I distrusted the idea when I wrote the 3rd edition, so I left it out. Since mid-1999, the idea has been polished and extended, but in my view it has also been fatally compromised by new evidence. There was certainly one major glaciation, probably more than one, but the worst scenario that is believable is more like "Slushball Earth" than "Snowball Earth".
Go to Planktonic Paradise on Slushball Earth: a scenario for the metazoan radiation for my semiformal essay on this topic.
But it would be really good to check that story using a completely different approach. How about genomics? the study of the genetic make-up of choanoflagellates versus sponges and other protists. In 2001 and 2002, it was established that both nuclear and mitochondrial genes confirm that choanoflagellates and metazoans are closely related. However, the underlying reasons for that genetic similarity remained unclear until a beautifully logical study was published in 2004, from Sean Carroll's lab at the University of Wisconsin.
This study revealed that choanoflagellates have the genes to produce a variety of proteins that are used in metazoans to hold cells together and to transmit signals between cells. Of course, choanoflagellates use such proteins rarely, since many of them are single-celled all the time, and those that form colonies do so with only a few cells. But choanoflagellates, apparently alone among protists, had the genes that made it possible for them or their close relatives to experiment with multicellularity in a way that was not possible for other protists.
This study strengthens the choanoflagellate model for the origins of sponges in particular and metazoans in general. And some day, when the entire choanoflagellate genome is sequenced, we will have a much better story.
The paper was in Science. and is now freely available on the Web. (Thank you!).
King, N., et al. 2003. Evolution of key signalling and adhesion protein families predates animal origins. Science 301: 361363.
When you look at fossil corals, you see right away that the Paleozoic rugosan corals have a skeleton that is bilaterally symmetrical. Other coral groups, including the living hexacorals, are less clearly bilateral, but you find that they are if you look carefully.
So why are cnidarians not part of the Bilateria? The answer is probably to do with the way the cnidarians are otherwise structured. They are sheet animals, not 3-D. They run a way of life where radial disposition of structures is favored. They don't look bilateral, and they certainly don't have the complexity of structure that all the Bilateria share. Remember that Bilateria is only a name.
The reason for even bringing up this topic is that there are a few cnidarians with rather pronounced bilateral symmetry (they are burrowing anenomes). One might ask how they do it? Do they share some genes with bilaterians that we can study and understand?
And the answer is that they do share some Hox genes with bilaterians, presumably genes that organize bilateral symmetry as the larve develops into an adult. Thus the basics of bilateral symmetry were present in the ancestor of cnidarians and bilaterians. But they were and are expressed in very muted fashion in cnidarians, while they dominate the morphology of Bilateria (both for very good functional reasons).
So there are two main conclusions:
The paper was Finnerty, J. R., et al. 2004. Origins of bilateral symmetry: Hox and Dpp expression in a sea anenome. Science 304: 13351337, and comment, 1255-1256. The paper is available on the Web. Thank you!.
The reference list for Chapter 4, 4th Edition, with associated Web links
Page last updated, October 5, 2004.
All links last checked, October 1, 2005.
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