“Earth Pop.1000 to Outer Space”: A Single Genome Reveals Entire Span of Human History

Astronaut_spacesmells Stored inside your genome are clues to the history of humankind, including global migrations and population crashes. So say researchers who have analyzed DNA pioneer, Craig Venter's publicly published DNA sequence, and those of 6 others, to reveal major milestones in human history.

The analysis, published in Nature, suggests that descendants of the first humans to leave Africa shrunk to as few as 1,000 reproductively active individuals before rebounding. The study also suggests that, contrary to popular theories, these early humans continued to breed with sub-Saharan Africans until as recently as 20,000 years ago.

Genetic researchers have traditionally compared DNA sequences from populations around the world to determine how populations relate to one another and when they might have branched off. Studies of DNA from maternally inherited cell structures called mitochondria, for example, established that all humans can trace their maternal lineage back to one mitochondrial Eve who lived in Africa around 200,000 years ago.

"Each little piece of the genome has its own unique bit of history and goes to a unique ancestor as you go further and further back," explained John Novembre, a population geneticist at the University of California, Los Angeles, to Nature.com. "As you look at different parts of the genome, you get access to different parts of history."

On the basis of this principle, Richard Durbin, a genome scientist at the Wellcome Trust Sanger Institute near Cambridge, UK, and his then post-doc Heng Li determined a way to calculate, from the ages of different segments of a single person's genome, changes in the population size of their ancestors.

The genomes of Venter and two others of European ancestry, two Asian men and two West African men all tell the same story up until about 100,000 years ago, when their populations began to split and then plummet in size, reflecting, it is believed, the first human migrations out of Africa.

The ancestors of Asians and Europeans dwindled by a factor of ten to roughly 1,200 reproductively active people between 20,000 and 40,000 years ago, Durbin and Li calculate. African populations also crashed, but by nowhere near the same extent, dropping to around 5,700 breeding individuals. Other studies have recorded population crashes at around the same time, Reich says.

In a seperate analysis, Durbin and Li compared an X chromosome from an African with one from a non-African to determine when their ancestors stopped interbreeding after the first humans left Africa and colonized other parts of the world. Human remains and artefacts unearthed in Europe, Asia and Australia seem to suggest humans rapidly colonized these places by about 40,000 years ago, diminishing the opportunities to interbreed with Africans.

Durbin and Li suggest that these groups continued to interbreed until as recently as 20,000 years ago. One possible explanation, Durbin says, is that after the first humans left Africa some 60,000 years ago, successive waves of Africans followed suit, interbreeding with the ancestors of the earlier migrants.

Mining individual genomes can't reveal every chapter of human history, notes David Reich, who works with Li at the Broad Institute of Harvard and MIT in Cambridge, Massachusetts. The approach reveals little about upheavals of the last 20,000 years, such as the peopling of the Americas, because few chunks of the genome are young enough. Similarly, Durbin and Li's method can't deduce the history of human ancestors who existed before about 2 million years ago because few regions of the genome are much older.

Reich, a geneticist and professor in the department of genetics at the Harvard Medical School. told nature.com that he plans to lean heavily on the new approach, not least for work on ancient genomes belonging to Neanderthal  and a mysterious sister population, known as Denisovans, discovered through DNA recovered from a 30,000–50,000-year-old finger bone found in a Siberian cave. Reich and his colleagues have been unable to determine when Neanderthals and Denisovans stopped breeding with one another, and the new approach has the potential to answer that question.

The Daily Galaxy via nature.com/news


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