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prof_pangaea) wrote2005-03-20 03:24 pm
prof_pangaea) wrote2005-03-20 03:24 pm
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Wondeful Life (Part V)
Excerpt from:
Wonderful Life: The Burgess Shale and the Nature of History
by Stephen Jay Gould
V. The Origin of Terrestrial Vertebrates
Our traditionalist is now reeling. He is ready to abandon virtually all of life to contingency, but he will make his last stand with vertebrates. The game, after all, centers on human consciousness as the unpredictable product of an incidental twig, or the combination of an ineluctable, or at the very least, a probable, trend. To hell with the rest of life; they aren't on the lineage leading to consciousness in any case. Surely, once vertebrates arose, however improbable their origin, we could then mount confidently from ponds to dry land to hind legs to big brains.
I might grant the probability of the most crucial environmental transition -- from water to land -- if the characteristic anatomy of fishes implied, even for incidental reasons, an easy transformation of fins into sturdy limbs needed for support in the gravity of terrestrial environments. But the fins of most fishes are entirely unsuited for such a transition. A stout basal bar follows the line of the body axis, and numerous thin fin rays run parallel to each other and perpendicular to the bar. These thin, unconnected rays could not support the weight of the body on land. The few modern fishes that scurry across mud flats, including Periophthalmus, the "walking fish," pull their bodies along and do not stride with their fins.
Terrestrial vertebrates could arise because a relatively small group of fishes, only distantly related to the "standard issue," happened, for their own immediate reasons, to evolve a radically different type of limb skeleton, with a strong central axis perpendicular to the body, and numerous lateral branches radiating from the common focus. A structure of this design could evolve into a weight-bearing terrestrial limb, with the central axis converted to the major bones of our arms and legs, and the lateral branches forming digits. Such a fin design did not evolve for its future flexibility in permitting later mammalian life; (this limb may have provided advantages, in superior rotation, for bottom-dwelling fishes that used the substrate to aid in propulsion). But whatever its unknown advantages, this necessary prerequisite to terrestrial life evolved in a restricted group of fishes off the main line -- the lungfish-coelacanth-rhipidistian complex. Wind the tape of life back to the Devonian, the so-called age of fishes. Would an observer have singled out these uncommon an uncharacteristic fishes as precursors to such conspicuous success in such a different environment? Replay the tape, expunge the rhipisdistians by extinction, and our lands become the unchallenged domain of insects and flowers.
Wonderful Life: The Burgess Shale and the Nature of History
by Stephen Jay Gould
V. The Origin of Terrestrial Vertebrates
Our traditionalist is now reeling. He is ready to abandon virtually all of life to contingency, but he will make his last stand with vertebrates. The game, after all, centers on human consciousness as the unpredictable product of an incidental twig, or the combination of an ineluctable, or at the very least, a probable, trend. To hell with the rest of life; they aren't on the lineage leading to consciousness in any case. Surely, once vertebrates arose, however improbable their origin, we could then mount confidently from ponds to dry land to hind legs to big brains.
I might grant the probability of the most crucial environmental transition -- from water to land -- if the characteristic anatomy of fishes implied, even for incidental reasons, an easy transformation of fins into sturdy limbs needed for support in the gravity of terrestrial environments. But the fins of most fishes are entirely unsuited for such a transition. A stout basal bar follows the line of the body axis, and numerous thin fin rays run parallel to each other and perpendicular to the bar. These thin, unconnected rays could not support the weight of the body on land. The few modern fishes that scurry across mud flats, including Periophthalmus, the "walking fish," pull their bodies along and do not stride with their fins.
Terrestrial vertebrates could arise because a relatively small group of fishes, only distantly related to the "standard issue," happened, for their own immediate reasons, to evolve a radically different type of limb skeleton, with a strong central axis perpendicular to the body, and numerous lateral branches radiating from the common focus. A structure of this design could evolve into a weight-bearing terrestrial limb, with the central axis converted to the major bones of our arms and legs, and the lateral branches forming digits. Such a fin design did not evolve for its future flexibility in permitting later mammalian life; (this limb may have provided advantages, in superior rotation, for bottom-dwelling fishes that used the substrate to aid in propulsion). But whatever its unknown advantages, this necessary prerequisite to terrestrial life evolved in a restricted group of fishes off the main line -- the lungfish-coelacanth-rhipidistian complex. Wind the tape of life back to the Devonian, the so-called age of fishes. Would an observer have singled out these uncommon an uncharacteristic fishes as precursors to such conspicuous success in such a different environment? Replay the tape, expunge the rhipisdistians by extinction, and our lands become the unchallenged domain of insects and flowers.