Evolutionary Legacies February 9, 2005

Is Natural Selection Still Working? YES! How we have come to be the way we are How we are still being affected.

Is Natural Selection Still Working? Infectious Disease Chronic Diseases –Hypertension –Obesity/Heart Disease/Diabetes –Arthritis

Evolutionary Legacy Evolution of Human Sexuality

Some Intriguing Questions: Why do males have nipples? Of the 5 ape species, which has the largest penis? Of the 5 ape species, which has the largest breasts? Why do humans have sex in private when all other social animals do it in public?

Sources “Male Nipples and Clitoral Ripples” Stephen J. Gould “The Evolution of Human Sexuality” Jared Diamond

But Why?!?!?! Is a feature adaptive (functional), maladaptive, or neither? Example: Humans have big butts –Explanation: those are important muscles that hold us upright—FUNCTIONAL

Male Nipples Early theories: hermaphroditic pre- human ancestor

Male Nipples Developmental explanation: –Embryos are generalized before sexual differentiation –Males have nipples because they are functional in females –Evolutionary side effect, neither adaptive nor maladaptive

Clitoral Ripples Freud: female sexual stimulation begins in clitoris and moves to vagina WRONG! Clitoris and Penis develop from the same embryonic tissue

Other Confounding Factors Concealed estrus Concealed ovulation Linked to: –Social food gathering –Male role in child rearing

Why is human sexuality so bizarre? Social group (like wolves or lions) Pair bonding Enormous Penises Enormous Breasts How is any of this adaptive?

Testes More sex = Bigger Testes Larger Testes = Greater Chance of Pregnancy Cost/Benefit

Penis Adaptive to human sex? –No. Organ of Display –Why? –For Whom?

Phallocarp

Breasts Human female breasts are large and permanent Enlargement occurs before first pregnancy

Concealed Estrus, Sex Female primate genitals swell and color (red, pink, blue) to indicate estrus Humans: –Concealed ovulation –Constant sexual receptivity –Brief period of fertility during menstrual cycle

Why is human sexuality so bizarre? Ape sex: dangerous luxury –Burns valuable calories –Takes time that could be used gathering food Human sex for conception: –Waste of time –Waste of energy –Maladaptive

Why is human sexuality so bizarre? Human sexuality is adaptive for reasons other than conception

Theory 1 Concealed ovulation/copulation reduces competition and enhances cooperation among male hunters

Theory 2 Concealed ovulation/copulation enhance bond between man and woman by making woman continuously receptive

Theory 3 Male chimps are more likely to share food with females in estrus. Concealed estrus may ensure steady food supply

Theory 4 Concealed estrus exploits male paranoia over paternity, enhances pair bond

Human Paradox Men and women desire their children to survive and must cooperate in child rearing Must also cooperate with other pairs in economic activity Sexual relations Economic relations

Human Paradox Concealed ovulation/copulation reinforce the distinction between sexual partners and economic partners Sex as social cement rather than simple procreation

So, why do we have such large penises/breasts? Arbitrary sexual display symbols

Example 1: Saltshakers Curse African-Americans: –Higher blood pressure –Twice the risk of hypertension –Ten times more likely to die –By age 50, half of black men are hypertensive –Can cause heart disease, kidney failure or stroke

Example 1: Saltshakers Curse High blood pressure caused by –Genetics –Environment (intake of salt, fat, alcohol, high stress)

Salt Needed in body Excreted in urine White and Black Americans consume about the same amount of salt Black Americans consume less potassium and calcium –Experience more stress –Are less likely to have medical care

Example 1: Saltshakers Curse Black Americans retain salt longer than do White Americans Hyptertension is more likely to be “salt sensitive” in Black Americans Labeled as a “defect”

Selecting Agent: Slave Trade 70% of captured Black Africans died between capture and sale In between: long marches, heavy burdens, Middle Passage –Sweat –Vomit –Diarrhea (Cholera)

Selecting Agent: Slave Trade Slave trade selected for “superefficient” kidneys that retain salt Salt retention was adaptive until modern medicine Saltshaker’s “Curse”: proliferation of salt in our diet makes salt retention maladaptive.