▁be ▁distinct ly ▁fl atten ed . ▁A si atic ▁heads ▁of ▁straight ▁hair ▁are ▁formed ▁from ▁almost - round ▁hair s , ▁and ▁C au cas ian ▁hair ' s ▁cross ▁sections ▁form ▁o val ▁shapes . ▁Af ro - text ured ▁hair ▁has ▁a ▁fl atten ed ▁cross - section ▁and ▁is ▁fin er , ▁and ▁its ▁ring lets ▁can ▁form ▁tight ▁circles ▁with ▁diam eters ▁of ▁only ▁a ▁few ▁mill im eters . ▁In ▁humans ▁world wide , ▁A si atic ▁hair ▁is ▁the ▁most ▁common , ▁whereas ▁af ro - text ured ▁hair ▁is ▁the ▁least ▁common . ▁This ▁is ▁because ▁the ▁former ▁hair ▁texture ▁is ▁typical ▁of ▁the ▁large ▁populations ▁inhab iting ▁East ▁Asia ▁as ▁well ▁as ▁the ▁ind igen ous ▁pe op les ▁of ▁the ▁Amer icas . ▁ ▁Af ro - text ured ▁hair ▁str ands ▁can ▁possess ▁" t ors ion ▁tw ists ", ▁where ▁the ▁hair ▁str and ▁turns ▁around ▁itself . ▁These ▁t ors ion ▁tw ists ▁may ▁prevent ▁the ▁hair ▁str ands ▁from ▁co iling ▁into ▁tight ▁curl s , ▁instead ▁separ ating ▁them ▁and ▁allowing ▁the ▁hair ▁as ▁a ▁whole ▁to ▁have ▁a ▁fl uff ier , ▁more ▁undefined ▁look . ▁ ▁E volution ▁ ▁Clar ence ▁( 2 0 1 2 ) ▁suggests ▁that ▁af ro - text ured ▁hair ▁may ▁have ▁initially ▁evol ved ▁because ▁of ▁an ▁adapt ive ▁need ▁amongst ▁humans ' ▁early ▁hom in id ▁ancest ors ▁for ▁protection ▁against ▁the ▁inten se ▁U V ▁radiation ▁of