The Inheritance of Mitochondrial dna EGG FERTILIZED EGG 37 GENE MITOCHONDRIAL DNA MITOCHONDRION NUCLEAR DNA Ⅳ ost of an individuals genes are located on DNA mole- cules in the cell nucleus. Mitochondria, the specialized structures that provide cells with energy, also carry some genes for their own manufacture on a ring of DNA. When sperm and an egg cell unite, they contrit DNA in the nucleus of the resulting cell. All the mitochondn and the dNa they contain, however, derive from the egg Studies of mitochondrial dna can reveal an individuals aternal ancestry. ges. Any two humans a static population that always con- cal trees could give rise to any set of randomly from anywhere on th tains 15 mothers. Every new generation genetic data. To recognize the one that are so alike in most of their must contain 15 daughters, but some is most probably correct, one must ap quences that we can measure evolution mothers w-ill fail to produce a daugh- ply the parsimony ple, which re- in our species only by concentrating on ter, whereas others will produce two or quires that subjects be connected in the the genes that mutate fastest. Genes more Because maternal lineages die out simplest possible way. The most effi- controlling skeletal characters do not whenever there is no daughter to carry cient hy pothetical tree must be tested data to Second, unlike nuclear DNA, mito. but one lineage disappears. In a stable whether it is consistent with them. If chondrial dNa is inherited from the population the time for this fixation of the tree holds up it is analyzed for evi- mother alone, unchanged except for the maternal lineage to occur is the dence of the geographic history inher chance mutations. The father's contri- length of a generation multiplied by ent in elements bution ends up on the cutting-room twice the population size In 1988 Thomas d. Kocher of Berke. floor, as it were. The nuclear genes, to ley(now at the University of New Hamp- which the father does contribute. de- ne might refer to the lucky shire) applied just such a parsimonious scend in what we may call ordinary lin- woman whose lineage survives interpretation to the interrelatedness eages, which are of course important to as eve. Bear in mind however. of the mitochondrial dna of 14 huma the transmission of physical character- that other women were living in Eve's from around the world. He determined istics. For our studies of modern hu- generation and that Eve did not occupy 13 branching points were the fewest man origins, however, we focus on the a specially favored place in the breed- that could account for the differences mitochondrial, maternal lineages ing pattern. She is purely the beneficia- he found. Taking the geographic consid- Maternal lineages are closest among ry of chance. Moreover, if we were to erations into account, he then conclud- siblings because their mitochondrial reconstruct the ordinary lineages for ed that Africa was the ultimate human DNA has had only one generation in the population, they would trace back homeland: the global distribution of which to accumulate mutations. The de- to many of the men and women who mitochondrial DNA types he saw could fir e moves along the pedigree, from tion geneticists Daniel L. Hartl of Wash- sult of no more than three migration ternal grandmother, to second cousins and Andrew G. Clark of Pennsylvania A crucial assumption in this analysis descended from a common matenal State University estimate that as man) is that all the mitochondrial lineages reat-grandmother and so on. The far-, as 10,000 people could have lived then. evolve at the same rate. For that reason, the circle of maternal relatives becomes leading-she is not the ultimate source son of the human mitochondrial dnas until at last it embraces everyone alive. of all the ordinary lineages, as the bibli- he also included analogous sequences Logically, then, all human mitochon- cal Eve wa from four chimpanzees. If the human drial dNA must have had an ultimate From mitochondrial dNa data, it is lineages had differed in the rate at common female ancestor. But it is easy possible to define the maternal lin. which they accumulated mutations to show she did not necessarily live in eages of living Individuals all the way then some of the 14 human sequences a small population or constitute the back to a common ancestor. In theory, would be significantly closer or farther only woman of her generation. Imagine a great number of different genealogi away from the chimpanzee sequences CIENTIFIC AMERICAN April1992ic changes. Any two humans chosen randomly from anywhere on the planet are so alike in most of their DNA se￾quences that we can measure evolution in our species only by concentrating on the genes that mutate fastest. Genes controlling skeletal characters do not fall within this group. Second, unlike nuclear DNA, mito￾chondrial DNA is inherited from the mother alone, unchanged except for chance mutations. The father’s contri￾bution ends up on the cutting-room floor, as it were. The nuclear genes, to which the father does contribute, de￾scend in what we may call ordinary lin￾eages, which are of course important to the transmission of physical character￾istics. For our studies of modern hu￾man origins, however, we focus on the mitochondrial, maternal lineages. Maternal lineages are closest among siblings because their mitochondrial DNA has had only one generation in which to accumulate mutations. The de￾gree of relatedness declines step by step as one moves along the pedigree, from first cousins descended from the ma￾ternal grandmother, to second cousins descended from a common maternal great-grandmother and so on. The far￾ther back the genealogy goes, the larger the circle of maternal relatives becomes, until at last it embraces everyone alive. Logically, then, all human mitochon￾drial DNA must have had an ultimate common female ancestor. But it is easy to show she did not necessarily live in a small population or constitute the only woman of her generation. Imagine 70 S~~ENTIFI~~~ERI~AN April1992 The Inheritance of Mitochondrial DNA EGG FERTILIZED EGG MITOCHONDRIAL DNA NUCLEAR DNA ost of an individual’s genes are located on DNA mole￾cules in the cell nucleus. Mitochondria, the specialized Qa ? structures that provide cells with energy, also carry some ‘-4 ) genes for their own manufacture on a ring of DNA. When a sperm and an egg cell unite, they contribute equally to the SPERM DNA in the nucleus of the resulting cell. All the mitochondria and the DNA they contain, however, derive from the egg. 4 Studies of mitochondrial DNA can reveal an individual’s maternal ancestry. a static population that always con￾tains 15 mothers. Every new generation must contain 15 daughters, but some mothers w-ill fail to produce a daugh￾ter, whereas others will produce two or more. Because maternal lineages die out whenever there is no daughter to carq’ on, it is only a matter of time before all but one lineage disappears. In a stable population the time for this fivation of the maternal lineage to occur is the length of a generation multiplied b). twice the population size. 0 ne might refer to the luck). woman whose lineage survii’es as Eve. Bear in mind, however, that other women were living in Eve’s generation and that Eve did not occup)- a specially favored place in the breed￾ing pattern. She is purely the beneficia￾ry of chance. Moreover, if we were to reconstruct the ordinary lineages for the population, they would trace back to many of the men and women who lived at the same time as Eve. Popula￾tion geneticists Daniel L. Hart1 of Wash￾ington Universib School of Medicine and Andrew G. Clark of Pennsylvania ! State Universiw estimate that as man) as 10,000 people could have lived then. ’ The name “Eve” can therefore be mis￾leading-she is not the ultimate source of all the ordinary lineages, as the bibli￾cal Eve was. From mitochondrial DNA data, it is possible to define the maternal lin￾eages of living Individuals all the wa)p back to a common ancestor. In theov, a great number of different genealogi￾cal trees could gi\*e rise to an)’ set of genetic data. To recognize the one that is most probably correct, one must ap￾ply the parsimony principle, which re￾quires that subjects be connected in the simplest possible \\.a)‘. The most effi￾cient hypothetical tree must be tested by comparison Mith other data to see whether it is consistent with them. If the tree holds up, it is analyzed for evi￾dence of the geographic historI. inher￾ent in elements. In 1988 Thomas D. Kocher of Berke￾ley (now at the Uni\,ersity of New Hamp￾shire) applied just such a parsimonious interpretation to the interrelatedness of the mitochondrial DNA of 14 humans from around the world. He determined 13 branching points were the fejvest that could account for the differences he found. Taking the geographic consid￾erations into account, he then conclud￾ed that Africa was the ultimate human homeland: the global distribution of mitochondrial DNA t)-pes he saw could then be esplained most easily as the re￾sult of no more than three migration events to other continents. A crucial assumption in this analysis is that all the mitochondrial lineages evolve at the same rate. For that reason, when Kocher conducted his compari￾son of the human mitochondrial DNAs, he also included analogous sequences from four chimpanzees. If the human lineages had differed in the rate at which they accumulated mutations, then save of the 14 human sequences would be significantly closer or farther away from the chimpanzee sequences