Figures
The evolution of animal limbs.
Bilaterian animals have both an anterior-posterior and dorsal-ventral axis: they encompass lophotrochozoans (e.g. cuttlefish), gnathiferans, ecdysozoans (e.g. arthropods) and deuterostomes (e.g. vertebrates). Tarazona et al. imply that an appendage program originated in the common ancestor, urbilateria (red box at bottom of the bilaterian phylogenetic tree; after Marlétaz et al., 2019). This program would still exist in the lophotrochozoans, ecdysozoans and deuterostomes – the situation in the gnathiferans has not been studied yet. In each group, the program has been co-opted to create new appendages such as vertebrate limbs, arthropod legs or cephalopod tentacles and arms (arrows and corresponding colored boxes). These lineages are strongly divergent, yet the appendage program is conserved in each of them. This prompted Tarazona et al. to conclude that the program has remained active during evolution. Interestingly, pairs of frontal appendages (shown in red) have been found in fossils belonging to all four branches. Did the appendage program initially evolve to build these structures? Image credit: Nectocaris: after Smith and Caron, 2010; Amiskwia: after Caron and Cheung, 2019; Anomalocaris: after Collins, 1996; Pikaia: after Morris and Caron, 2012.
