Zimmer has a summary of the latest discoveries in the evolution of the baleen whales. It’s beautiful stuff, with the lineage showing their origin from toothed whales, through a phase where they had both teeth and baleen, to their current condition lacking teeth and having only baleen.
I’ve been getting swamped with links to this hot article, “Evolution reversed in mice,” including one from my brother (hi, Mike!). It really is excellent and provocative and interesting work from Tvrdik and Capecchi, but the news slant is simply weird—they didn’t take “a mouse back in time,” nor did they “reverse evolution.” They restored the regulatory state of one of the Hox genes to a condition like that found half a billion years ago, and got a viable mouse; it gives us information about the specializations that occurred in these genes after their duplication early in chordate history. I am rather amused at the photos the news stories are all running of a mutant mouse, as if it has become a primeval creature. It’s two similar genes out of a few tens of thousands, operating in a modern mammal! The ancestral state the authors are studying would have been present in a fish in the Cambrian.
I can see where what they’ve actually accomplished is difficult to explain to a readership that doesn’t even know what the Hox genes are. I’ve written an overview of Hox genes previously, so if you want to bone up real quick, go ahead; otherwise, though, I’ll summarize the basics and tell you what the experiment really did.
Darren Naish has a fine summary of the hypothesis that the megabats are flying primates. It’s a tangled tree we’re flitting about in here.
Often, as I’ve looked at my embryonic zebrafish, I’ve noticed their prominent median fins. You can see them in this image, although it really doesn’t do them justice—they’re thin, membranous folds that make the tail paddle-shaped.
These midline fins are everywhere in fish—lampreys have them, sharks have them, teleosts have them, and we’ve got traces of them in the fossil record. Midline fins are more common and more primitive, yet usually its the paired fins, the pelvic and pectoral fins, that get all the attention, because they are cousins to our paired limbs…and of course, we completely lack any midline fins. A story is beginning to emerge, though, that shows that midline fin development and evolution is a wonderful example of a general principle: modularity and the reuse of hierarchies of genes.
If you’re at work, I hope you have headphones; if you don’t, check in once you get home. Here are a couple of audio recordings of good science.
Carel Brest van Kempen has posted one of his paintings of Cambrian animals—be sure to click on it to get the larger size. I wish I had a pet anomalocarid in my aquarium.
Assuming that none of my readers are perfectly spherical, you all possess notable asymmetries—your top half is different from your bottom half, and your front or ventral half is different from you back or dorsal half. You left and right halves are probably superficially somewhat similar, but internally your organs are arranged in lopsided ways. Even so, the asymmetries are relatively specific: you aren’t quite like that Volvox to the right, a ball of cells with specializations scattered randomly within. People predictably have heads on top, eyes in front, arms and legs in useful locations. This is a key feature of development, one so familiar that we take it for granted.
I’d go so far as to suggest that one of the most important events in our evolutionary history was the basic one of taking a symmetrical ball of cells and imposing on it a coordinate system, creating positional information that allowed cells to have specific identities in particular places in the embryo. When the first multicellular colony of identical cells set aside a particular patch of cells to carry out a particular function, say putting one small subset in charge of reproduction, that asymmetry became an anchor point for establishing polarity. If cells could then determine how far away they were from that primitive gonad, evolution could start shaping function by position—maybe cells far away from the gonad could be dedicated to feeding, cells in between to transport, etc., and a specialized multicellular organism could emerge. Those patterns are determined by interactions between genes, and we can try to unravel the evolutionary history of asymmetry with comparative studies of regulatory molecules in early development.
I am going mildly nuts right now—somehow, I managed to arrange things so multiple deadlines hit me on one day: tomorrow. I’ve got a new lecture to polish up for our introductory biology course, a small grant proposal due, and of course, tomorrow evening is our second Café Scientifique. Let’s not forget that I also have a neurobiology lecture to give this afternoon, and I owe them a stack of grading which is not finished yet. I’m really looking forward to Wednesday.
Anyway, so my new lecture for our introductory biology course is on…creationism, yuck. What I’m planning to do is to describe some of the most common creationist arguments and then give a biologist’s rebuttal. Creationism is really a waste of our class time, but using it to explain some general concepts that any informed biologist should understand (and that the creationists, including Mike Behe, are astonishingly clueless about) will make it a little more productive, I hope. We’ll find out tomorrow.
One of the common creationist claims I plan to shoot down is the whole idea of “irreducible complexity” as an obstacle to evolution. I was going to bring up two ideas that invalidate it: the principle of scaffolding (which I discussed here), and exaptation, in which features evolved for some other purpose than the one that they play in an organism we observe today. I was looking for a good example, and then John Wilkins fortuitously sent me a paper that filled the bill (we evilutionists, you know, are sneakily sending each other data behind the scenes to help in our assault on ignorance. We’re devious that way.)
Yunnanozoans and Xidazoon…there are some very pretty early Cambrian critters on display at Sinanthropus.
Mark Chu-Carroll has a post up that does two admirable things: it deflates yet another creationist and his grandly fallacious claims, and it gives me a new toy to play with.
