Evolution Across the Human Life Course Flashcards
(53 cards)
1
Q
Life history
A
- predicts how natural selection shapes an organism’s ability to convert energy into making babies
- energy trade-off between somatic maintenance, growth and reproduction
2
Q
Extrinsic mortality
A
- high EM = more energy into reproduction
- earlier age at weaning, earlier age at sexual maturation, shorter pregnancies, small litters and small offspring
3
Q
Primate slow life history
A
grade shift to another life history course with longer lifespan and reduced fertility
why?
- brain size?
- slow growth? beneficial if resources are low
- unpredictable juvenile survival?
- unpredictable environment? if have longer lifespan, can wait till better conditions to reproduce
4
Q
Human life history
A
- slightly longer gestation though within variation across great apes
- significantly larger babies
- significantly larger newborn brain size
5
Q
Human - slow life history traits
A
- long gestation
- large babies
- reduced mortality rates so don’t need to rush fertility
- long lifespan
- long period of parental investment
6
Q
Human - fast life history traits
A
- earlier weaning = allows birthstacking and helping at the nest
- shorter interbirth intervals = increased fertility
- high fertility = increased incidence of twins and greater interest in sexual behaviours (long-term partnerships)
grade-shift in primates and then again in great apes to reduce offspring but humans are in the primate range
7
Q
Variation in human life history courses
A
- more deprived neighbourhoods = mothers younger at 1st birth, reduced birthweights, earlier weaning and grandmothering less likely
- switch to fast life history course due to increased extrinsic mortality
- reduced paternal investment as seeks other partners to increase reproductive success
- reduced parental investment negatively impacts child development e.g. literacy tests
8
Q
Primate brain size
A
- large variation
- grade shift from primates to great apes
- most variation is allometric = due to variation in body size
- larger brain size represents a functional difference
- EC = observed/predicted brain size
- EC>1 = encephalised, human EC ~7.5
- not all brains of the same size are the same e.g. different proportions of neocortex
9
Q
Theories of primate brain size
A
- social brain hypothesis
- visual specialisation hypothesis
- social innovation hypothesis
10
Q
Social brain hypothesis of primate brain size
A
- cognitive demands of living in complex social groups selects for increased neocortex
- doesn’t deny existence of ecological issues but suggests that these are solved socially
- social interactions determine foraging patterns
- predation solved in groups
- neocortex limits number of relationships that can be formed = determines group size
11
Q
Visual specialisation hypothesis of primate brain size
A
- transition to eating fruit
- visual areas larger in frugivorous species
- larger visual areas for processes such as colour acuity and fine detail
- allows perception of fruit
12
Q
Social innovation hypothesis of human brain size
A
- brain size and intelligence linked
- behavioural innovation, social learning and tool use correlate with relative size of the neocortex
13
Q
Why are Hominins further encephalised?
A
same selection pressures as on primates but intensified?
- greater focus on cognitive-demanding foods
- food sharing requires social cognition
novel selection pressures?
- dietary niche with technology
release of constraints on brain size?
- shift to food which is difficult to acquire but energy-rich
- cooking and expensive tissue hypothesis
14
Q
Expensive tissue hypothesis
A
gut versus brain energy trade off
high quality diet = faster meat digestion = shorter gut = quicker energy assimilation = increased brain size
AND higher quality diet = more complex feeding strategy = increased brain size
- meat made us human hypothesis
- evidence of cooking (flint tools, bones and controlled fire use)
15
Q
ETH - cats
A
- gut increased in length to consume starchy foods from humans
- altered microbiota alters neuronal connectivity in the brain
- reduced energy consumption of the brain
- reduced brain, increased gut
- opposite of encephalisation
16
Q
Trade-offs other than the ETH
A
- mammals trade fat storage for brain
- maybe we have both due to efficient bipedalism
- reproductive function = high level athletes pause menstrual cycle
- reduced inflammation (bad if extreme)
- reduced sperm count
17
Q
Why are humans fat?
A
- starvation = reduction in adipose to save other organs like brain
- surviving famine and food shortages = cycles of feast and famine
- surviving infection
- funding reproduction
- thermal insulation
- funding growth (brain)
- sexual selection
18
Q
Determinants of height
A
- height more variable in West where environmental conditions less likely to limit
- greater stature associated with increased income, IQ, reproductive success, reduced CV disease, cancer etc
- associations do not equal causation
- associations with mate preference, life history strategy, subsistence strategy and climate?
19
Q
Determinants of size
A
- adaptation = long term selection on size e.g body size, brain size, lifespan etc
- plasticity to environmental experience e.g. nutrition, disease load, activity and stress
20
Q
Why are the Dutch so tall?
A
- hidden genetic variants? may have been masked by poor environmental conditions but now improved
- natural selection?
- sexual selection?
- milk consumption? Dutch drink a lot of milk which affects IGF1 levels
21
Q
Gestation
A
- primates have longer gestation, highly altricial offspring, single litters, reduced daily energy cost of fetal growth
- human babies much larger relative to adult body size
- deep placentation required to provide nutrients for e.g. larger brain
22
Q
Energy costs of gestation
A
late gestation, BMR is constrained
capital gains:
- increased tissue mass
- fetus and associated tissues
- maternal tissue hyperplasia
- maternal fat gain
running costs
- increased BMR
- increased cost of activity due to increased body mass
23
Q
Obstetric dilemma
A
- obstructed labour is major cause of maternal mortality
- bipedalism selects for narrow pelvis as increased hip width = increased cost of locomotion
- costs of locomotion similar between sexes, suggests selection has been successful
- increased pelvic width = increased pelvic floor disorders (could be fatal in past and low income countries)
- inflexible pubic symphyses as flexibility would increase PFDs further (only flexible in small mammals with reduced PFDs)
- large brain and small birth canal = tight fetopelvic fit
- secondary altriciality as solution to the dilemma = head size determines timing of parturition
24
Q
Disputes of the obstetric dilemma
A
- some debate about whether bipedalism actually selects for a narrow pelvis
- timing of parturition due to overwhelming maternal BMR (energetics of gestation and growth) not fetal head size
25
Culture influencing fetopelvic fit
- switch to supine birthing position during transition to medical setting
- use of forceps = trained on general pelvic shapes = pelvic shapes differ in and between populations = increased mortality in non-white women
- Western culture = corsets and poor nutrition
- improving nutrition = increased fetal growth = maternal dimensions lag behind a generation = tighter fetopelvic fit
26
Infant phase
- breastfeeding
- ends with weaning = transition to solid foods
- weaning earlier in humans and even earlier in Western cultures
- other primates are self-sufficient at weaning
27
Infant growth
- rapid growth velocity
- deciduous teeth come in by age 3
- bipedal walking begins at approx 15 months
- learning motor skills, language and social relationships
- theory of mind
- growth controlled by insulin and IGLF-1 (nutritional control)
28
Energetics of lactation
- more demanding per day than pregnancy
- and longer duration than gestation
- fat gain in pregnancy to fund lactation?
- hip fat easy to mobilise for this purpose
- lactation costs also met through increased energy intake and reduced physical activity compared to pre-pregnancy levels
29
Breastmilk
primate breastmilk
- reduced energy content = due to slower, extended period of growth
- increased carbohydrate but reduced protein and fat = demands of the brain
30
Altricial
large litter
short gestation
closed eyes and ears at birth
naked skin at birth
cannot walk at birth
rich milk
31
Human secondary altriciality
humans at birth
- skeletally less mature
- brain growth not completed
- lack of motor development
4th trimester/extrauterine gestation period of infancy
32
Childhood - why?
- longer period of growth
- allows learning of complex skills
- increased total maternal fertility as weaning resumes fertility = birth stacking
- period of low nutritional requirement and low mortality so don't threaten adult food resources
- allows brain and body to develop in concert
- helping at th nest
33
Childhood growth
- moderate and stable
- age 3-7
- brain growth nearly completed by end of childhood
- adrenarche = activation of adrenal glands, low levels of sex hormone release
- ends with eruption of first permanent molar
- IGF-1 hormonal control
34
Juvenile period
- weaned and self-sufficient in primates
- humans do not gain nutritional independence as other primates do
- ends with puberty
35
Puberty and hormones
- neuroendocrine event
- pulse of sex hormones
- kickstarts somatic changes associated with secondary sex characteristics
- change in activity at hypothalamus
- reactivation of GnRH pulse generator
36
Adolescence and puberty
- menarche = first menstrual period
- spermarche = first mobile spermatozoa present in the urine
- epiphyseal fusion of long bones
- permanent teeth eruption complete
- growth spurt in both sexes (usually not at all or only males in other species)
37
Behavioural changes in adolescence
- improvements in physical and cognitive levels of work capacity
- increased interest in social, economic and sexual behaviours
- organisation of brain = increased risk-taking behaviour and changes in language
- post-fertile physically developed but still socially dependent
38
Timing of menarche
- secular trend of decreasing age at menarche
- body mass might be cue for menarche
- improving living conditions = reduced age at menarche
- better nutrition = increased fat triggers menarche
- trade-offs = can have impacts in later life!
39
Senescence
- progressive loss of function, accompanied by a decrease in fertility and increase in mortality
- theories of senescence: mutation accumulation, antagonistic pleiotropy and disposable soma theory
40
Mutation accumulation theory of senescence
- selection decreases with age
- selection shadow once post-fertility
- so mutation stays in population if affects post-reproductive state
- may be cause of death
41
Antagonistic pleiotropy theory of senescence
- pleiotropy = when a gene has 2 or more separate effects
- antagonistic pleiotropy when these effects work against one another or where one is beneficial and the other is harmful
- if positive early in life, will be selected for
- but may be negative later in life, post-fertility so cannot be selected against
42
Disposable soma theory of senescence
- selective pressure to invest in somatic repair is limited
- organism invests in good condition only for as long as would live in wild
- investment in repair always less than needed for indefinite survival
- genes-eye-view = selection acts on genes which survive, not individuals which survive
43
Hypotheses of post-reproductive longevity
adaptive theories:
- mother hypothesis
- grandmother hypothesis
- reproductive conflict model
non-adaptive theories:
- physiological trade-off hypothesis
- selection shadow by-product of recently increased longevity
- embodied capital
44
The mother hypothesis of post-reproductive longevity
- menopause to avoid higher reproduction-mediated mortality in later life
- increased survival and outcomes of existing children
- cost of reproduction on existing children
- but only when young
45
The grandmother hypothesis of post-reproductive longevity
- kin selection model
- post-reproductive women increase fertility of their daughters
- only works due to genetic relatedness
- allows daughters to breed earlier, more frequently and more successfully
- shared energy pool
- reduced when daughter is post-reproductive = correlates with increased mortality of grandmother
46
Critique of the grandmother hypothesis
- is it just correlation?
- maybe its just because the same factors which increase survival of grandmother also increase fertility of daughter
- e.g. wealth, health, genetics
47
Physiological trade-off hypothesis of post-reproductive longevity
- extension of disposable soma hypothesis
- selection for front-loaded fertility
48
Selection shadow theory of post-reproductive longevity
- selection can't act on post-reproductive state
- doesn't explain why reproductive senescence is before that of other systems
49
Embodied capital model of post-reproductive longevity
- invest in large brain, slow growth and longer learning
- live longer to produce food etc.
- food sharing supports those still reproducing
50
Food sharing
easy diet = no food sharing
difficult diet = sharing with offspring
sharing with adults
between or within sex food sharing (mating or coalition)
risky foraging niche = interdependence = widespread food sharing
51
Risk-reduction reciprocity
- individual hunter's expected return rate (mean) is same alone or with food sharing
- but variance is significantly reduced
- and reduced chance of having nothing to eat
- but vulnerable to free-loaders
52
Paternal care
- hormonal shift in men from mating effort to parenting effort
- high testosterone = mating effort
- low testosterone = parental effort
53
Intergenerational transfers
- selective shadow hypothesis for evolution of senescence assumes strength of selection decreases with reproductive value
- in humans, productive values matter = ability to provision kin
- generalisation of the grandmother hypothesis
