Protein evolution is irreversible

In their paper "An epistatic ratchet constrains the direction of glucocorticoid receptor evolution", Bridgham, Ortlund and Thornton show that sequence evolution of glucocorticoid receptor is irreversible. Although they only show it for one protein, it may be a general principle. This means that even if evolution was repeated in exactly the same conditions, a completely different world would result, by chance.