▁evidence ▁from ▁mit och ond rial ▁DNA ▁that ▁modern ▁humans ▁have ▁passed ▁through ▁at ▁least ▁one ▁gen etic ▁bott l ene ck , ▁in ▁which ▁gen ome ▁divers ity ▁was ▁dr ast ically ▁reduced . ▁Henry ▁Har p ending ▁has ▁proposed ▁that ▁humans ▁spread ▁from ▁a ▁ge ograph ically ▁restricted ▁area ▁about ▁ 1 0 0 , 0 0 0 ▁years ▁ago , ▁the ▁passage ▁through ▁the ▁ge ographic ▁bott l ene ck ▁and ▁then ▁with ▁a ▁dram atic ▁growth ▁amongst ▁ge ograph ically ▁dispers ed ▁populations ▁about ▁ 5 0 , 0 0 0 ▁years ▁ago , ▁beginning ▁first ▁in ▁Africa ▁and ▁th ence ▁spread ing ▁elsewhere . ▁Cl imat ological ▁and ▁ge ological ▁evidence ▁suggests ▁evidence ▁for ▁the ▁bott l ene ck . ▁The ▁explos ion ▁of ▁T oba , ▁the ▁largest ▁vol can ic ▁er u ption ▁of ▁the ▁Qu atern ary , ▁may ▁have ▁created ▁a ▁ 1 , 0 0 0 ▁year ▁cold ▁period , ▁potentially ▁reducing ▁human ▁populations ▁to ▁a ▁few ▁tropical ▁ref ug ia . ▁It ▁has ▁been ▁estimated ▁that ▁as ▁few ▁as ▁ 1 5 , 0 0 0 ▁humans ▁surv ived . ▁In ▁such ▁circumstances ▁gen etic ▁dr ift ▁and ▁founder ▁effects ▁may ▁have ▁been ▁maxim ised . ▁The ▁greater ▁divers ity ▁amongst ▁African ▁genom es ▁may ▁be ▁reflect ▁the ▁extent ▁of ▁African ▁ref ug ia ▁during ▁the ▁T oba ▁incident . ▁However , ▁a ▁recent ▁review ▁highlight s ▁that ▁the ▁single - source ▁hypothesis ▁of ▁non - A fr ican ▁populations ▁is ▁less