AEC 360  Ecology

Lecture 28: Human evolution

I.  Human divergence from other great apes

    A.  Important differences between humans and our closest relateive, chimps
        - Fully upright posture
        - Hairless, smaller incisors, fully opposable thumbs
        - Brain size, which is needed for language, recognition of complex cause and effect
        - Development of complex culture

II.  Human evolution
    A.  Key events
        1.  Shrewlike ancestors climbed shrubs and trees ~80 Mya
        2.  Arboreal ancestors endowed humans with binocular vision and adept hands
        3.  As African climate dried, ancestors descended from trees 5-10 Mya

    B.  Early fossil discoveries linked chimps to humans through the evolution of upright posture, increased brain size
        1.  There are costs and benefits (tradeoffs) to bipedalism
             --  Bipedalism has costs: unstable, poor physiological architecture
             --  And benefits:  Ability to carry things while walking, development of hands, others

    C.  The hominin line is characterized by the evolution and extinction of many species

III.  The geographic origin of Homo sapiens

    A.  Prior to 2 Mya, all of our ancestors lived in Africa
        -- As well as H. sapiens, H. neanderthalis, H. erectus have been found out of Africa

    B. Competing models of location and pattern of human evolution, which make different assumptions about migration, hybridization, and gene flow
        1.  African replacement hypothesizes that H. sapiens left Africa and replaced others

        2.  Multiregional assumes there was independent evolution and gene flow among regions

    C.  Relatively recently, H. sapiens co-occurred with H. neanderthalensis
        1.  H. sapiens and H. neanderthalensis separated about 600,000 years ago
        2.  H. neanderthalensis was mainly European, and died out 30,000 years ago
        3.  H. neanderthalensis was probably better adapted physically to norther climates, but had poorer technologies, and was outcompeted by H. sapiens

    D.  Evidence generally supports the African replacement hypothesis
        1.  All present-day people are descended from African ancestors
        2.  Hypothesis supported by small amount of genetic variation among humans
        3.  These ancestors left Africa in the last hundred thousand years or so
        4.  May be some intermediate model, with African descent dominating, but occasional gene flow

    E. But a better model might be "leaky replacement" 

IV.  Many uniquely human characteristics are associated with the evolution of larger brain sizes
    A.  Use of stone tools
        1.  Humans have three muscles that chimps lack
        2.  Australopitecines lack some human thumb characteristics
        3.  Even H. neandertalensis were more poorly adapted to tool use

        4.  There is still debate about whether Australopithecines developed stone tools. Probably limited to Homo

        5.  2015: discovery of 3.3 million year old stone tools.  Nearly certainly Australopithecines.


    B.  The development of language

        1.  Again, associated with brain size
        2.  Other physical changes in speech apparatus
        3.  Complex speech arose at least 30,000 yrs ago, but less than 60,000 yrs ago

V.  The Great Leap Forward Cultural evolution and human behavior

    A.  Language and tools were precursors to massive cultural changes, like advent of agriculture

    B.  Agriculture allowed people to concentrate, necessitating the formation of complex societies

    C.  The human conundrum
        1.  Evolutionary hangovers
        2.  Humans evolved as hunter-gatherers at low population densities
        3.  Rapid human cultural evolution has led to massive population sizes and complex societies
        4.  How do we reconcile relatively slow genetic evolution with rapid cultural evolution?