1. Grandmothering
By Sarah Ford

2. What is the Grandmothering Hypothesis?
The theory to explain the existence of menopause in humans’ evolutionary history by identifying the value of extended kin networking.
Builds from the ”mother hypothesis” that the costs of reproducing become greater as the mother ages and energy should be further devoted to ensuring the survival of her existing offspring rather in their reproductive efforts.

3. Kin Selection
Serves as an adaptive strategy for related individuals to exhibit altruistic behaviors in order to ensure the survival of their own genetic material being passed on.
Hamilton’s rule: RB > C
Evidence emerged around 1.7 to 1.8 million years ago demonstrated in food sharing and female foraging practices due to climate changes.

4. Parental Investment
Trivers defines parental investment is any investment that a parent contributes to an offspring that will increase the offspring’s survival.
Females are the more investing sex due to 100% maternal certainty and tending to be the sex that does the majority of the childrearing.
Paternal uncertainty is much higher; therefore, they are the less investing sex.

5. The Grandmother Effect
Due to parental investment, females invest more into their children due to the inter birth interval (IBI). Which limits the number of children a woman can have. 
IBI - The period of time between births is longer for humans than some species- perhaps due to the extended childhood - which is unique to humans.. 
IBI - longer in larger species and shorter for smaller primates.
Grandmothers are able to help facilitate shorter IBIs for mothers by helping the mother with childrearing. Grandmothers are alloparents that can provide provision and protection for their grandchildren which helps the mother reproduce more children. This benefits the grandmother because her genetic material is more likely to be passed on. 
It has also been found that grandmothering can decrease senescence (Hawkes & et al., 2004).

6. Origins of Grandmothering and Menopause
Menopause is the cessation of ovulation which occurs in a female human’s life around 45 to 50 years of age.  It is estimated that 60% of a woman’s life is spent in her post- reproductive life span.
Extended PRLS is unique to humans in this sense, because typically female nonhuman primates do not live for an extended amount of time after they stop reproducing.
Evidence for menopause has been exhibited in rhesus macaques and Japanese macaques as well as other species such as guppies and whales.

7. Origins of Grandmothering
Evidence for grandmothering dates back to 1.9 million yrs ago with Homo Erectus. As we have learned H. Erectus were the first of our ancestors to begin hunting  for “big game” when they moved into colder drier climates.
We began eating meat helped us evolve larger brains by promoting more cooperation and dependence on our social groups. 
Larger brains have lead to slower maturation and increases in extended childhood.
Grandmothers were perhaps able to facilitate the evolution of human life span by providing for the children.

8. Caspari and Lee Tooth Study
Rachel Caspari and Sang-Hee Lee examined the teeth of Australopithecines, early Homos, Neanderthals, and Early Upper Paleolithic homos.
They determined how old the teeth were based on wear seriation and the eruption of molars.
Found that there was an increase in longevity amongst all the groups, which indicated survivorship amongst older adults in human evolution.
The greatest increase was with modern homos in Upper Paleolithic era, where there was a larger amount of older adults than young adults in the death distribution.

9. Table 1. Sample summary: Sample sizes and OY ratio for each group
Old
Young
Total OY
Australopithecines 37
316
353
0.12
Early Homos 42
166
208
0.25
Neanderthals 96
113
0.39
Early Upper Paleolithic 50 24 74
2.08
All groups (combined)
602
768
0.28

10. The Hazda
In the 1980’s, Kristen Hawkes and James O’Connell studied the Hazda, a traditional hunting-gathering tribe in Tanzania, Africa. They are known for their foraging with tubers.  
Hawkes discovered that mothers who were caring for their infants spent less time foraging and contributed less to the nutrition of their weaned children. They also noticed older women were spending their days collecting tubers and other food for their grandchildren.
Hawkes suggested that grandmothers increase their inclusive fitness by doing this decrease weaning ages of their daughter’s children thus shortening IBI.
Grandmothers increase the survival of their young weaned grandchildren by supplying them with food when their mother is busy taking care of a new infant. 

11. Longevity and Grandmothering
Kim, Coxworth, and Hawkes examined findings from three hunter-gatherer groups: Dobe! Kung, Ache, and Hazda.
Female reproductive output as a function of expected life span.

12. Conclusion
Grandmothering has made us uniquely human, because it allowed for ”a whole array of social capacities that are then the foundation for the evolution of other distinctly human traits, including pair bonding, bigger brains, learning new skills and our tendency for cooperation”
Grandmothering has developed from “big game hunting” which has allowed for our increase in brain size. Increase in brain size has led to slower developing infants and the need for alloparenting, social dependence, and pair bonding amongst groups.
Grandmothers allow for longevity by decreasing senescence and allowing for natural selection to allow for older adult genetic material to be passed on.
Increase in longevity has allowed for the assimilation of culture in human evolution over time. rM