Geneticists have traced similarities in our mitochondrial DNA back to a single shared mother of us all and have also determined the hypothetical father of all the males on Earth today by tracing back through their Y-chromosome variation.
Most people have only heard of mitochondrial DNA because of the sensation "mitochondrial Eve" caused in the 1990s. mtDNA is not the same as nuclear DNA, which is found in the nucleus of the cell. MtDNA is located outside of the nucleus in energy-producing organelles called mitochondria. For several reasons mtDNA is an excellent source of evolutionary information. First of all the genome for mtDNA is smaller than nuclear DNA and it is therefore easier to analyze than nuclear DNA. Second, each cell has thousands more copies of mtDNA than copies of nuclear DNA so it is more amenable to work with in the laboratory. Third, mtDNA does not accumulate mutations as fast as nuclear DNA. And finally, mtDNA is only inherited maternally, meaning that everyone receives their mtDNA from their mother. There is no recombination with the father's mtDNA because the mtDNA in sperm are destroyed at the time of egg fertilization. Because of the hereditary properties of mtDNA, we can trace every living person's genes along a maternal lineage back to a single mother of all modern human mtDNA. This ancestor is known as "mitochondrial Eve."
Rebecca Cann and her colleagues were the first to estimate a common mtDNA ancestor for all living humans. Since then others have tried to answer the same question and have reported varying but overall similar results. Based on comparing sequences between different populations across the globe, they concluded that we share a common mtDNA ancestor who lived about 200 Kya in sub-Saharan Africa.
Of populations all over the world, the African samples show the most variation in the mtDNA genome. In other words, the populations outside of Africa only exhibit a subset of the variation seen in the African sample. According to molecular clock theory, the sequence with the most variation caused by mutations has had the longest time to evolve or to accumulate those mutations. MtDNA analysis confirms that people have been evolving in Africa for a longer portion of human evolutionary history than anywhere else.
MtDNA and Y-chromosome studies as well as those on nuclear genes support a population expansion out of Africa beginning 100 Kya. Through statistics and computer simulations based on sequence analyses of different populations around the world, geneticists from many different laboratories have concluded that Homo sapiens experienced a bottleneck between 150
and 50 Kya. At that point, human variation was particularly low. Studies of genetic variation within and between populations suggest that at some point, the global human population could have been as small as 10,000, which would have been an endangered level, on par with bonobos and mountain gorillas today. Since that loss of genotypic diversity at the bottleneck, humans have experienced phenotypic diversification and have developed regional variation to make us the polytypic or variable species we are today. --
Of course, "Eve" was not the only living hominin female at the time. Other women were alive and reproducing. The special point of interest about our Eve is that she is the only woman whose entire descending lineage had surviving and reproducing females in every generation. All the other lineages that were plodding along 200,000 years ago have since ended and contributed nothing to our mtDNA.
By comparing worldwide variation at the Y chromosome, geneticists were able to corroborate the mitochondrial Eve hypothesis. Since there is no female counterpart to the Y chromosome it, like mtDNA, does not undergo recombination. So the Y chromosome is passed, as is, from father to son. All the world's human Y chromosomes converge on a 'Y-Chromosome Adam" who lived about 100 Kya. And just like mtDNA, because African Y chromosomes show the most variation, it follows that those lineages have been evolving the longest and that the human Y chromosome originated there.
The recent age of our common ancestors at 200 Kya and 100 Kya is strong evidence against the Multiregional model for human origins and supports the Out of Africa model, which is also called the Garden of Eden hypothesis (see Chapter 6). Mitochondrial Eve and Y-chromosome Adam should not be taken literally as if they are the biblical couple. According to strict interpretation ofthe Bible, Adam and Eve were the very first humans on earth. However, mitochondrial Eve and Y-chromosome Adam are consistent with evolutionary theory which assumes that they had ancestors and contemporaries. They are simply the only woman and the only man to have descendents that survived to the present. It is important to remember that Y-chromosome Adam and mitochondrial Eve need not have lived at the same time. They each signify the roots of different aspects of our biology, which evolution has been building upon for eons.
The gene pool of mtDNA and Y chromosomes in Africa contains more variation than anywhere else and the genetic variation outside of Africa represents only a subset of that found within the African continent. Therefore, from a genetic perspective all humans are Africans. Because of their tropical habitat in which they would have experienced high levels of UV radiation, "Adam" and "Eve" probably had darkly pigmented skin. --
The genetic root of all the strains of human gut bacteria (Helicobacterpylori) is located in Africa. About every other human carries H. pylori, that is, the bacteria responsible for stomach pains and ulcers and is the only known microorganism that can survive in such a highly acidic environment. Based on genetic differences among populations' strains of the pathogen, the initial H. pylori infection in humans is estimated to have occurred around 60 Kya. Because the sequences of the African strains of the bacteria are more diverse, it is assumed that evolution has had longer to act in Africa, so the ancestral root is located there. The date of 60 Kya implies a later "Out of Africa" scenario than that constructed by the evidence for mitochondrial Eve since humans carried H. pylori out of Africa with them.
Molecular phylogenetics also show that H. pylori is a distant relative of a similar ulcer-causing species of bacteria in lions, tigers, and cheetahs. The genomes of these microbes show that some of the genes in the human form are still functioning while their counterparts in the feline form (Helicobacter acinonychis) are not. Since it is unlikely (i.e., not parsimonious) that these genes would have been lost and then restored in the human form, it is probable that the human form, rather than the big cat form, is more like the ancestral microbe. Therefore, it is quite possible that the bacteria was transmitted from humans to big cats and, based on molecular clock estimates of the two species' sequences, the transmission occurred 200 Kya. Big cats could have contracted the microbe after eating a human and then spread it to other cats the way humans spread it to one another through close physical contact. --
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