THE EDITORS REPLY: Polonium 210 emits alpha radiation, which loses energy rapidly in the air and is blocked by clothing or by human skin. Thus, it is harmless when outside the body. The isotope does pose some cancer risk when ingested, but according to Argonne National Laboratory, the risks from inhalation—as through cigarette smoke—are about six times higher than for dietary ingestion.
SEX AND BONES
Although the discovery of internal fertilization in 375-million-year-old fossils is as important as John A. Long makes it out to be in “Dawn of the Deed,” his article made the leap from placoderms to tetrapods without mention of lobe-finned fish. The fossil fish Tiktaalik, discovered in 2006, dates back to the same period, and its skeleton bears many more similarities to tetrapods than to the placoderms described in Long’s article—including homologous arm bones and shoulder, neck and ear features. If, as his article suggests, claspers are the progenitors of tetrapod limbs, then where do nearly amphibious lobe-finned fish such as Tiktaalik, of the same age as his placoderms, fit in this phylogeny?
Salt Lake City
LONG REPLIES: Tiktaalik is as close as a fish can get to being a tetrapod; the only things it lacks are fingers and toes. Unfortunately, the available fossils of tetrapodlike fish such as Tiktaalik and early tetrapods carry no evidence of how they reproduced. The closest living relatives of these transitional creatures are the lobe-finned fishes, including the lungfishes, which spawn in water, and the coelacanth, which has internal fertilization despite lacking claspers. Internal fertilization by copulation has evolved independently many times during vertebrate evolution, with many lineages retaining simple spawning. But whether these modern-day species spawn or copulate, the equipment they use to get the job done derives from the same embryonic tissue under the direction of the same so-called hox genes that formed claspers in placoderms.
PRIVACY NOT PROTECTED
In “Don’t Worry about Who’s Watching” [TechnoFiles], David Pogue assures us that there is little to fear about the potentially vast database that an interested party might assemble about any or all of us in a matter of days. Governments are not always as benign as the ones that some of us now enjoy. History offers plenty of examples where a regime bent on total domination found it worthwhile to assemble dossiers on tens or hundreds of thousands of innocent individuals. The time, difficulty and expense required to gather information on masses of citizens have decreased exponentially, and the motivation to use such information for evil purposes can be aroused from dormancy just as easily.
David A. Burack
Many readers wrote that they were confounded by John Allen Paulos’s description of the Monty Hall Problem in “Animal Instincts” [Advances]. A full description of the paradox would take up too much space here, but you can read more on this topic at www.ScientificAmerican.com/may2011/monty-hall.