Asymmetry is a fundamental aspect of the biology of all organisms, and has a deep evolutionary history. The fossil record contains evidence of both morphological and behavioural asymmetries. Morphological asymmetry is most commonly expressed as conspicuous, directional asymmetry (either lateral asymmetry or spiral asymmetry) in body fossils. Few examples of fluctuating asymmetry, a form of subtle asymmetry, have been documented from fossils. Body fossil evidence indicates that morphological asymmetry dates to the time of the appearance of the first life on Earth (Archaean Eon). Behavioural asymmetry can be assumed to have been concomitant with conspicuous morphological asymmetry, but more direct evidence is in the form of trace fossils. Trace fossil evidence suggests that behavioural asymmetry, including nervous system lateralization, was in existence by the beginning of the Palaeozoic Era.
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