Mapping Human History:
Discovering the Past through Our Genes
Mapping Human History follows humans from Africa to Europe, Asia, the Middle East, and the Americans, tracking their movements across continents, exploring regional genetics at every stop. Reconstructing the history of various peoples, the book points out how the historical path of humans constantly overlaps.
In the book, Steve Olson makes a very big claim such as “Every single one of the billion people on the planet today is descended from the small group of anatomically modern humans who once lived in eastern Africa”. Africans with high foreheads, sharp chins, and light graceful bodies much like today. But before they are markedly different. Certainly people would say they are extraordinary looking human beings. They are the so called archaic humans. Today they are gone. Modern humans substitute their ancestors.
Olson claims that genetics confirms that human groups are all closely related and acquire only the most external genetic differences. Human DNA passes on genetic information from one age group to the next, bears indentation of human history. It documents the appearance of modern humans. It records the diversifications of modern humans into “races” and “ethnic groups” that we are familiar with today. Due to the human affinity to interbreed and our genus times past of resettlement from continent to continent, everyone is connected to a common band of ancestors. Genetics are beginning to understand the story written in our DNA, They can map out the association of modern humans out of Africa to the rest of the world. They piece together when and how people acquire their distinctive physical appearances. With continual mixing of people, physical attributes intermingle. As a result individuals differ from each other. The differences are all between individuals. The genetic variations accountable for our diverse emergences tend to remain localized within groups. Groups keep hold of some measure of physical individuality. History reveals the many ways in which we are genetically linked. Human beings are all related, all different.
Olson contends that everyone inherit his or her DNA from the same people, and everyone gets the rest of DNA from the people who lived sometimes in the past yet everyone’s DNA is not exactly the same. Whenever cell divides it has to copy its DNA so that each daughter cell has a complete set of chromosomes. This process is amazingly accurate. The molecular machinery that reproduces DNA can continue for millions of nucleotides without making mistakes. But the process is not perfect. Occasionally a mistake occurs. Any mistake in the copying of DNA is known as a mutation. Mutations – created when cells reproduce their DNA – are the key to reconstructing of our genetic history, writes Olson. Parents hand down mutation to their children, creating a unique genetic pattern that spreads throughout certain populations. By counting the mutations that differ between two distinct DNA sequences, genetics can find out who is related to whom and estimate the number of generations that passed since a common ancestor existed. All living humans share common ancestors, the so-called “mitochondrial Eve” and “Y-chromosome Adam”. Each of us has two parents, four grandparents and so on. Follow this back through several hundred generations of human history, and we each have more ancestors by far than there were people alive on earth – so it follows that we are likely to have ancestors in common. The evidence that lead us back to the original figures, however, is indeed based on genetics. When sperm fertilizes an egg, the egg gets its share of DNA in the sperm’s nucleus to pair with its own. But egg’s are more than just DNA- they are cells with enzymes, and especially the tiny particles called mitochondria, which serves as cellular energy converters. Humans received their mitochondria from their mothers. Since mitochondria are transmitted from the mother, Olson writes that all “human mitochondrial DNA sequences that exist in the world today are descended from the mitochondrial DNA of a single woman.” a mitochondrial Eve. Similar for the Y-chromosome that male inherit from their fathers. Olson also writes there was an Adam and all men are his descendants. It is based on such analyses that it can be calculated that all modern human DNA is derived from something like 86,000 individuals, living in Africa, of whom the mitochondrial Eve and Y-chromosome Adam were the two whose lineages made it into the present day. Our mitochondria come from Eve and all Y chromosomes come from the Adam, but does not all of our DNA, Olson write. All human DNA comes from approximately 86, 000 people who lived on Earth. If this is so, then why is everyone’s DNA not the same? That question writes Olson, is the single most important question in all of human biology. The question evokes issues of how different appearances of human groups occurred, why genetic diseases happen, and what role our genetic heritage plays in determining who we are. Olson describes the different types of mutation and how mutation have caused one of Eve’s daughters to produce a different mitochondrial sequences. A distinct mitochondrial sequence is called a haplotype, and a group of related haplotypes that descend from single ancestral haplotype is termed a haplogroup. This is the starting point for one of the more fascinating applications of modern genetics- the mapping of human history in terms of gene flow. Because although all humans are overwhelmingly genetically similar, there are subtle differences in the frequency of particular genes between populations. Knowing approximately the frequency with which genetic mutations occur and the number of these tiny genetic differences between one population group and another enables one to guess when the populations diverge.
South Africa is a good example of how people from different continents can produce highly varied populations. The Africans in search for better jobs, or better education for their children, merchants, sailors, servants, soldiers, international slave trade, oceangoing ships gave people new way to move between continents. The slave trade was responsible for one of the largest human migrations. As stated, some of these communities have survived to the present, but in most places Africans gradually intermarried with non- Africans and blended into the surrounding populations. Their ancestors were one member of an openly interracial marriage. Africans quickly became the major portion of population in the Americas. They have been a powerful force for demographic mixing not only in United States. Many Africans settled in Europe and Asia and married non- Africans and their descendants gradually were absorbed into the population.
Olson says that human beings have never been able to resist the urge to merge. Consequently our species has interbred too enthusiastically to develop substantial genetic differences. The author’s enthusiasm for this idea overreaches in a passage that addresses a period of cohabitation between Neandertals and modern humans in Europe. He rationalizes that humans must have interbred with local Neandertals.
The uncovered bones at Skhul cave, one of the largest collections of prehistoric human fossils found. After then the excavators found at a much large cave named Tabun. They found fossils that were clearly the remains of Neandetals – the limb bones were short and thick, an adaptation to the cold that has gripped the Neandertals European homelands for much of the past years. Olson states that perhaps they had moved into the Mideast during the most part of the last Ice Age to escape the relentless advance of the modern glaciers. Neandertals seem to have occupied the site both before and after modern humans. The interaction between the two usually center on Europe, where modern humans clearly had a more advanced culture. According to the author the two groups encountered each other first in the Middle East. The issues have a relevance that extends far beyond the shores of the ancient Mediterranean. They involved the origin of modern language, which may well have happened in or near the Middle East. They relate whether modern humans mated only among themselves or whether they mixed with the archaic humans they encountered outside Africa. The way in which these groups interacted in the Middle East can tell us much about what it means to be human today.
These divergences can in turn be related to population movements, the great migrations which carried our many ancestors from humanity’s birthplace in Africa into Asia, Europe, Australia, and America.
In investigating and studying these haplogroups, Olson starts in Africa and discusses very early man and his migration outward. He travels to the Middle East and discusses Aaron, Moses’s brother. Aaron’s male lineage was decreed by the Jewish God to become the high priest, the Kohanim, of Israelites. Surnames of Cohen, Cohn, and Kahn are derivatives of Kohan, A few years ago geneticists found about 5o percent of Jews who presently identify themselves as Kohanim as having a set of particular genetic markers. Today this is called the Cohen Modal Haplotype. Thus Aaron may have been a real person.
As Olson sets his sails to Asia up to Australia, he devotes a chapter to “Genes and Languages” and how they may have sprung from a common source. In Europe, he relates how every present-day European has some DNA from a Middle Eastern farmer and a Stone Age hunter-gatherer. Not only does population genetics inform our understanding of the great sweep of human history, but it also shows extraordinary parallels with the evolution of languages. Languages and genes, co-migrated along roads and around seacoast, carried by the migrants as changing climate, population pressures, or the sheer urge to explore drove them on.
Americans followed and Olson relates that not all of the Native American ancestors may have come from just the Bering Strait pass. Some may have come by boat.
Olson finishes his travels and his book with a stop in Hawaii. He describes how the Hawaiian “genetic history” is occurring backwards. In the single-origin mentioned, human groups become differentiated in appearances as they move across the globe and undergo some measure of reproductive isolation. In Hawaii, the reverse is happening. Physical distinction that took thousands of generations to create is being undone with a few generations of intermarriages.
In the final chapters, Olson questions the practice of studying the genetics of ethnic groups; He worries that although “the only way to understand how similar we are is to learn how we differ… studies of human differences can seem to play into the hands of those who would accentuate those differences.” Some may benefit from tracing their lineage back to royalty, but others could be stigmatized by possessing genes associated with disease, for example.
He predicts that global rates of intermarriage will increase. As this happens, it will become increasingly harder to identify a person as belonging to a single racial category. But he doesn’t think this will eliminate racial tensions.
Olson admits that pursuing genetic knowledge implies both risks and opportunities. His lasting vision is “a world in which people are free to choose their ethnicity regardless of their ancestry.”
Olson, Steve. Mapping Human History: Discovering the Past through Our Genes.
New York: Houghton Mifflin Books, 2002.