Making the break: the origin of the reptiles
Amphibians only made it halfway on to land, and they still produce swimming tadpoles.
Box 16.7 The first tetrapods had seven or eight toes
New studies of Ichthyostega and Acanthostega from the Late Devonian of Greenland (Coates et al. 2002), and of other animals of the same age, including the "limbed fish" Tiktaalik from Arctic Canada (Daeschler et al. 2006; Shubin et al. 2006), show that the first tetrapods had more than five fingers and toes, indeed as many as seven or eight (Fig. 16.14b, c). It is possible to draw comparisons between the bones of the pectoral fin of a sarcopterygian (Fig. 16.4a) and those of the forelimb of an early amphibian (Fig. 16.14b). This caused a major rethink of the classic story of the evolution of vertebrate limbs: five digits must have become standard only after the origin of tetrapods. What if tetrapods had settled on seven, rather than five, fingers? Probably we would not use the decimal counting system, and imagine the changes to musical instruments and computer keyboards!
The implications are wider, because the new evidence suggests that particular features of an organism may not all be preprogrammed in the genetic code of the developing embryo. In other words, there is not a single gene that codes for each finger and toe. It seems that aspects of the developmental environment, rather than genetic programming, determine some details of adult structure: as a limb develops in the embryo, at first it has no fingers or toes, and then a pulse of information triggers the sprouting of digits at a particular time. In rare cases, humans may be born with a sixth finger, perhaps a genetic memory of our condition 400 Myr ago. Also, many tetrapods have only four (frogs), three (rhinos), two (cows) or one (horses) finger - perhaps losing digits is associated with the "switching" on or off of particular controlling genes.
Read more about the basal tetrapods and the fin to limb transition in Zimmer (1999), Clack (2002) and Shubin (2008) and at http://www.blackwellpublishing.com/paleobiology/.
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