Pentadactyl Tetrapods
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http://www.corante.com/loom/archives/000767.html
(Interesting discussion of why we have five fingers (well, four and a thumb) as opposed to three or seven or whatever.)
Eight Little Piggies Redux
- Posted by Carl at 10:38 PM
In a post last month, I pointed out how aerospace engineers can learn a lot from looking at the fossils of ancient flying reptiles. Today's issue of Nature contains a variation on that theme: ancient swimming reptiles can teach geneticists a lot as well.
Almost all humans have five fingers. Genetic disorders can produce extra fingers and toes, but only rarely. Five fingers is generally the upper limit not just for humans, but for all vertebrates on land. You can find plenty of tetrapods whose ancestors lost one or more of those five fingers. Horses have just one; snakes none. But tetrapods with more than five digits are incredibly rare. In most cases, these aren't true digits--wrists bones or other parts of limbs have evolved into finger-like appendages (the panda's "thumb" made so famous by Stephen Jay Gould, for example). But if you're looking for seven or eight real digits--made of three or four rod-shaped bones extending from the wrist or ankle, you're out of luck.
As Gould pointed out in his essay "Eight Little Piggies" (from his book of the same name), nineteenth century biologists treated this pattern as a geometrical law. Five digits were part of the tetrapod "archetype"--the divine blueprint on which all variations were built. But that turned out not to be the case. In the 1980s, the paleontologists Jenny Clack and Michael Coates discovered that the earliest tetrapods that lived some 360 million years ago--vertebrates with legs and toes--had six, seven, or even eight toes.
(More at site...)
(Interesting discussion of why we have five fingers (well, four and a thumb) as opposed to three or seven or whatever.)
Eight Little Piggies Redux
- Posted by Carl at 10:38 PM
In a post last month, I pointed out how aerospace engineers can learn a lot from looking at the fossils of ancient flying reptiles. Today's issue of Nature contains a variation on that theme: ancient swimming reptiles can teach geneticists a lot as well.
Almost all humans have five fingers. Genetic disorders can produce extra fingers and toes, but only rarely. Five fingers is generally the upper limit not just for humans, but for all vertebrates on land. You can find plenty of tetrapods whose ancestors lost one or more of those five fingers. Horses have just one; snakes none. But tetrapods with more than five digits are incredibly rare. In most cases, these aren't true digits--wrists bones or other parts of limbs have evolved into finger-like appendages (the panda's "thumb" made so famous by Stephen Jay Gould, for example). But if you're looking for seven or eight real digits--made of three or four rod-shaped bones extending from the wrist or ankle, you're out of luck.
As Gould pointed out in his essay "Eight Little Piggies" (from his book of the same name), nineteenth century biologists treated this pattern as a geometrical law. Five digits were part of the tetrapod "archetype"--the divine blueprint on which all variations were built. But that turned out not to be the case. In the 1980s, the paleontologists Jenny Clack and Michael Coates discovered that the earliest tetrapods that lived some 360 million years ago--vertebrates with legs and toes--had six, seven, or even eight toes.
(More at site...)
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