Geographic patterns of nasal morphology in Homo

Abstract

Nasal morphology represents a classic example of skeletal variation long depicted as reflecting adaptation to regional climatic regimes in both modern and fossil humans. Temperature, and especially humidity, have been implicated in strongly affecting nasal form, with longer, narrower and more projecting noses associated with dry, cold climates, and shorter, broader noses associated with hot, humid climates.

In this paper, we use 460 crania of known provenience from the Morton skeletal collection to test the hypothesis that nasal breadth associates with temperature and humidity across the five continents. Climatic data spanning the past 100+ years were collected for each cranium’s specific locality. Removing the effect of nasal height on nasal breadth via partial correlation, we confirm that human populations demonstrate a global gradient of decreasing relative nasal aperture as one moves to colder and dryer regions. However, analysis within continents reveals conspicuous exceptions to this pattern. For example, while nasal morphology on the African continent significantly associates with local climate (p=.001), no such association was found in Asia (p=.63) or in Europe (p=.74). Similar results were found using nasal index, although this parameter of nasal form is not independent of nasal height.
We suggest that observed regional patterns of nasal morphology may be explained by the effects of population history, cultural practice, and by the fact that the respiratory functional matrix is not independent of selective forces acting on contiguous functional matrices. Neandertal nasal morphology is discussed in light of these results.

Full text

Nasalmorphologyrepresentsaclassicexampleofskeletalvariationlongdepictedas
reflectingadaptationtoregionalclimaticregimesinbothmodernandfossilhumans.
1

Thatis– exceptforsomenotableexceptions.
2

Theeffectofclimateonnasalmorphologymaybemorepronouncedthanmanyother
somaticfeatures,sinceitisprominentlyexposedwhileotherbodypartsmaybecoveredin
clothingorheadwear.Accordingly,climaticvariablessuchastemperatureandhumidity
havebeenimplicatedinstronglyaffectingnasalform,withlongnarrownosesassociated
withdry,coldclimates,andshortbroadnosesassociatedwithhot,humidclimates.But
interestingly,thisisexactlytheoppositeofwhatAllen’srulepredicts,asinendothermic
species,therelativesizeofexposedportionsofthebodyshoulddecreasewithdecreasing
meantemperature.
3

Theeffectofclimateonnasalmorphologymaybemorepronouncedthanmanyother
somaticfeatures,sinceitisprominentlyexposedwhileotherbodypartsmaybecoveredin
clothingorheadwear.Accordingly,climaticvariablessuchastemperatureandhumidity
havebeenimplicatedinstronglyaffectingnasalform,withlongnarrownosesassociated
withdry,coldclimates,andshortbroadnosesassociatedwithhot,humidclimates.But
interestingly,thisisexactlytheoppositeofwhatAllen’srulepredicts,asinendothermic
species,therelativesizeofexposedportionsofthebodyshoulddecreasewithdecreasing
meantemperature.
4

However,otherresearchersarguedthatsinceaprimaryfunctionofthenoseistomoisten
andhumidifyinspiredair,humidityshouldaffectnasalformfarmorethantemperatureto
preventcold‐inducedrespiratorydamageandtomaintainproperfunctionofthemucosal
membraneandcilia.
5

Subsequentlyhumannasalmorphologyhasbeenshowntoassociatecloselywiththe
amountofwatervaporintheair.Forexample,studieshavefoundapatternofdecreasing
nasalbreadthasonemovestocolderanddryerregionswithinpopulationsofEskimos,
NativeAustralians,EuropeandtheMediterranean,andindigenousPacificCoastpeoples.
6

Twohypotheseshavebeenputforwardtoaccountfortheassociationbetweennasalform
andhumidity.Thefirstisthatlongthinnosesmaymoisturizeinspiredairinarid
environments,asanarrownosemaybettermoistenairbeforeitreachesthelungs.
7

Thesecondhypothesisisnasalmorphologyindryclimatesisprimarilyanadaptationto
reducewaterlossfromexpiredair.Thismodelpositsthatwatervaporcondensesasit
coursesthroughlongnarrownasalpassagesuponexpiration,andisrecycledduring
subsequentinspiration,whereaswatervaporexperiencesismorereadilylostwithbroad
nasalpassages.
8

Whethernasalforminhumansreflectsananatomythathumidifiesinspiredairor
promotestheretentionofwatervapor,both climaticmodelsexplainnarrownosesasan
adaptationthatultimatelypromoteshumidityintherespiratorytractandultimatelyinthe
lungs.
9

Despitetheveneratedassociationbetweenclimateandnasalform,weareremindedthat
nasalstructureisanontogeneticcompromisebetweentheolfactory,respiratory,visual,
andmasticatoryfunctionalmatrices,andassuch,non‐respiratoryfactorshavealsobeen
suggestedstronglyinfluencenasalform.
10

Schwalbefoundastructuralcorrelationbetweennasalbreadthandintercaninebreadth,
allowingfortheargumentthatthewidthoftheanteriordentitionisthemaindeterminant
ofnasalbreadth.Inthismodel,selectionforthesizeofthenasalapertureisconstrained
bythecaninerootsatitslateralmargins,andaccordingly,linkedtomasticatoryfunctional
matrixintheformoftissueresponsetoanteriorloading.
11

Similarly,Huntsuggestedthatincreasedmasticationduringgrowthcouldresultinan
increaseinlowerfacialbreadth,andhenceanincreaseinnasalbreadth.Huntarguedthat
thiswasaresultofagreaterlevelofactivityofthemedianpalatinesutureinthegrowthof
themaxillaryarch.
12

Inthispaperweexaminedbothclimateandmasticationtofactoreachoftheirrelative
contributionstonasalbreadth.Firstwe(1)assessthehypothesisthatnosewidthona
globallevelisafunctionofclimate,and(2)thenwetesttoseewhetherchangesinnasal
breadthareindependentofthoseinthemasticatoryfunctionalmatrix.Inshort,weaimto
determinewhethernasalmorphologyisbestpredictedbythebiomechanicalmasticatory
modelorbythehomeostaticrespiratorymodel.
13

OurskeletalsampleforthisstudywastheMortonCollection,whichiscomposedof
approximately2000humancraniafromover100globalpopulations,initiallycollectedby
SamuelGeorgeMortonintheearlypartofthe19th century.WhileMorton’suseofthe
collectionhasbeencriticizedforhisuseofcranialcapacitytorankhumanraces,the
collectionofferspresentresearchersconsiderablevalue.ThisisbecauseofMorton’s
meticulousdocumentationoftheage,sex,geographicandpopulationalderivationofeach
cranium,oftendetailingthelatitudeandlongitude,city,ortownoforigin.
14

InmanycasesMorton’sdocumentationincludesthepersonalhistoryofanindividual,as
wellasahistoryofmorbidityandmortality,ascanbeseenhereinoneofMorton’scatalog
entriesofaDutcharmycaptainborninUtrechtbutwhodiedinJava…
15

Andshouldserveasawarningtothosewhoaretoodevotedtoconvivialityand
dissipation…
16

Thecollection’sutilityisalsorootedinthefactthatitpredatestheadventofmodern
antibiotics,culturalandtechnologicalglobalization,andmanyreportedseculartrends,and
isderivedfromaperiodwhenpopulation‐mixingwaslesslikelytooccurthanitistoday.
TheMortoncollectioniscuratedattheUniversityofPennsylvaniaMuseumof
AnthropologyandArchaeologywheredatacollectionforthisstudywasconducted.Since
theseconfoundingfactorsarelikelyreducedrelativetomoremodernsamples,theMorton
Collectionmaybeespeciallyrelevantintheinvestigationofgeographic,climaticand
populationalvariationamongmodernhumans.
17

Inadditiontostandardanthropometricvariablesofthenose,anteriordentition,faceand
skull,wecollectedclimaticdata*spanningmorethan100yearsforthespecificlocalityfor
460craniarepresenting97populationstotestthehypothesisthatnasalformassociates
withclimate.Wecollecteddatafor9climaticvariablessuchasvaporpressure,
precipitation,mean,maximumandminimumtemperature,fromtheWorldMeteorological
OrganizationandNationalGeophysicalDataCenterwebsites.
*TheclimatedatausedinthisstudywasobtainedawebsiteestablishedbytheWorld
MeteorologicalOrganization
(http://ipccddc.cru.uea.ac.uk/asres/baseline/climate_download.html),andisconstrained
toninesurfacevariables:precipitation[PRE]((millimeters/day)*10),andwet‐day
frequency[WET](Days*10);mean,maximumandminimumtemperature[TMP,TMX,
TMN](Deg.C*10);diurnaltemperaturerange[DTR](Deg.C*10);vaporpressure[VAP]
(hPa*10);cloudcover[CLD](%);andfrostfrequency[FRS](Days*10).Thestationdata
wereinterpolatedasafunctionoflatitude,longitudeandelevationusingthin‐platesplines.
DataonlocalelevationwasalsorecordedfromdatacollectedbytheNationalOceanicand
AtmosphericAdministration/NationalGeophysicalDataCenterwebsite
(http://geodata.grid.unep.ch/).
18

Climaticdatapointsattributedtoeachcraniuminthisstudygreatlyimproveouranalytical
resolutionrelativetostudieswheredatafrompopulationsarepooled.Thus,inthisstudy
theunitsofanalysisareindividualsandtheirspecificlocalenvironments,ratherthan
populationsacrossbroadregionalclimaticloci.
19

Analysis:Whenweplotthelogofnasalindexagainstthelogofmeanannualtemperature
weseesomewhatweakbutsignificantcorrelationshowingthatastemperatureincreases
nasalindexincreases,confirmingaglobalassociationofrelativelywidenosesandwarm
climates,andnarrowernosesincoldregions.
20

Andwhenweexaminetheassociationbetweennasalindex&annualvaporpressurewe
alsoseeaweakbutsignificantcorrelation.Thesedataconfirmotherstudiesusingnasal
indexshowingaglobalgradientofdecreasingrelativenasalbreadthasonemovestocolder
anddryerregions.
21

Buttheuseofnasalindextocharacterizenasalformhasbeencriticizedonthegrounds
thattheuseofnasioninthecalculationofnasalindexhaslittlefunctionalassociationwith
noseform.Instead,Wolpoffpointstonasalbreadthasthemorerelevantadaptive
variable.
22

Yetwhenweexaminenasalbreadthwefindthatitdoesnotassociatewithmeanannual
temperatureorvaporpressure,suggestingthatnasalbreadthhaslessutilityasaskeletal
variablethanthenasalindex.
23

Thismaybeattributedtothefactthatnasalindexexpressesnasalshapewithoutthe
influenceofsize,whereasnasalbreadthisnotimmunefromtheconfoundingfactorof
differentialsize,andsaysnothingaboutrelativenasalshape.However,ifweremovethe
effectofnasalheightonnasalbreadth,thenwemaycontrolforthisshortcomingand
eliminatesomeofthecriticismconcerningusingthenasalindexandineffect,capturea
morerelevantfunctionalparameterofthenose.
24

Whenweremovetheinfluenceofnasalheightonnasalbreadthbypartialcorrelation,we
findthattheassociationbetweennasalbreadthandclimateslightlyimproves,andallof
ourclimaticvariablesweaklycorrelatewithnasalbreadthinourworldwidesample*.
*However,analysiswithincontinentsrevealsconspicuousexceptionstothispattern..For
example,whilenasalbreadthontheAfricancontinentsignificantlyassociateswithlocal
climate(p=.001),nosuchassociationwasfoundinAsia(p=.63)orinEurope(p=.74).The
sameresultswerefoundusingnasalindex,althoughthisparameterofnasalformisnot
independentofnasalheight.
25

Soatthispointwecanconcludethatnasalformhasaweakbutsignificantassociationwith
climaticvariables.Wealsoseeinoursamplethatnasalindexandcorrectednasalbreadth
relatetoclimate,whilerawnasalbreadthmeasurementsdonot.
26

Butnowweturnourattentiontothequestionofwhetherchangesinnasalformare
independentofthoseinthemasticatoryfunctionalmatrix.Wetestedtoseewhether
variationinnasalformisbetterexplainedbyitsassociationwiththeanteriordentitionand
thepalate.
27

Andinfact,thatisexactlywhatweseeinourdata.Nasalbreadthisbetterpredictedby
intercaninebreadthandmoresobythewidthofthepalate.Buttocompare“appleswith
apples”,weneedtoremovetheeffectofclimateonthemasticatorystructurestoinsure
theirindependenceandaccuratelyassesstheircontributiontonasalbreadth.
28

Weperformedprincipalcomponentanalysisofourclimaticvariablestocondensethem
intototwofactorsloadingontemperature&humidityrespectively.
29

Whenweassesstheindependentcontributionsofintercaninebreadth,temperature,and
humiditytopredictnasalbreadththroughmultipleregression,ourbetascoresindicatethat
theindependentcontributionofintercaninebreadthismuchstrongerthanhumidityand
thantemperature.Thus,theinfluenceofintercaninebreadthonnasalbreadthishighly
significant,whileclimaticvariablesarenot.
30

Whenwelookattheinfluenceofpalatebreadthonnasalbreadth,it’sindependent
contributionissignificant,andalmost3timesstrongerthanhumidityand4timesstronger
thantemperature.Thesedatasuggestthatnasalbreadthisbetterexplainedbythe
masticatoryfunctionalmatrix,whiletheeffectsofclimateonnasalbreadtharenot
significant.
31

Thesedatasuggestthattherespiratoryfunctionalmatrixisnotindependentofselective
forcesactingoncontiguousfunctionalmatrices.
32

And respiratory morphology in the form of nasal breadth is subordinate to masticatory
morphology.
33

While our global study does not standardize for cultural or dietary differences, our data
suggest that measures of nasal breadth are not effective indices for respiratory adaptation.
This may be in part that as a two-dimensional parameter, the nasal index may belie the
three-dimensional nature of the nose. This can be seen in these two skulls from the Morton
Collection with nearly identical nasal indices, but with very different nasal angles.
34

If nasal breadth is somewhat determined by masticatory structures as our data suggest, then
climatic vectors of selection may principally operate on nasal bone projection to increase
the ratio of surface area to volume of air passing through the nose for air temperature and
water regulation. Similarly, selection on the internal nasal capsule must be considered.
35

Selection along these structural axes would explain the projecting nasal morphology in
Neandertals as a climatic adaptation, while the wide nasal aperture reflects the functional
primacy of their anterior dentition.
36

This pattern may reflect a deep evolutionary trajectory attested to by patterns of heavy
anterior loading and wear in Neandertals and their ancestors.
37

And consideration of the nasal aperture in three dimensions rather than two may explain
why broad nasal apertures predominate throughout the terminal Lower and Middle
Paleolithic, and help to explain a seemingly contradictory feature among our fossil
ancestors.
38

We would like to acknowledge the following people for their constructive comments on this
presentation.
39