Why you must never marry a second wife in order to get a son.
AFRICANS are more genetically diverse than the rest of the world combined.
Modern humans evolved in Africa about 200,000 years ago, and began moving out of the continent into Eurasia and beyond about 60,000 years ago.
Referred to as the “Out of Africa theory”, migration is understood to have happened by the movement of small bands of people away from Africa, as populations break away, the number of mating partners available is smaller, and so genetic diversity narrows.
It explains why the further away from Africa you go, the less genetically diverse populations are; it means that two Africans might have less in common – genetically – than an African and a European.
Although the word “mutation” conjures up the image of a freakish, perhaps even grotesque abnormality, the word is neutral in science-speak, simply denoting a permanent change in the genetic sequence of an individual.
Mutations are often harmful, even deadly, but sometimes, they confer special evolutionary advantages in a particular environment; the mutant organism is better able to withstand particular environmental stresses, and so the mutation eventually becomes common in the population through natural selection.
Here are ten genetic mutations found in Africans that have persisted due to the beneficial traits they confer, as well as some surprising down sides:
1. Malaria resistant, but HIV susceptible…
95% of black Africans are resistant to two strains of malaria called Plasmodium vivax and Plasmodium knowlesi, by a genetic mutation that makes their red blood cells lack a receptor called the Duffy antigen.
Without the receptor, the malarial parasite cannot bind onto the red blood cells, and so most black Africans are naturally resistant to these strains of malaria; just 3-5% of Caucasians are Duffy-negative.
But a recent study revealed the dark side of malaria resistance – the Duffy mutation makes people more prone to HIV infection. Although the researchers are not quite sure how exactly this happens, they reckon that up to 11% of HIV infections in Africa could be down to this single mutation alone.
It offers a possible explanation as to why HIV Aids has had such a dramatic impact in Africa compared to other continents.
2. …but you live longer
Still, the Duffy mutation causes those infected with HIV to live two years longer on average than those without the mutation, a counter-intuitive finding that researchers are hoping to exploit in the search for a cure – most other risk factors that enhance HIV infection are also associated with faster, not slower, development of the disease.
3. Sickle cell but malaria resistant
Even deadlier than P. vivax and P. knowlesi is Plasmodium falciparum, which causes a more debilitating form of malaria with a much higher mortality rate.
Somewhere in Africa, genetic mutation in which the red blood cells adopt a curved “sickle” shape – as opposed to the normal circular shape – emerged; the sickle mutation caused anaemia and premature death, but also made it much more difficult for P. falciparum to bind onto the red blood cells. The sickle trait is common where P. falciparum is endemic in Africa, with 10-40% of the population carrying this trait.
Employees at a textile mill manufacture durable mosquito nets for distribution to high-risk areas for malaria. (Photo: Flickr/ Gates Foundation)
Ordinarily, people who have two copies of the mutation (one from their mother and the other from their father) suffer from sickle cell anaemia, but if you have just one copy of the mutation, you don’t have anaemia but are highly protected against P. falciparum malaria.
It speaks to the huge toll that malaria has on Africa, that evolutionarily speaking, you are better off risking anaemia and an early death just to escape malaria.
4. Can’t drink milk
In most mammal populations, children lose the ability to digest milk at the end of infancy, in order to allow the mother to bear more young without the older offspring competing for the mother’s milk.
Without lactase – the enzyme that digests milk sugar, or lactose – drinking milk causes stomach cramps and potentially life-threatening diarrhea, which is a good evolutionary trick to ensure that milk is left to the babies in the family.
Predominantly agricultural societies in Africa have a high level of lactose intolerance — up to 90%, according to a study published in research journal Nature Genetics.
But lactase persistence is common in some pastoralist communities, where the ability to digest milk can be the difference between life and death – milk is a highly nutrient-dense food that keeps you going in arid environments, and importantly, a source of water during droughts.
The study showed that lactose persistence co-evolved with cattle domestication in Africa and around the world, only 10-12% of the Tutsi of central Africa and the Beja of Sudan are lactose intolerant, while its over 80% for the traditionally hunter-gatherer Sandawe of Tanzania.
5. Can eat vegetables…
A recent study published in the Public Library of Science (PLOS) journal suggested that early humans were able to move out of Africa after a single genetic mutation allowed them to become vegetarians.
The especially complex brains of humans require a fatty acid called DHA (Docosahexanioc acid) to grow and develop. Early humans who evolved in Africa lived around water because their primary source of DHA was fish and shellfish.
But a genetic mutation allowed some to convert plant-based fatty acids into DHA, which allowed them spread out and to live further away from water, eating domesticated grains and vegetables.
6. …but will get hypertension, stroke, diabetes
Researchers found that people of African descent have a significantly higher frequency of the gene that allows conversion of plant-based fatty acids into food for human brains, but that the gene also causes inflammation, which may be why they have higher rates of hypertension, type 2 diabetes, stroke, coronary heart disease and certain types of cancer.
7. Scooping all the medals
The majority of Kenyan top middle and long-distance runners come from the Kalenjin community, who make up just 0.06% of the world’s population, but have won about 70% of all the elite races since they began competing in them.
Their complete dominance in the sport has confounded sports scientists for decades, with various theories thrown in – the high altitude in their Rift Valley homeland primes them for better oxygen utilisation, their slender, lanky build makes them resist fatigue longer, a simple, wholesome maize based diet is the key or that they had simply spent their childhood running to and from school.
Kenyan runners at the London marathon in 2012. Kenyans dominate all the top middle and long distance races. (Photo/ Sue Kellerman).
Many of these theories have been debunked, but one theory threw some light: there are two types of skeletal muscle fibres, type I, or slow-twitch muscles, and type II, fast-twitch muscles. Endurance runners like the Kenyans have more type I fibres, which allows them to take up more oxygen and keep running for longer.
But Kenyan pioneering marathoner Kip Keino dislikes these “natural advantages” theories, condemning them as racist and arguing running just takes hard work and mental discipline.
8. Lightning bolts
Whereas East Africans dominate long-distance running, athletes of primarily West African descent have surged to the fore in short-distance events.
Jon Entine in his book “Taboo: Why Black Athletes Dominate Sports and Why We’re Afraid to Talk About It” says that athletes of primarily West African descent—which includes the majority of U.S. blacks—hold all but six of the 500 best times in the 100-metre race, “the purest measure of running speed,” says Entine, whose book set off a fiery debate on the subject.
Sprinters, as opposed to marathoners, have mostly type II fibers, which hold lots of sugar as well as enzymes that burn fuel in the absence of oxygen, meaning they can perform anaerobically at very high power output. But the jury is still out on Africans’ evolutionary advantages for sport, and for good reason.
9. Super-dense bones in Afrikaners
As people grow old, their bones lose density in what is called osteoporosis, which is what causes elderly people to stoop over and makes them susceptible to fractures.
But Afrikaners in South Africa tend to have a mutation that causes people to gain bone mass throughout their lives instead of losing it.
Like in sickle cell, if a person inherits two copies of the mutated gene, the resulting condition is serious and life threatening- the bone disorder here is called sclerosteosis, which leads to severe bone overgrowth, gigantism, facial distortion, deafness, and early death.
However, if they only inherit one copy of the gene, they don’t get sclerosteosis and simply have especially dense bones throughout their lives. Researchers are studying the DNA of Afrikaners in the hope that they can get some clues on how to reverse osteoporosis and other skeletal disorders.
10. Don’t marry a second wife, hoping for a son
In all societies around the world, more boys than girls are born; the sex ratio is 102 to 108 male for every 100 female live births. Because baby boys are more fragile than girls, have less developed immune systems than girls and are more likely to die in infancy, it’s evolution’s way of compensating for the “anticipated” loss.
But research has shown that women in sub-Saharan Africa go against the trend and give birth to more girls than non-Black women in all other parts of the world, at about 99 boys for every 100 girls.
There are several possible reasons for this: first, African women (and men) tend to have many babies, and the more children a woman has, the more likely that each additional child will be a girl; older men are also likely to father girls as opposed to boys. Each year in the parent’s age decreases the odds of having a son as the first child by 1%.
The question is, why? One article in Psychology Today suggests a simple, economic reason – sons’ reproductive success largely hinges on the status and resources that they inherit from their parents, particularly, their fathers.
In sharp contrast, daughters’ future reproductive success is largely determined by their youth and physical attractiveness. Once they are conceived with particular genes that influence their physical attractiveness, there is very little that parents can do to increase their daughters’ future reproductive success, beyond keeping them alive and healthy.
The problem with older parents, of course, is that they are more likely to die sooner. If the parents die before the children reach sexual maturity, it will have a greater negative impact on sons’ future reproductive success than on daughters’.
(Photo: Flickr/ DFID).
It suggests parents may be evolutionarily designed to have more daughters when they are older, so that, when they die, they are less likely to leave sons who have not sexually matured. Being orphaned young is bad both for boys and girls, but it’s much worse for boys than for girls, the article concludes.
[BONUS] Why bats are so deadly
Bats appear to carry more scary, human-killing diseases than pretty much any other animal; Ebola, Marburg, SARS and Hendra are all traced to bats as vectors. Researchers propose surprisingly simple reason for this: it’s because they are the only flying mammals.
The act of flying generates so much heat in a bat that it is called flight-induced fever, typically reaching 40C. Thus viruses that live inside bats are primed to survive at that high temperature, while killing all other mammals whose normal temperature is around 37C.
Plus bats are the perfect carriers of a virus – they love company, and will often roost with hundreds or thousands of their peers in close quarters, allowing the viruses to spread, sometimes even across species.