Human Evolution

I. Evolutionary Ecology of Early Hominids

A. Context (circa 7.0 MYA) -- Socioecology of the great apes and the last common ancestor of chimpanzees, bonobos, and humans --

1. Female dispersal -- characterizes all African great apes (but not orangutans)

2. Male retention (philopatry) and kin based coalitions of males (chimpanzees, bonobos, and humans)

B. Drying out of sub-Saharan Africa (circa 4 to 3.5 MYA)

1. Resulted in more open, less wooded habitats -- more patchy resource distribution

2. Increased home range size and day range length requirements created selection pressure for more efficient locomotion

3. BIPEDALITY is an efficient form of distance locomotion -- compare chimpanzee quadrupedalism and human bipedality

a. Also frees hands for other activity

b. Possible thermoregulatory advantage

c. Major COST of bipedality is loss of adaptations for arboreality which may restrict diet

C. Plio-Pleistocene hominid radiation (circa 2.0 to 1.5 MYA) -- Several hominid species living at same time and coexisting in some habitats -- Divergent specializations

1. Robust australopithecine lineage -- shifted to lower quality diet which => increase in group size, decrease in home range size and day range length; with females more localized in space, monopolization of females (female defense) by males within group could => harem social organization within a female-dispersal, male-philopatric species

2. Early Homo lineage -- more reliance on animal resources, made possible by tool use; resources more patchily distributed so increase in home range size and day range length; male kin coalitions still important (as in LCA) in jointly defending territory used by multiple females; coalitions perhaps important for joint hunting

a. INCREASING TOOL USE and INCREASING BRAIN SIZE characterize Homo lineage -- Probably acted synergistically

b. Increased brain size may have permitted more coordinated hunting and communication or may have been associated with tracking resource conditions or may have allowed better storage and processing of spatial knowledge

c. Greater encepahlization is costly and may have demanded more male investment in kids leading to formation of particular male-female bonds within the context of female-dispersal, male-philopatric groups

II. Applying Evolutionary Theory to Modern Humans

A. Humans are "just another unique species" -- There is no reason to imagine that human behaviors and psychological process have not been shaped by natural selection in exactly the same way as seen in other species

B. Adaptive explanations of human sexual behavior

1. Evolutionary theory predicts that females should be CHOOSIER about potential mates than are males -- Who should females choose? Perhaps males that can provide RESOURCES that improve chance of offspring survival. If this is true, then males with greater resources, and females who marry these men, should have greater reproductive success (more kids)

2. Men should be less discriminating about their potential mates

3. Given the option, men should should mates with the greatest reproductive value

Among Kipsigis, men pay a "brideprice" to the family of their brides. Higher brideprices are paid for women who reach menarche earlier (and thus have a longer reproductive lifespan ahead of them). Women are generally circumcised around menarche.

4. Predictions extend to stated preferences in modern human populations. Buss (1989) took a survey of 37 different cultures from 33 countries around the world and asked men and women to rank the importance of a variety of characteristics about potential mates on a 4-point scale from "indispensible" to "irrelevant"

a. In 36 of 37, females valued "good financial prospects" as a characteristic of potential males significantly more than did males

b. In 29 of 37, females valued "ambition/industriousness" significantly more than did males

c. In 34 of 37, males valued "good looks" significantly more than did females

d. In 23 of 37, males valued "chastity" significantly more than did females

e. In all 37, males preferred younger partners and females preferred older partners

C. Adaptive explanations of human parenting behavior

1. Given that human kids are costly to raise, evolutionary theory predicts that parents should only invest in kids that are their own as a defense against cuckoldry -- Applies particularly to males who are (by virtue of internal fertilization) less certain of paternity than females are of maternity

a. In some sexually permissive societies in Oceania, males invest in sister's kids rather than wife's kids

b. Female adultery more commonly leads to divorce than does male adultery

2. A corrolary of this hypothesis is that abuse is typically directed against nonkin

a. Stepfathers have more conflicts with stepchildren than do genetic fathers with their genetic children

b. Stepchildren are much more likely than natural children to be abused

D. A final IMPORTANT point -- Just because humans behave adaptively, in accord with the predictions of evolutionary theory, in many circumstances does not mean that these behaviors are directly coded for by genes -- i.e. there are no specific GENES for child abuse or for sexual promiscuity, for example. Moreover, remember that just because a behavior is adaptive (i.e. leading to increased fitness) does not mean it is "good" or "moral" or "right"