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Comparative Primatology and the Evolution of Human Life History
Shannen Robson
Department of Anthropology
What is a primate?
Primate distribution worldwide
Humans expanded into non-tropical habitats
Are humans primates?
Or are we too distinct?
Behavioral similarities ….
Primate phylogeny
1. reflect shared ancestry
2. branch lengths reflect time
Eg., Trichromatic vision
Bichromatic: blues/greens
Trichromatic: blues/greens & reds
Ancestral state: Strepsirhini & Aotus mono- or bi-chromatic
Polymorphic:Platyrrhini (NWM) -- X-linked
Trichromatic:Howling monkeys & all catarrhines
Life histories … “.. lie at the heart of biology; no other field brings you closer to the underlying simplicities that unite and explain the diversity of living things and the complexities of their life cycles”
“LH theory evolution makes the simplifying claim that the phenotype consists of demographic traits connected by constraining relationships, trade-offs”.”
Stearns, 1992
Life history traits
• size at birth
• growth pattern
• age at maturity
• size at maturity
• number, size, &
sex ratio of offspring
• reproductive span
• length of life
Charnov’s Life History Invariant Model
Compare great ape life histories
slow slow short short equal slow
1. Longer lifespan & subadult period = larger body size
2. Age at last birth neutral
3. Reproductive rate faster than expected
1. Humans have the slowest life history
Are old people a recent novelty?
Oeppen & Vaupel (2002) Science 296:1029
Stability of the old/young ratio
Change life expectancy 3-fold &% of senior women in population varies little
N Howell Demography of the Dobe !Kung (Academic, 1979)
K Hill & AM Hurtado AcheLife History (Academic, 1996)
NG Blurton Jones et al AJHB 14, 184 (2002)
ee00 = 30 & 33 = 30 & 33..5% adults >455% adults >45 ee00 = 37 & 39% adults >45 = 37 & 39% adults >45 ee00 = 33 & 40 = 33 & 40..4% adults >454% adults >45
Survival of hunter-gatherers0
0.0
5
0.1
0.1
5
0.2
0.2
5
Fraction of population
0 - 4 5 - 9
10 - 1415 - 1920 - 2425 - 2930 - 3435 - 3940 - 4445 - 4950 - 5555 - 5960 - 6465 - 6970 - 7475 - 7980 - 84
!Kung C&D West 5
West 5 seniors (>45)
West 5 childbearing (15-45)
West 5 juveniles (<15)
0
0.0
5
0.1
0.1
5
0.2
0.2
5
Fraction of population
Ache
Ache seniors (>45)
Ache childbearing (15-45)
Ache juveniles (<15)
0
0.0
5
0.1
0.1
5
0.2
0.2
5
Fraction of population
ag
e c
lass
Hadza
Hadza seniors (>45)
Hadza childbearing (15-45)
Hadza juveniles (<15)
0%
5%
10
%
15
%
20
%
25
%
Percent of population
0 - 4 5 - 9
10 - 1415 - 1920 - 2425 - 2930 - 3435 - 3940 - 4445 - 4950 - 5555 - 5960 - 6465 - 6970 - 7475 - 7980 - 84
Chimpanzees (5 sites)
Hill et al. 2001
Chimp seniors (>45)
Chimp childbearing (10-45)
Chimp juveniles (<10)
0%
5%
10
%
15
%
20
%
25
%
Percent of population
Hadza
Blurton Jones et al. 2002
Hadza seniors (>45)
Hadza childbearing (15-45)
Hadza juveniles (<15)
ee00 = 33 & 40 = 33 & 40..4% adults >454% adults >45ee00 = 15 & 2 = 15 & 2..2% adults >452% adults >45
Compare chimp-human age-specific survival
1. Similar age decline in female fertility2. Longer adult lifespans, later first birth3. Much older population age structures
Age structure difference part of wider regularity:compare humans & chimps to macaques
Macaca fuscata (Arashiyama West) MSM Pavelka & LM Fedigan AJPA 109, 455 (1999)
0
0.1
0.2
0.3
0.4
0.5
0.6
0 - 4 5 - 9
10 - 1415 - 1920 - 24
25 - 2930 - 34
35 - 3940 - 4445 - 49
50 - 5555 - 5960 - 6465 - 6970 - 7475 - 79
80 - 84
Hadza seniors (>45)
Hadza childbearing (15-45)
Hadza juveniles (<15)0
0.1
0.2
0.3
0.4
0.5
0.6
0 - 4 5 - 9
10 - 14
15 - 1920 - 2425 - 29
30 - 3435 - 3940 - 44
45 - 4950 - 5555 - 59
60 - 6465 - 6970 - 74
75 - 7980 - 84
Chimp seniors (>45)
Chimp childbearing (10-45)
Chimp juveniles (<10)
0
0.1
0.2
0.3
0.4
0.5
0.6
0 - 4 5 - 9
10 - 14
15 - 1920 - 2425 - 29
30 - 3435 - 3940 - 44
45 - 4950 - 5555 - 59
60 - 6465 - 6970 - 74
75 - 7980 - 84
M. f. past reproductive age (>25)
M. f. reproductive age (5-25)
M. f. juveniles (<5)
2. Neutral age at last birth
Human reproductive pattern (Holman 1996)
0
0.1
0.2
0.3
0.4
0.5
Bir
ths/
Wo
ma
n/ Y
ea
r
10 t
o 1
4
15 t
o 1
9
20 t
o 2
4
25 t
o 2
9
30 t
o 3
4
35 t
o 3
9
40 t
o 4
4
45 t
o 4
9
Age
Human Age-specific Fertility
Hadza 1980s-2000
!Kung 1963-73
Ache Forest
Canadian 1700s
US 2001
0
0.1
0.2
0.3
0.4
Bir
ths/
Fe
ma
le/ Y
ea
r
6 t
o 1
0
11 t
o 15
16 t
o 2
0
21 t
o 2
5
26 t
o 3
0
31 t
o 3
5
36 t
o 4
0
41 t
o 4
5
46 t
o 5
0
Age
Chimpanzee Age-Specific Fertility
Tai
Bossou
Mahale
Chimp Average 3 Sites
0
0.05
0.1
0.15
0.2
0.25
0.3
Hu
nte
r-G
ath
erer
Av
erag
e 3
Po
pu
lati
on
s
0 t
o 5
6 to
10
11 t
o 15
16 t
o 2
0
21 t
o 2
5
26 t
o 3
0
31 t
o 3
5
36 t
o 4
0
41 t
o 4
5
46 t
o 5
0
Age
3 species fertilities.data
Chimpanzee Average 3 Sites
Hunter-Gatherer Average 3 Populations
Birt
hs/fe
mal
e/ye
ar
Compare chimp-human age-specific fertility
Similarity suggests conserved, ancestral pattern
How can a female primate increase quantity?
By increasing her reproductive rate By increasing her litter size
3. Fast reproductive rates
Both result in multiple dependent offspring at the same time that she must …
… nurse
… and wean
How can a mother manage?!
… carry
Usually she can’t
For most primates, stacking is unsuccessful
Increased quantity compromises quality
Callitrichids & humans increase quantity
because they have help
These primates are called ‘cooperative breeders’
Callitrichids shorten intervals by …
increasing litter size postpartum estrus
Humans shorten interbirth intervals by …
‘stacking’ sequential dependents
through early weaning
via nonmaternal
food-sharing
Who helps human moms feed kids?
Male hunting?
• Success is unpredictable
• Hunter not “owner”
• Wives & kids not favored
• Must eat everyday!
Gotta eat everyday!
Postmenopausal females - Grandmothers
Grandmother hypothesis
2. More use of resources difficult for juveniles
1. Chimp-like life history; PlioPleistocene ecology
3. More maternal provisioning
4. Females with declining fertility help
5. Vigorous perimenopausal females leave more
descendants6. Selection for slower aging, greater longevity
7. Longer adult lifespans; so later maturity, larger
size8. Expand into previously unoccupied habitats