ANP 200 did they evolve in the savanna or the forest

savanna hypothesis

long-held idea (Darwin?)

• morphological change (bipedalism) as adaptation to savanna habitat

• cultural change caused by expanding grasslands

climate drying and cooling, savanna expanded

→ bipedalism evolved in open habitat - 2 scenarios

• ancestors of hominini were the losers who were forced by the ancestors of panini out into savanna - bias

• ancestors of hominini were the opportunist; actively left forest to explore new and promising environments

forest model

rather new idea

• morphological change (bipedalism) evolved before expanding grasslands

climate drying and cooling, savanna expanded

→bipedalism preadaptation; evolved in forest - 1 scenario

Hominini evolved in forest; actively left later

• again, viewing human ancestors as opportunists /investors

Reconstructing paleo-environments

• adaptive traits (fossils) and fossil assemblages

• plant remains (plant parts or pollen)

  • plants - from wet or dry habitats

  • % wind distributed

• sediments and soil

defining environments

forest - continuous tree stand, interlocked crowns

woodland - open tree stand; >40% (80%) woody cover

wooded grassland - grasses and herbs; woody cover 10-40%

grassland - grasses and herbs; woody cover <10%

Reconstructing environments vis soils

measure current habitats

• amount of tree cover

• variation of carbon isotope ratios in soils

• match trees cover carbon isotope ratio

compare to

• carbon isotope ratio in paleosols

photosynthesis

• different types (C3, C4/CAM)

• reflected in ratio carbon 12 / carbon 13

carbon photosynthesis

• more carbon - 12 in soil (negative &¹³C

  • more C3 plants (=”forest”)

• more carbon - 13 in soils (close to zero &¹³C)

  • more C4 plants of C4 animals (grassland)

step 1 at ca. 8 ma BP

tectonic event separating LCA Panini - Hominini

• Western part rift valley

  • humid, forest, ancestor panini

• eastern part rift valley

  • dry, open savanna, ancestor hominini

• early hominini through allopatric speciation

• bipedalism evolved as adaptation to open habitat

step 2 at ca. 3-2.5 ma BP

• cooling and drying of climate

• habitat becomes more open (more grass, less trees)

reconstruction for one particular area in East Africa(Omo river)

• oldest (4 ma BP, Mursi formation)

  • animals w/forest adaptions

  • many tree pollen, few grass pollen

• youngest (1 ma BP, Shungura formation

  • many animals w/ savanna adaptations

  • few tree pollen, many grass pollen

step 1 of critique

unconvincing

• tectonic event only (but is this convincing?)

• no climate data

• habitat is unclear (4ma BP was likely woodland)

step 2 of critique

• fine (though habitat is not habitat use)

Australopithecines

previous habitat reconstructions

• mixed habitats

  • woodland, wooded grassland, savanna

  • validity debated

characteristic early hominini

• perhaps bipedal; climbing ability

• small brain

• reduced canines

• body size dimpophism

  →status as hominins debated

example - ardipithecus spp.

• “ground ape”

• East Africa

• ca. 4.4 ma BP

habitat of Ardipithecus

woodland

• 150,000

coexisted with animals which prefer woodland

→ similar reconstruction than for australopithecines (which were perhaps less arboreal than Ardi)

in which habitat did hominins evolve

hominin environment

carbon isotope ratios in soil

• ca. 2 ma BP onward

  • more open again

• 3-5 ma BP

  • more trees / less open

• 7-6 ma BP

  • open habitat rather common

climate during human evolution

at ca. 7 ma BP no strong climate signals

at ca. 3-2.5 m BP more variable

• grassland is expanding, more grassland species

caveat

or be VERY careful

• environment is NOT habitat use

if multiple habitats are available

• what is used often and why

open woodland (light green) - macaques only

forest (dark green) - both leaf monkeys and macaques

Reconstructing diet via tooth enamel

measure

• carbon isotope ratios in tooth enamel

• more carbon - 12 in soil (negative &¹³C

  • more C3 plants (=”forest”)

• more carbon - 13 in soils (close to zero &¹³C)

  • more C4 plants of C4 animals (grassland)

problem - meat from animals - same signature as plants they ate

fish - more carbon-13 (similar to C4 plants/animals

early hominini, australopith and early homo diets

carbon isotope ratios from tooth enamel

• early hominini & some australopith

  • mostly C3 foods

• some australopith and early homo

  • mix of C3 and C4

• 1 paranthropus & 1 australopith

  • mostly C4

overview hominin diet

7 - 4 ma BP (early hominini)

• mostly C3 (fruits, leaves; meat unclear)

4-2.5 ma BP (most australopithecines)

• mix of C3 (fruits, leaves) and some C4 food(grassland plants); little meat

2.5 - 1.7 ma BP (Early homo)

• mix of C3 (fruits, leaves) and some more C4 food (some meat and grassland plants)

overall conclusion early hominini

habitat: mixed; little forest/ woodland, rather open

habitat use: stayed (fed) mostly in forest/ woodland

contrasts with savanna hypothesis

in which habitat did hominins evolve

perhaps forest

general categories of suggested explanations

1. bipedalism results from pre-adaptation*

   • function of a characteristic changes during evolution

     → existing behavior facilitates bipedalism

2. bipedalism is an adaptation

   • a characteristic that provides a selective advantage (increases fitness)

     → bipedalism provides a fitness benefit

     • fitness(survival, mate finding, reproduction) can also be indirect benefit: energy

also called exaptation

overall conclusion (australopithecines)

habitat: mixed; little forest/ woodland, rather closed

habitat use: stayed (fed) often in forest/ ventured into grassland

overall conclusion (early homo)

habitat: mixed; little forest/ woodland, rather open

habitat use: used both forest woodland and grassland

learning objective or what you should know