"Who We Are and How We Got Here: Ancient DNA and the New Science of the Human Past"

by David Reich


(genetics, aDNA)
It has arrived.
“Ancient DNA is rewriting human (and Neanderthal) history. The genomes of the long dead are turning up all sorts of unexpected and controversial findings.”
“In just five years, the study of ancient DNA has transformed our understanding of world prehistory.”
“The story of human populations, as [David Reich] shows, is ever one of widespread and repeated mixing, debunking the fiction of ‘pure’ populations.”
“In the 1990s and 2000s, they [Cavalli-Sforza and others] brought their work to a new level by moving beyond protein variation and directly examining DNA, our genetic code.”
“They [Cavalli-Sforza and others] analyzed a total of about one thousand individuals from around fifty populations spread across the planet, examining variation at more than three hundred positions in the genome.”
“When [a computer]—which had no knowledge of population labels—[clustered humans] into 5 groups, the results corresponded uncannily to commonly held intuitions about deep ancestral divisions among humans (West Eurasians, East Asians, Native Americans, New Guineans, & Africans).”
“It took the revolution wrought by the ability to extract DNA from ancient bones—the “ancient DNA revolution”—[to realize that] the present-day structure of human populations cannot recover the fine details of ancient events.”
“The problem [i.e., ] is not just that people have mixed with their neighbors, blurring the genetic signatures of past events. It is actually far more difficult […].”
“[ ] we now know, from ancient DNA, that the people who live in a particular place today almost never exclusively descend from the people who lived in the same place far in the past.”
“Under these circumstances [i.e., ], the power of any study that attempts to reconstruct past population movements from present-day populations is limited.”
“We [David Reich and others] discovered that the population of northern Europe was largely replaced by a mass migration from the eastern European steppe after five thousand years ago”
“We [David Reich and others] discovered that […] farming developed in the Near East more than ten thousand years ago among multiple highly differentiated human populations that then expanded in all directions and mixed with each other along with the spread of agriculture”
“We [David Reich and others] discovered that […] the first human migrants into the remote Pacific islands beginning around three thousand years ago were not the sole ancestors of the present-day inhabitants.”
“A great surprise that emerges from the genome revolution is that in the relatively recent past, human populations were just as different from each other as they are today, but that the fault lines across populations were almost unrecognizably different from today.”
“DNA extracted from remains of people who lived, say, ten thousand years ago shows that the structure of human populations at that time was qualitatively different.”
“Present-day populations are blends of past populations, which were blends themselves.”
“The African American and Latino populations of the Americas are only the latest in a long line of major population mixtures.”
“To understand why genetics is able to shed light on the human past, it is necessary to understand how the genome—defined as the full set of genetic code each of us inherits from our parents—records information.”
“The Age of Modern Humans”
“The higher the density of differences separating two genomes on any segment, the longer it has been since the segments shared a common ancestor as the mutations accumulate at a more or less constant rate over time.”
“[ ] So the density of differences provides a biological stopwatch, a record of how long it has been since key events occurred in the past.”
“The first startling application of genetics to the study of the past involved mitochondrial DNA.”
“The earliest human skeletons with “anatomically modern” features—defined as falling within the range of variation of all humans today with regard to having a globular brain case and other traits—date up to two hundred to three hundred thousand years ago and are all from Africa.”
“Outside of Africa and the Near East, though, there is no convincing evidence of anatomically modern humans older than a hundred thousand years and very limited evidence older than around fifty thousand years.”
“Archaeological evidence of stone tool types also points to a great change after around fifty thousand years ago, a period known to archaeologists of West Eurasia as the Upper Paleolithic, and to archaeologists of Africa as the Later Stone Age.”
“After this time [ ], the technology for manufacturing stone tools became very different, and there were changes in style every few thousand years, compared to the glacial earlier pace of change.”
“Humans in this period [ ] also began to leave behind far more artifacts that revealed their aesthetic and spiritual lives”
“[ ] beads made of ostrich eggshells, polished stone bracelets, body paint made from red iron oxide, and the world’s first [known] representational art.”
“The dramatic acceleration of change in the archaeological record after around fifty thousand years ago was also reflected by evidence of population change.”
“Population turnovers […] occurred [many places] in Eurasia, as well as in southern Africa, where there is evidence of abandonment of sites and the sudden appearance of Later Stone Age cultures.”

“The natural explanation for all these changes was the spread of an anatomically modern human population […] who practiced a sophisticated new culture, and who largely replaced the people who lived in each place before.”

“The intensification of evidence for modern human behavior after fifty thousand years ago is undeniable”
“[In 2001] medical geneticists had identified FOXP2 as a gene that, when mutated, produces an extraordinary syndrome whose sufferers have normal-range cognitive capabilities, but cannot use complex language, including most grammar.”
“Pääbo and his colleagues showed that the protein produced by the FOXP2 gene has remained almost identical during the more than hundred million years of evolution separating chimpanzees and mice.”
“[ ] However, two changes to the protein occurred on just the human lineage since it branched out of the common ancestral population of humans and chimpanzees”
“Later work by Pääbo and his colleagues found that engineered mice with the human versions of FOXP2 are identical to regular mice in most respects, but squeak differently, consistent with the idea that these changes affect the formation of sounds.”
“Pääbo and his colleagues later identified a third mutation that is found in almost all present-day humans and that affects when and in what cells FOXP2 gets turned into protein. This change is absent in Neanderthals”
“Pääbo’s papers [between 2010 to 2013] highlighted an evolving list of about one hundred thousand places in the genome where nearly all present-day humans carry genetic changes that are absent in Neanderthals.”
“[Human] Females create an average of about forty-five new [DANA] splices when producing eggs”
“[Human] males create about twenty-six [DNA] splices when producing sperm”
“One generation back, a person’s genome is derived from about 118 (47 plus 71) stretches of DNA transmitted by his or her parents.”
“Two generations back, the number of ancestral stretches of DNA grows to around 189 (47 plus 71 plus another 71) transmitted by four grandparents.”
“Ten generations back, for example, the number of ancestral stretches of DNA is around 757 but the number of ancestors is 1,024, guaranteeing that each person has several hundred ancestors from whom he or she has received no DNA whatsoever.”
“Twenty generations in the past, the number of ancestors is almost a thousand times greater than the number of ancestral stretches of DNA in a person’s genome, so it is a certainty that each person has not inherited any DNA from the great majority of his or her actual ancestors.”
“These calculations [of the number of ancestral stretches of DNA versus the number of ancestors, for the number of generations going back] mean that a person’s genealogy, as reconstructed from historical records, is not the same as his or her genetic inheritance.”
“The number of ancestors you have doubles every generation back in time. However, the number of stretches of DNA that contributed to you increases by only around seventy-one per generation.”
“if you go back eight or more generations, it is almost certain that you will have some ancestors whose DNA did not get passed down to you. Go back fifteen generations and the probability that any one ancestor contributed directly to your DNA becomes exceedingly small.”
“the times in the past when the population size was low can be identified based on the periods in the past when a disproportionate fraction of lineages have evidence of sharing common ancestors”
“after the separation of non-African and African populations, there was an extended period in the shared history of non-Africans when populations were small, as reflected in evidence for many shared ancestors spread over tens of thousands of years”
“evidence of an extended period of small population size [of humans outside of Africa] was also hard to square with the idea of an unstoppable expansion of modern humans both within and outside Africa around fifty thousand years ago”
“[ ] Our history may not be as simple as the story of a dominant group that was immediately successful wherever it went.”
“the earliest branching modern human lineage that has contributed a large proportion of the ancestry of a population living today […] contributed the lion’s share of ancestry to the San hunter-gatherers of southern Africa”
“[ ] the separation had begun by around two hundred thousand years ago and was mostly complete by more than one hundred thousand years ago”
“[ ] “Pygmy” groups from Central African forests harbor ancestry that is arguably just as distinctive.”
“Expanding our analysis to the whole genome, we could not find any location—apart from mitochondrial DNA and the Y chromosome—where all people living today share a common ancestor less than about 320,000 years ago.”
“genome scans of present-day human genetic variation will miss most instances of natural selection because they simply will not have the statistical power needed to detect it, and that scans of this type will also have more power to detect some types of selection than others”
“only a small fraction of evolution in humans has likely involved intense natural selection for advantageous mutations that had not previously been present in the population”
“By testing whether the mutations identified by genome-wide association studies as affecting particular biological traits have all tended to shift in frequency in the same direction, we can obtain evidence of natural selection for specific biological traits.”
“But even if genetic changes—through coordinated natural selection on combinations of many mutations simultaneously—did enable new cognitive capacities, this is a very different scenario from Klein’s idea of a genetic switch.”
“[ ] Genetic changes in this scenario are not a creative force abruptly enabling modern human behavior, but instead are responsive to nongenetic pressures imposed from the outside.”
“the genetic formula that may have been necessary to drive the striking advances in human behavior and capacities that occurred during the Upper Paleolithic and Later Stone Ages […] were already in place [long long before this], and …”
“[ ] … and many alternative combinations of these mutations could have increased in frequency together due to natural selection in response to changing needs imposed by the development of conceptual language or new environmental conditions”
“it is unlikely that the first occurrence of these mutations triggered the great changes that followed”

“It is in the area of shedding light on human migrations—rather than in explaining human biology—that the genome revolution has already been a runaway success.”
“human populations are related to each other in ways that no one expected”

“massive mixtures of differentiated populations; sweeping population replacements and expansions; and population divisions in prehistoric times that did not fall along the same lines as population differences that exist today”

“The Age of Neanderthals”
Missing some Tweet in this thread?
You can try to force a refresh.

Like this thread? Get email updates or save it to PDF!

Subscribe to @reiver ⊼ (Charles Iliya Krempeaux)
Profile picture

Get real-time email alerts when new unrolls are available from this author!

This content may be removed anytime!

Twitter may remove this content at anytime, convert it as a PDF, save and print for later use!

Try unrolling a thread yourself!

how to unroll video

1) Follow Thread Reader App on Twitter so you can easily mention us!

2) Go to a Twitter thread (series of Tweets by the same owner) and mention us with a keyword "unroll" @threadreaderapp unroll

You can practice here first or read more on our help page!

Did Thread Reader help you today?

Support us! We are indie developers!

This site is made by just three indie developers on a laptop doing marketing, support and development! Read more about the story.

Become a Premium Member and get exclusive features!

Premium member ($3.00/month or $30.00/year)

Too expensive? Make a small donation by buying us coffee ($5) or help with server cost ($10)

Donate via Paypal Become our Patreon

Thank you for your support!