The East Asian summer monsoon and desertification in Eurasia is driven by fluctuating Northern Hemisphere ice volume and global sea level during the Ice Age, as shown in a study published in Nature Communications. Today, two thirds of the world's population is dependent on agriculture sustained by rains of the East Asian summer monsoon, and future climate change in this region can therefore have a major impact on global food production.

The East Asian summer monsoon and desertification in Eurasia is driven by fluctuating Northern Hemisphere ice volume and global sea level during the Ice Age, as shown in a study published in Nature Communications. Today, two thirds of the world's population is dependent on agriculture sustained by rains of the East Asian summer monsoon, and future climate change in this region can therefore have a major impact on global food production [Credit: Thomas Stevens]

Huge areas of central China is covered by a plateau consisting of a fine grained soil type called loess - a sediment deposited here by winds during the Ice Age. The soils formed on loess are very fertile and have been one of the key factors driving cultural development and population growth in China for thousands of years. Additionally, the loess plateau also contains a geological archive that can be used to decipher past climate changes.

Through detailed examination of the loess sediments, a group led by researchers at Uppsala Univirsity together with colleagues from Denmark has identified how changes in climatological phenomena such as ice volume and sea level also affected the extent of deserts in China, as well as the behavior of the East Asian summer monsoon.

"We have conducted the most detailed dating of the loess to date, which has enabled us to identify changes in the monsoon and desertification processes in more detail and with much greater accuracy than previously possible. We can now compare these changes to other known climate changes such as variation in ice volume, sea level and even the Earth's orbit during the Ice Age", says Dr. Thomas Stevens, first author and researcher at Uppsala University.

"We can now show that when ice volume decreased and sea level rose, the summer monsoon rainfalls in East Asia intensified and spread further inland, while sandy deserts in China retreated", says Dr. Stevens.

With today's shrinking ice caps and rising sea levels, this has implications for how the Eurasian continent will once again experience changes in the summer monsoon rainfall and desertification.

Source: Uppsala University [March 07, 2018]

Scientists once could reconstruct humanity's distant past only from the mute testimony of ancient settlements, bones, and artifacts.

The use of stylized bell-shaped pots like this one from Sierentz, France spread across Europe beginning about 4,700 years ago. DNA analysis show that this so-called Bell Beaker culture was brought to Britain by people who largely replaced the island's existing inhabitants  [Credit: Anthony Denaire]

No longer. Now there's a powerful new approach for illuminating the world before the dawn of written history - reading the actual genetic code of our ancient ancestors. Two papers published in the journal Nature, more than double the number of ancient humans whose DNA has been analyzed and published to 1,336 individuals - up from just 10 in 2014.

The new flood of genetic information represents a "coming of age" for the nascent field of ancient DNA, says lead author David Reich, a Howard Hughes Medical Institute investigator at Harvard Medical School - and it upends cherished archaeological orthodoxy. "When we look at the data, we see surprises again and again and again," says Reich.

Together with his lab's previous work and that of other pioneers of ancient DNA, the Big Picture message is that our prehistoric ancestors were not nearly as homebound as once thought. "There was a view that migration is a very rare process in human evolution," Reich explains. Not so, says the ancient DNA. Actually, Reich says, "the orthodoxy - the assumption that present-day people are directly descended from the people who always lived in that same area - is wrong almost everywhere."

Instead, "the view that's emerging - for which David is an eloquent advocate - is that human populations are moving and mixing all the time," says John Novembre, a computational biologist at the University of Chicago.

Stonehenge's Builders Largely Vanish

In the first new Nature paper, Reich and a cast of dozens of collaborators chart the spread of an ancient culture known by its stylized bell-shaped pots, the so-called Bell Beaker phenomenon. This culture first spread between Iberia and central Europe beginning about 4,700 years ago. By analyzing DNA from several hundred samples of human bones, Reich's team shows that only the ideas - not the people who originated them - made the move initially. That's because the genes of the Iberian population remain distinct from those of the central Europeans who adopted the characteristic pots and other artifacts.

But the story changes when the Bell Beaker culture expanded to Britain after 4,500 years ago. Then, it was brought by migrants who almost completely supplanted the island's existing inhabitants - the mysterious people who had built Stonehenge - within a few hundred years. "There was a sudden change in the population of Britain," says Reich. "It was an almost complete replacement."

For archaeologists, these and other findings from the study of ancient DNA are "absolutely sort of mind-blowing," says archaeologist Barry Cunliffe, a professor emeritus at the University of Oxford. "They are going to upset people, but that is part of the excitement of it."

Vast Migration from the Steppe

Consider the unexpected movement of people who originally lived on the steppes of Central Asia, north of the Black and Caspian seas. About 5,300 years ago, the local hunter-gatherer cultures were replaced in many places by nomadic herders, dubbed the Yamnaya, who were able to expand rapidly by exploiting horses and the new invention of the cart, and who left behind big, rich burial sites.

DNA from people from the Bell Beaker culture (illustration of one man shown) reveal that they descended from nomadic herders who migrated from the steppes of Central Asia [Credit: Manuel Rojo-Guerra/Luis Pascual-Repiso]

Archaeologists have long known that some of the technologies used by the Yamnaya later spread to Europe. But the startling revelation from the ancient DNA was that the people moved, too - all the way to the Atlantic coast of Europe in the west to Mongolia in the east and India in the south. This vast migration helps explain the spread of Indo-European languages. And it significantly replaced the local hunter-gatherer genes across Europe with the indelible stamp of steppe DNA, as happened in Britain with the migration of the Bell Beaker people to the island.

"This whole phenomenon of the steppe expansion is an amazing example of what ancient DNA can show," says Reich. And, adds Cunliffe, "no one, not even archeologists in their wildest dreams, had expected such a high steppe genetic content in the populations of northern Europe in the third millennium B.C."

This ancient DNA finding also explains the "strange result" of a genetic connection that had been hinted at in the genomes of modern-day Europeans and Native Americans, adds Chicago's Novembre. The link is evidence from people who lived in Siberia 24,000 years ago, whose telltale DNA is found both in Native Americans, and in the Yamnaya steppe populations and their European descendants.

New Insights from Southeastern Europe

Reich's second new Nature paper, on the genomic history of southeastern Europe, reveals an additional migration as farming spread across Europe, based on data from 255 individuals who lived between 14,000 and 2,500 years ago. It also adds a fascinating new nugget - the first compelling evidence that the genetic mixing of populations in Europe was biased toward one sex.

Hunter-gatherer genes remaining in northern Europeans after the influx of migrating farmers came more from males than females, Reich's team found. "Archaeological evidence shows that when farmers first spread into northern Europe, they stopped at a latitude where their crops didn't grow well," he says. "As a result, there were persistent boundaries between the farmers and the hunter-gatherers for a couple of thousand years." This gave the hunter-gatherers and farmers a long time to interact. According to Reich, one speculative scenario is that during this long, drawn-out interaction, there was a social or power dynamic in which farmer women tended to be integrated into hunter-gatherer communities.

So far that's only a guess, but the fact that ancient DNA provides clues about the different social roles and fates of men and women in ancient society "is another way, I think, that these data are so extraordinary," says Reich.

Advanced Machines

These scientific leaps forward have been fueled by three key developments. One is the dramatic cost reduction (and speed increase) in gene sequencing made possible by advanced machines from Illumina and other companies. The second is a discovery spearheaded by Ron Pinhasi, an archaeologist at University College Dublin. His group showed that the petrous bone, containing the tiny inner ear, harbours 100 times more DNA than other ancient human remains, offering a huge increase in the amount of genetic material available for analysis. The third is a method implemented by Reich for reading the genetic codes of 1.2 million carefully chosen variable parts of DNA (known as single nucleotide polymorphisms) rather than having to sequence entire genomes. That speeds the analysis and reduces its cost even further.

The new field made a splash when Svante Pääbo of the Max Planck Institute for Evolutionary Anthropology, working with Reich and many other colleagues, used ancient DNA to prove that Neanderthals and humans interbred. Since then, the number of ancient humans whose DNA Reich has analyzed has risen exponentially. His lab has generated about three-quarters of the world's published data and, included unpublished data, has now reached 3,700 genomes. "Every time we jump an order of magnitude in the number of individuals, we can answer questions that we couldn't even have asked before," says Reich.

Now, with hundreds of thousands of ancient skeletons (and their petrous bones) still to be analyzed, the field of ancient DNA is poised to both pin down current questions and tackle new ones. For example, Reich's team is working with Cunliffe and others to study more than 1,000 samples from Britain to more accurately measure the replacement of the island's existing gene pool by the steppe-related DNA from the Bell Beaker people. "The evidence we have for a 90 percent replacement is very, very suggestive, but we need to test it a bit more to see how much of the pre-Beaker population really survived," explains Cunliffe.

Beyond that, ancient DNA offers the promise of studying not only the movements of our distant ancestors, but also the evolution of traits and susceptibilities to diseases. "This is a new scientific instrument that, like the microscope when it was invented in the seventeenth century, makes it possible to study aspects of biology that simply were not possible to examine before," explains Reich. In one example, scientists at the University of Copenhagen found DNA from plague in the steppe populations. If the groups that migrated to Britain after 4,500 years ago brought the disease with them, that could help explain why the existing population shrank so quickly.

With the possibility of many such discoveries still ahead, "it is a very exciting time," says Cunliffe. "Ancient DNA is going to revitalize archeology in a way that few of us could have guessed even ten years ago."

Source: Howard Hughes Medical Institute [February 21, 2018]

Until recently, very little was known about the genetic relationship between modern humans of the Upper Palaeolithic age (the period of time between 50,000 and 10,000 years ago, also called the Late Stone age) and today's populations. But with direct DNA sequencing, researchers are discovering unexpected genetic connections between individuals on opposing sides of Eurasia. These suggest a complex history that may represent early gene flow across Eurasia or an early population structure that eventually led to Europeans and Asians.

Schematic of populations in Eurasia and the Americas from 45,000 to 7,500 years ago. A summary of major events  in each of the time periods is on the left [Credit: Melinda A.Yang and Qiaomei Fu]

In a review published in the journal Trends in Genetics, scientists at the Chinese Academy of Sciences in Beijing discuss what we know about the genetics of ancient individuals from Eurasia (Europe and Western Asia) between 45,000-7,500 years ago. The authors summarized work that investigated the genomes of more than 20 ancients in the Eurasian family tree, including the 45,000-year-old Ust'-Ishim individual from Central Siberia, for their paper.

"Aside from these individuals, it is a fact that sampling for the Eurasian region is sparse for all time periods except the present-day," says co-author Qiaomei Fu, a paleogeneticist at the Chinese Academy of Sciences. "But with the information from the several individuals available for ancient DNA sequencing we do have hints at interesting population structure, migration and interaction in East Asia."

The researchers learned that in Eurasia between 35,000 and 45,000 years ago, at least four distinct populations were present. These were early Asian and Europeans, as well as populations with ancestry hardly found or not at all in modern populations. By 15,000-34,000 years ago, however, DNA sequencing showed that modern humans in Eurasia are similar to either Europeans or to Asians, suggesting that a genetic Asian-European separation likely occurred prior to 40,000 years ago. By 7,500-14,000 years ago, the populations across Eurasia shared genetic similarities, suggesting greater interactions between geographically distant populations.

These analyses also revealed at least two Neanderthal population mixing events, one approximately 50,000-60,000 years ago and a second more than 37,000 years ago. This Neanderthal ancestry gradually declined in archaic ancestry in Europeans dating from ~14,000-37,000 years ago.

"Genetic studies of ancient individuals have become more frequent in recent years because of technology," says Fu. "As a result, we can now see the presence of multiple distinct subpopulations in Europe and in Asia, and these in turn contribute different amounts of ancestry to more recent subpopulations."

"Right now is a great time to study human evolutionary genetics because the development of sequencing technology and computing resources minimizes destruction of samples and maximizes data generation and storage," Fu says. "With large present-day genomic datasets and increased international collaboration to handle the many newly sequenced ancient datasets, there is huge potential to understand the biology of human prehistory in a way that has never been accessible before."

Looking ahead, Fu and colleagues hope to extend this type of sequencing and analysis to learn more about the genetic prehistory of East Asia and other regions, including Oceania, Africa, and the Americas. "All of those areas have a rich human prehistory, particularly in Africa, so any ancient DNA from those continents will likely resolve some major questions on human migration," she says.

Source: Cell Press [January 25, 2018]