Figure 1. The origin of vertebrates.

The evolution of jawed vertebrates is illustrated against an approximate time-scale of millions of years ago (Mya). The taxa considered in this Review are indicated with an asterisk and are accompanied by schematics and diagrams of the ‘eye’ region. The earliest chordates, represented by extant cephalochordates and tunicates, are thought to have appeared around 550 Mya. Jawless craniates (agnathans) were present in the early Cambrian, by 525 Mya, and a time of 530 Mya has been indicated for their presumed first appearance. As elaborated on in BOX 1, there is considerable controversy as to whether myxiniformes (solely represented by extant hagfish) diverged before or after the separation of lampreys from jawed vertebrates (shown as dashed black and grey lines). Numerous lines of jawless fish evolved between 500 and 430 Mya ago, although none have survived to the present day. The first jawed vertebrate arose around 430 Mya, and this line is represented today by cartilagenous fish, bony fish and tetrapods. Six ‘stages of interest’ in vertebrate eye evolution correspond to the time intervals between the divergence of important surviving taxa. This diagram does not include the evolutionary changes that have occurred in the last 400 million years. The presented timeline is based primarily on evidence from the fossil record; see REFS 2,13,15,17,18,144,160–163. The schematics are modified, with permission, from REF. 11 © (1996) Oxford University Press (lancelet, sea squirt, hagfish and lamprey) and REF. 164 © (2004) Academic Press (jawed vertebrate). The eye images are reproduced, with permission, from the following references: lancelet, REF. 165 © BIODIDAC (1996) University of California Museum of Paleontology; sea squirt, REF. 63 © (2006) Blackwell Publishing; hagfish, REF. 166 © (2006) Australian Museum. Lamprey and jawed vertebrate eye images are courtesy of G. Westhoff and S. P. Collin).