Digit (anatomy) - Evolution

Evolution

Two ideas about the homology of arms, hands and digits have existed in the past 130 years. First that digits are unique to tetrapods and second that antecedents were present in the fins of early sarcopterygian fish. Until recently it was concluded that "Both genetic and fossil data support the hypothesis that digits are evolutionary novelties".p. 640.

However new research that has created a three-dimensional reconstruction of a Panderichthys, a coastal fish from the Devonian period 385 million years ago, shows that these animals already had many of the homologous bones present in the forelimbs of limbed vertebrates. For example, they had fin radials, bones similar to rudimentary fingers but positioned in the arm-like base of their fins. Thus there was in the evolution of tetrapods a shift such that the outermost part of the fins were lost and came to be replaced by early digits. This change is consistent with additional evidence from the study of actinopterygians, sharks and lungfish that the digits of tetrapods arose from pre-existing distal radials present in more primitive fish.

Controversy still exists since Tiktaalik, a vertebrate often considered to be the missing link between fishes and land-living animals, had stubby leg-like limbs that lacked the finger-like radial bones found in the Panderichthys. The researchers of the paper commented that "It is difficult to say whether this character distribution implies that Tiktaalik is autapomorphic, that Panderichthys and tetrapods are convergent, or that Panderichthys is closer to tetrapods than Tiktaalik. At any rate, it demonstrates that the fish–tetrapod transition was accompanied by significant character incongruence in functionally important structures.".p. 638.