Table 1 Examples of historical holdovers, phylogenetic and ontogenetic, revealing traces of prior conditions

Original function altered or absent (=non-functional)

Vestiges: Cormorant wing-drying & dog circling behaviors; mammalian dive reflex; dewclaw; cave animal eyes; coccyx; vermiform appendix; piloerection

Circulatory shunts including fossa ovalis & ligamentum arteriosum; male nipples; inguinal canal & looped vas deferens from descent of testes

Function, if any, continues, yet reveal past states of evolution (via phyletic analysis) or development

Primate fingernails & stereoscopic vision as synapomorphies (shared, derived characters); cephalization with terminal rather than central head

Cranial sutures & epiphyseal joints; umbilicus (navel) & other scars

Function lost or shifted, yet compound clues reveal joint evo-devo history within lineage

Atavisms: Sudden reversion to retention of tail in humans or of limbs in snakes or cetaceans; bird teeth; Exaptations: fish lungs before swim bladders; feathers for insulation before flight

Homologies: Pharyngeal glands (e.g., tonsils) & chordate gill arches; male & female genitalia

May be altered or non-functional; development in divergent lineages reveals clues to history/homology (in common origins & developmental processes)