A schematic representation phylogeny of main vertebrate groups. Timescales are based on molecular clock (Kumar and Hedges 1998). Some of the proposed vertebrate whole genome duplications are marked by asterisks. The connected asterisks denote the range of the proposed genome duplication time. The references are as follows: 1, Ohno 1970; 2, Ohno 1973; 3, Lundin 1993; 4, Holland et al. 1994; 5, Sidow 1996; 6,Kasahara et al. 1996; 7, Spring 1997; 8, Ohno 1998; 9, Meyer and Schartl 1999; 10, Amores et al. 1998 and Mayer and Schartl 1999.