What is a bottleneck? It’s a narrowing in something. Here the narrowing is in the human population. The big question is, what might have caused that narrowing?
I became interested in this question about what caused the winnowing of the human population while listening to my writing coach, Lola Sparrowhawk Kohen, in a presentation of her heavily researched book into the possibilities of early human migrations and cultures from very far into the past. She looked deeply into accounts of past events (for example: Did Civilization X culture exist? And, if it did, who might they have been?)
What I’ll be referring to here is a long-recovery bottleneck may have occurred just after a volcanic or supervolcanic explosion (creating a volcanic winter in sub-Saharan Africa around 70,000 years ago) and reducing the human population there to perhaps 2000 individuals.
I had written a series of paranormal romances, LEGENDS OF THE GOLDENS (a cross-over series of urban fantasy and sci-fi romance). The main characters are dark-phase albinos from Tanzania at the foot of Mount Kilimanjaro. The literature likes to suggest that the reason more albinos are born in Tanzania than the rest of the world is because of something called CONSANGUINITY.
Consanguinity, briefly, is the habit of Africans to intermarry within their families which will concentrate the two recessive genes responsible for complete albinism. In ecological circles, ISOLATION, of small populations, tend to intermarry (or interbreed) resulting in the expression of recessive genes. Thus, the consanguinity concept for why more albinos are born in Africa would have to be considered, but that wasn’t what came to mind listening to Kohen speak about her Arch of History.
Having been trained as a scientific generalist, the first thing that came to my mind about the human bottleneck was the intersection of African albinism and Earth’s climate (more specifically, volcanic winters). So what would make volcanoes erupt more than they have, thus blocking out insolation from the surface of the Earth? And how can that affect the color of a person’s skin?
First, our sun is a variable star, even though it varies little from year to year (the variation is evidenced by the amount of insolation and the sunspot cycle). Right now we’re going through a solar minimum, but our sun has been getting brighter and warmer. How does this warming create a difference in the ability of the crust of our Earth to prevent volcanic eruptions?
“Every so often, shifts in Earth’s orbit lead to rapid warming of the planet, massive melting of glaciers and a quick rise in sea levels. The team found that much more tephra, or layers of volcanic ash, appeared in the sediment cores after those periods [during warming].” LIVESCIENCE
Today, we are experiencing a warming of the Earth due, in part, to increased insolation because the Earth’s orbit is perfectly circular (equal seasons, northern hemisphere close to the sun in the winter (perfectly circular orbit eccentricity = 1)). Though we are in a solar minimum (few high heat sunspots and reduction of insolation by as much as 50%), high temperatures are setting records on the continents and frozen ice at the poles is taking liquid form, resulting in ocean flooding, especially of archipelagos. The LIVESCIENCE article suggests that melting of the polar caps seems to historically have resulted in volcanic activity (maybe because of excess water loading the ocean’s crust).
Today the Ring of Fire around the Pacific Ocean is quite active, possibly because the Pacific basin has a thinner crust than the Atlantic.
So, there have been times when the Earth warmed and ash covered the continents and volcanic winters resulted in flooded continental shorelines. So, what does this say about the kind of UVB solar rays responsible for creating vitamin D within the human skin? (It is said that few other means of getting vitamin D from other foods is possible. So, if the UVB light cannot penetrate the skin, then the human cannot manufacture their own life-giving vitamin D).
See where we’re going with this? The Earth gets hotter. The ice caps melt. The released water, more dense than ice, puts pressure on oceanic crusts. the crust fissures, releasing pressure and melting the underlying rock (sometimes catastrophically in stratovolcanoes). Lots of volcanic dust and noxious gases are released into the upper atmosphere where they cut off sunlight (UVB) to the resident humans down below. During volcanic winters, human populations are not able to produce vitamin D with low insolation of UVB rays. Lack of vitamin D can result in. Without vitamin D (vitamin D deficiency: VDD) all systems/organs in the body are at risk, and so is the individuals immune system . (http://www.scielo.br/pdf/jped/v90n1/0021-7557-jped-90-01-00004.pdf)
So our conclusion might be that out-of-Africa humans migrated northward with some form of albinism gene, and very dark skinned individuals meeting with volcanic winters were severely compromised, creating, perhaps, a bottleneck in the number of individuals on Earth (as few as 1000 males and females, each).
References for Global Climate Change and The Incidence of Phenotypes and Genotypes of Albinism and Reasons for:
“The team found that when glaciers melt, they reduce the pressure on continents, while sea-level rise increases pressures on the ocean floor crust. In the computer model, the change in pressures on the Earth’s crust seem to cause increases in volcanism.”
“Vitamin D insufficiency affects almost 50% of the population worldwide. An estimated 1 billion people worldwide, across all ethnicities and age groups, have a vitamin D deficiency (VDD). This pandemic of hypovitaminosis D [VDD] can mainly be attributed to lifestyle (for example, reduced outdoor activities) and environmental (for example, air pollution) factors that reduce exposure to sunlight, which is required for ultraviolet-B (UVB)-induced vitamin D production in the skin.”
Variable expression of albinism within a single kindred.
(Castronuovo S1, Simon JW, Kandel GL, Morier A, Wolf B, Witkop CJ, Jenkins PL.)
We studied the albinotic characteristics in 13 members of a white family (age range, 2 to 73 years), which were graded according to severity and were correlated with visual acuity. Clinical, electrophysiologic, and biochemical characteristics of this family do not fit any known category of human albinism. The degree of heterogeneity in expression of albinotic features was unexpected. The correlation between visual acuity and nystagmus was particularly strong. The brown-haired propositus had severe skin involvement, iris transillumination, fundus hypopigmentation, and foveal hypoplasia. He had no manifest nystagmus, however, and his visual acuity was nearly normal. These observations suggest that nystagmus imposes a visual deficit beyond that related to foveal hypoplasia alone.”
4) OCA is Oculocutaneous albinism.
“[Genetically, OCA]…affects approximately 1 in 20,000 people….” To read more about the complexity of OCA genetics and how some of these combinations result in disease and some in variable hair, eye, and skin color (what we, here, refer to as dark-phase).”
5) Colin Davies, Natal RN, answer in Quora https://www.quora.com/Why-are-there-so-many-albinos-in-Tanzania
“In Tanzania OCA2 is highly common compared to other locations”
“The Heterozygote advantage assumes that the 1 in 75 or 76 persons in Tanzania have some form of genetic advantage over other members of the population. And why the number is so high in Tanzania is because the ancestral pool was located in an area where this advantage was necessary.
6) One such assumption http://www.sciencedirect.com/sci… suggests that persons with a OCA2 mutation are protected from a form of leprosy.
However the leprosy might be a form that does not exist today, which makes this impossible to test for now.”
Above references are the state of understanding of this “genetic disease” today, but more may be learned about how skin color evolved under the influence of volcanic winters and latitudinal reduction in UVB.