Transcript Evolutionary Legacies

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 prehuman 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.