Woodpeckers and Evolution – Part 2
(This took longer than I expected to put together, so the first post has rolled off of the front page and may be found here.)
In the first post on woodpeckers and evolution, I touched on some background material on how a woodpecker's tongue is put together and how the whole mechanism operates. Ordinarily, I wouldn't give much consideration to a woodpecker's tongue, but as it happens, it's one of those things that certain creationist writers like to point at and say "Evolution can't explain that, so creationism must be true!"
Ignoring for a moment that even if evolution couldn't explain the woodpecker's tongue, it wouldn't automatically imply that creationism (or any other alternative) was true,1 the fact of the matter is that the woodpecker's tongue is quite readily explained in an evolutionary context, and that's what I'll be discussing here.
The original article at Mental Floss that inspired me to write this included a link to a write-up at TalkOrigins that get into the guts and feathers (if you'll pardon the expression) of woodpecker tongues. I'll be discussing that article a lot here, so I'd suggest having that article open in another tab or window for reference. Rusty Ryan does a first-rate job of discussing the material, but the article seems to assume a bit more background than a lot of people might have, so I'm going to attempt to fill in the blanks.2
Despite the tendency of creationist writers to portray the woodpecker's tongue as some mysterious, freakish organ for which there's no good evolutionary explanation, it's really pretty easy to characterize in very basic evolutionary terms.
Let's start with what I consider to be a key concept in evolutionary discussions.
The raw material of evolution is heritable variation amongst individuals within a population. This variation is everywhere you look, too - your siblings don't all look just like you, the squirrels in your back yard don't all have the exact same color fur, some of the kittens in the litter grow to be slightly larger than the others, and some woodpeckers have slightly longer tongues than others of the same species.
Another key concept is that variations that tend to confer an advantage in a given environment will, on average, increase within a population.3
For example, to stick with woodpeckers, a slightly longer tongue allows a bird to get a few more insects per day than his neighbors since his reach is deeper.4 This could easily be an advantage in times of drought where food is relatively scarce, but even in times of plenty, the extra nutrition could make him healthier and more likely to find a mate. If he passes the genes for a longer tongue to his offspring, that trait increases within the population, perhaps to the point that none of the shorter tongued birds can find mates.
There's no reason to suspect that the process stops - longer tongues, more feeding opportunities, more reproductive success. Where it gets interesting in the case of the woodpecker isn't so much that the tongue got long, but how characteristics that were already present were tinkered with to arrive at the somewhat exotic anatomy that we see today (and we will see that the tinkering is actually quite minimal).
And on that note, let's dive into Rusty Ryan's article.
After a bit of preamble, Ryan notes that while human tongues are mostly muscular, bird tongues are supported by the cartilage and bone of the hyoid apparatus. I discussed this some in the previous post, but Ryan does a much better job of detailing just how this works.5
One of the most important points to keep in mind comes next, and it's worth quoting in its entirety:
The Y- shaped hyoid apparatus of birds, however, extends all the way to the tip of their tongues. The fork in the "Y" sits just in front of the throat, and it is in this area that most of the muscles of the hyoid attach. Two long structures, the "horns" of the hyoid, grow backwards from this area and provide insertion sites for protractor muscles which originate on the lower jaw.The hyoid horns of some species of woodpeckers are quite startling in appearance, as they can grow all the way up to the top of the head and, in some species, grow around the eye socket, or even extend into the nasal cavity. (Sources: Goodge, WR. 1972. Anatomical Evidence for Phylogenetic Relationships Among Woodpecker, The Auk, 89: 65-85; Short, LL. Woodpeckers of the World. 1982, Delaware Museum of Natural History)
Take special note of the third and fourth sentences. The hyoid horns grow backwards from the throat area. Not just in woodpeckers, but in birds in general. The interesting thing about woodpeckers is how long the horns can get and how they manage to accommodate that extra length.
What happens isn't very mysterious or unexpected when you think it through. Juvenile woodpeckers have hyoid horns that aren't particularly long, and the bone is exactly where it should be in a bird - anchored in the front of the throat with the horns pointing backwards. Note that the fork of the hyoid can't move backwards - the internal structure of the throat is in the way.
As the woodpecker matures, the hyoid horns grow in the only direction they can - towards the back of the skull. (The horns are surrounded by a sheath of tissue in which they can slide.)
Despite the length, the horns are still attached to the mandible of the bird via the branchiomandibularis muscles (remember those muscles from Part 1?), and when those muscles contract, they act on the hyoid, pulling the horns tight against the skull. As the slack is taken up, the tongue is extended. (You can see this in Ryan's Figure 4, most clearly in the two skulls on the right.)
Hopefully the mechanics of the woodpecker's tongue are now clear, and we can turn our attention to the detail that makes some folks think that the whole Theory of Evolution has cracked a wobbly when it comes to woodpeckers: the seemingly drastic routing of the horns up around the back of the skull and down the front, in some cases even into the nasal cavity, as the woodpecker matures.
Ryan summarizes it thus:
It is only with age that the hyoid horns of the flicker grow up to the top of the head, forward, and into the nasal cavity, where the sheath fuses to the nasal membrane. This makes adaptive sense, since the young flicker is fed by its parents, and a long tongue would only get in the way.
The genetic changes necessary for such a modification are quite minor. No new structures are required, merely an extended period of growth to lengthen an existing structure. It is likely that in ancestral woodpecker species which began to seek grubs deeper in trees, those woodpeckers with mutations for increased hyoid horn growth had a fitness advantage, as they could extend their tongue farther to reach prey.
In other words, you can get to a hyoid bone with extremely long horns simply by delaying the cue to stop growing. This is such a basic and obvious concept that it's difficult to express it more simply. The technical term for this process is heterochrony, and it's the same process that has allowed dog breeders to select for variations that lead to such diverse breeds as Chihuahuas and Great Danes. There is no need to introduce any novel structure to the bird's anatomy.
Now, at this point, one might wonder why the hyoid horns of some woodpeckers manage to route around one eye socket and into their nasal cavity, rather than, say, one horn routing around each eye socket and into the nasal cavity on either side in a symmetrical manner.6
Ryan doesn't address this specific question directly, but I strongly suspect that the routing of the horns is a spandrel. By this I mean that there isn't really any functional reason that the horns would have taken that path, but as they grew, they had to go somewhere and the routing that we see now is just coincidence rather than the result of any particular selection pressure . They could just as easily have taken a different route. In point of fact, Ryan's only reference to this matter, (and it's a somewhat oblique acknowlegement) cites John James Audubon's comments on the variation in routing present in different species of woodpeckers:
"There is a very curious gradation in the degree of elongation of the horns of the hyoid bone in the different American Woodpeckers, some of which consequently have the power of thrusting out their tongue to a much greater extent than others. Thus: In Picus varius [Yellow-bellied Sapsucker], the tips of the horns of the hyoid bone reach only to the upper edge of the cerebellum, or the middle of the occipital region. In Picus pubescens, they do not proceed farther forward than opposite to the centre of the eye. In Picus principalis, they reach to a little before the anterior edge of the orbit, or the distance of 1/2 inch from the right nostril. In Picus pileatus, they extend to half-way between the anterior edge of the orbit and the nostril. In Picus erythrocephalus, they reach to 3 twelfths of an inch from the base of the bill. In Picus tridactylus, they reach the base of the ridge of the upper mandible. In Picus auratus [Nothern Flicker] , they attain the base of the right nasal membrane. In Picus canadensis, they curve round the right orbit to opposite the middle of the eye beneath. Lastly, in Picus villosus, they receive the maximum of their development, and, as represented in the accompanying figures, curve round the right orbit, so as to reach the level of the posterior angle of the eye." [Note - many of the latin names used by Audabon do not reflect current usage and classification].
Audubon, John James. 1840-1844. The Birds of America - As an addendum to the entry for "golden-winged woodpecker" (Northern Flicker), Audubon writes:
The last thing to point out is that we aren't discussing fossils here. We're talking about living birds, whose anatomy can be observed in real-time, from hatching to adulthood. Consequently, we can watch how the tongue and its supporting structures develop and don't have to speculate.
In this post, we've looked at Rusty Ryan's article and picked out a few of the most significant points. Namely, we've revisited the anatomy of the woodpecker's tongue, considered how that anatomy is simply explained by an extended period of growth for the hyoid apparatus, and that the specific routing is somewhat variable in woodpeckers and doesn't appear to serve any particular purpose in and of itself. In short, we've seen that the woodpecker's tongue is in no way a challenge for the Theory of Evolution to explain, and is actually a very straightforward anatomical feature that is very easily accounted for without any head scratching whatsoever.
The next post will take a look at several creationist claims about the woodpecker's tongue, and will point out where those claims fall short.
CB
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1Remember when I talked about false dilemmas?
2TalkOrigins should be on the bookmark list of anyone who has any interest in the Evolution/Creationism issue. There's a wealth of great information at the site itself, and the sites linked from there are as comprehensive as you're likely to find in any one place. That said, a lot of the articles assume some degree of familiarity with evolutionary concepts. This can present a problem in that folks that come to the discussion from an anti-evolution background tend to have a lot of incorrect ideas about evolution that have to be corrected first, and that's not something that can be done in 10 or 15 minutes.
3This concept is frequently oversimplified to "survival of the fittest", which is a workable approximation so long as you keep in mind that "fittest" isn't as intuitive a term as you might like to think, and that what makes one "fittest" for one environment might very quickly get one eaten in another.
4This can lead to an evolutionary arms race of sorts. Such arms races are interesting for a lot of reasons, not the least of which is that they demonstrate the intimate interconnectedness of life. In this example, birds that feed on insects that live in wood put a selective pressure on the insects, namely the pressure to avoid that type of bird. One thing that might result is that insects that tend to live deeper in the wood get eaten less frequently than insects that live near the surface. It doesn't have to be much deeper - just slightly deeper than the bird's tongue can reach. An unusually long-tongued bird makes that pressure that much stronger. This sort of situation is called a Red Queen race.
5I'm not going to quote too liberally from Ryan's article because I want folks to read the whole thing for themselves. I will mostly be calling attention to what I think are the key points in his presentation.
6We're so used to the visible external symmetry of animals that we find asymmetries to be uncomfortable. As it happens, a lot of the asymmetries we might think of, like the fact that our heart is to the left of center or that our digestive tract twists all over the place go away if you unpack everything. Spend a little time thinking about it and you'll figure out that our body plan is basically a donut with a few infoldings and outgrowths here and there.
It Must Be a Slow News Day
A short Monday morning rant.
The MSNBC science section had an article a couple of days ago reporting on a paper by David Hone at the Institute of Vertebrate Paleontology and Paleoanthropology of China in Beijing and Oliver Rauhut at Ludwig Maximilian University of Munich in Germany.
The paper, Feeding behavior and bone utilization by theropod dinosaurs, sits behind a pay wall at the Lethaia Journal site1
MSNBC goes on for some length about the paper's conclusions about T. rex (remember T. rex?).
T. rex
According to MSNBC, the paper concludes that T. rex largely preyed on juvenile animals and ate them whole (NB - the paper appears to focus on theropod dinosaurs in general, of which T. rex is just one example. MSNBC locks onto T. rex presumably because it's well known). This is based on the observation that fossilized bones bearing toothmarks are rare. The researchers also cite the rarity of juvenile dinosaurs in the fossil record as possible evidence suggesting that they were frequently eaten.
The thing that strikes me as odd about the article isn't so much that the researchers are claiming these things, but that MSNBC seems to be reporting them as if they are surprising or unexpected discoveries.2
They're not, and the researchers themselves say as much when they mention the possibilities that (for example) juvenile bones simply might not have been preserved well (fossilization, despite what some folks think, is a relatively rare event) or that such bones may have been entirely digested.3
It's not remarkable in the least to suggest that large, predatory animals (like T. rex) would go after young, old, or incapacitated prey. Such behavior is extremely common among predators - it's much more efficient to eat something that's easy to catch and relatively defenseless than to chase down and kill a strong, experienced adult. (This is why you never want to be the slowest zebra.)
In the bigger scheme of things, the points that MSNBC reports don't reveal anything other than the fact that theropod dinosaurs apparently acted like predatory animals generally act - they ate what they could catch and probably tended to pick off the easiest prey they could find.
I'm still interested in reading the full paper, but I can't help but wish that the mainstream media would do a better job of reporting the actual research instead of giving in to the trend of sensationalizing it.
CB
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1Check with your local public or university library and see if they have an institutional subscription to Lethaia. Currently, my library in Louisville is drying out all of their servers following last week's flooding, so I don't yet have a copy of the article itself.
2MSNBC also makes a very lame attempt at humor with the following opening paragraph:
Although past research has suggested Tyrannosaurus rex was related to chickens, now findings hint this giant predator might have acted chicken too.
In this case, "acting chicken" apparently means "preferring easy to catch prey", in which case every predator "acts chicken".
3MSNBC muddies the waters here. They cite crocodiles as having strong stomach acids, and claim that crocodiles are "among the closest living relatives of dinosaurs". That's a true but misleading statement, as (particularly with respect to the theropod dinosaurs) birds are closer.
Woodpeckers and Evolution – Part 1 (Edited)
A couple of weeks back, Miss Cellania wrote a Mental_Floss article describing the some of the weirdness to be found in birds. One of the birds she mentioned was a particular species of woodpecker, that has a bizarre tongue. She mentioned that this woodpecker has been used by both evolutionists1 and creationists in support of their positions, and linked to an essay from each side.
I asked Miss C if she would mind if I used her article as a starting point for some analysis of the different perspectives on this bird, and she graciously agreed. The reason I made a point of asking is that the evolution/creation issue is a hot-button topic, and I didn't want to drop Miss C into the middle of it, even by association. (Her article didn't take a strong position on the matter. Mine will.)
That preamble out of the way, let's look at woodpeckers.
Firstly, we're not worried about this guy:
Amusing, but not relevant
We'll be considering woodpeckers of the less slapstick variety, like this one:
Now we're talking
Specifically, we're going to be talking about the freakishly long tongue that certain woodpeckers have, and how it got to be what it is. Miss C had a really good photo of our subject of discussion, which I've reproduced below:
Jenkies!
Now, before we proceed, we need to be familiar with a little piece of anatomy called the hyoid bone. The hyoid bone serves as an anchor for the muscles of the tongue (among other things). In people, it's small and U-shaped, and sits in your throat:
The human hyoid bone
It's an easily broken bone, and it's often fractured in strangulation victims. (The next time you're watching NCIS, and Ducky turns to Gibbs and gravely intones "I can tell she was strangled, Jethro. Her hyoid bone is fractured", this is what he's talking about.)
Birds have hyoid bones, too, but they're quite a bit different than those of humans. A bird's hyoid is shaped like the letter Y, with the horns of the Y branching to either side of the throat and pointing toward the back of the skull and the base of the Y actually extending into the bird's tongue. (See Figure 2 in this article at talkorigins.org.) Actually, a bird's hyoid looks somewhat like the threaders my kids' orthodontist gives them to floss their braces (the right end of the loop would be open, and at the back of the skull - we'll refer to that as the proximal end - and the single filament at the left would be interior to the bird's tongue - we'll refer to that as the distal end.):
Orthodontic floss threader
You may have noticed that I said (twice) that the bone goes into the bird's tongue. Our tongues are basically all muscle. A bird's tongue is more like a flesh covered bone spear. The bone, by the way, is thin enough to be flexible, which is important to keep in mind when we consider what actually happens when the bird extends its tongue.
Each one of the hyoid horns is surrounded by a sheath of tissue in which it can slide. There are some muscles called the branchiomandibularis muscles which connect the proximal ends of the horns to the bird's jawbone. When these muscles contract, they pull the ends of the horns forward, which moves the entire hyoid/tongue apparatus forward, tightening the hyoid horns against the skull, which in turn extends the tongue. So, all other things being equal, longer hyoid horns paired up with correspondingly longer branchiomandibularis muscles enable the tongue to be extended farther m out of the beak. It's worth noting here that juvenile woodpeckers have relatively modest hyoid horns, and the horns grow as the bird grows, and in species with the freakishly long tongues such as the one in Miss C's article, the process is no different. The horns just grow longer and continue the typical march up the back of the skull, and happen to end up growing all the way around.
In the next entry, I'll work through Rusty Ryan's post at talkorigins and highlight the reasons why biologists aren't at all puzzled by the woodpecker's tongue and why conventional evolutionary explanations of it's development are perfectly sufficient to explain it.2
CB
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1I don't like the term evolutionist. Henceforth, I'm going to use the more accurate term, biologist. Working biologists accept evolution. Period. I likewise don't like the term Darwinist, since it implies that the evidence for and knowledge of evolution circa 2009 hasn't changed a whit since Darwin published Origin in 1859. Darwin didn't know genetics. Genetics was a game changer for evolutionary theory, since it provided the physical mechanism for the processes that Darwin envisioned.
2A tactic that turns up repeatedly in creationist literature is to pick some odd feature of some organism, assert that the feature can't be explained by evolution (or, more dramatically, that the feature is an embarrassment that biologists don't want to address), and then declare that "since evolution can't explain" it, the creationist position must perforce be correct. That simply isn't true, and in a sense it's just another manifestation of the fallacy of false choice that I talked about here. But it has a bigger problem - it assumes that scientific knowledge doesn't grow. It assumes that because biologists haven't figured out how some feature arose, they never will, and that's a very poor assumption.
Views on Accommodationism
James McGrath has an interesting and lively comment thread at Exploring Our Matrix that raises some complex questions.
By way of (very high level) background, Jerry Coyne recently wrote a review for The New Republic of a couple of books - Saving Darwin: How to be a Christian and Believe in Evolution by Karl W. Giberson, and Only a Theory: Evolution and the Battle for America's Soulby Kenneth R. Miller. (ObDisclosure1: I've read Miller's book, and generally liked it, and I've read Coyne's Why Evolution is True, and found it to be an excellent presentation.) Chris Mooney at The Intersection took issue with some of Coyne's comments, and then Jason Rosenhouse at Evolutionblog took exception to Mooney's issues, at which point James took umbrage to Rosenhouse's exception to Mooney's issues with Coyne. Whew. (ObDisclosure2: I also read Chris's and Jason's blogs fairly regularly, and usually find them to be of very high quality.)
The comments at James' place have shifted to some interesting discussion on what early Christians would have believed, and in particular on the distinction between their beliefs qua articles of faith vice their beliefs qua social norms. (For example, Paul believed that consciousness and reasoning was in the heart, vice the brain, but that's an Aristotelian concept which would have been a commonly held view at the time, and not an article of faith that seems key to Paul's christology and theology.)
But, the topic that got everything started was, at the core, Coyne's apparent opinion that science and religion, specifically evolution and Christianity, are incompatible. (The books that Coyne reviewed both take the position that science and religion can and should coexist. This position is called Accommodationism, and it seems to generate opinions almost as strong as the subject of evolution does.)
I consider myself to be an accommodationist, with some caveats. The first should be almost trivial: All religions fall along a continuum with respect to how literally they consider their scriptures to be.1 There are groups that consider the Bible to be word-for-word literally true and inerrant, and at the other end of the spectrum are groups that view the Bible to be an extended metaphor interspersed with some historically accurate sections. It's naive to assume that all denominations of Christians are interchangeable.2 The second caveat is that "coexist" doesn't imply "seamless integration". Science tends to be very good at answering questions of what or how, and religions tend to focus more on questions of why. The late Stephen Jay Gould used the term Non-overlapping Magisteria to capture this idea.3 The third caveat is that someone who seeks to reconcile science4 and religion needs to be willing to learn accurate information about both. This may mean moving outside a comfort zone and finding out that things you've previously been taught are incomplete or wrong.5 The net result of all this is that I think that it's more constructive to explore the ways that science and religion can complement each other than to spend time building hard walls between the two realms. Our human experience is a composite of our tangible experiences and our emotional experiences, and I'm not sure it's possible to completely separate them.
Does this mean that I advocate teaching Creationism or Intelligent Design in public school biology classrooms? Of course not. Attempts to do this have been struck down in the courts numerous times, most recently in Kitzmiller et al. v. Dover in 2005, with the important finding that Creationism and ID are not science, but rather attempts to introduce religion into science classrooms.6 What I do advocate is offering comparative religion classes at the High School level. As a practical matter I think that would be difficult (but not impossible) to do in a way that doesn't violate the Establishment Clause, because here in the U.S. people are overwhelmingly Christian, and it would be natural to teach a comparative religion class in a way that presents (the teacher's brand of) Christianity as true and everything else as false.7
It's a difficult course to navigate. On the one side, the evidence supporting evolution is overwhelming - perhaps moreso than for any other scientific theory - and the insights it has provided have improved the lives of millions, if not billions.8 On the other, religion can be an extremely powerful thing, inspiring unparalleled acts of creativity and beauty. I think it's possible to strike a balance that both acknowledges and respects both.
CB
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1I'm going to use Christianity as the example going forward, because that's what I'm most familiar with.
2There is a huge number of Christian denominations. The Wikipedia listing shows hundreds.
3NOMA is not a perfect concept, largely because some religions do attempt to explain the what and the how, and some scientists attempt to get into the why.
4When I say science, I'm talking mostly about modern evolutionary theory. Very few people have a problem with the germ theory of disease, or the atomic theory, Newtonian physics, or even Einstein's theory of relativity. Evolution is personal, though, because it ultimately deals with how we got here. Consequently lots of people have a very strong emotional reaction to the subject, maybe even a physical aversion. But stop and think about it for a minute. The physics that people have no problem with underlies the chemistry. The chemistry, in turn, underlies the biology. I've never seen a court case grow out of a school board trying to get a physics text thrown out.
5This is usually neither easy nor pleasant. And it cuts both ways. I can say with a fair degree of confidence that if your concept of evolutionary theory comes from materials published by the Discovery Insititute, Answers In Genesis, Ray Comfort, or any of the conservative Christian textbook publishing houses, you've been taught something that is evolutionary theory in name only. The distortions and misunderstandings in sources like these culminate in a version of evolution that no working biologist would agree with. (Honestly, I'm not convinced that secular texts, at the Middle and High School level anyway, do much better - reducing discussion of evolution makes books more palatable to textbook adoption committees, and improves book sales for the publishers.) The flip side is that most people aren't familiar with faith traditions beyond that in which they grew up. I grew up Catholic, and even though I went to public schools, I just assumed everyone else was Catholic too. It was only when I was about 8 that I realized that there were people that weren't Catholic, and I was older still when I began to appreciate the breadth and depth of the differences among different Christian denominations. I know a lot more now than I did then, which only reinforces in my mind how much more there is to learn, and how dangerous broad-brush generalizations can be.
6I personally think that if a court were to find that Creationism or ID were appropriate topics for a science classroom, the logical extension of this would be that all Creation stories were appropriate, and I suspect that isn't the goal.
7And such preference of one religion over others is precisely the sort of thing that the Establishment Clause seeks to prevent.
8The next time you get a flu shot or a Z-pack for an infection, think about how those medicines are developed.
Missing Links, Transitional Fossils, and Media Hype
I've seen some articles recently regarding a recently described early primate fossil found in Germany. Primate fossils are of particular interest because we humans are primates, and understanding primate evolution in general helps shed light on human evolution in specific. (The paper describing the specimen is available here. The species has been designated Darwinius masillae, and has been nicknamed Ida.)
That topic alone would be worth a plethora of blog posts, but that's not what I want to get into just now.
What I want to discuss are two terms that tend to crop up around discussions of fossils, and why those terms are, at best, misapplied (and at worst outright deceptive). Those terms are "transitional fossils (or forms)" and "missing link".
First some discussion on the fossil record. (Note: I'm going to make some significant oversimplifications in the discussion that follows. I'm trying to get across a couple of concepts, and I think I can do that more effectively if I stay out of the weeds. and for a more detailed and much better written treatment, I recommend Douglas J. Futuyma's Evolution for a textbook treatment of the subject, or Donald Prothero's Evolution: What the Fossils Say and Why it Matters for a more readable treatment. In particular, I'm going to play rather loose with the idea of ancestor species, even though I'm fully aware that what I should be saying is "this older specimen shares a lot of characteristics with the true ancestor species of the more modern specimen, but is more than likely not the literal direct ancestor of it.)
The fossils we're most familiar with are the mineralized remains of dead organisms. They can be remains of either the organism itself (e.g. the skeleton), or of the organism's presence (e.g. a trackway or a chemical signature). Note that a fossilized skeleton is no longer made of bone - the organic material having been replaced by minerals. (There are several different methods by which remains may be preserved, but the specifics aren't terribly important here.)
Despite the large number of fossils housed in museums, fossilization is relatively rare for a number of reasons. Soft tissues don't fossilize well, dead things get eaten, and not all environments are conducive to the process of fossilization. Consequently, the fossil record is (and always will remain) incomplete - it's like a photo album with some of the pictures removed.
The fossil record that we do have is pretty good though. Significantly, organisms are represented predictably1 within the record, and we can make very good determinations of the age of an organism within the fossil record.
The point here is that while it would be wonderful to be able to assemble a complete record of every organism that ever lived, and sort it so that we could walk the lineage of every organism that ever existed, that's just not going to happen - ever.
What is the implication of this?
First, let's consider the concept of a family tree. If you're the sort that has an interest in geneaology, you may have found that it's pretty easy to trace back through the most recent few generations, but before that the records get spottier - maybe your family came over from Ireland in the late 1800's with few possessions and fewer documents, or the family birth records were kept in the heirloom Bible that was destroyed in a fire - and you end up with gaps in the tree. Sometimes you might locate a distant relative2 who has records that intersect with yours, and perhaps you can fill in some of the missing pieces, but you'll probably always have some question marks.
Now, let's bring this back to fossils. Any given fossil is a little like a node on a family tree. As we move back in the tree, we find (indeed, we expect to find) gaps. That doesn't invalidate the whole tree, of course, any more than not knowing the name of your 5-greats grandmother means that she never lived.3 It does mean that sometimes we have to re-think some relationships when we uncover new information, and it also means that the particulars of some relationships remain somewhat speculative.
I think that's enough background. I'll turn back now to the primate of interest, a lemur-like critter that has been named Darwinius masillae. The run-up around the publication of the description of this guy has frequently used the term "missing link", as in "new primate may be missing link in human evolution". What, exactly, is a missing link, anyway? Well, just about everyone has probably seen an illustration called The March of Progress (or a parody of it - there are many, just one of which is shown below). The idea of a missing link is that there should be something that's "half ape, half man" that would bridge the gap between humans and our apelike ancestors. That sounds reasonable as far as it goes, but what we need to consider is the fact that evolution in general and speciation specifically doesn't work the way a lot of people think it does.
One frequent objection to evolution is "if man came from monkeys, why are there still monkeys?4" That's a lot like saying "if I'm descended from my grandparents, then why are there still grandparents?" Let's look at why.
If I were able to line up the last 5 generations or so of my bloodline, starting with my kids and working back to, say, Great Grandpa Badger, you could pretty easily look at us and pick out the family resemblances (or run DNA tests, if you were inclined) and see that my kids are, without a doubt, related to GGpB. Likewise, if you started with cousin Skunklet, you could work back to GGpB and similarly establish, without a shadow of a doubt, the relatedness. There's no point in the tree at which GGpB became a badger cub or a skunklet, and if he were still alive today, he'd be just as much GGpB as he ever was. There's similarly no point at which you could look at one of us and say "he's not a badger, but his son is a badger". You could go back to the 1000-Greats Grandpa Badger, and the same would hold true - consecutive generations are recognizably similar. Now, the picture might change if you took my son and placed him next to his 1001-Greats Grandpa. Those two, separated in time by 20,000 years, give or take, probably don't look much alike apart from both being human.
At a species level, the same basic concept applies. Speciation doesn't occur at a single birth. If you look at, say, a domestic cat, and were able to follow it's lineage back through time at each previous generation, you'd see much the same thing that you saw with my family line in the example above: extreme similarity between any two adjacent generations, but if you look at two generations a thousand times removed from each other, the differences start to become apparent. With the fossil record, you're much more likely to be seeing representatives of lineages hundreds or thousands (or more) generations removed from each other, so the differences are more apparent. The important thing to take away from this part of the discussion is that species arise gradually, and the fossil record doesn't generally capture that process completely. Even if it did - even if you could line up the skulls of every generation - you wouldn't be able to draw a line and say "before this line, these creatures weren't cats, and after this line they are."5
But that's what the concept of a "missing link" implicitly requires you to do. As it applies to humans, it requires you to draw a line in time and say "everything that came before wasn't human, and everything that came after was", and then it requires you to find something that straddles that line - to be "half ape and half man", for example (whatever that means).6
Related to the "missing link" concept is that of the "transitional form".
I would argue that every fossil represents a transitional form, since evolution is a continuous process, and every species came after some earlier species and at least had the potential to give rise to subsequent species.
That's not the way some people use the term, though. Often, when someone starts talking about transitional forms, it's in the following context: "There's no way that whales evolved from land animals! There aren't any transitional forms!" The implication here is that there aren't any fossils that demonstrate, say, a reduction in rear limb size and changes from front limbs to flippers. (This is nonsense, by the way. There are numerous fossils demonstrating the evolution of whales from land-living ungulates.7)
Let's think about this for a moment. Suppose you've got a fossil of some animal, and you claim that it's an ancestor of some more modern animal. You'd make this claim because of features you could identify and document.8 Those features woudln't necessarily be the most conspicuous features like limbs. They may be very subtle things - dentition, an articulating surface between two bones, a bony process - that would be easy for a non-specialist to miss. If you have a long series of such fossils, you could see a lot of change between the earliest and the latest (in other words, you'd be seeing the transitions), but between any two that are relatively close in time. The folks who complain about the lack of transitional fossils are caught in the same trap as the missing link folks - they want to see three fossils - one with (say) an arm, one with a flipper, and one in the middle with something halfway between (A flarm? An arlipper?). But what if we found such a series? What happens next is that someone wants to see the fossil between the arm and the flarm, and the one between the flarm and the flipper. When you fill in one gap, you create two more. Taking this to the logical conclusion, we're back at the concept of a complete fossil record that has every organism that ever existed, and that's just not possible.
So what can we say about D. masillae? Well, it's an early primate, and those are very interesting. Is it a "missing link"? Well, it's a missing link between whatever came before it and whatever came after it, but that's a pretty trivial thing - it's like saying "it's old". Is it "transitional"? Again, trivially yes, but that in and of itself isn't a big deal, either. I'm afraid that the media has vastly overhyped this find, and has created a lot of unrealistic expectations around it. What will happen is that the specimen will be studied for years, and will likely yield some good data on early primates from around the point at which lemurs split off from the main lineage. It'll also become iconic in textbooks, because it's so complete and well-preserved, and that's a positive. However, in my opinion, one of the most significant things about this little critter is that the descriptive paper for it is available to the public at PLoS. Most people have never read a paper describing a new species. Now you can! It's a very interesting paper, and gets into extremely minute detail regarding what sort of indications scientists look for when evaluating a previously unidentified species. There's a lot more involved than most folks realize. I highly recommend clicking over to PLoS, downloading the paper, and read through it. Just try to set aside the media hype first.
CB
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1Human remains, for example, are never found in the same layers as non-avian dinosaurs (a term which I only use in order to keep someone from calling me on it). The Flintstones was not a documentary.
2Several years ago, I was sitting in the waiting room of my doctor's office, and noticed a woman on the other side of the room looking intently at me. When we made eye contact, she asked if I was related to a certain family, and when I said I was, she said that she recognized me by my earlobes. At least she wasn't staring at me because I had taco sauce on my shirt...
3I don't want to overplay the analogy between the fossil record and a family tree. When I look at my family tree, I know that my mom and her mom and her mom are literally my direct ancestors. Fossils are a little different. In the fossil record, it's not the individual that's relevant, it's the species. The word ancestor here is, of necessity, rather less specific than when we're talking about direct ancestors of individual people. The likelihood of any particular fossil actually being the direct ancestor of any other particular fossil is vanishingly small. It's more correct, though a little less intuitive, to think in terms of the older fossil belonging to an earlier species along the same (or a close) line with the more recent one. A more thorough discussion of this can be found in Richard Dawkins' The Ancestor's Tale. Also, John Wilkins at Evolving Thoughts put up a post as I was writing this that explains things a little more.
4Nobody who knows what they're talking about claims that man evolved from monkeys, anyway. We're apes. (We don't have tails, remember...) Anyone who makes the monkey statement is showing an incorrect understanding of evolution.
5This is also why statements like "well nobody has ever seen a cat give birth to a dog" are just absurd, and anyone who puts a statement like that in a book and expects his readers to believe it is (at best) sadly misinformed or (at worst) lying.
6Speciation is normally observed through time, so most of the time we have to determine relationships by comparing dead organisms. There are, however, some special cases where we can observe speciation through space. Ring species are groups of species that are geographically distributed around some natural barrier, such as a crater or a lake. If you start at one point on the ring, you'll observe that the birds (for example) can freely interbreed with the birds in an adjacent area, which can freely interbreed with the birds in the next adjacent area, and so forth, all the way around until you get to the other side. However, by the time you get all the way around, the birds at the other side of the ring can't (or don't) interbreed with the birds at the starting point. A more complete explanation of this may be found here or here (scroll to "Tale of the warblers")
7Prothero spends a lot of time on whale evolution, dedicating Chapter 14 in Evolution: What the Fossils Say and Why it Matters to the topic. Before anyone brings up horses or the dinosaur/bird connection, there's a lot of very good fossil and molecular evidence supporting the evolution of those animals, too.
8And not because you'd decided ahead of time that you wanted the species to be related and interpretted the data in light of the conclusion you wanted to see.
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