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