Today’s falsehood1 is the idea of “The Missing Link.” You’ve heard about The Missing Link. You’ll hear that some palaeontologist has discovered something and they tell us it is “The Missing Link.” Often, it is a supposed “link” between some ancestor of humans (a fossil ape, a monkey, whatever) and us humans. And often, you’ll also find that when the press reports a “missing link” the science blogosphere erupts with a torrent flowing over the phrase and the concept, about how there really is no such thing as “The Missing Link,” or that this particular report of such a link is spurious, or something else bad.
This is one of the great things about the blogosphere. In the old days, reporters would use terms or phrases that were inaccurate, misleading, or otherwise annoying and the rest of us … the scientists and others with direct interest … would cringe. If teaching a class at that moment, we might have ranted to our students about how we don’t like the term or phrase. But today, with the blogosphere up and running in all its glory, we find a totally new phenomenon: The mis-users of terminology get very publicly raked across the coals like so many burning marshmallows. Bwahahaha!!!
This can be powerful and it can be real. For instance, last year, a new and very cool fossil (a new genus, Darwinus) was discovered and called a “missing link.” The blogosphere went nuts, and the finders and reporters of Darwinus got totally raked like marshmallows over the coals. Around the same time, a hominid-related research project was reported with the term “missing link” used in a way that fostered a certain degree of blogspheric ire, and again, there was raking. So, when more recently a fossil that in any reasonable person’s view could actually be called a missing link was reported, nobody used the term “missing link.” Too much bad blood … marshmallow blood … had been spilled over the term. No one was going near it this time.
And that, of course, got me thinking. Was there now a consensus that the term “missing link” was always wrong, always incorrect, that there was no such thing as a missing link, and that using the term did little other than making it clear that they … the users of the phrase … are stupid marshmallows? I knew this could not be entirely true. I knew this was wrong partly because I have seen the term “Missing Link” used correctly, and I understood it to have a potentially accurate, if somewhat fluffy and not critically important, meaning. Well, the meaning is critically important but using this phrase to represent that meaning may not be important. I also knew something was up because during this last round of missing link bashing some very smart and well educated people were saying things about the concept that were partly true and partly incorrect. They were throwing the marshmallow out with the ashes, as it were. And during the course of this discourse, it dawned on me that “[Fill in the blank] is the missing link!” and “There is no such thing as the missing link!” are BOTH falsehoods. And when that happens, I go to work, because I write about falsehoods and stuff.
The term “Missing Link” is actually central to evolutionary thinking, and Darwin referred to the concept, although often using different words (“intermediate link” for example). “Linking forms” both interested and vexed Darwin. If enough intermediate links could be found between two established species, were they really different species? If evolution proceeds by tiny incremental changes, why does the fossil record not display the intermediate links? He addressed these questions with a number of suppositions, but we now understand that the rate of physical evolution is often variable, with very little visible change occurring for long periods, and rapid change occurring for short periods, and thus difficult to see in the fossil record.
Evolutionary stories are often about adaptive change over time, in which a feature changes in its function, or a new feature emerges. These stories were addressed by Darwin using a combination of fossil evidence, biogeography and comparison of modern species, and information from development (later, DNA would be added as a primary source of evidence). But sometimes, there was a step that must have occurred but that was not indicated by any of these lines of evidence. There would be a species, extinct or not yet discovered, that filled the gap in the sequence of events. These were not always actual historical events, but often simply a sequence of variants that demonstrated the possible modes of transition from one form to another.
Here, Darwin points out important gaps in one part of the fossil record, oppine about what the various missing links could demonstrate, and indirectly sets up a number of hypotheses that could be tested by further fossil discoveries or even ontogenetic studies:
… look at the Galeopithecus or flying lemur, which formerly was falsely ranked amongst bats. It has an extremely wide flank-membrane, stretching from the corners of the jaw to the tail, and including the limbs and the elongated fingers: the flank membrane is, also, furnished with an extensor muscle. Although no graduated links of structure, fitted for gliding through the air, now connect the Galeopithecus with the other Lemuridae, yet I can see no difficulty in supposing that such links formerly existed, and that each had been formed by the same steps as in the case of the less perfectly gliding squirrels; and that each grade of structure had been useful to its possessor. Nor can I see any insuperable difficulty in further believing it possible that the membrane-connected fingers and fore-arm of the Galeopithecus might be greatly lengthened by natural selection; and this, as far as the organs of flight are concerned, would convert it into a bat. In bats which have the wing-membrane extended from the top of the shoulder to the tail, including the hind-legs, we perhaps see traces of an apparatus originally constructed for gliding through the air rather than for flight.*
Not all “missing links” need to be fossil forms in sequence. It is conceivable that links, missing or known, can exist in a hypothetical continuum of adaptive forms that do not represent a real history, but rather, a range of variants that model potential transitions. The various eyes that numerous organisms sport these days can be ‘lined up’ in a sequence from light sensitive cells to compound eyes or vertebrate-like lensed eyes. Both of those complex and highly effective organs evolved multiple times, but that is not the point. The point is that if we look at all the eyes and eye-like things, there are no meaningful or important gaps in which must exist some form of the organ that we can’t demonstrate empirically, no matter how easily we may imagine it.
The term “Missing Link” dates back quite a few years but does appear to have sprung from it’s use by Darwin and his contemporaries. According to Google Ngram, which is an excellent if rather coarse and shotgun style data mining tool, the term shows up in any numbers just after 1840, which accords with my thinking that it was first used by Darwin and/or his correspondents or colleagues prior to the Origin. Ngrams, which tracks the appearance across time of words or phrases in the Google Books collection, shows the phrase reaching a huge peak in 1859 (when that book was published) and holding strong since then. Both Ngram and other sources actually show the use of the term in low levels far, far back in time, but I suspect that many of those uses, if not all, are coincidental juxtapositions of the two words, and not the palaeontological term of interest here (but corrections to this guess will be welcome!).
At this writing, the biggest and one of the most recent blogospheric reactions to “Missing link” was probably to the use of the term to describe the fossil known as Ida (Darwinus, mentioned above). First the term was used by the usual media outlets: Fossil Ida: Extraordinary find is ‘missing link’ in human evolution, Is fossil Ida a missing link …?, “MISSING LINK” FOUND: New Fossil Links Humans, Lemurs?. Then, almost as quickly as the metaphorical air was alive with the sound of that phrase, the critiques started to roll in: Controversial “Ida” Fossil No Missing Link, Why Ida fossil is not the missing link, The “Ida” fossil: on missing links and media circuses, this critique by PZ Myers, Poor, poor Ida … Overselling an adapid, and so on.
It is true that despite their denials, the rhetoric controlled by the research team used the word “link” to describe how “Ida,” an ancient monkey skeleton, demonstrated the link between the modern primate lineage that includes humans and one (but not the other) of the two major primate lineages that were extant tens of millions of years ago. That has been a major controversy for some time: How do the early primate lineages go together on a properly constructed phylogenetic tree of living species and fossils? Ida the fossil primate actually does address that question, and as such, could be thought of as a link, which was missing, and thus a missing link. But is it?
In the early days, or should I say hours because it happened so quickly, of the discussion of Ida, I believe it was the primatologist Gingerich who noted that the real importance of Ida was “linking” the head to the body. Prior to Ida, there was a phylogeny of fossil primates of the era based mainly on dental remains and other skull fragments, but some of the most important transitions and features of the whole primate evolutionary story have to do with postcrania (the non-head) bones. There were postcranial remains from the early days of primate evolution, but it was impossible to link specific postcranial characteristics to the phylogeny because no head bones were ever found connected (linked!) to the body bones. Ida is a complete skeleton found with the head on the body and the body attached to the head. Gingerich literally stated that Ida was a “missing link” because her head and her body linked the cranial (phylogenetic and functional) and post cranial (important functional) issues of early primate evolution. And thus, he was using “Missing Link” as a literary allusion.
It turns out that among the earliest uses of the term “Missing Link,” in the middle 19th century, was the application of the term by a creationist giving lectures in Ohio to describe Darwin’s work in evolution in a very negative light. He makes the claim that Mr. Darwin is “of so mean and degraded a presence as to justify the hypothesis that he may himself be the veritable missing link of his own philosophy.” The British writer who reported that notes the widespread frequency of drawings that depict an ape-like figure and/or a Darwin-like figure mixed up in one or the other parody of Darwinian theory, which apparently were misinterpreted by the hapless American creationist as literal depictions. “… [t]he comic pictorial papers in England have several times encouraged such an idea by depicting Mr Darwin’s head fastened to the body of an ape.” (Chiefton Union clipping found among Darwin’s notes.)
There is a bit of an irony here in that the most legitimate use of the term in the case of Ida, as a literary allusion linking the head and the body, was not the first allusion to which the phrase has been applied, and that allusion rather than direct palaeontological status dates to the very early days of the term’s use. Another use of the term in a non-biological way is found in a letter probably written in 1866 from a Mrs. Boole to Darwin. Mrs Boole is bothered by all the usual things that bother people when they first encounter a truly naturalistic version of the world, regarding the role of a god or spirit, the role of fee will, and so on. She asks Darwin about these issues, and suggests that this is an appropriate thing to ask because she felt he had “… supplied one of the missing links–not to say the missing link–between the facts of science and the promises of religion.”2
So, are missing links real or not?
Some claim that missing Links are never real because evolution is bushy (variant: There can be no missing links because evolution is never a ladder). This is wrong. Yes, evolution is bushy, but MOST of the notable evolutionary changes among animals and plants, where one species “evolves” to another, happen not between branches, or in some vague way distributed vaguely among untold vague branches, but rather, along a branch, where the branch has a time element, and as we look at the lifeforms along the branch we see one, then a different one, and that was a speciation event, or at least an important and interesting change. It is quite possible that such an event may invoke an interesting evolutionary question about change, that our spotty view of the branch may limit our understanding of it, and that discovery of a certain fossil may answer the question. In fact, this is often the most interesting sort of question in evolution: The transition from aquatic to terrestrial, the origins of flight in each lineage that has it, the beginnings of echolocation, the origin of the flagellum, and so on. There is a lot of bushiness, yes, and how much one sees depends on the level at which one looks, but when it comes to species arising from former species or novel adaptations arising, there can be a step not observed at present but still of interest. A link between two qualitatively different forms, which is missing, until we find it.
The problem with the term “missing link” is that it does invoke a simplified and, to people who study evolution, hideous version of the past. We use boldly insulting terms like “ladder” and “chain” when we speak of this hideous concept of linearity in the fossil record. A “missing link” seems to demand a chain, or at least, emphasizes a simplified linear sequence which may, in turn, invoke an even more hideous concept at which we hurl invectives such as “teleology” or “intentionality” or, even “intelligent design.”
The problem with this problem is that when we (science communicators) scream at the average person for not rejecting the term “missing link” because of it’s role in reifying the Baltimore Catechismesque Great Chain of Being, suggesting a teleological model of biotic change and underscoring an inaccurate and misleading ladder-like view that falls into the hands of nefarious Behe-ites who are mere puppets of the Discovery Institute, we are making a good point and they are often hearing “bla bla bla.”
But the term, while usable under certain circumstances, probably has a range of uses that is much preferred, and a range of uses that should be avoided. Ida is not a missing link connecting a particular ancient form of primate with a particular modern group of primates (which happens to include us) because, as John Wilkins would put it, Ida is one of many possible missing branches (see There is no missing link). But this is true to some extent of all fossils. What is important is to know when this is true in a trivial way vs. an important way.
We often find a tip of a branch that, when you follow the branch back, you would get to a “link” along a sequence of change, with that particular linky organism not represented in the fossil record, but rather, inferred. Ida lived many thousands of years after the population that we presume linked early and later primates, but is not from that population. However, Ida informs us of what that population looked like. To call Ida a link is to claim that Ida is a rung in a particular ladder, which she is not. Yet, she speaks to the link. “Missing link” is not a good phrase to use in describing Ida, other than in the meta-metaphorical way mentioned earlier: Ida’s head is linked to Ida’s body, and that is spectacularly important.
The fact that for many fossil problems we don’t have the actual transition populations does not mean that a) it is not possible to ever have the transitional population or that b) we can’t view a particular fossil find as highly informative of such a transition and thus roughly equivalent to a missing link. For many later events we have excellent samples of one population, and excellent examples of a later, still extant population, and can go to sleep at night knowing that one population pretty much evolved into the other, or at least, something so close to that happened that we may as well not worry about it. For other organisms the fossil record is so inherently good that we can assume that we are not missing a lot of important detail (fresh water snails in regions with abundant lake fossils, for instance). In all cases, one can never be sure that a particular species that has a certain set of traits really and truly evolved into a second species with only slightly different traits (with many identical) always found in the layer above and slightly later in time compared to the first species (etc. etc.) … one may not assume one has seen the actual transitional population no matter how good the fossil record is … but there are a lot of other things in life with less certainty that we don’t worry about. I mean, really, how often have you tried to get into your car and discovered that someone else has parked an identical car to yours right where you usually park yours, and you mistook theirs for yours? If you’re like me, fewer than a half dozen times, I’d say. Yes, you can sleep at night knowing that certain parts of the fossil record are pretty complete and easy to read, links and all.
So, are “missing links” ever real? Yes, in my view they can be, but the boundary between something being a valid “missing link” and misuse of the term is vague. The link must be a transition of interest, and the fossil must reveal something previously unknown about that transition. Say we have fish and we have amphibians, and we have reason to believe that amphibians arose from a post-fish/pre-amphibian population that demonstrates somehow the way tetrapods went from fins to feet. That is a missing link. We can make assumptions, guesses, models for what form or forms may have existed during this transition, and some of those will be based on the ontogeny of the living forms and information from what fossils we do have. But only by finding fossils that represent (hopefully not too distantly) the actual transitional forms (the ones with the features that are not all fin or all feet) can we test our hypotheses.
It could be said that everything is a transitional form, and it could also be said that every new “transitional form” or “missing link” simply creates two new missing forms … the one that would show the transition from A to B and the one that shows the transition from B to C (A is the fish, C is the footed form, and B is the “missing link.”) But that is not really true. By my definition, if you have a transition from A to C and you find B, there may or may not be a new question regarding A to B or B to C. It is not invertible. It may be that there was a single major intermediate form between A and B (and yes, yes, A is also an intermediate form between two other things, bla bla bla, but we were talking about the A-C transition, not that other stuff). Or, there could be several distinct forms. If there were several, then finding B does indeed create two new missing forms, but if not, it doesn’t. Subjective, but definable.
But do notice that I’ve used the word “distinct” here to signal you that yes, there may be a zillion little transitions from one point to another. But, we should learn from philosophy here. Zeno’s paradox can not be solved given only the conditions provided by Zeno, yet Zeno’s paradox never happens in real life. It is simply not the case that there is an infinite, or even large, number of distinctly different forms of bat-like creature marking out the transition from a bat without echolocation to a bat with echolocation. We might argue about how many distinct forms there were, but the argument will be about thee or four, or one vs. three, or five vs. seven, not one vs. infinity.
It turns out that in the transition between fishy things and footy things there was a form of fish that had feet but not for walking on land (I oversimplify), and then later, a form of animal that walked on land using those fishy feet. That implies at least two forms intermediate between a fish with fins and a land animal that may still lay its eggs in water but that locomotes using limbs on land. The point is, there is not an arbitrarily large number of intermediate forms, with two new ones being generated every time one is found. Not really, not for all practical purposes, not in any non-trivial way. Which means that there can be and in fact are “missing links” which we often seek and occasionally find. Which raises the question, of course, of what to call them once we do find them (… found links? … the link formerly known as missing?).
Is the phrase “missing link” problematic, something to be avoided, misleading, or incorrect? Yes, yes, sometimes, not always. The biggest problem that I see with the term is that when it is used in the rhetoric linked to a new fossil find, a lot of brain power and excellent writing is expended on the misuse of “missing link” that could otherwise be spent on talking about the science itself. Which is a shame.
A brief summary of problems with the term “Missing Link” might look something like this:
1) The things being linked are way far apart. A fossil is found that connects living horses to living elm trees. Is that really a link? I think not. A more likely, and in fact real, example is when Ida is called a missing link between monkeys and humans. Meh on that one.
2) The link wasn’t missing. The term “missing link” is often used for no other reason than that a fossil has been found that relates to human evolution. Many human-ancestry related fossils are found that look a lot like the other fossils we already have and that do not speak to any important transitions, though they may add important knowledge regarding bio-geography or variation in ancient hominids, or they may be from a time period earlier or later than previously expected. But since they look just like some other fossil in the ways we think of as important, they are not “missing links” but rather “newly discovered species.”
3) The term “missing link” may be more plausible or useful than “The missing link.” There may in fact be many missing links in a certain evolutionary setting, or if there is something missing, we can not a priori be sure that there is only one step not yet demonstrated by fossil evidence. The The in “The Missing Link” may be the problem.
4) Scientists and science communicators worry about giving ammunition to creationists. A missing link is a lack of evidence, or a “gap” in the fossil record, or the seeming admission that there is a missing ‘transitional form’ (the alleged lack of transitional forms is one of the pieces of ‘evidence’ used by creationists to suggest that there is no such thing as macro-evolution). Furthermore, once a “missing link” is found, that is seen as creating two more missing links, one before vs one after the newly found fossil form (This is disputed in the main body of this post) so things just get worse. Also, if we really must rid ourselves of terms that are misused or abused by creationists, then we need to give the old Gold Marshmallow award to one of our most cherished terms: Theory. I don’t think so.
4) The term overstates or too strongly implies simple lineal ladder-like teleological goal directed evolution. Although this is the main argument against the term “missing link” it is not, in my reading, the most common reason it is wrong when it is used.
Is the term “missing link” ever OK? Not any more, but not strictly for the reasons often cited. The term, or lack thereof, is almost as much a marker of group membership than it is a term of art in science journalism. Old school science journalists will use it at every opportunity, the new guard of science bloggers will bludgeon anyone who utters the term under any circumstances. (Expect me to get bludgeoned for having suggested that the term can be used meaningfully and correctly.) The term can have meaning, but like the child who decided that marshmallows taste like dirt after hers caught on fire, the marshmallow tossed to the ground, the fire stomped out vigorously, then the marshmallow (and the dirt) eaten, we have found the term lacking then sullied it further with our zeal. Alas, “Missing Link,” you are no longer worthy as a phrase.
What do we lose by not being able to use the term again? Probably nothing or next to nothing, though the way the term has been pushed out of use is not the best example of how these sorts of conversations should go, in my opinion. But, I suppose it is all a rather Darwinian process. Survival of the fittest, don’t you know!
1A falsehood, in the parlance of this blog, is a belief that is not entirely accurate (perhaps quite inaccurate) but usually in a way that is nuanced or complex, so there may well be truth within it. Most importantly, a good falsehood is a belief that, when examined thoughtfully, leads to a better understanding of … something.
2Darwin’s answer: “I cannot see how the belief that all organic beings, including man, have been genetically derived from some simple being, instead of having been separately created, bears on your difficulties.” Source: Darwin, Francis ed. 1887. The life and letters of Charles Darwin, including an autobiographical chapter. London: John Murray. Volume 3.