The recent Y-chromosome bottleneck in the human genome may have been caused by the low tempertures of the Younger Dryas cooling event 12,900 years ago.
Y-chromosome bottleneck – Part two. Several months ago, I came across a paper on genetics that documented a bottleneck in the human Y-chromosome during the past 10,000 years. The authors of the paper suggested that the bottleneck was caused by changes in human culture that affected variance of reproductive success among males, but I noticed that the effective population size started decreasing at the Younger Dryas Boundary 12,900 years ago, and I questioned whether the bottleneck could have been triggered by the low temperature of the Younger Dryas cooling event.
The paper, published in 2015 by Monika Karmin and one hundred co-authors is entitled "A recent bottleneck of Y chromosome diversity coincides with a global change in culture". The decrease in effective population size for all regions in the Y-chromosome graph starts gradually at the onset of the Younger Dryas 12,900 years ago, then it dips suddenly and it continues to decrease well beyond the 1300-year duration of the cold event. The increase of the effective population size starts to recover abruptly about 7,000 years ago.
The inset on the left shows a bottleneck in the Y chromosome. The mitochondrial DNA on the right does not have a similar bottleneck for the female population. This prompted the researchers to conclude that the Y chromosome bottleneck was not caused by natural selection, so they hypothesized that the drop of the male effective population size was due to cultural changes associated with the spread of Neolithic cultures, demographic changes, as well as shifts in social behavior.
The authors referenced a paper by Zerjal et al. that described an example of competition through a male-driven conquest that left a mark in the human genome. Increased reproductive fitness, transmitted socially from generation to generation, of males carrying the same Y chromosome would lead to the increase in frequency of their Y lineage, and this effect would be enhanced by the elimination of unrelated males.
Genghis Khan, who lived from 1162 to 1227 AD, and his male relatives established the largest land empire in history and often slaughtered the conquered populations. Genghis Khan and his close male relatives had many children. Although the Mongol empire soon disintegrated as a political unit, the male-line descendants ruled large areas of Asia for many generations. However, the Genghis Khan slaughters did not create even a small change in the male to female ratio.
The graphs in the paper do not have the same vertical scale. The shaded portion shows the relationship of the effective population scale. The Y chromosome on the left is responsible for the development of male reproductive organs, and it is passed only from father to son. The mitochondrial DNA (mtDNA), on the right, is passed from mother to both male and female offspring.
The difference in effective male and female population means that approximately 7000 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.
The Y chromosome bottleneck requires a decrease in male genetic diversity. Cultural practices that reduce the male gene pool include: selective killing of males, castration, and jailing of males to prevent reproduction. Jailing can be immediately discounted because the number of jailed men would be several times greater than the number of free men. It would be very difficult to feed them and control them. It would be more practical to kill them to reduce the gene pool. Ten thousand years ago, there were eight females for every male. Assuming normal secondary birth ratios, one man would have had to kill 8 guys to keep 8 females for himself.
According to the graph, the selective slaughter of males in the conquered populations would have had to increase for several thousand years. Seven thousand years ago, the conquerors would have had a harder job. There were 17 females for every male. Again, assuming normal birth ratios, one man would have had to kill 17 guys to keep 17 females for himself.
This progressively vicious slaughter is supposed to have happened on a global scale for more than three thousand years in order to justify the Y chromosome bottleneck as the result of global cultural changes and shifts in social behavior. During the tremendous massacre some of the people killed would have been related to the killer. It should be obvious by now that this explanation for the Y chromosome bottleneck is preposterous because it means that the humans who lived just a few thousand years ago were devoid of compassion, empathy and other social feelings that are common in our species. The fact that the Y chromosome bottleneck started simultaneously in all parts of the word is another argument against this hypothesis.
Castration was not practiced on a global scale to have contributed to the Y chromosome bottleneck. The first records of deliberate castration to produce eunuchs in the Sumerian city of Lagash date back approximately 4,000 years. In China, the eunuch system was firmly entrenched in the imperial culture and persisted through two dozen dynasties until 1911, when the last emperor was deposed. Castration was performed as a punishment and also as a prerequisite for entering imperial service. Towards the end of the Ming Dynasty in the mid-17th century, approximately 70,000 eunuchs were employed to serve the emperor.
Castration of males before puberty prevents development of a deep male voice. Many young boys with beautiful voices were castrated to retain the infantile voice. A castrato is a type of classical male singing voice equivalent to a soprano, mezzo-soprano, or contralto. After the unification of Italy in 1861, castration for musical purposes was made illegal. The last Sistine castrato to survive was Alessandro Moreschi, the only castrato who made solo recordings. His discs give us a glimpse of the castrato voice.
It is difficult to believe that a global change in human culture was responsible for the Y-chromosome bottleneck. The graph shows that male effective population sizes for all regions of the world reached a maximum at the Younger Dryas Boundary 12,900 years ago, as indicated by the red arrow. A simultaneous global change in culture would mean that people in Europe, Southeast Asia, East Asia, the Near-East, Central Asia, and all other regions of the world coordinated their culture and sexual practices in lands separated by oceans, jungles, deserts and mountains. This level of coordination would be extremely difficult even today with modern transportation and electronic media.
Some researchers in Finland have found a temperature-related birth sex ratio bias in historical Sami people. Warm years bring more sons. The effect is quantifiable so that an increase of one degree Celsius during two years corresponds to approximately 1 percent more sons born annually. So, if warm years bring more sons, a long period of cold years may decrease the relative male population.
This is confirmed in a paper by Catalano, Bruckner and Smith that found cold ambient temperatures during gestation predict lower secondary sex ratios. They conclude that ambient temperature affects the characteristics of human populations by influencing who survives gestation, a heretofore unrecognized effect of climate on humanity.
The paper by Catalano says 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 a prolonged period of cold weather, like the Younger Dryas, could have had a devastating effect on the survivability of human males.
This chart combines the temperature record with the ratio of females to males for the past 100,000 years. The ratio of females to males increases for several thousand years after the Younger Dryas extraterrestrial event, and then returns to more normal levels.
Notice that about 55 thousand years ago there was a similar but less intense increase in the female to male ratio. It is also associated with a sharp decrease in temperature about 62 thousand years ago. Could this Y chromosome bottleneck be attributed to another extraterrestrial impact?
The Odessa Crater in Texas is 550 feet or 168 meters in diameter. Its age is estimated to be around 63,500 years. There are several other craters at the site, and over 1200 meteorite pieces have been collected from the surrounding area, the largest of which weighs around 135 kilograms. These fragments could be just a portion of a large meteorite cluster impact that disturbed the Earth enough to cause another Y chromosome bottleneck.
Thus far, geneticists have been looking for cultural changes to explain the Y chromosome bottleneck that occurred within the last 10,000 years. The long duration of the bottleneck and its simultaneous onset in all regions of the world makes it more likely that a severe cooling event affected the survivability of human males. Our genetic record indicates that at least two ecological catastrophes have affected humans during the last 65 thousand years. This may imply that Earth encounters killer asteroids with higher frequency than is currently estimated.