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Justhowsaltbecamesocrucialtoourmetabolismisamystery;oneappealingtheorytracesourdependenceonittothechemistryofthelateCambrianseas.Itwasthere,ahalfbillionyearsago,thattinymetazoanorganismsfirstevolvedsystemsforsequesteringandcirculatingfluids.Thewateroftheearlyoceansmightthushavebecomethechemicalprototypeforthefluidsofallanimallife-themediuminwhichcellularoperationscouldcontinuenomatterhowtheexternalenvironmentchanged.

Thisspeculationisbasedonthefactthat,eventoday,thebloodserumsofradicallydivergentspeciesareremarkablysimilar.Lizards,platypuses,sheep,andhumanscouldhardlybemoredifferentinanatomyoreatinghabits,yetthesaltcontentinthefluidsurroundingtheirbloodcellsisvirtuallyidentical.Asearlymarinespecicsmadetheirwaytofreshwaterandeventuallytodryland,sodiumremainedakeyingredientoftheirinterior,ifnottheirexterior,

milieu.Themostsuccessfulmammalianspecieswouldhavebeenthosethatdevelopedefficienthormonalsystemsformaintainingtheneededsodiumconcentrations.Thehumanbody,forexample,usesthehormonesrenin,angiotensin,andaldosteronetoretainorreleasetissuefluidsandbloodplasma.Theresult,underfavorableconditions,isadynamicequilibriuminwhichneitherfluidvolumenorsodiumconcentrationfluctuatestoodramatically.Butifthebodyisdeprivedofsalt,theeffectssoonbecomedangerous,despitecompensatorymechanisms.