drial DN. 4 was difficult to obtain in the of Africa; the), too, found that the mi. trol we arrived at a more an required quantities. Fortunately, the re- tochondrial lineages led back to Africa. cient date for the common mother cent development of a technique called We estimate the odds of their arriving That less certain, however, be the polymerase chain reaction has at that conclusion accidentally were cause questions remain about how to eliminated that constraint. The reaction only four in 100. Although these statis- correct for multiple mutations that oc- makes it possible to duplicate DNA se- tical evaluations are not strong or rig- cur within the control region quences easily, ad infinitum; a small orous tests, they do make it seem likely One might object that a molecular starting sample of DNA can espand that the theory of an African origin for clock known to be accurate over five into an endless supply [see" The Un- human mitochondrial DNA is now fair- million years could still be unreliable usual Origin of the Poly merase Chain ly secure for shorter periods. It is conceivable Reaction, by Kary B. Mullis: ScIENTIrIC for example, that intervals of genetic AMERICAN, April 19901 ccause our comparisons with the stagnation might be interrupted by The poly merase chain reaction en chimpanzee data showed the hu- short bursts of change when, say, a abled Linda vigilant, now at Pennsylva B man mitochondrial dna clock new mutagen enters the environment nia State University, to redo our study has ticked steadily for millions of ycars, or a virus infects the germ-line cells, or using mitochondrial DN.4 data from we knew it should be possible to calcu- intense natural selection affects all seg- parts of the sub-Saharan region. Vigi- manity lived. Wc assumed the human sibility that the clock might run by fits lant traced a genealogical tree whose and chimpanzee lineages diverged five and starts, we ran a test to measure 1+ dccpcst branches leadexclusively to million years ago, as Sarich's work had how much mitochondrial DNA has Africans and whose 15th branch leads shown. Wc then calculated how much evolved in populations founded at a to both africans and non -Africans The humans had diverged from one anoth- known time non-Africans lie on shallow secondary er relative to how much they had di- The aboriginal populations of New branches stemming from the 15th verged from chimpanzees-that is, we Guinea and Australia are estimated to branch. Considering the number of found the ratio of mitochondrial dna have been founded less than 50,000 to African and non-African mitochondrial divergence among humans to that be- 60,000 years ago. The amount of evolu- DNAs surveyed, the probability that tween humans and chimpanzees tion that has since occurred in each of lusively African is one in 10,000 for a determined the ratio was less than that shown by the whole human species tree with this branching order 1: 25. Human maternal lineages there- Accordingly, we can infer that Eve lived Satoshi Horai and Kenji Hayasaka of fore grew apart in a period less than three times 50,000 to 60,000 years ago the National Institute of Genetics in / 25th as long as five million years, or or roughly 150,000 to 180,000 years Japan analogously surveyed popula- less than 200,000 years. with a third ago. All our estimates thus agree the tion samples that included many more set of data on cl s in a section of split happened not far from 200,000 Asians and individuals from fewer parts the mitochondrial DNA called the con. years ago Those estimates fit with at least one H line of fossil evidence. The remains of anatomically modern people appear first in Africa, then in the Middle east and later in Europe and east Asia. An- thropologists have speculated that in east Africa the transition from anatom- ically archaic to modem people took lace as recently as 130,000 years ago see "The Emergence of Modern Hu On the other hand a second line of evidence appears to conflict with this view. The fossil record shows clearly that the southern parts of Eurasia were occupied by archaic people who had mi- grated from Africa to Asia nearly a mil- AFRICAN Tion years ago. Such famous fossils as NON AFRICAN PEOPLE Java man and beijing man are of this type 口 PYGMY CHIMPANZEES hat the archaic Eurasian population un derwent anatomic changes that made them resemble more modem people led y to the m multiregional evolutio similar evolutionary changes in sepa- ate geographic regions converted the INTERRELATEDNESS of 14 huma d four chimpanzees inhabitants from archaic small-brained was inferred from similarities in ochondrial dna se. to modem big-brained types. when various divergences in the human lineages occurred e uences. The chimpanzee data Huge levels of gene flow between 3 continents. however, would be neces- 72 SCIENTiFIC AMERICAN April 1992drill DN.4 was difficult to obtain in the required quantities. Fortunately, the re￾cent development of a technique called the pol).merase chain reaction has eliminated that constraint. The reaction makes it possible to duplicate DNA se￾quences easily, ad infinitum; a small starting sample of DNA can espand into an endless supply [see “The Un￾usual Origin of the Pollmerase Chain Reaction,” by Kary B. Mullis; Scrr~rtrrc RWERICAN, April 19901. The pol},mcrasc chain reaction cn￾abled Linda vigilant, now at Pcnns),lva￾nia State Univcrsit),, to redo our stud). using mitochondrial DN.4 data from 130 Africans, representing six dh,crsc parts of the sub-Saharan region. Vigi￾lant traced a genealogical tree whose 1 -L dccpcst branches lead csclusivcly to Africans and whose 15th branch leads to both Africans and non-Africans. The non-Africans lie on shallow secondar) branches stemming from the 15th branch. Considering the number of African and non-African mitochondrial DM4s surveyed, the probability that the 14 deepest branches would be ex￾clusively African is one in 10,000 for a tree with this branching order. Satoshi Horai and Kenji Hayasaka of the National Institute of Genetics in Japan analogously surveyed popula￾tion samples that included many more Asians and individuals from fewer parts I’ l--l u of Africa; the),, too, found that the mi￾tochondrial lineages led back to Africa. We estimate the odds of their arrikjng at that conclusion accidentally tvere only four in 100. Although these statis￾tical evaluations are not strong or rig￾orous tests, they do make it seem like11 that the theory of an African origin for human mitochondrial DNA is now fair- 1). secure. B ecause our comparisons bc-ith the chimpanzee data shorved the hu￾man mitochondrial DNA clock has ticked steadily for millions of years, ivc knew it should be possible to calcu￾late kvhcn the common mother of hu￾manity lived. WC assumed the human and chimpanzee lineages diverged fi\,c million years ago, as Sarich’s bvork had shown. WC then calculated how much humans had diverged from one anoth￾er relative to how much they had di￾verged thorn chimpanzees-that is, we found the ratio of mitochondrial DNA divergence among humans to that be￾tween humans and chimpanzees. Using two different sets of data, we determined the ratio was less than 1:25. Human maternal lineages there￾fore grew apart in a period less than ‘/,:th as long as five million years, or less than 200,000 )-ears. With a third set of data on changes in a section of the mitochondrial DNA called the con￾rv+: 4\ n AFRICAN NON AFRICAN PEOPLE n CHIMPANZEES q PYGMY CHIMPANZEES trol region, we arrived at a more an￾cient date for the common mother. That date is less certain, however, be￾cause questions remain about holv to correct for multiple mutations that oc￾cur within the control region. One might object that a molecular clock known to be accurate over fi1.e million years could still be unreliable for shorter periods. It is conceii,able, for example, that inten.als of genetic stagnation might be interrupted b). short bursts of change ivhen, say, a new mutagen enters the emironment, or a virus infects the germ-line cells, or intense natural selection affects all seg￾ments of the DNA. To rule out the pos￾sibilit), that the clock might run by fits and starts, we ran a test to measure how much mitochondrial DNA has evolved in populations founded at a knobin time. The aboriginal populations of New Guinea and Australia are estimated to have been founded less than 50,000 to 60,000 years ago. The amount of evolu￾tion that has since occurred in each of those places seems about one third of that shown by the lvhole human species. Accordingly, we can infer that Eve lived three times 50,000 to 60,000 years ago, or roughly 150,000 to 180,000 years ago. All our estimates thus agree the split happened not far from 200,000 years ago. Those estimates fit riith at least one line of fossil evidence. The remains of anatomically modern people appear first in Africa, then in the Middle East and later in Europe and east Asia. An￾thropologists have speculated that in east Africa the transition from anatom￾ically archaic to modem people took place as recently as 130,000 years ago [see “The Emergence of Modern Hu￾mans,” by Christopher B. Stringer; SCI￾ENTIFIC A~IERK~, December 19901. INTERRELATEDNESS of 14 humans and four chimpanzees Ll L--r, On the other hand, a second line of evidence appears to conflict with this L-iew. The fossil record shows clearI), that the southern parts of Eurasia were occupied by archaic people who had mi￾grated from Africa to Asia nearly a mil￾lion years ago. Such famous fossils as Java Man and Beijing Man are of this Q-pe. This finding and the hypothesis that the archaic Eurasian population un￾derwent anatomic changes that made them resemble more modem people led to the multiregional evolution model: similar evolutionary changes in sepa￾rate geographic regions converted the inhabitants from archaic small-brained to modem big-brained t)pes. 3 3 I Huge levels of gene flow between ; continents, however, would be neces￾was inferred from similarities in their mitochondrial DNA se￾quences. The chimpanzee data help researchers measure when various divergences in the human lineages occurred. 72 Scmmmc &RICAN April 1992