and chimps are not related, they should not share the same history of virus infections. . . [Scientists] found that [humans and chimps] share . . . two hundred and five [endogenous retroviral insertions] out of two hundred and fourteen [insertions] for this particular virus group. This makes perfect sense if we consider the evolutionary view of life. The two hundred and five viruses were inserted some time before the chimp-human split. . . In contrast, if we want to believe the fixed species view, we’re forced to conclude that these viruses are simply shared by coincidence. When we do the math . . . the chance of this happening by coincidence is less than one in [5.88 X 10^1418]. This evidence should be enough for the most reluctant yet rational person to carefully set aside the fixed species view. . . What we’ve seen here is just the tip of the iceberg. There are many other lines of DNA evidence available, together they demonstrate beyond all reasonable doubt that humans and chimpanzees did evolve from a common ancestor.

The “fixed species view” is code for Divine special creation – after all, unchanged species don’t just appear all of a sudden, they have to be miracled into existence supernaturally. I assume the video kept this language out of their presentation to remain religiously neutral, but the connection is unmistakable. For my purposes and for clarity, I’ll be re-stating the video’s claims in the more explicit God-language from now on.

There are two things I want to point out about this reasoning -

1. The argument here makes assumptions about what a “fixed species view” - i.e. Divine special creation – would look. To put it more bluntly, the video is claiming, without any warrant, that God would not create humans and chimps with the genomes they in fact have. In fact, if God created fixed species, their genomes would look a certain way (random). The video seems to be engaging in a bit of theology in the guise of science.
2. The conclusion that’s being drawn based on gratuitous similarity is common ancestry, but as we’ll see – it should more appropriately be to a common cause.

Let’s explore these points in turn.

1.1. Common descent as a theological explanation

I’m sure that the readership of this blog has heard the phrase “Nothing in biology makes sense except in the light of evolution”. This sentence, especially oft-quoted in the context of evolution-creation debates, is the title of a journal article by the geneticist Theodosius Dobzhansky. The paper itself has been cited with abandon, and it’s often one of the first assigned readings in an evolution class to acquaint folks with the evidence for evolution.

When I first read this paper a couple years ago, I was surprised at how theological it was. The author professes quite clearly that he’s a believing Christian, going so far as to say that he’s a creationist and an evolutionist – meaning, he believes evolution is God’s preferred way of creating biodiversity (a position we’d call theistic evolutionism nowadays). As for the arguments for evolution themselves, virtually all of them are in the general shape of – we find pattern X in nature, X can be explained by process A or process B, but process B is impossible because God would never do it that way. In this context, process A is natural evolution, process B is supernatural fiat. Here are three of the first quotes I found in the article that demonstrates this:

“[B]ut what a senseless operation it would have been, on God's part, to fabricate a multitude of species ex nihilo and then let most of them die out!”

“Was the Creator in a jocular mood when he made Psilopa petrolei for California oil-fields and species of Drosophila to live exclusively on some body-parts of certain land crabs on only certain islands in the Caribbean?”

“[Creationists] must insist that He [God] deliberately arranged things exactly as if his method of creation was evolution, intentionally to mislead sincere seekers of truth.”

And so on. God wouldn’t do things this way, the argument goes, so things must not have happened this way. The argument the paper seems to be making is unmistakably theological.

The Stated Clearly video represents Dobzhansky on steroids. Let’s consider the odds they mention at the end of their video: human and chimp viral elements lining up by coincidence. Why are we calculating odds for this view? Because this is what the video believes a “fixed species view” would predict: If humans and chimps were indeed fixed species (i.e. if they were miraculously brought about in that way), their endogenous retroviral elements would be random, meaning their lining up in the way that they do would be nothing but a brute coincidence. Of course, these odds look pathetic compared to common ancestry, especially since we know how retroviral infections happen and are retained through generations.

If you think about it, this is some pretty heavy-handed theological speculation. The video isn’t just claiming that God – the presumable Author of “fixes species” - would decidedly not create humans and chimps with shared endogenous retroviral elements, but goes one step further to claim if God created us, such elements would be randomly strewn about in our genomes.

Of course, there’s something peculiar about this line of reasoning – for one, they seem to only be able to justify common descent at the expense of supernatural creation, which entails dabbling in theology. Could they not have presented the evidence for common descent on its own? Must there be such a comparison with supernatural alternatives to make a positive case for common descent?

The answer to this question is yes. Actually, it’s almost trivially the case.

One might think that this conclusion is too strong – Dobzhansky’s article, for example, was written to convince a lay, churchgoing audience; plus the journal where the article appeared was more of a teaching journal than a research one (The American Biology Teacher) - so the author can definitely be forgiven for indulging in a few rhetorical flourishes in such a context. However, despite those considerations, what Dobzhansky lays out is in fact the argument for common descent – presented in a rhetorically effective, lay-friendly way to be sure.

The argument for common descent between two or more creatures – between humans and chimpanzees, say – is essentially a comparison between likelihoods. Observation A is more probable on hypothesis B than it is on hypothesis C. Observation A obtains. We’re therefore more warranted in accepting hypothesis B than hypothesis C. In the case of common descent, the observation is gratuitous similarities, e.g. endogenous retroviral elements.

This sort of contrastive explanation is at the heart of common descent reasoning. More strongly, a case for common descent cannot be made without making an inter-hypothesis comparison. This is an important point, because a popular misconception is that the evidence for common descent is somehow intrinsic to the data – that we can somehow “read off” the theory of common descent from the gratuitous similarities we find in nature. In fact, two organisms being gratuitously similar to each other, taken on its own, is only consistent with them having a shared ancestry. Given no other information, the shared pattern of viral elements is consistent with infections in the common ancestor of humans and chimps. But for the hypothesis to have justifying reasons for acceptance, that likelihood needs to be compared with other, competing explanations to show that it’s superior. In our case, the competing explanation is special creation, or “fixed species” view as Stated Clearly put it. This would, in turn, mean calculating odds about what would the world look if God was acting supernaturally.

For a thorough demonstration of the need for such a theological comparison, I refer the reader to papers like this and this, where the authors show that contrasting with creationist hypotheses is how common descent has always been justified; that it’s fundamental to the logic of the argument, and not merely a rhetorical flourish. I cite papers to demonstrate this point, but really, it should be something obvious and uncontroversial. Let’s say you find feature X shared between humans and chimps that serve no adaptive purpose, and you think this suggests common ancestry. However, I can very easily counter that claim by saying: hold on, how do you know God didn’t create humans and chimps separately with shared feature X? The only way to move forward in this conversation for the common descent proponent is to show that the separate creation hypothesis is explanatorily inferior – i.e. that there’s some reason to go with the common descent hypothesis than the special Divine action one. That might be easy or hard, but the point is that obstacle needs to be removed for the common descent inference to go through.

Is that a problem? We’ll return to that shortly, but let’s look at the second gap in the video’s logic.

1.2. Equivocating common cause with common ancestor

The philosopher of science Eliot Sober painstakingly lays out the logic of evolutionary inferences in his book Evidence and Evolution: The Logic behind the Science. In the chapter on common descent, for example, he shows why similarities between organisms are evidence for common descent through a 9-step Bayesian decomposition. However, careful logician as he is, Sober also mentions this curious detail afterwards:

There are three ways in which a similarity observed to unite organisms X and Y might trace back to a common cause:

• ·       The common cause is a common ancestor.
• ·       The common cause is an organism though not a common ancestor.
• ·       The common cause is not an organism.

The second of these possibilities might involve lateral gene transfer; the third would be true if X and Y originated from the same slab of rock but lacked a common ancestor. When the human beings and chimps alive now share a characteristic (and propositions 1–9 are true), this is evidence that there was a common cause. But what characteristics did that common cause have? Biologists do not doubt that it was an organism, but they go much farther – it was an animal, a vertebrate, a mammal, and a primate.

Sober, therefore, distinguishes two steps in the common descent inference: one, establishing a common cause; and two, identifying what that common cause is. Meaning, the similarities between humans and chimps, strictly speaking, only show that these two organisms share a common cause. However, it might be that this common cause is “some slab of rock”. Maybe – as in it’s logically possible that - there’s a slab of rock with magical properties, which can produce similar organisms like humans and chimps. However, Sober quickly moves away from such a possibility and concludes that we know that there were no magic rock involved, and in fact the common cause was a biological common ancestor.

One might think that we’re being pedantic by talking about magic rocks, and they might be right. But it’s important to acknowledge that this gap in logic exists. Here’s Sober a few pages back, being even more explicit about the two separate steps of the common ancestry inference:

The first order of business is to determine whether there was a common cause. If the answer is yes, the second question is whether that common cause was a common ancestor, an organism that was not a common ancestor, or no organism at all. [emphasis in original]

To sum up, then, any argument from gratuitous similarities – including the one presented by the video – can only go so far as to establish that these organisms have a common cause, not that they had a shared biological ancestor. So why is it that scientists, Sober and our friends at Stated Clearly are so confident in concluding that the common cause is “an animal, a vertebrate, a mammal, and a primate” as opposed to “some slab of rock”? Is this inference significant?

As it happens…

2. All of this is trivial

The reader might be willing to grant that these are indeed gaps in the video’s logic – it assumes too much about what God might do (i.e. what “fixed species” would look like), and conflates common cause with common ancestor. In what the cool kids call an epic twist, I’ll now demonstrate that the common descent proponent need not lose sleep over this, since the gaps are trivial and can be filled with some smart framing.

First, however, I need to make clear that these two gaps in fact represent the same problem. For the first one, the common descent proponent is assigning probabilities to God’s supernatural action. Likewise for the second, the hasty move from cause to ancestor also represents some assigning of probabilities: the idea being, the odds of God having caused two gratuitously similar species are lower than that of common ancestry leading to such an outcome. Put differently, the common cause being God (or to use Sober’s example, some slab of rock) is lower than the common cause being a primate ancestor. So while the gap in logic can be fleshed out in two ways, both of them involve the same thing: assigning probabilities to the God hypothesis.

In other words still, the charge we’re hurling against our common ancestry proponent is: they need to justifiably “rule out” (i.e. probabilistically) supernatural Divine action before his inference can move through. Dobzhansky and Stated Clearly, as well as evolutionary inferences in general, don’t seem to be providing such justification, and instead taking these theological views for granted.

Conceptualized this way, this demand seems to be quite trivial.

Imagine, if you will, that we find a pawprint in the sand, and want to conclude that this has been made by an animal with paws. Now imagine the irritation we must feel if our pedantic philosopher friend appears from behind the bushes and says – ah, but is it not possible that God supernaturally arranged the local structure of the sand in such a way which would resemble a paw? Are you really warranted in moving ahead with the pawprint hypothesis before refuting the special-paw-shaped-sand-hole-creation hypothesis? One might argue that we’re doing something very similar in the case of the common descent inference.

And so, the response goes, there might indeed be a leap in logic in popular common ancestry inferences, but it’s trivial and filled readily. Let’s use this line of thought to steelman common descent inferences.

2.1. Steelmanning common descent

The question of whether the triviality problem is a genuine one, I think, boils down to how probabilities are assigned to the Divine special creation hypothesis.

One is the way Theodosius Dobzhansky and Stated Clearly do it – to say that such probability (i.e. of God creating gratuitously similar creatures) is low (actually, non-existent. For Stated Clearly, fixed species means Divine action means completely random viral elements. For Dobzhansky, even the suggestion is blasphemous). I think this is actually a pretty strong claim – by saying that God would not do things in a certain way, we’re engaging in substantive theological reasoning. Not only are we claiming to know things about God’s character, we’re also claiming that such character is inconsistent with acting in a certain way.

I think this is an unnecessarily difficult route for the common descent arguer to take. This is because, all the creationist would need to do to counter this claim is to show that, for all we know, God might have reason for allowing gratuitous similarity in nature. These don’t even have to be good reasons necessarily, but just reasons we can’t confidently state are bad.

As an example, consider this argument made in a biology textbook, quoted in this paper I cited above:

An engineer would never use the same underlying structure to design a grasping tool, a digging implement, a walking device, a propeller, and a wing. Instead, the structural homology exists because mammals evolved from the lungfish-like ancestor that had the same general arrangement of bones in its fins.

Really, all one would need to do to counter this claim is to show that some engineer might not never do such a thing. One can point to Richard Owen for example, the British naturalist contemporary to Darwin, who basically did all of the hard work of unearthing patterns of homology in vertebrate bone structures but, came to a very non-naturalistic conclusion. According to him, these similarities pointed to some primeval archetype – either a pantheistic force of nature, or plan in a Divine mind – on which the design of organisms is based. So here we do have an example of someone showing that God might have some design-theoretic reasons for creating a grasping tool, a digging implement, a propeller, and a wing out of the same underlying structure. Regardless of whether that’s a good reason or not, this possibility means one can no longer claim that an engineer would “never” create biodiversity in this way.

As mentioned above, the route taken by the Stated Clearly video coincides with this. The only thing one would need to do to counter their assertion is to provide a marginally plausible reason as to why God might not create fixed species completely randomly. For example, similarity between creatures has aesthetic value, and by making human and chimp genomes similar to each other – God is increasing the aesthetic value of creation, even though the arrangement itself isn’t adaptive. Alternatively, consider the old school great chain of being idea – apparently God created all possible forms of biology, so on this “chain of being”, there would be things that look almost exactly alike. If I’m not wrong, the first reports of chimpanzee sighting were met with positivity for this very reason: ah, finally, another link in God’s great chain of being. On this view of creation, then, the fact that humans look so similar with chimpanzees actually has a high probability. I’m not saying these are good responses by any means, but it just goes to show that strong claims about Divine motivations are ill-advised and easy to shoot down.

So this, I think, is a non-starter for the common descent proponents. A much simpler (and I think more convincing) approach is to adopt what Dilley and Tafacory call indeterminate theology. This approach wouldn’t say that God has no reasons for allowing gratuitous similarities, but would only claim that we don’t know how God would do things. He might allow gratuitous similarities, or He might not, but there’s no reason to prefer either possibility above the other. I think this sort of skeptical theistic approach has a much more manageable burden of proof.

Consider this quote from another evolution textbook, again quoted in the same paper:

Similarity of structure despite differences in function follows from the hypothesis that the characteristics of organisms have been modified from the characteristics of their ancestors, but it is hard to reconcile with the hypothesis of intelligent design. Design does not require that the same bony elements form the frame of the hands of primates, the digging forelimbs of moles, the wings of bats, birds, and pterosaurs, and the flippers of whales and penguins. [emphasis mine]

The authors are being extremely careful in their wording. They’re not saying a Designer would not create things this way, but only that – there’s no reason to think that He would (i.e. Design “does not require” that these things obtain). He might, or He might not, we’re in no position to adjudicate. The great chain of being might be a thing, or it might not. The probability of gratuitous similarity on a special creation hypothesis, then, is indeterminate. On the other hand, the probability of such features on common descent is decent. Faced with the choice of two hypotheses – one with a knowable, high likelihood and the other with an indeterminate one – we would be warranted in accepting the former over the latter.

I think this argument is convincing as far as it goes. This is also the argument made by Sober against a certain formulation of Intelligent Design in his book Evidence and Evolution. In order for a hypothesis to be successful, really, for it to get off the ground – we need to be able to assign probabilities to it. But if God’s reasons are inscrutable, and we have no idea whether He would create earthly biodiversity in a certain way over others, then that’s essentially a death knell to any design inference. More relevant to our case, if we don’t know if God would have any motivation either way to allow gratuitous similarities between humans and chimps, then that’s a battle common descent would win easily – because common descent does give us reasons to suspect such patterns.

If the argument for common descent in framed in these terms, I would agree that the gaps become trivial and the inference goes through. If you’re not convinced, consider the pawprint case: it might be somewhat weird for someone to claim to have positive reasons as to why God might not allow a footprint in the sand, but such weirdness would be amplified further in a human origins discussion. For whatever reason – be it sociological or religious biases or otherwise – humanity is really hesitant to say God would have no reason to become supernaturally involved in an event like the origin of humans. In the language of probability, we’re more confident in assigning a low probability to the Divine action hypothesis in the pawprint case, than we are in the human origins case. So if that’s the burden of proof the creationist is supposed to meet, I would think it would be met rather easily. A more clever tack would be just to profess ignorance: God might have reasons to become supernaturally involved in creating gratuitously similar organisms or a pawprint in the sand, but we have no idea if He does. Let’s keep theology indeterminate.

With this change in framing, it seems like the common descent argument has indeed been vindicated. Unless.

3. The solution is a theodicy

One of the most discussed arguments in philosophy of religion is the problem of suffering: the idea that God would not allow (at least certain kinds of) pain and suffering in the world. It’s important to note that even if the argument were successful, it wouldn’t lead to atheism per se. The argument only attempts to show that a deity in charge of our affairs cannot have certain attributes – they cannot be powerful enough to prevent the occurrence of suffering, or benevolent enough to have the intention to do so. The problem of suffering is consistent with certain forms of deism, for instance, the idea that the creator of the universe doesn’t really care very much about our pains and pleasures. So what the problem of suffering targets is not God, but God’s providence – His protective care over His creation (of course, one could argue whether such an amoral or non-omnipotent God even deserves the name to begin with, since the God of theism is defined as the greatest possible being. But still, one would hardly call belief in a deistic “god” atheism). Put differently, the argument targets one type of divine action (providence), while leaving other types of action unscathed (creation and sustenance).

The most popular versions of this argument are probabilistic (also called evidential): the occurrence of widespread pain and suffering has a low probability on the hypothesis of Divine providence. However, it has a decent probability on the “indifferent universe/deity” hypothesis: pain and suffering, after all, are readily explicable as parts of the natural biological responses of animals. This means we should probably opt for the second hypothesis.

I hope the analogies with the common descent problem are clear. Like the problem of suffering, the common descent hypothesis is aimed at a particular type of Divine action: special creation of humans. And like the problem of evil, it also involves assigning probabilities to the Divine action hypothesis.

So, how do the theistic philosophers of religion respond to the problem of evil?

By far the most popular tack is to demonstrate that God, in His providence, in fact has a decent probability of allowing pain and suffering in the world. This is not merely skeptical theism – this is not saying for all we know God could’ve had reasons to allow suffering. Rather, the theist here is making a stronger claim: given what we know about God’s character, we would actually expect Him to allow suffering in the world to attain some of His purposes.

I think in the common descent discussion, the proponent of special Divine action needs to accept a similar burden of proof. We need to demonstrate that the probability of gratuitously similar organisms, far from being low or indeterminate, is in fact high on a special Divine action hypothesis. In other words, given what we know about God’s character and motivations, this sort of pattern in nature is exactly what we should expect. That’s what effectively blocks the common descent inference.

A theodicy, essentially. Of course, I attempted to develop just such a theodicy in my most recent article on the blog.

That’s pretty much all I wanted to say in this framing post, but one other thought for general consideration: in addition to explaining the evidence for common ancestry on a special Divine action hypothesis, we would also be benefited from a positive case for the hypothesis: i.e. observations that are readily explained on a uniquely supernatural human origin scenario, but unlikely on a naturalistic common ancestry one. I might write more in the future about such approaches.