The ancestor of all living vertebrates may have had gills, a finding that adds to a long-standing debate about the evolutionary history of gills.
In jawless animals such as lampreys, gills form from the embryo's innermost layer of cells, or 'endoderm', whereas in jawed vertebrates, including many fish species, gills were thought to develop from the outermost layer, or 'ectoderm'. This led scientists to think that gills evolved separately in the two lineages.
Andrew Gillis and Olivia Tidswell at the University of Cambridge, UK, studied embryonic gill formation in the little skate (Leucoraja erinacea; pictured), a jawed vertebrate related to sharks and rays. They found that most of the gill tissue developed from the endoderm — as it does in jawless vertebrates.
The discovery that gills seem to grow from the same tissues in both jawless and jawed vertebrates suggests that gills may have evolved only once — in the vertebrates' common ancestor.
Curr. Biol. http://dx.doi.org/10.1016/j.cub.2017.01.022 (2017)