a figure from the essay
Fossils, Genes and the Evolution of Animal Limbs
by Neil Shubin, Clif Tabin and Sean Carroll
Figure 5 (from page 112 of Shaking the Tree: Readings from Nature in the History of Life edited by Henry Gee)
The evolution of the arthropod limb and the origin of the insect wing. a-e, Some of the major transitions in limb architecture. a, A simple unjointed, annulated lobopodium; b, separate lateral lobes may have served a gill-like function; c, a jointed biramous limb in which the two limb branches are joined at the base, an upper branch is often derived from lateral lobes; d, a multibranched limb found in the branchiopod crustacean Artemia; e, the insect wing and uniramous leg appear to derive from a polyramous limb in an aquatic ancestor. Box: left, a developing polyramous limb and corresponding differentiated structure. The apterous gene is expressed (solid block) in a dorsal respiratory lobe, whereas the Distal-less gene is expressed (edges) in the other limb branches.54 An ancestor-descendent relationship between the limbs in the box is not implied. Right, separation of the dorsal respiratory lobe from the ventral limb primordium in a primitive pterygote such as a Palaeodictyoptera nymph. The proto-wing at this stage was probably a gill-like structure on all trunk segments and still attached to the base of the limb. The apterous and Distal-less genes play critical roles in wing and leg formation in Drosophila.
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About the book: Shaking the Tree: Readings from Nature in the History of Life edited by Henry Gee, published by the University of Chicago Press.