A recent bottleneck in the genetic diversity of the human Y chromosome indicates that as recently as 6,000 years ago women outnumbered men by a ratio of 17 to 1. This presentation examines the collapse of the ecosystems after the Younger Dryas and the effect that it had on humans.
An extraterrestrial impact in North America 12,900 years ago has been proposed as the cause of the megafaunal extinctions, the disappearance of the Clovis culture, and the onset of the Younger Dryas cooling event. Recent studies have found a bottleneck in the genetic diversity of the human Y chromosome, which indicates that as recently as 6,000 years ago, women outnumbered men by a ratio of 17 to 1. This video examines the collapse of the ecosystems after the Younger Dryas and the effect that it had on humans.
I will discuss five stages of the ecological collapse at the Younger Dryas Boundary and the subsequent recovery. The extraterrestrial impact 12,900 years ago had some immediate effects on the animals and people of North and South America. The onset of the Younger Dryas cold event affected the whole planet with colder than usual weather for 1,300 years. The survivors became isolated in areas where there were sufficient resources to sustain life. When the ecosystem recovered, renewed interaction became possible between the groups that had been isolated. We will examine these stages in more detail by trying to correlate geological and genetic data with historical accounts.
Many scientists still dispute that an extraterrestrial impact was the cause of the extinctions in North and South America. Some still propose that overhunting by the humans who colonized America and climate change caused the extinctions. However, a large platinum anomaly and a wide field of impact microspherules provide strong evidence that the earth was hit by an extraterrestrial object 12,900 years ago. The secondary impacts of glacier ice chunks ejected by the extraterrestrial impact on an ice sheet created the Carolina Bays and Nebraska Rainwater Basins and simultaneously killed the megafauna in a large area from the Rocky Mountains to the East Coast of the United States.
The Carolina Bays and the Nebraska Rainwater Basins are shallow elliptical geological depressions with raised rims whose major axes converge by the Great Lakes. The best explanation is that they were created by secondary impacts of glacier ice boulders that were ejected when an extraterrestrial object crashed into the Laurentide Ice sheet that covered North America. The secondary impacts of the glacier ice boulders produced inclined conical cavities that became shallow elliptical features through viscous relaxation. Elliptical features with similar characteristics as the Carolina Bays can be produced by experimental impacts.
The Carolina Bays are the most prevalent geological structures in the Atlantic Coastal plain. This image shows the Carolina Bays 25 kilometers southwest of Fayetteville, North Carolina. The thin wavy line on the right of the image is the I-95 highway. The largest bays have a major axis exceeding four kilometers.
The energy required to create the Carolina Bays can be calculated using equations that correlate energy with crater size. The inset shows that some of the larger bays were made by impacts of more than 60 Megatons. Each of these impacts is at least 4000 times more powerful than the bomb dropped on Hiroshima. The saturation bombardment by glacier ice pieces that made these scars was fatal to the megafauna that inhabited this region.
This is an image of the United States showing the circular area that was subjected to the saturation bombardment by the glacier ice boulders in the ejecta curtain of the extraterrestrial impact on the Laurentide Ice Sheet. Any animals within this circle with a radius of 1,500 kilometers would have been killed by the ice boulders that landed about 6 to 9 minutes after the extraterrestrial impact. All the vegetation in this area would have been crushed by the impacts, and the land would have been uninhabitable for many years.
Megafauna are large animals weighing more than 45 kilograms or 100 pounds. Animals that had inhabited North America for thousands of years suddenly disappeared along with the Clovis people who populated America at the time. Around 45 genera became extinct, including mastodons, camels, mammoths, lions, giant armadillos, and saber-toothed tigers.
South America also suffered a megafaunal extinction around 12,900 years ago. The fossil record in southern Chile indicates a massive extinction at the Younger Dryas Boundary. This is too far south to have been caused by the impact in the Great Lakes in North America, so some scientists have suggested that the Earth was hit by multiple fragments of a disintegrating comet.
The extraterrestrial impact ejected debris high into the atmosphere, and the light of the sun was partially blocked leading to an impact winter. This dark period temporarily inhibited photosynthesis, causing a global collapse of terrestrial and marine food chains. The Greenland Ice Sheet Project 2 shows that toward the end of the Ice Age, approximately 12,900 years ago, the Earth was warming and the ice sheets were retreating. But suddenly, the climate became colder and returned to glacial conditions for approximately 1300 years. This cold interval is called the Younger Dryas cooling event after a flower called Dryas octopetala that grows in cold conditions and became common in Europe during this time. The cold interval is more recent, or younger, than other cold events during the Ice Age.
The onset of Younger Dryas cold event was abrupt and extreme. The average global temperature dropped 8 degrees Celsius within 200 years of the extraterrestrial impact. The ecosystems collapsed, but the effects varied by region. The cold ambient temperature caused a shift in the human subsistence from harvesting to hunting and fishing. The rivers and the sea still provided sources of food. Some people became nomads and followed animal migrations to survive. The nomadic life style did not make it worthwhile to establish permanent settlements. Temperate zones far away from the extraterrestrial impact site recovered sooner from the effects of the cold weather.
We can get an idea of the suffering of the humans who lived during the Younger Dryas by looking at what happened during the Little Ice Age in the 14th century. This was just a drop in average temperature of less than half a degree Celsius, but it led to crop failures, epidemics and famine. The resulting social instability also triggered conflicts and war.
The sea level rose 120 meters during the last 20,000 years. This chart from the US Geological Survey shows the meltwater pulses. I put some vertical lines to indicate the Younger Dryas Boundary 12,900 years ago at the time of the extraterrestrial impact, and the start of the Holocene Epoch 11,600 years ago, which marks the end of the Younger Dryas cooling event 1,300 years later. Notice that during the Younger Dryas cooling event the sea level rose by 17 meters. This means that the survivors of the extraterrestrial impact had to cope with colder climate as well as with a landscape where the seashores were always changing.
The conditions of genetic survival depend on having reliable food sources and a large group. Without a reliable food source, there is only starvation and a genetic dead end. A lone survivor can live to be one hundred years old, but without children, it is a genetic dead end. The same is true for people too old to reproduce. A group of males and females of reproductive age can have genetic survivors, but if the group is small, it will not have genetic diversity. Small groups are subject to genetic drift. A large group of males and females can maintain genetic diversity, and it has the greatest chance of genetic survival.
A paper published by Ralph Catalano, Tim Bruckner and Kirk Smith in 2007 concluded that ambient temperature affects the characteristics of human populations by influencing who survives gestation.
The paper states that low temperatures may cull males in utero and leave a more robust cohort compared with males born in years with warmer mean temperature. This means that the cold climate of the Younger Dryas may have substantially reduced the birth ratio of males to females.
Genetic drift is an evolutionary process caused by random events that change the frequency of a gene variant in a population to create a genetic bottleneck. The genetic drift game illustrates how the distribution of food affects evolution. A link to this game is provided in the description below. At the beginning of the game, there are five planaria worms of different colors. During each generation, five pieces of food are distributed at random among the five worms. The worms that don't get any food die. The worms reproduce based on the number of pieces of food that they get.
This is a sample run of the genetic drift game. We start each iteration by feeding the worms. The red worm does not get any food during the first generation and it dies. In the second generation the brown worm gets more food and increases its presence in the ecosystem. In the third generation, all the worms get fed and live to see another day. During the fourth generation, the yellow worm does not get any food and becomes extinct. The green worm starves during the fifth generation. In the sixth generation the brown worms outnumber the blue worms by a 3-to-2 ratio, but the blue worms get most of the food. By the seventh generation the blue worms are the only survivors.
The Near East which is in a temperate zone saw the construction of Göbekli Tepe and the beginning of agriculture of barley and oats approximately 11,500 years ago. By 10,000 years ago, sheep, goats and cattle were being domesticated in regions that had stable grasslands. During the period from 10,000 years ago to 5,000 years ago, the global temperature warmed further and became more stable. Glaciers receded, and the climate became suitable for cultivation.
During this 5,000-year period, the sea level rose by about 42 meters due to meltwater pulses 1B and 1C. Meltwater pulse 1B has been associated with the melting of the glaciers in northern Europe. Approximately 7,500 years ago, during meltwater pulse 1C, water from the Mediterranean Sea spilled into the Black Sea. Such sudden rises in water level may have given rise to stories such as Noah's flood in the Bible. The rise of the sea level covered coastal areas and land bridges thus blocking previous migration routes. Many of the dwellings and artifacts in the previous coastlines that could shed light on human history now rest beneath the sea.
Humans evolved the ability to digest lactose in adulthood about 9,000 years ago in areas where the milk from cattle, goats, and sheep was used as a large source of food. Evolution can change the characteristics of a population very fast through natural selection. More than 70% of western Europeans can drink milk as adults, compared with less than 30% of people from eastern Asia. People who do not produce the lactase enzyme are lactose intolerant and experience bloating, diarrhea and gastrointestinal discomfort if they drink milk. Processing milk into cheese and yogurt reduced the lactose. These milk products became suitable foods for a larger percentage of the population. Adapting to the new food source was a very favorable genetic mutation for Neolithic humans at a time when the cold ambient temperature made it difficult to have reliable agriculture. By 8,000 years ago, lactose tolerance was well established in Europe, the Middle East and Africa.
A paper published in 2020 reported the discovery of remains of two prehistoric hunters at an Early Holocene site in highland Peru, including the 9,000-year-old remains of a young woman buried with a well-stocked big game hunting toolkit. Her burial included a comprehensive array of hunting and animal processing tools that support her hunter status, including stone projectile points for killing large animals, a knife and flakes of rock for removing internal organs, and tools for scraping and tanning hides. The authors concluded that between 30% and 50% of Late Pleistocene and Early Holocene big game hunters in the Americas may have been women.
The active participation of women in hunting 9,000 years ago is consistent with the high ratio of women to men during the Y-chromosome bottleneck and may have given rise to the veneration of Diana as goddess of the hunt in Roman mythology. Women who brought food to the table were universally respected and worthy of elaborate burials after their deaths.
The stress of surviving the Younger Dryas is recorded as a human Y-chromosome bottleneck at the end of the Pleistocene. The Y chromosome is responsible for the development of male reproductive organs, and it is passed only from father to son. The paper by Karmin with 100 authors proposes that the bottleneck was caused by a global change in culture. However, the Y-chromosome diversity started to decrease in all regions of the world exactly at the Younger Dryas boundary about 12,900 years ago, and it is not likely that similar cultural changes would have occurred simultaneously in regions widely separated by oceans, deserts, mountains and jungles. The mitochondrial DNA which is passed from a mother to her offspring indicates that the worldwide human population continued to increase in spite of the reduction of male population size.
The difference in effective male and female population means that approximately 6000 years ago, during the minimum of the Y chromosome bottleneck, there were 17 females for every male. This graph also shows a smaller bottleneck 55 thousand years ago when there were 6 females for every male. If the male-to-female birth ratio had remained at 1-to-1 after the Younger Dryas, the cultural changes proposed by Karmin would have involved a culture of killing about 90% of the male descendants of each generation to maintain the 17-to-1 female-to-male ratio on a global scale for several thousand years. It is more plausible that natural selection caused by the cold ambient temperature was responsible for the skewed birth ratio.
The continued increase in human population in spite of the large ratio of females to males can be explained if the males were able to mate with multiple women so that the total number of births would compensate for the reduced number of male births.
The Y-chromosome data has enough temporal resolution to show that the Near-East and Caucasus region, in orange, recovers starting about 7,000 years ago, which is at least one thousand years before other regions. At this time humans started using copper tools. This is followed by recovery in southeast and east Asia in blue, and then by South Asia, in green.
The domestication of horses and the start of the Bronze Age coincide with the time when Europe, in yellow, reaches the lowest effective population size at around 5,500 years ago. The effective population of Europe has one thousand years of genetic stagnation before diversity increases again 2,500 years ago. Siberia loses genetic diversity until 1000 years ago when it experiences a rapid rise in effective population size.
This chart puts all the different graphs together on the same time scale. The top graph indicates that approximately 6,000 years ago, the ratio of women to men was 17-to-1, which corresponds to the time of the greatest bottleneck of Y chromosome diversity. The graph on the right shows that the Y chromosome diversity stopped increasing in all areas of the world at the Younger Dryas Boundary, but the decrease in male genetic diversity did not bottom out until 5,000 years after the end of the Younger Dryas. The fact that such a long time elapsed between the onset of the cold period and the genetic bottleneck brings up the question of whether the Younger Dryas cooling event was the cause of the Y chromosome bottleneck. This requires further research, but it is undeniable that the cooling event had a world-wide effect, including the change in human male-to-female birth ratios that in combination with genetic drift for populations isolated by newly created geographical barriers had long-lasting effects on the human genome.
Woolly mammoths were among the most abundant cold-adapted species during the Pleistocene. Their once-large populations went extinct in two waves, an end-Pleistocene extinction of continental populations followed by the mid-Holocene extinction of relict populations ~5,600 years ago on St. Paul Island, off the coast of Alaska, and ~4,000 years ago in Wrangel Island, which is in Siberia. The mammoths in Wrangel Island experienced an episode of rapid demographic decline coincident with their isolation, leading to a small population, reduced genetic diversity, and the fixation of harmful mutations.
Wrangel Island is located approximately 5,700 kilometers from the site of the extraterrestrial impact by the Great Lakes. A population of woolly mammoths became trapped on Wrangel Island due to rising sea levels. This population was so isolated and so small that it didn't have much genetic diversity. Harmful genetic mutations accumulated as these woolly mammoths inbred, and this contributed to their extinction. This is an example of what probably happened to human populations that became isolated after the Younger Dryas cataclysm and the floods that followed.
One would expect that, after having survived the Younger Dryas cataclysm, the human population would rejoice and try to prepare to overcome future natural calamities. But no! The encounters of the people that had been separated by natural boundaries generally resulted in wars. Different languages and religions created mental barriers of suspicion, envy and hatred that we have not been able to overcome to this day. We will probably die fighting each other before the next natural disaster strikes the Earth. Pardon my pessimism, but our history books are full of wars, conquests and advances in science to create better and more destructive weapons.
This video has tried to combine geological, genetic and historical information to put in perspective the reasons for the human Y chromosome bottleneck. The Younger Dryas extraterrestrial impact 12,900 years ago killed the megafauna from the Rocky Mountains to the East Coast of the United States. There were many survivors outside this area, but they became extinct from the global winter that followed. The global winter is called the Younger Dryas cooling event, and it lasted 1,300 years. Melting of the glaciers in North America and in Europe after the extraterrestrial impact increased the sea level by 70 meters and flooded many coastal areas and land bridges.
The pockets of isolated survivors lost genetic diversity through genetic drift and inbreeding. The ecosystem recovery stimulated development of agriculture and animal domestication. The renewed interaction between groups resulted in wars and conquests, but it also restored genetic diversity in the human population. The question of why there is a 5000 year delay between the end of the Younger Dryas cooling event and the bottom of the human Y chromosome bottleneck still requires further research.