Ten millennia ago, there were about six million people on Earth. Today, there are six billion.

This thousandfold increase in the global population is often thought to be linked to the invention of farming and the domestication of animals about 13,000 years ago in the Near East. Growing crops and raising live animals requires a larger work force than hunting or gathering, so as agriculture took hold, families grew in size. These families grew into villages, villages grew into towns and cities, which eventually consolidated into the vast civilizations in ancient Mesopotamia, Egypt, and elsewhere.

Was this surge during the early days of agriculture the ONLY time in the history of our species that the worldwide population size of humans experienced a substantial boost? The traditional answer was yes. Early human hunter-gatherer populations were relatively stable, their overall size not changing much from generation to generation. Stone-age humans had a limited number of gazelles to hunt, berries to pick and eggs to snatch. Those ecological limitations suggest that pre-agricultural communities would have grown only modestly, even when the hunting was good.

But a new DNA analysis published in the July 29 issue of PLoS One suggests that the world’s human population grew dramatically even long before the development of farming.

Earlier analyses of mitochondrial DNA (mtDNA) have suggested that there were bursts of population growth much further back in time, but those studies have been unable to pinpoint a specific time span.

Geneticist Michael Hammer and his colleagues looked at nuclear DNA from four African populations: the San hunter-gatherers of the Kalahari Desert, the Biaka Pygmies of the Central African Republic, the Mandenka of Senegal and the Yorubans of Nigeria. Nuclear DNA is useful here because it is more sensitive to population size changes than mtDNA.

The authors found genetic evidence for a surge in human population size about 40,000 to 50,000 years ago. This period, just after humans first set foot outside Africa, is of great interest to archaeologists because it coincides with a dramatic increase in the sophistication of human behavior. People began crafting tools from bone, burying their dead and fashioning clothing to keep themselves warm in cool climates. They developed complex hunting techniques, and created great works of art in the form of cave paintings and jewelery.

The archaeological record also shows that during this time, humans began hunting more dangerous prey and more easily exploiting small game like rabbits and birds. They traveled farther than they had before, perhaps due to the growth of long-distance trade routes – the first of their kind. Jared Diamond, author of The Third Chimpanzee, calls this period “The Great Leap Forward,” when humans burst forth culturally – finally separating themselves from their evolutionary cousins.

The exact cause for these changes in human behavior may never be known. Some believe a simple genetic mutation or that the evolution of language could have sparked such a dramatic change. But what we do know now, thanks to this new genetic research, is that like the invention of agriculture this explosion of innovation was accompanied by population growth.

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Most experts agree that the earliest examples of farming and animal domestication lie in the aptly named Fertile Crescent, in present day Iraq.  But still many questions have lingered over the years, especially with regard to remnants of farming or animal domestication that have not survived to the present day.  What kind of tools did they use to farm the earliest crops? How did they transport these crops to neighboring communities? Now a new archaeological discovery in the mountains of Turkmenistan has finally given us more answers than questions, and has shed light on some of earliest farming communities in western Asia.

The finds center around the discovery of several model-sized carts at the archaeological site of Altyndepe, a Bronze Age settlement near the city of Ashgabat in southern Turkmenistan. These tiny carts may have been used in ritual ceremonies, or may have simply been the toys of young children.  But the most interesting aspect of these carts is that they depict camels as the main beasts of burden. Archaeologists are always interested in artifacts that reveal clues about daily life from ancient civilizations, and these camel-pulled carts are a comparative jack-pot.

An article about the carts recently appeared on the Discovery News website. They were documented by Lyubov Kircho of the Institute for the History of Material Culture at the Russian Academy of Sciences and are described (in Russian) in the journal Archaeology, Ethnology and Anthropology of Eurasia. An English version will be published in the Proceedings of the 19th International Conference of the European Association of South Asian Archaeologists.

Archaeologists have discovered cattle-pulled carts from the region, dating to about 6,000 years ago. These carts were originally used for transporting necessities like grain, but later carried other items like alabaster and the prized stone lapis lazuli from hundreds of miles away. Trade networks with neighboring communities began to spring up, and by 3,500 BC one of the first dedicated ‘highways’ for vehicles ran between Altyndepe and nearby towns in present-day Iran and Afghanistan.

But by 3,000 BC, the climate was becoming more arid and the people of Altyndepe could not longer trust their cattle-pulled carts to make the long journeys. Archaeologists already suspected that the communities must have switched to camels, which were better able to handle the drier climate. Now these model carts show that their suspicions were correct; camel-pulled carts were the standard for this region, and were such an integral part of daily life that miniature versions were created as children’s toys.

The presence of these carts – combined with previous ideas on the sharing of ideas and culture throughout this part of western Asia – also have implications for the genetic history of the region. It is well documented that about 10% of modern Europeans contain a genetic signature of the early agriculturalists who arrived from the Near East beginning about 9,000 years ago, bringing their farming techniques and DNA with them. Scientists now believe that while some of these Near Eastern farmers did travel west into the heart of Europe, others headed into the plateaus and foothills east of the Caspian Sea. Like their western counterparts, these farmers brought their farming tools and techniques to the indigenous people of southern Turkmenistan. And like their western counterparts, they probably brought their genes. In fact, some genetic studies have examined the genetic make-up of modern day residents of Turkmenistan, and have found that many of these people also bear the genetic signatures of the early farmers of the Fertile Crescent.

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Almost since the 1871 publication of “The Descent of Man,” in which Charles Darwin applied his theory of natural selection to the human species, biologists have argued over whether the dramatic series of evolutionary events that led to the emergence of Homo sapiens continues to this day.

Some have argued that culture and technology have eclipsed the powerful biological forces that shaped our species in its formative years. In their view the species, no longer faced with a daily struggle for survival, is adrift in an evolutionary Sargasso Sea.

“There’s been no biological change in humans in 40,000 or 50,000 years. Everything we call culture and civilization we’ve built with the same body and brain,” the famed evolutionary biologist Stephen J. Gould once said in an interview.

In their new book “The 10,000 Year Explosion,” anthropologists Henry Harpending and Gregory Cochran argue the contrary position. They claim that in fact, far from grinding to a halt, human evolution has accelerated dramatically since the origins of agriculture about 10,000 years ago.

“We intend to make the case that human evolution has accelerated in the past 10,000 years, rather than slowing or stopping, and is now happening about 100 times faster than its long-term average over the 6 million years of our existence,” they write.

In evolutionary terms, 10,000 years is no time at all — about 400 human generations. Rabbits can go through 400 generations in not much more than a century — can you imagine rabbits being substantially different than they were 100 years ago?

Far from ending the chain of dramatic evolutionary changes that led to upright walking, advanced cognitive abilities and spoken language, Cochran and Harpending argue, the adoption of agriculture so dramatically changed the human environment that a new wave of genetic innovations flourished. These new genetic variants thrived because they helped people cope with the challenges an agricultural way of life presented, such as the shift to a low protein, high carbohydrate diet; the creation of an organized, stratified society and the rise of infectious diseases in response to increased population density.

In fact, many of the genetic variations that 23andMe provides information about are relics of those evolutionary changes. The SNP that confers lactose tolerance, for example, appears to have arisen in Europe about 8,000 years ago among the first people to herd cows and other milk-producing animals. The lactose-digesting variant quickly spread throughout the parts of Eurasia that were ecologically suited to pastoralism.

There are also a number of genetic variations covered by 23andMe that cause physiological problems when two mutated copies are present, but provide protection against infectious disease when a person has one of each version of the gene. For example, the genetic variations that cause sickle cell anemia and G6PD deficiency confer resistance to malaria. Geneticists call this situation balancing selection; over the entire population, the reproductive cost to those who end up with the genetic disease is outweighed by the benefit to others who are resistant to the infectious one.

At the end of the book, Cochran and Harpending make the controversial argument that balancing selection is responsible for the increased incidence of a number of genetic diseases among people of Ashkenazi Jewish descent — and for their higher intelligence relative to other groups.

The authors do raise some interesting points about the anomalously high frequency among Ashkenazi of genetic disorders that stimulate the growth of neurons in the brain. And they cite studies that have shown increased intelligence among people with some of these diseases.

But genetic explanations for between-group differences in intelligence are best taken with a whopping dose of skepticism. Even the definition of intelligence is a matter of intense debate, not to mention the degree to which it can be inherited through genetics. in the end, their case is little more than a just-so story.

In telling it Cochran and Harpending blunt the rest of their book’s powerful message: human evolution is not over by a long sight.

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Before genetics came into the picture, researchers interested in the introduction of agriculture to Europe had only the archaeological record to go on — a limited collection of primarily stone and bone artifacts that left much room for interpretation. But as researchers began applying population genetics to the question of how farming spread across Europe, beginning with the work of Luca Luigi Cavalli-Sforza nearly 40 years ago, there began to be hope for a better understanding of how agriculture spread through the continent beginning about 10,000 years ago.

Decades of genetic research have produced some clarity. But geneticists have also created their own new sources of new debate surrounding the spread of agriculture.

Over the years, two schools of thought have jockeyed for dominance: one argues that farmers from the Near East came into Europe around 10,000 years ago, spreading both their knowledge of farming AND their descendants throughout the continent.  Known as the ‘Demic Diffusion Model’, this hypothesis predicts that the majority of today’s Europeans trace their ancestry to early farmers from the Near East.  The alternative hypothesis, dubbed the ‘Cultural Diffusion Model’, argues that while the practice of farming likely spread from the Near East into Europe, the farmers themselves did not.  Instead, the technology was passed along from settlement to settlement, until it reached northern Europe 5,000 years later. It predicts most modern Europeans should trace their ancestry to the original Stone Age inhabitants of Europe.

As the debate has continued, newer studies have begun breaking the puzzle down into more manageable pieces, looking at particular regions within Europe to see how farming changed the demographics on a more regional level.  This is exactly what new research published in the European Journal of Human Genetics seeks to do, by examining the genetic history of the Balkans in Eastern Europe in an effort to work out what was happening there nearly 10,000 years ago.

The authors of the study, including Stanford professors and 23andMe scientific advisers Drs. Peter Underhill and Roy King, focused on the Balkans chiefly because the archaeological record in this area hints at interactions between farmers from the east and the hunter-gatherers who were already living there.  The researchers chose to examine the Y-chromosome, which is passed down from father to son, to see if any traces of such interactions remained in people living in the Balkans today.  They sampled more than 1,200 men from Greece, Albania, Slovenia, and many other countries on the Balkan Peninsula and throughout Eastern Europe.

What they found were three major Y-chromosome haplogroups (clusters of men who share deep ancestry along the paternal line). One group pre-dated the arrival of agriculture into the region, a second arrived along with agriculture, and a third fell somewhere in between.

Specifically, more than 60% of those sampled belonged to branches of either haplogroup I or haplogroup R, both of which are believed to have arrived in Europe tens of thousands of years ago.  In addition, a unique branch of haplogroup E is found at levels of 15-25% among the men tested, but only in those people hailing from the southern Balkans (such as Greece or Macedonia).  Finally, up to one-fifth of the men tested fell into haplogroup J, which is common in the Near East and thought to be closely associated with both Mediterranean seafarers and the origin and spread of agriculture.

What do these figures mean?  First of all, it is clear that over half of the men in the Balkans and Eastern Europe belong to haplogroups that pre-date the arrival of agriculture in the area.  These mens’ paternal ancestors likely trace back to the Mesolithic hunter-gatherers.  On the other hand, the 20% of men belonging to Haplogroup J likely trace their ancestry to the same Near Eastern populations where agriculture originated.

So it stands to reason that – because the authors’ samples contain both hunter-gatherer and farmer haplogroups – there was likely some kind of interaction between the two populations when they came into contact with each other nearly 10,000 years ago.  More importantly, it reveals that the spread of farming into Europe cannot be pigeonholed into either the Demic Diffusion or Cultural Diffusion Model.  Its spread is far more complex – and intriguing – than many initially thought.

But what about the men who traced their ancestry to that sub-branch of haplogroup E, which is typical of North or East Africans; what is it doing in the Balkans?  Haplogroup E has a unique history, stretching back some 40,000 years to East Africa. From where men carrying it expanded into the Nile Valley.  The authors believe this branch of Haplogroup E exited Africa almost 16,000 years ago, well before the arrival of farming, making it to the southern Balkans a few thousand years later.  Men bearing the haplogroup may have been among the first converts to agriculture when Neolithic farmers arrived in the area a few thousand years after that.

So it would seem that by using genetics to piece together ancient interactions between Mesolithic hunters and Neolithic farmers, we are actually left with new questions surrounding the arrival of Haplogroup E.  Who were these men who carried it?  What made them expand from the Nile Valley into the Balkans, just prior to the arrival of farming?  And did their presence in the Balkans influence interactions between the hunter-gatherers and the farmers from the Near East?  Once again, research into the genetic signatures of the first farmers has given us both new answers, and new questions.

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Archaeologists rarely agree on anything.  So it’s no surprise that for years two groups of scholars have drawn completely opposite conclusions about the relationship between the ancient people of Thailand and China.

Some experts argue that, thousands of years ago, people from Thailand migrated into East Asia, becoming the ancestors of present-day Chinese peoples.  Others argue the exact opposite: that it was people from East Asia who migrated south into Thailand.  Unfortunately, the sheer complexity of languages and cultures in the region, combined with conflicting archaeological reports, makes it next to impossible to draw concrete conclusions.

In situations like this one researchers increasingly turn to the field of genetics to help fill in the gaps and piece together the history of various populations — and that is just what a team from Thailand has done.  In this month’s issue of the American Journal of Physical Anthropology, a bipartisan team of archaeologists and geneticists from across Thailand describes how the mitochondrial DNA (mtDNA) of both ancient and modern Thai populations can help us understand the peopling of the country.

Because the demographics of Thailand are so complex, the authors felt that just sampling modern Thai populations would not give a clear picture into the peopling of the region.  So, the researchers dug deeper by sampling archaeological remains as well.  They extracted DNA from 43 skeletons excavated at two sites in northeastern Thailand that date to about 3,500 years ago.  The authors chose these particular sites, called Noen U-loke and Ban Lum-Khao, because they represented typical ancient farming communities – some of the earliest settlements in existence.

Right away, the authors noticed the ancient and modern samples were very similar genetically — an indication that within the last 3,500 years, people haven’t moved around very much in northeastern Thailand.  Some of the modern people tested may in fact be descendants of these early Thai farmers – a very exciting prospect.

More importantly, however, the authors believe they have worked out the peopling of Thailand from the genetic information they gathered.  Their data support the hypothesis that many present-day Thai populations are actually descended from people who migrated from southern China only a few thousand years ago – an idea that has gained prominence in recent years among many archaeologists.

These results are compelling for a number of reasons — most notably the fact that the authors were able to use genetic data to resolve uncertainties about the peopling of Thailand when other types of data could not.  It also shows how useful it can be to compare ancient and modern DNA when tracing prehistoric migrations.  Many studies use only one or the other.  Finally, these authors were able to utilize genetics in a region — Southeast Asia — that has not seen much genetics research compared to other regions.  Hopefully future research on the genetic history of this region will continue down the path laid out by these researchers, following their lead of taking evidence from all disciplines in reshaping theories on ancient civilizations.  Perhaps we will even see more agreement from archaeologists.

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Throughout the history of our species there has been one constant:  movement.  Since the origin of Homo sapiens nearly 200,000 years ago in East Africa, humans have journeyed around the globe, ultimately inhabiting every continent save Antarctica.

Scientists have traditionally used archaeology, and more recently genetics, to understand the timing and scope of these ancient migrations.  The field of historical linguistics has also been used in the same way,  reasoning that if people migrated to new regions, they would have brought their languages with them.  In this way, linguistics can be used as an additional resource in understanding our species’ past movements.

However, this concept is rarely – if ever – that straightforward.  For example, a language might spread from local population to local population, while the original speakers of the language stayed put.  This concept, called ‘cultural diffusion’ is at the core of many debates about our species’ prehistory:  did people (and therefore their genes) migrate to a new region, or was it just a transfer of cultural traits (such as language) from one region to the next?

There are two famous examples in human prehistory that delved deep into this very question.

Bantu Expansions in sub-Saharan Africa

About 5,000 years ago, the majority of sub-Saharan African peoples still relied on hunting, gathering, and foraging as their main source of food.  But some people in west-central Africa were developing new techniques of survival.  They began to experiment with herding and agriculture, cultivating the yams, legumes, peppers, and gourds that would became staples of a sub-Saharan African diet.  Then, about 4,000 years ago, they began to move.  As they traveled over a period of centuries, they both displaced and absorbed groups that were already living throughout Africa. 

The languages that they brought with them from their ancestral homeland, belonging to the Bantu family, also spread throughout sub-Saharan Africa.  Today the majority of sub-Saharan African languages are Bantu.

What both genetics and linguistics have told us about the Bantu expansions is that it was an expansion of both peoples and cultures.  Though these early Bantu speakers may have intermarried as they expanded,  their genetic signature still shows West African ancestry.  The distribution of Bantu languages throughout sub-Saharan Africa confirms this prehistoric migration.  In fact, this example marks one of the most straightforward instances of a combined genetic and cultural expansion.  

The Spread of Agriculture and Indo-European Languages

Unfortunately, the spread of agriculture from the Near East into Europe and its connection to the spread of Indo-European languages is far more complicated.  

Agriculture is believed to have originated in the Near East about 13,000 years ago.  Starting about 10,000 years ago, the archaeological record indicates that the practice expanded westward into Europe. By 9,000 years ago, agriculture existed in Greece.  By 5,000 years ago it had reached Scandinavia.

But was this spread of technology accompanied by farmers themselves, as in the Bantu migrations?  Linguists felt they had the answer: the vast majority of languages spoken in Europe today are members of the Indo-European language family.  Languages such as Greek, Latin, Celtic, and English all belong to this group.  In fact, only a few isolated European languages (such as Basque and Finnish) are unrelated.  

Linguists argued that the geographical origin for this language family was probably somewhere in the Near East or Caucasus Mountains, and that these languages spread – along with the farmers themselves – into Europe, replacing the older languages that were already in existence.  The archaeology appeared to support this idea, and soon many scholars had argued for an expansion of agriculturalists from the Near East.

However, the genetics told a somewhat different story.  Though there are genetic footprints in Europe of these early Near Eastern agriculturalists, there are pre-agricultural genetic footprints as well.  In fact, more Europeans trace their ancestry back to ancient European hunter-gatherers, who survived the harsh Ice Age in the southern fringes of Europe 20,000 years ago, expanding into northern Europe as glaciers receded several thousand years later.  They were not replaced or marginalized by the arrival of agriculturalists.

This discordance has made the question of the arrival of agriculture and Indo-European languages in Europe one of the most contentious in the fields of archaeology, genetic anthropology, and linguistics.  It remains unresolved, though many scientists now argue for lower levels of genetic diffusion into Europe than originally thought.

There are countless other examples in human prehistory of cultural vs. genetic diffusion, with different disciplines often yielding different hypotheses.  The connection between genetic diffusion and cultural diffusion is anything but straightforward, and the best research will take into account not only genetic and linguistic evidence, but evidence from the archaeology, human skeletal remains, and paleobotany.  As noted scientist and author Jared Diamond has put it, “It is quite a challenge, but a uniquely fascinating one.”

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The question of how agriculture first arose and spread in Europe has perplexed archaeologists and geneticists alike for decades. Evidence for farming communities in Greece, Crete, the Balkans, Southern Italy, and Mediterranean France and Spain first appears in the archaeological record eight to nine thousand years ago with the appearance of domesticated wheat, barley, sheep, goats and cows, and a related culture featuring pottery and anthropomorphic figurines. Before this period, most of the indigenous people of Europe fished, hunted small game or foraged for their livelihood.

But how did farming get to Europe in the first place? We know that 2,000 years before agriculture hit Europe, just after the end of the Ice Age, people living in the Near East had already developed farming, with the domestication of wild species of grasses, goats and cattle likely beginning in the fertile river valleys in present-day northern Iraq, Syria and southeastern Turkey. Near Eastern farmers also settled in villages and produced pottery and ceramic human figurines similar to the ones later found across Mediterranean Europe.

So did the first farmers of the Near East hop into boats with their domesticated plants, animals and artistic motifs and colonize Mediterranean Europe? Or did native Europeans learn about farming through trade with the Near East and decide to adopt this agricultural economy too?

Genetic studies are starting to provide answers to this enduring question. Not only do genetic studies of sheep, goats, cows and wheat demonstrate that the European varieties are subtypes of the species found in the Near East, but data from the human Y chromosome suggest that people currently living in Southern Europe are descended in large part (at least through the paternal line) from the Near East. This suggests, at least for the Mediterranean areas of Europe, that the most probable scenario is that Near Eastern farmers did actually move into Europe, bringing farming with them.

23andMe has a Y chromosome marker on its custom chip, rs34126399, which captures the spread of agriculture from the Near East to Europe. The G state at rs34126399 is found in most individuals carrying paternal haplogroup J2a, whose origin can ultimately be traced to Turkey 15,000 to 20,000 years ago. Southern Turkey is the likeliest source for the initial domestication of wheat.

In the millennia after its appearance in Turkey, rs34126399G then experienced a population growth and spread westward into the eastern Mediterranean and eastward into Mesopotamia, Iran and northern Egypt. The map to the right shows the present-day distribution of rs34126399G in Europe and the Middle East.

In addition to its high frequency in the Near East, rs34126399G is present in 10 to 25% of the population of southern Greece and Italy, Crete, Sicily, Bulgaria and Romania. The concentration is an indication of the early spread of farming to these regions, where the climate of wet winters and hot dry summers suited the varieties of winter wheat that were first domesticated.

Since the initial movement of people associated with the origins of agriculture nearly 10,000 years ago, there has been persistent contact and trade throughout the Mediterranean. Just think of the Mediterranean diet — rich in olive oil, goat cheese, and red wine — all of whose origins can be traced to these regions. One archaeologist described the Mediterranean Sea figuratively as a giant bathtub that for millennia has permitted easy seafaring transit from one area to another. The genetic evidence appears to support that analogy.

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