Transitional Fossils

Ron Britton, one of the Masters o’ the Smack-o-Matic, recreated this wonderful poster that really says everything one needs to say when arguing evolution’s case. He says:

I’ve always liked this poster. The evidence for evolution goes way beyond the fossils, but this poster summarizes the fossil evidence quite well. One of the things creationists are always claiming is that there are “no transitional fossils!” Well you’re staring right at one!

All of you evilutionists should know that transitional fossil by sight. For the rest of you, we’ll be revealing its identity shortly.

If the creationists were right, I wouldn’t be able to write this post for lack of fossils. In fact, I have the exact opposite problem. Here’s a partial list of transitionals. Keep scrolling. And scrolling. And scrolling. And once you’ve finally reached the bottom of that partial list, keep something in mind: it was last modified in 2001, before many of the spectacular finds I’ll be highlighting in this post were made.

That’s a hell of a lot of transitional fossils the creationists have to pretend don’t exist, isn’t it?

I’m currently re-reading Richard Dawkins’s wonderful book The Ancestor’s Tale, and before we dive into the transitionals themselves, I want to share several insights I’ve gleaned. One of the major confusions, and the hardest thing to explain to folks who don’t know much about evolution, is how species transition from one to another. People tend to see evolution as a series of discrete steps instead of cumulative, gradual changes. That’s why you get IDiots demanding to know why there’s no cat-dog in the fossil record.

Dawkins handles this rather nicely:

It is true that when we look at living species, we expect members of different genera to be less alike than members of different species within the same genus. But it can’t work like that for fossils, if we have a continuous historical lineage in evolution. At the borderline between any fossil species and its immediate predecessor, there must be some individuals about whom it is absurd to argue, since the reductio of such an argument must be that parents of one species gave birth to the child of the other. It is even more absurd to suggest that a baby of the genus Homo was born to parents of a completely different genus, Australopithecus. These are evolutionary regions into which our zoological naming conventions were never designed to go.

When we consider transitional fossils, then, it’s best to remember that they’re capturing a moment in time. They’re like snapshots. There were plenty of individuals both before and after the transition that were shading from one thing to another.

The diagram below captures the essence of this fairly well, albeit without the shading. Notice the relationships. We’re not walking a straight line from Australopithecus to Homo erectus to Homo sapiens here.

Here’s another evolutionary fact to keep in mind. Dawkins, tracing humanity’s ancestry back to a small shrew-like mammal he calls Henry, shows how species diverge:

Long-distance ancestry, of a particular group of descendants such as the human species, is an all-or-nothing affair. Moreover, it is perfectly possible that Henry is my ancestor (and necessarily yours, given that you are human enough to be reading this book) while his brother Eric is the ancestor of, say, all the surviving aardvarks. Not only is it possible. It is a remarkable fact that there must be a moment in history when there were two animals in the same species, one of whom became the ancestor of all humans and no aardvarks, while the other became the ancestor of all aardvarks and no humans. They may have met, and may even have been brothers. You can cross out aardvark and substitute any other modern species you like, and the statement must still be true. Think it through, and you will find that it follows from the fact that all species are cousins of one another. Bear in mind when you do so that the “ancestor of all aardvarks” will also be the ancestor of lots of very different things besides aardvarks (in this case, the entire major group called Afrotheria… which includes elephants and dugongs, hyraxes and Madagascan tenrecs).

By now, you should be getting the feeling that finding a transitional fossil isn’t necessarily like filling in a gap on the family tree. The gaps in our fossil record aren’t so concise as, say, Grandmother, blank spot, granddaughter. In the case of a genealogical chart, we know there’s one and only one way to fill in that blank: with a mom. In the fossil record, it’s more like a series of “moms,” all a little bit less grandmother and getting closer to granddaughter. And, just like mothers can give birth to more than one granddaughter (or grandson), one species can give rise to several, who eventually become so different from one another that it’s hard to comprehend that they were ever related at all, even distantly. (I’m sure many of us feel that way about our distant ancestors, comes to that – I’m sure that if I ran into the cave-dwelling couple who, 20,000 years ago, started the lines that eventually led to me and, say, the Kiowa, we wouldn’t have plenty to talk about. We’d seem absolutely alien to each other. Yet, there we are – related. They’re my ancestors, while not a single Kiowa is. Interesting, eh?)

Now that you’ve been hedged about with all of the proper caveats and such, I feel comfortable showing you a pretty straightforward evolutionary line.

We were fishes once, and young. At the bottom, you see an undeniable lobe-finned fish, Eusthenopteron. These darlings had all of the necessary equipment to start us on the road to land – nostrils (in their case, internal), “a distinct humerus, ulna, and radius (in the fore-fin) and femur, tibia, and fibula (in the pelvic fin).” Those bits probably sound familiar, especially to those of you who have broken any of those bones.

Onward we go, through Panderichthys, whose fins are still very much fins but showing clear signs of headed toward tetrapod limbs, and whose spiracle, a nifty little bit of anatomy that allowed it to breathe water through the top of its head, eventally became our ear’s stirrup bone. We’re getting closer to land, and then, ZOMG, there’s a gap!!1!11!

You can see it in that red bit in the line, there. Acanthostega used to be our next fossil in line, and it had true limbs, complete with really-real toes. How the hell did we get there from the mostly-finny fins of Panderichthys? And that’s where one of the greatest demonstrations of the predictive power of evolution comes in:

The scientists who discovered Tiktaalik made a threefold prediction, based on evolutionary theory: that such a creature would exist, that it would be found in rocks in a certain location, and that it would be found in rocks of a certain age. They went to this area explicitly because other primitive tetrapods had been found there, and searched in Late Devonian strata because more fish-like creatures were known from earlier strata and more tetrapod-like creatures were known from later strata. And all three of these predictions were borne out by the evidence.

Tiktaalik, it turns out, has plenty to tell us about the transition from water to land, which is why it’s the perfect subject for Ron’s evolution poster:

Meticulous studies of the internal structure of the cranium from several fossil fishapods, T. roseae, reveal the step-wise process that morphological changes followed as terrestriality evolved in tetrapods. [snip] “The braincase, palate and gill arch skeleton of Tiktaalik have been revealed in great detail,” reports Jason Downs, a research fellow at the Academy of Natural Sciences who is the lead author of the study. “By revealing new details of the pattern of change in this part of the skeleton, we see that cranial features once associated with land-living animals were first adaptations for life in shallow water.”

Well, that’s what makes transitionals so interesting, innit? They demonstrate how we got here from there. And if we have a here without a there, we know that all we have to do is go look – there will be a there there. In fact, don’t be surprised if we find several fossils that have paleontologists arguing over whether they’re Tiktaalik or Panderichthys – there’s bound to be some hard-to-classify versions that represent transitions between the two. Many fossils have led to constant quibbling, reshuffling, redefining, and a variety of interpretations that might drive you absolutely nuts until you realize that kind of uncertainty is a good thing. Here’s Richard Dawkins again:

If you think about it, we should be worried if there
was not disagreement over the divisions. On the evolutionary view of life, a continuous range of intermediates is to be expected.

Conversely, of course, we shouldn’t panic if we can’t find those intermediates right off – we’re lucky, between the vagaries of decomposition, critters scavenging other critters, and chemical and mechanical weathering, not to mention the grand finale of plate tectonics, that we have any fossils at all. There are some transitionals we may never find, simply because they didn’t get preserved. Put it like this, though: we know they lived. You wouldn’t claim you didn’t have a great-great-great-great-grandmother just because all the records about her got destroyed in fires and floods and various other mishaps, now, would you? She obviously existed – otherwise you wouldn’t be here. Same thing with the transitionals.

And there’s always the chance we’ll come across her records in unexpected places, which would leave those arguing against her existence looking rather foolish. Check out these beauties, found just within the last decade:

Yanoconodon

The latest Nature reveals a new primitive mammal fossil collected in the Mesozoic strata of the Yan mountains of China. It’s small and unprepossessing, but it has at least two noteworthy novelties, and first among them is that it represents another step in the transition from the reptilian to the mammalian jaw and ear. [snip] In us, the old articular and quadrate bones have completely lost their role in supporting the jaw as a joint and instead have become imbedded in the middle ear of mammals, suspended with the stapes between two delicate membranes to specialize in conducting sound vibrations to the inner ear. What does the hearing apparatus look like in Yanoconodon?

I’m going to be cruel and force you over to Pharyngula to find out.

Heteronectes

Now, Matt Friedman from the University of Chicago has described a new transitional fossil that is one of the most dramatic yet. Its name is Heteronectes (meaning “different swimmer”) and it’s a flatfish, but not as you know it.

You’ve probably eaten flatfish before but tasty fillets of plaice, sole or halibut give few hints about their extraordinary physical specialisations. They are fish that live on their sides and their flat profiles make them both efficient hunters and difficult prey. For other fish, lying sideways would give one eye a useless view of sand but flatfish have adapted accordingly. Their fry resemble those of other fish but as they grow, one of their eyes makes an amazing journey to the other side of its head. The adults look like they’ve swum out of a Picasso painting.

But Heteronectes is a half-committed flatfish. Like modern representatives, its skull is asymmetrical and one eye has begun migrating to the other side of its head. But it hasn’t made it all the way round and stops near the midline without crossing to the other side. No living flatfish has eyes arranged in such a way. We couldn’t have wished for a better intermediate form – it’s a marvellous missing link between the standard fish body plan and the distorted visages of flounders and soles.

Ed Yong’s post is wonderful, but don’t stop there – click the photo for some classic snark from the University of Chicago’s news room. Take that, creationists!

There’s so much more where that came from – I mean, just a single page search on Pharyngula turns up a snake with legs, a transitional turtle, a water-going whale ancestor that gave birth on land, ancient arachnids… and that’s not even beginning to scratch the surface of what comes up when you search for transitionals on ScienceBlogs. You could fossilize a creationist underneath that strata of fossils.

So I think we’ll just end with some videos from Afarensis instead:

Not a bad haul from an evening’s digging, eh? If this post got you as excited over transitional fossils as it should have, do Ron Britton a favor – copy that wonderful Tiktaalik poster at the beginning and post it on your own blogs. It nearly went extinct, but, as Ron says, ” If we can get enough members of this species onto the internet, it will reestablish itself as a self-sustaining population.”

Conservation is always a worthy goal. Especially when the message is so damned true.