Modern-Day Human Evolution

I love Mental Floss. I love the gadgets they advertise in the back of the magazine, and the awesome gifts that will make happy the nerdiest birthday-girl in your life. A few highlights: Te (Tellurium) and A (Admantium) teacup coasters, Be (Beryllium) and Er (Erbium) beer coasters, a Darwin tote bag, hilarious bookends, a glass pie pan with pi etched into it (my birthday is in September, guys), typewriter key cuff-links, and a pencil case with the words “Choose your weapon.”

I also love how, every question I have, they have already covered in a user-friendly numbered list. For example, they have an itemized article on signs that humans are still evolving. I’ll summarize them briefly here:

  1. As heartthrob Calvin & Hobbes author Bill Watterson once wrote in his strip, “The more you think about things, the weirder they seem. Take milk for example. Why do we drink COW milk? Who was the guy who first looked at a cow and said. I think I’ll drink whatever comes out of these things when I squeeze ‘em!” As it turns out, this is a really valid point. Before cows, goats, etc. were domesticated, humans lost the ability to metabolism lactose (milk sugar) after they were weaned off their mother’s breast milk. But the added nutritional benefits we got from drinking other animals’ milk proved to be selectively advantageous. Though lactose intolerance was the norm around 3,000 years ago, the mutations that allows us to digest lactose our whole lives are now known to be carried by approximately 95% of Northern European descendants!
  2. Have you had your wisdom teeth removed? If so, you’re in the majority, according to this NY Times article that estimates the number of Americans with jaws too small to accommodate them to be around 70%. While our ancestors needed them in order to masticate foodstuff of hunter-gatherer variety, we with our fancy forks, knives, and food processors don’t have a use for a third set of molars. Because of this, people born without wisdom teeth (especially common among Inuits) have no disadvantage that prevents them from passing on their genes. And oral surgery makes it possible for those of us born with painfully impacted wisdom teeth to remove them, so we can pass on our small jaws to our babies.
  3. Mutations that make people more resistant to malaria, tuberculosis, leprosy and HIV are becoming more and more common. Incidentally, it’s nothing other than the sickle cell gene that gives an increasing population of Africans resistance to malaria. When people have one allele with the sickle-cell mutation and the other without, they have a much smaller chance of dying from Plasmodium falciparum infection of their blood cells. Also, since they still have one good copy of normal hemoglobin, they do not suffer from sickle cell anemia (SCA). As a result, Africans with this heterozygous mutation do not die of either disease, and therefore live to pass on their genes. Of course, as a result of this selection, it is also more common for people living in places at a high risk for malaria transmission to be homozygous for the sickle cell gene (having it in both alleles), and thus to die of SCA.
  4. Our brains are getting smaller. Fossil records show that the human brain has gone from 1,500 cubic centimeters to 1,350 cubic cm over the last 30,000 years. Why would a smaller brain be advantageous? The theory I like says that our brains are becoming more efficient. We also may be getting less aggressive and more tolerant, which affords us a social advantage. 
  5. Blue eyes–what’s up with that? Scientists think that prior to 10,000 years ago, our ancestors only had brown eyes. Around that time, genetic mutations popped up in people near the Black Sea that gave them blue eyes. Because blue eyes are recessive, two blue-eyed partners can only have blue-eyed children (unless a very unlucky mutation occurs in the parents’ germ cells). This helps ensure fidelity, because if a blue-eyed man’s blue-eyed partner has a brown-eyed baby, he’s not likely to make any more babies with this partner. Interestingly, this is proven to play a significant subconscious role in mate-selection by blue-eyed males. This study by researchers in Norway showed that, while brown-eyed males show no bias toward women of a certain eye-color, blue-eyed males do rate blue-eyed women as more attractive.

Now, I think there’s one more item that should be added to the list: the evolution of the hymen. Unfortunately, there are no hard statistics on the hymens of the past, as tissues are obviously not preserved in the fossil record. But we can make some compelling deductions from our current numbers if we factor in the importance of the hymen in some ancient cultures, and in more conservative cultures today.

Theories about how the hymen originally arose in primates generally converge on: it protects against microbial infection of the vagina and womb, preserving reproductive viability right up to the moment reproduction first occurs. After it showed up, it began to play a role in sexual selection; i.e. males might preferentially choose to mate with a virgin to ensure that the first offspring is his.

It’s now common knowledge that the presence of an unbroken hymen isn’t always an accurate way to determine whether a girl is a virgin or not. The hymen can break during activities like bike-riding, horse-back riding, tampon insertion, etc. Interestingly, the opposite is also true: a 2004 study found that 52% of adolescent girl subjects who admitted to previous intercourse still had an intact, non-disrupted hymen. This is not extremely surprising, as only 1 in 2,000 females are born with imperforated hymens (having no opening whatsoever).

I imagine that in cultures where women were/are punished for failing to bleed during consummation of marriage, an ideal hymen would evolve that does not break easily, but which reliably does break during first coitus. Take a moment to read Deuteronomy 22:13-21:

“If any man takes a wife and goes in to her and then turns against her, and charges her with shameful deeds and publicly defames her, and says, ‘I took this woman, but when I came near her, I did not find her a virgin,’ then the girl’s father and her mother shall take and bring out the evidence of the girl’s virginity [i.e. the sheets from the marriage consummation] to the elders of the city at the gate. And the girl’s father shall say to the elders, ‘I gave my daughter to this man for a wife, but he turned against her; and behold, he has charged her with shameful deeds, saying, “I did not find your daughter a virgin.” But this is the evidence of my daughter’s virginity.’ And they shall spread the garment before the elders of the city. So the elders of that city shall take the man and chastise him, and they shall fine him a hundred shekels of silver and give it to the girl’s father, because he publicly defamed a virgin of Israel. And she shall remain his wife; he cannot divorce her all his days.
But if this charge is true, that the girl was not found a virgin, then they shall bring out the girl to the doorway of her father’s house, and the men of her city shall stone her to death because she has committed an act of folly in Israel, by playing the harlot in her father’s house; thus you shall purge the evil from among you.”

Clearly, the women who had hymens weak enough to break prior to marriage or perhaps flexible enough to remain intact during intercourse would often not survive long enough to reproduce and pass on their genes. Several world cultures, including the Yungar people of Australia, starved, tortured or even killed girls found to be lacking an intact hymen prior to marriage, according to a 1990 textbook cited by this article–and this was the 20th century! But in present-day cultures where it’s socially standard to be sexually active out of legal covenant (largely due to invention of reliable birth control), there is no sexual selective pressure on the preservation of the hymen. Women who don’t bleed on their wedding nights are not stoned to death, and thus are free to pass on their loser hymen genes to progeny. Additionally, if in fact it’s true that the hymen protects the womb from infection that eliminates reproductive viability, then only in untreatable cases will the hymen provide a selective advantage. This doesn’t necessarily mean that the hymen is being phased out–polymorphisms can occur that make it weaker or stronger, and neither will affect the female’s ability to survive and reproduce.

Even in cultures where virginity is still highly valued and impurity is punished, it doesn’t matter whether women would naturally have an intact hymen on her wedding night, because she can undergo hymenoplasty. In Marjane Satrapi’s (Iranian author of Persepolis) graphic novel Embroideries–highly recommend!-she mentions that sometimes Iranian women undergo this surgical procedure in which the remnants of a torn hymen are stretched and stitched to the vaginal orifice, in order to fake their virginity and avoid divorce or other consequences. Actually, the word “embroidery” is the euphemism for this procedure. It allows women to be as sexually active as men, while still having the selective advantage of a virgin. And thus, even in these countries, women can pass on whatever hymen genes they have to give.

Discussion Topic: We’re still evolving! What are some other human traits that you predict are changing or increasing in variability due to medical/technological breakthroughs?

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