any of the African apes at all. More- from the observ ation that coated double helix the mitochondrial age of some sivapitheclne fos- netic change from point DNA comes in small, two-strand rings sils was downgraded to only about six (changes in individual dNA Whereas nuclear dNa encodes an esti million years. By the early 1980s al- were so steady over long mated 100.000 genes-most of the in most all paleontologists came to accept one could use them to time divergences formation needed to make a human be Sarich's more recent date for the sepa- from a common stock ing-mitochondrial DNA encodes only ration of the human and ape lines 7. In this handful of genes, every one e could begin to apply these is essential: a single adverse mutation en reauc ed to arguing that methods to the reconstruction in any of them is known to cause some Sarich arrived at the right answer purely of later stages in human evolu- severe neurological diseases tion only after 1980, when DNA restric. For the purpose of scientists studyir Two novel concepts emerged from tion analysis made it possible to ex. when lineages diverged, mitochondrial the early comparisons of proteins from plore genetic differences with high res- DNA has two advantages over nuclear of inconsequential, or neutral, muta- Wes Brown, Mark Stocking and us, ap- drial dNA that interest us accumulate tions. Molecular evolution appears to bc plied thetechnique to trace the mater. mutations rapidly and steadily, accord dominated by such mutations, and they nal lineages of people sampled from ing to empirical observations. Because accumulate at surprisingly steady rates around the world many mutations do not alter the mito in suniving lineages In other words, The dNa we studied resides in the chondrion's function, they arc effec evolution at the gene level results mitochondria, cellular organelles that tively neutral, and natural selection mainly from the rclcntlcss accumula- convert food into a form of energy the, does not eliminate them tion of mutations that seem to be nel- rest of the cell can use Unlike the dna This mitochondrial dNA therefore ther harmful nor beneficial. The second of the nucleus, which forms bundles of behaves like a fast-ticking clock, which concept, molecular clocks, stemmed long fibers, each consisting of a protein. is essential for identifying recent genet- 190 目 AFRICAN D ASIAN▲ AUSTRALIAN▲ NEW GUINEAN■ CAUCAS|AN AFRICAN ORIGIN for all modem humans is indicated by the genet- ic evidence. A genealogy based on 182 current mitochondrial DNA types (outer edges points to the existence of a common female an. cestor who lived recently in Africa. The arrows on the maps(center 6 DIVERGENCE IN DNA indicate the minimum number of unrelated females who colonized DIVERGENCE IN DNA SEQUENCE(PERCENT) major geographic areas, as inferred from the branching pattern. SCIENTIFIC AMERICAN April 1992 69any of the African apes at all. blore￾o\‘er, the age of some sivapitheclne fos￾sils was downgraded to only about six million years. By the early 1980s al￾most all paleontologists came to accept Sarich’s more recent date for the sepa￾ration of the human and ape lines. Those ivho continue to reject his meth￾ods have been reduced to arguing that Sarich arrived at the right ansiver purely, b\. chance. T\vo no\?1 concepts emerged from the early comparisons of proteins from different species. One was the concept of inconsequential, or neutral, muta￾tions. bIolecular c\,olution appears to bc dominated by such mutations, and the), accumulate at surprisingly stead), rates in suniving lineages. In other rvords, evolution at the gene 1~~~~1 results mainly from the rclcntlcss accumula￾tion of mutations that seem to be nei￾ther harmful nor beneficial. The second concept, molecular clocks, stemmed from the obsen.ation that rates of ge￾netic change from point mutations (changes in individual DNA base pairs) were so steady over long periods that one could use them to time divergences from a common stock. Iv e could begin to apply these methods to the reconstruction of later stages in human e\,olu￾tion onl}, after 1980, when DNA restric￾tion analysis made it possible to es￾plore genetic differences with high rcs￾olution. kk’orkers at Berkclc); including Lk’cs Brobin, Mark Stoncking and us, ap￾plied the tcchniquc to trace the matcr￾nal lineages of pcoplc sampled from around the world. The DNA WC studied rcsidcs in the mitochondria, cellular organcllcs that convert food into a form of energy the, rest of the cell can use. Unlike the DK.4 of the nucl&s, which forms bundles of long fibers, each consisting of a protcin￾coated double helix, the mitochondrial DIV.A comes in small, two-strand rings. LYhereas nuclear DNA encodes an esti￾mated 100,000 genes-most of the in￾formation needed to make a human be￾ing-mitochondrial DNA encodes only 37. In this handful of genes, e\er). one is essential: a single ad\,erse mutation in an)’ of them is knorkn to cause some sct’ere neurological diseases. For the purpose of scientists studying ivhen lineages diverged, mitochondrial DN.4 has t\\‘o advantages o\‘er nuclear DX-\. First, the sequences in mitochon￾drial DN.4 that interest us accumulate mutations rapidl}, and steadill,, accord￾ing to empirical obscn,ations. Because many mutations do not alter the mito￾chondrion’s function, the), arc effcc￾tivcly neutral, and natural sclcction does not eliminate them. This mitochondrial DNA therefore beha\,es like a fast-ticking clock, kvhich is essential for identifying recent genet- 0 AFRICAN DASIAN A AUSTRALIAN A NEW GUINEAN n CAUCASIAN AFRICAN ORIGIN for all modem humans is indicated by the genet￾ic evidence. A genealogy based on 182 current mitochondrial DNA , . . _ _ . I I I I I I I 0 0.2 0.4 0.6 DIVERGENCE IN DNA SEQUENCE (PERCENT) types (outer edges1 points to the existence of a common female an￾cestor who lived recently in Africa. The arrows on the maps (cented indicate the minimum number of unrelated females who colonized major geographic areas, as inferred from the branching pattern. 0 . 6 DIVERGENCE IN DNA SEQUENCE (PERCENT) SCIENTIFIC AMERICAN April 1992 69