While we obsess about minor wars and Janet Jackson’s left nipple, serious history rolls on in its subterranean channels. In the last couple of weeks, for example, three exceedingly important scientific papers appeared that bear on the origins and nature of animal life. A run down and some discussion:
Bejerano et. al. announced in the 28 May issue of SCIENCE the discovery of 481 DNA segments that are 100% conserved in the human, rat, and mouse genomes and are also extraordinarily similar to corresponding stretches in the DNA of chickens and fugu fish. Analogies between DNA segments are not surprising, but absolute identity is amazing because of the well-known near impossibility of perfectly faithful transmission—even the best Xerox machine will eventually produce a garbled image if you keep copying the copies of the copies. The authors of the paper point out that their findings imply that the duplicated stretches are extremely important to the survival of the organisms and therefore subject to powerful stabilizing selection or that some sort of exceptionally effective and so far unknown editing mechanism is at work on just those base sequences. Interestingly, many of the conserved segments do not code for messenger RNA—only 111 overlap the mRNA of a known protein-coding gene. Presumably they are important because of their role in the regulation of gene expression and RNA processing rather than specifying a protein.
The same issue of SCIENCE includes a paper by Finnerty et. al. that also deals with conserved genes, in this case the Hox genes that pattern the front-to-back axis of development in all bilateral animals, including us. The authors demonstrate that Hox genes underlie the development of the sea anemone, Nematostella vectensis, a bilateral organism in a phylum (Cnidaria) whose members mostly have radial symmetry. Remarkably, the top-to-bottom axis of Nematostella is established by the action of the same gene (decapentaplegic) that patterns the top-to-bottom axis in Bilateria. The authors conclude that their data “suggest that bilateral symmetry arose before the evolutionary split of Cnidaria and Bilateria.” Now that was a very long time ago indeed—Cnidarians and bilaterians appear together in Precambrian fossils from 560 to 570 billion years ago. Meanwhile.
A June 3 paper published in SCIENCE on line (Chen, et.al) reports the earliest known bilaterian fossils. The authors date these tiny forms (<150 microns) to 580-600 million years before present, just after the end of a period of intense glaciation, hence the name of the organism, Vernanimalcula guizhouena, spring animalcule. The date matches up pretty well from guesses on the age of the earliest bilateral animals derived from phylogenetic analyses, but other scientists are sure to second guess this research. The authors also claim the animals “had paired coeloms extending the length of the gut; paired external pits that could be sense organs; bilateral, anterior-posterior organization; a ventrally directed anterior mouth with thick walled pharynx; and a triploblastic structure,” That’s a lot of structure to identify from minute and ancient specimens. It remains to be seen if these conclusions hold up. However
Taken together, these three papers emphasize how little wobble there has been in the history of animals. It appears that over the last 600 million years or so evolution has been playing with a very small toolkit indeed. Of course it may be possible that there is just one way to put together a mobile, multicellular organism and that the leaping green mice of Mars will turn out also to have Hox genes and hyperconserved DNA; but it certainly looks as if what occurred was the selection of one possible system among many. Of course that version suits my own rather hobbyhorsical view of how the universe operates—on every scale and in every venue relentlessly thinning out an original plentitude of possibilities in favor of a vanishingly small set of survivors. That’s just my take. The results are remarkably suggestive even for those who don’t like to make generalizations in the style of Herbert Spencer.
In other news, Ronald Reagan died. Mr. Reagan is mostly significant as an example of how a handsome nonentity can be turned into a beloved and irreproachable icon by modern public relations techniques.