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Australian Geographic - - Geobuzz -

Dr Frank Tu­lenko, Aus­tralian Re­gen­er­a­tive Medicine In­sti­tute, Monash Univer­sity


Why do we have five fin­gers and five toes, and why do so many crea­tures also have five fin­gers? Wil­liam For­rest, Tamworth, NSW


Mod­ern te­trapods (limbed ver­te­brates) gen­er­ally have hands and feet with five dig­its. This was not the case for the first te­trapods that moved from wa­ter to land as early as al­most 400 mil­lion years ago – they had up to eight dig­its. It wasn’t un­til about 350 mil­lion years ago that a five-digit hand sta­bilised and was in­her­ited. We don’t re­ally un­der­stand why the five-digit model evolved, though biome­chan­i­cal ad­van­tages for walk­ing have been sug­gested. De­vel­op­men­tal bi­ol­o­gists have be­gun to tell a fas­ci­nat­ing story of the ge­netic changes un­der­pin­ning how limbs evolved from fins, and how these changes may have af­fected digit count. Dig­its have of­ten been lost within te­trapods, but they al­most never in­crease. This ob­ser­va­tion sug­gests a strong con­straint – per­haps re­lated to the use of the same genes for mak­ing dif­fer­ent body parts – on what is pos­si­ble through evo­lu­tion­ary tin­ker­ing.

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