This is a short video clip of myotome formation in a zebrafish embryo — it’s the subject of an upcoming column in Seed, so I’m putting a short visual aid here.
You can Download the Quicktime movie (620K), or you can watch it via YouTube. Watch closely, it’s short and it flies by!
People are always arguing about whether primitive apes could have evolved into men, but that one seems obvious to me: of course they did! The resemblances are simply too close, so that questioning it always seems silly. One interesting and more difficult question is how oysters could be related to squid; one’s a flat, sessile blob with a hard shell, and the other is a jet-propelled active predator with eyes and tentacles. Any family resemblance is almost completely lost in their long and divergent evolutionary history (although I do notice some unity of flavor among the various molluscs, which makes me wonder if gustatory sampling hasn’t received its proper due as a biochemical assay in evaluating phylogeny.)
One way to puzzle out anatomical relationships and make phylogenetic inferences is to study the embryology of the animals. Early development is often fairly well conserved, and the various parts and organization are simpler; I would argue that what’s important in the evolution of complex organisms anyway is the process of multicellular assembly, and it’s the rules of construction that we have to determine to identify pathways of change. Now a recent paper by Shigeno et al. traces the development of Nautilus and works out how the body plan is established, and the evolutionary pattern becomes apparent.
A little knowledge would short circuit a lot of strange speculation. That picture to the right is of a shark caught in Malaysia, and people are calling those odd dangly bits “legs”. Despite the fact that someone said what they actually are in a comment early on, there are people arguing both that a shark with legs is evidence for evolution, and that it is evidence for creation.
They’re both wrong.
It’s a male shark. Those are the shark’s claspers, or intromittent organs. The shark does the usual act you’d expect with a female of the species, and like many shark species, it has clasper spurs, or little poky bits that help lock the organ into the female’s cloaca while he gets happy.
They aren’t homologous to legs at all. We also wouldn’t expect to find legs on a shark — they aren’t in the lineage that led to tetrapods.
I’m actually most surprised that a worker at the Malaysian Fisheries Development Board, who found this animal, didn’t recognize that these were just ordinary claspers. Anyone who has worked with sharks for even a little while would know about these structures. I suspect someone at the board is pulling the media’s, and the public’s, leg.
You know you want to see this: here’s a photo of two sharks getting affectionate. Shark lovin’ often involves a fair amount of biting and tearing, and leaves scars that you can find in captured specimens, so I don’t recommend trying this at home. Especially since sharks don’t believe in safewords.
A few years ago, everyone was in a tizzy over the discovery of Flores Man, curious hominin remains found on an Indonesian island that had a number of astonishing features: they were relatively recent, less than 20,000 years old; they were not modern humans, but of unsettled affinity, with some even arguing that they were like australopithecines; and just as weird, they were tiny, a people only about 3 feet tall with a cranial capacity comparable to a chimpanzee’s. This was sensational. Then on top of that, add more controversy with some people claiming that the investigators had it all wrong, and they were looking at pathological microcephalics from an isolated, inbred population, and then there were all kinds of territorial disputes and political showboating going on, with the specimens taken out of the hands of the discoverers, passed off to a distinguished elderly scientist whose lab damaged them, etc., etc., etc. It was a mess of a story, and the basic scientific issues are still unsettled.
Now the leader of the investigators who found the specimens has written a book, A New Human: The Startling Discovery and Strange Story of the “Hobbits” of Flores, Indonesia(amzn/b&n/abe/pwll), by Mike Morwood and Penny Van Oosterzee. I’m coming to this a bit late — Afarensis reviewed it already this spring — but finally got far enough down in my pile of books to encounter it.
Next time GrrlScientist comes to visit, we’re going to have to record what she says early in the morning, and then play it back ten times faster — I have a suspicion that we’ll hear birdsong.
At least, that’s the way this video art installation by Marcus Coates works. He had people sing strange little nonsense tunes (you can hear one here) that, when played back at a greater speed, recreated the songs of wild British birds. Why, if GrrlScientist had only talked a little faster, I’m sure the whole house would have sounded like an exotic tropical island inhabited by parrots!
It’s like a flood of papers on that curious cnidarian worm, Buddenbrockia. Now you can also read about its thoroughly bizarre pattern of development — there are unicellular amoeboids, plasmodial masses, and syncytia involved, so be prepared to be titillated.
If you’ve been reading that fascinating graphic novel, Y: The Last Man(amzn/b&n/abe/pwll), you know the premise: a mysterious disease has swept over the planet and bloodily killed every male mammal except two, a human named Yorick and a monkey named Ampersand. Substantial parts of it are biologically nearly impossible: the wide cross-species susceptibility, the near instantaneous lethality, and the simultaneity of its effect everywhere (there are also all kinds of weird correlations with other sort of magical putative causes, which may be red herrings). On the other hand, the sociological part of the story seems very plausible. There is no feminist utopia, the world goes on in a traumatized and rather complicated way, and the reactions everywhere vary from crazed euphoria to a more common despair. One thing that isn’t at all implausible, and actually has been observed, is a plague that selectively exterminates males.
Since I recently pointed out the strange news reports of an “octosquid” that even went so far as to call it half squid/half octopus, I thought I’d show why the preliminary assignment to the genus Mastigoteuthis was suggestive. It probably did have 8 arms and 2 tentacles … before it got sucked up in a pipe and flung to the surface. Those two feeding tentacles are delicate.
Figure from Cephalopods: A World Guide (amzn/b&n/abe/pwll), by Mark Norman.
Sometimes, I confess, this whole common descent thing gets in the way and is really annoying. What we’ve learned over the years is that the evolution of life on earth is constrained by historical factors at every turn; every animal bears this wonderfully powerful toolbox of common developmental genes, inherited from pre-Cambrian ancestors, and it’s getting rather predictable that every time you open up some fundamental aspect of developmental pattern formation in a zebrafish, for instance, it is a modified echo of something we also see in a fruit fly. Sometimes you just want to see what evolution would do with a completely different starting point — if you could, as SJ Gould suggested, rewind the tape of life and let it play forward again, and see what novelties arose.
Take the worm. We take the generic worm for granted in biology: it’s a bilaterally symmetric muscular tube with a hydrostatic skeleton which propels itself through a medium with sinuous undulations, and with most of its sense organs concentrated in the forward end. The last common ancestor of all bilaterian animals was a worm, and we can see that ancestry in the organization of most animals today, even when it is obscured by odd little geegaws, like limbs and armor and regional specializations and various dangly spiky jointed bits. You’ll even see the argument made that that worm is the best of all possible simple forms, so it isn’t just an accident of history, it’s a morphological optimum.
But what if we could rewind the tape of life a little bit, to the first worms? Is it possible there are other ways such an animal could have been built? It seems nature may have carried out this little experiment for us, and we have an example of a reinvented worm, one not constructed by common descent from that initial triumphal exemplar in the pre-Cambrian — an alternative worm.
