Two approaches to dating evolutionary divergence times. Lineages x, y, z, i and j are shown going back (down) from the present day. Thick bars represent periods for which there is a fossil record for the lineage; dotted lines represent 'ghost' lineages, times when a group is inferred to have been present but left no record . Horizontal lines represent occurrences of a fossil from the lineage in the record; dt(x,y) indicates the date of divergence of lineage x from lineage y; i and j are lineages for which no fossil record is available. (a) Discovery of older fossils of one lineage (red) can alter our views of how the various groups evolved. (b) Calibration of divergence times from sequences using fossil record dates. First, rates of sequence divergence are calibrated using taxa for which a reliable fossil record is available. Gd represents the genetic distance of present-day species from each other, derived from sequence data. A mean rate of sequence substitution is then calculated from a regression of these calibration points, and is used (right) to compute divergence times (gd(x,i) and gd(x,j)) between taxa for which the fossil record is not reliable.