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A body plan with two sets of paired appendages, pectoral and pelvic, is a derived feature that first appears in later jawed vertebrates (gnathostomes) 2, 3, 4. The demands of feeding and locomotion in Ordovician and Silurian seas led to a surprising variability of the earliest known appendages: some forms possessed a continuous anterior fin that ran the length of the body, others had paired fins that projected immediately behind a head shield, and still other primitive vertebrates had no paired fins at all ( Fig.
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Vertebrate limb diversity was produced by changes in the number, position and shape of structures that can be traced to Ordovician 2, 3 (463–439 Myr) through Late Devonian 2, 3, 4 (409–362 Myr) fossils. These examples illustrate how comparative developmental genetics can provide a mechanistic explanation of the origin and evolution of structures when palaeontological data are robust and important new hypotheses about evolutionary history when the fossil record is silent. We will develop the hypothesis that the evolution of successively derived limb types, from lobopods to insect wings, and from agnathan fins to tetrapod limbs appears to be due, in part, to the successive cooption and redeployment of signals established in primitive metazoans. Then we will explore the significance of newly discovered genetic similarities between arthropod and vertebrate appendages-similarities that have been retained despite more than 500 million years (Myr) of independent evolution. We will integrate palaeontological and developmental evidence that suggests that major innovations are largely derived from pre-existing developmental systems and will illustrate the potential genetic regulatory changes that enabled appendage evolution.
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In both phyla we are confronted with a similar issue, namely the origin and adaptive modification of serially homologous organs. We will first examine the origin and evolution of vertebrate limbs and digits and of arthropod legs and insect wings. These genetic discoveries can be integrated with palaeontological data to address some of the principal events in the history of animal designs. Enormous progress has been made in the past few years in understanding appendage development in both phyla. The adaptive evolution of vertebrates and arthropods to aquatic, terrestrial and aerial environments was accomplished by the invention of many novel features, especially new types of appendages. How do new structures arise? Can they evolve de novo or are they generally derived from pre-existing structures? And what is the developmental and genetic basis for their origin and modification 1? The origin of evolutionary novelties raises some of the most fundamental questions of biology.