Then, once the rate of mutation is determined, calculating the time of divergence of that species becomes relatively easy. Broadly speaking, the evolution of important genes occurs more slowly than that of genes with less vital functions. More rapidly changing genes are used to date more recent evolutionary events, and slower evolving genes are used to map more ancient divergences, he explains. But we can take the rate of change of genes from vertebrates or plants, which have a decent fossil record, and apply it to the unknown group.
The molecular clock can also be used for putting a series of evolutionary events into chronological order. This is done by comparing sequences from different species to determine when they last shared a common ancestor, in effect drawing the family tree. Though the molecular clock is still regarded as somewhat controversial, says Hedges, it is gaining acceptance as our understanding of genome sequences improves.
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Molecular Anthropology
What is a molecular clock? Because there are so many different forms, these noncoding sequences are especially useful for determining kinship among closely related individuals, such as members of a tribe or extended family.
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When this occurs, two sequences may appear to be more closely related less separated in time than they really are, since the intervening mutation the change from C to T, in this case may not be apparent. Because of back mutation, the observed number of differences between sequences represents the minimum actual difference. Correction factors can be applied to estimate the true difference. Another potential problem with any sequence on a chromosome, whether or not that sequence codes for a protein, is that most chromosomes do not remain intact during meiosis. This is because crossing over occurs, in which homologous chromosomes recombine exchange segments.
After a few generations, it becomes very difficult to track individual sequences and compare them with any confidence to similar sequences in another person. To avoid this problem, molecular anthropologists focus on two sources of DNA that do not recombine: The Y chromosome , which determines male sex, does not undergo recombination along most of its length. Instead, it passes intact from father to son. A man's Y chromosome, therefore, is a more-or-less exact copy of the one possessed by his father, grandfather, great-grandfather, and so on back through time.
Like any other DNA segment, it may mutate, and any changes it accumulates are faithfully passed along as well. Two brothers are likely to have exactly the same Y chromosome sequence. Two men whose last common male ancestor was ten generations ago, however, are likely to have slightly different sequences.
Comparison of the sequences of two Y chromosomes, therefore, can show how closely related two males are. Y chromosome analysis has been used to track migration of human populations, and to study the relatedness of modern populations. For instance, Jews and Palestinian Arabs derive from a common ancestral population that lived in the Middle East about 4, years ago. Recent studies have linked the ancestors of American Indians to several small populations in Siberia , confirming the predominantly Asian origin of American Indians and refining the understanding of their migration history.
Many other similar studies have been performed, providing an increasingly clear and complex picture of human migration and mixture.
Probing Question: What is a molecular clock?
Mitochondria are energy-harvesting organelles in the cell. They are inherited only from the mother, and so track maternal inheritance in the same way that the Y chromosome tracks paternal inheritance. One of the earliest and most famous mitochondrial studies was used to address a central question in anthropology: Where and how did modern humans originate? The Homo genus itself is universally believed to have originated in Africa. Groups of Homo erectus are known to have migrated out of Africa, populating Europe and Asia between one and two million years ago.
Anthropologists call these groups "archaic" modern humans. They include the Neandertals, who lived in Europe and the Middle East from , to 28, years ago.
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Did modern humans evolve from these older populations in several different regions simultaneously? Or did they arise from a small group in Africa, and spread out from there? If so, did they mix with less advanced local populations such as Neandertals , or replace them entirely? The scientists who performed the mitochondrial DNA study Rebecca Cann, Mark Stoneking, and Allan Wilson reasoned that populations that had been in one place for only a short period of time would show very little variation in their mitochondrial DNA, since they all shared a relatively recent common ancestor.
This would be the case in a modern human population if it had only recently migrated into the area in which it is found. Such relative genetic homogeneity in newly formed populations is known as the founder effect. In contrast, populations that have remained in place for long periods have much more ancient common ancestors, and therefore have more mitochondrial DNA variations.
How DNA accumulates changes
To perform their analysis, the scientists collected samples from different ethnic groups from all over the world. They found that the populations with the greatest amount of sequence variation were in sub-Saharan Africa, indicating these were the groups with the most ancient ancestry. All other groups had much less variation, indicating more recent arrivals of those groups in those regions. Cann, Stoneking, and Wilson went on to estimate the date at which all these groups had their most recent common ancestor.
Using a figure of 2 to 4 percent sequence divergence per million years, they estimated that the most recent common ancestor lived approximately , years ago. The simplest explanation, they argued, was that ancestors of the non-African Homo sapiens migrated out of Africa about , years ago to populate other regions, over time replacing the nonmodern humans H.
They argued that the relatively short time since the divergence of all modern humans was too brief to support the alternative hypothesis, that each local group of archaic humans had independently evolved modern traits, a model called multiregional evolution. The conclusions drawn in this study are still controversial. Numerous other studies have been done since, and the data have been subjected to multiple different analyses.
Some studies suggest differing dates for the most recent common ancestor ranging from , to , years ago , and others suggest that an exclusive African origin is not the only possible interpretation of the data. It is important to keep in mind that the vast number of comparisons that must be made in such studies require computer programs, not only to make the comparisons, but to draw from them the simplest "family tree" that fits.
Much of the controversy surrounds the assumptions that must be built into these programs in order to generate results. The mutation rates by which events are timed the " molecular clock " are also not known with precision, leading to further uncertainties about the exact timing of migrations. In their study, Cann, Stoneking, and Wilson pointed out that the patterns of mitochondrial variations they saw suggested that all the mitochondria of all living groups could be traced back to a single woman who lived in Africa approximately , years ago.
Many people at the time of the original study and since have misinterpreted the results to claim there was a single female ancestor for all modern humans, dubbed "Eve. However, our nuclear DNA certainly does not derive exclusively perhaps even at all from this woman, and the thirty thousand or more genes in our nuclear DNA are far, far more important in determining our characteristics than the thirty-seven mitochondrial genes. Because of recombination, our nuclear DNA cannot be traced back to any single person. Rather, it is an amalgam of countless ancestors through time. Mitochondrial Eve was also not the first modern human woman, nor the only woman in existence at the time she lived.
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She was not even the only woman in her local population; it is estimated that Eve was one of about 10, people in her population. There was really nothing particularly special about her, except that, by chance, the descendants of her mitochondria happen to have ended up in the cells of every living human. Even this, which sounds remarkable, is pretty much what we should expect from small populations.
To understand why, consider four couples, each of which has two children. Remember that mitochondria are passed from the mother to each child. One couple has two boys. Each boy inherits the mother's mitochondria, but neither passes them on to his children. The mother's mitochondrial type thus becomes extinct in one generation. Two of the couples have a boy and a girl, while the fourth has two girls.
These four daughters go on to have children of their own, each with the same distribution according to sex. Whenever a family has only boys, a mitochondrial type becomes extinct. Any time a family has only girls, the mitochondrial type handed down from the mother becomes more common in the next generation.
In a small population, over time, it is highly likely that one type will become most prevalent, ultimately becoming the one type found in all the members of the population. Looking back, we would give the name "Eve" to the original mother of that line of mitochondrial genetic inheritance. A similar phenomenon occurs with the Y chromosome, for exactly the same reasons: Any family with only girls extinguishes that Y chromosome type. The "Y chromosome Adam" lived 60, to , years ago. There is no reason to expect that "Y chromosome Adam" would know "mitochondrial Eve"; indeed, even without the dates to make it impossible, it would be a remarkable coincidence if they had.
DNA can be extracted from some archaeological samples, allowing direct sequencing and comparison with modern DNA. This has so far been possible with specimens up to about 40, years old the dating of such samples is often inexact.
Molecular anthropology - Wikipedia
DNA is isolated, purified, amplified with the polymerase chain reaction , and sequenced. By this technique, DNA from extinct animals such as the woolly mammoth has been obtained, but not dinosaur DNA, which is millions of years old. The DNA that can be isolated is typically highly fragmented and incomplete, and unsuitable for cloning the whole organism.