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.
