Most common beliefs about human origins
Gallup poll data, 1982-2012
Consistent:
- God created humans in present form
- Humans evolved guided by God
- Humans evolved & God had no part in it
Competing theories on origin of species
Special creation (creationism)
- unchanging species
- independent creation
Descent with modification (Darwinian evolution)
- speciation changes with time
- species share common ancestors
Law of succession
Darwin noted trait similarities between fossils extinct species and modern animals
Fossil species adapted to environment –> inheritance of adaptive traits in modern species
Fossil succession
Specific fossils are found in specific layers of rock, no matter the location bc they lived at the same time
Early fossils (deeper layers of rock) are less complex
Shows that animals have changed over time
Transitional forms
Fossils that exhibit traits common to ancestoral & modern groups
Show transition between species
e.g. evolution of grey whale from Pakicetus
Pakicetus had nostrils at the front of its skull (50mil yrs)
Aetiocetus had nostrils at the middle of its skull (25mil yrs)
Gray whale has nostrils at the top of its skull (today)
Vestigial structures
Structures in th body that no longer have a purpose
e.g.
Whale pelvis & femur
Human tail bone
Wisdom teeth
Muscles once used to move our ears
Structural homology
Similarity due to a common ancestor
Similary structures with different functions
e.g.
- Whales’ fins
- humans’ arms
- bats’ wings
- cats’ legs
Structural analogy
Convergent evolution - different structures with same function
No common ancestor
e.g. wings of
- bats
- birds
- insects
e.g.
- sharks’ fins
- penguins’ wings
- dolphins’ flippers
Developmental homology
Embryos from different species (snake, chicken, possum, cat, bat, human) show similarities in early development (Richardson et al., 1998)
Molecular homology
Similarity of DNA shows how closely related species’ are
e.g. - chimps are humans’ closest relative, split off 4-6 My rs ago
Gorillas 6-8 M yrs ago
Orangutans 12-16 M yrs ago
Assumptions of evolutionary theory
examples
There is variation in individuals
Variation is heritable
Variation produces differential reproductive success - traits suited for success will dominate
Periwinkle
- introduction of crabs = change shell shape (1871-1982)
- more crabs = more individuals with new, rounder shell
- old shell more easily broken by crabs –> eaten
Darwin’s finches
- diet & habitat affect shape
- weather affects size (drought = bigger beak –> use to get to more difficult food)
What is a species?
related individuals that resemble one another, are able to breed among themselves, but are not able to breed with members of another species.
Speciation of chimps and bonobos
- congo split habitats
- chimps more aggressive, bonobos more sexual
- selection & mutation in new groups
- can’t reproduce together = different species
Human speciation
Primates
- social
- big brains
- grasping hands
- colour vision
Bipedalism (3.6mya)
- frees hands, allows more energy-efficient movement, better heat regulation :)
- Rosenberg & Trevathan (2002) narrow birth canal compared to baby head size :( BUT selection pressure on social bonds - help during childbirth bc child born facing away from mother = greater survival change for self and child
Diet
- plants - sagital cret on top of head for muscles used in chewing
- plants and animals - protein used for brain growth
- cooking - cooked food easier to digest –> efficient eaters –> divert energy to brain growth
- human brain 2% of body weight but uses similar energy to muscles which are 40%
Mirror neurons & human behaviour
Ramachandran
- human brain reached current size & perheps intellectual capacity ~250,000 yrs ago BUT ‘uniquely human’ attributes only appeared much later??
- “great leap” → explosion in tech sophistication, cave art, clothing ~40,000 yrs ago… why??
- Acquisition of language??
- emotional calls & musical sounds during courtship → language evolves
- Chomksy suggests LAD that emerged out of the blue
- mirror neurons in ventral pre-motor area of monkeys (Rizzolatti) - fire when monkey performs action & when it observes another performing action e.g. tasting a peanut
- empathy, imitation learning… evolution of language!
- allows development of “theory of other minds”
- innovations (e.g. maths, art, tool use, language) spread quickly bc env. change allows it to be ‘accidentally’ invented in one place by “pre-adapted” areas of the brain & mirror neurons spread it through the population
Criticisms of mirror neurons & human behaviour
- can’t be just mirror neurons - all rhesus monkeys and apes have them but don’t have human’s cultural sophistication
- argues that minimal culture development can exert selection pressures for additional mental traits
- “big bang” of human innovation happened over short period - why would chance discoveries all happen at the same time?
- inventions spread btwn 100k & 5k yrs ago - “great leap” may be sampling artefact of archaeological excavation
- innovations serve as catalysts for others
- genetic change? increase in sophistication of mirror neuron system
