9. Early Hominins

• Paleoanthropologists - Researchers that study human evolution

• Encephalization - The evolution of a big brain

• Bipedalism - The evolution of the way in which we move about on two legs

• Indications of dietary change - The evolution of our flat faces and small teeth

• Bipedal locomotion - One of the first things that evolved in our lineage

• Specimens have been found all along the East African Rift System (EARS) - Ethiopia, Kenya, Tanzania, and Malawi

• Site - Place in which evidence of past societies/species/activities may be observed through archaeological or paleontological practice

• In the past - taxonomy was primarily based on morphology

• Today - taxonomy is based on relationships of molecular phylogeny

• Cladistics - The field of grouping organisms into those with shared ancestry

• Phylogenetics - The study of phylogeny

• Cladistics groups - according to their last common ancestors based on shared derived traits

• Clade - A grouping based on ancestral relationships - branch of the evolutionary tree

• “Lumpers” - scientists argue that large variability is expected among multiple populations - lump specimens of subtle differences into single taxa

• “Splitters” - scientists argue that species variability can be measured - even subtle differences are extreme enough to mirror modern species differences

• Polytypic - Species that are capable of interacting and breeding biologically but having morphological population differences

• Chronospecies - “lineages” of species to determine when one species evolved into another over time

• Fauna - The animals of a particular region, habitat, or geological period

• Faunal assemblages - Collections of fossils of the animals found at a site

• Paleoenvironment - An environment from a period in the Earth’s geological past

• Analyzing pollen grains - shows which kinds of flora survived in an environment at a specific time period

• The environment has been interpreted as the following:

  • The driving force behind the evolution of bipedalism

  • The reason for change and variation in early hominin diets

  • The diversification of multiple early hominin species

• Aridification - Becoming increasingly arid or dry, as related to the climate or environment

• Ungulates - Hoofed mammals of various kinds

• Specialist eaters - Those who rely primarily on specific food types

• Generalist - Those who can eat more varied and variable diets

• High faunal turnover - Extinction of many species and the speciation, diversification, and migration of many others to occupy various niches

Savannah Hypothesis (or Aridity Hypothesis)

• Suggests that the expansion of the savannah (or less densely forested, drier environments) forced early hominins from an arboreal lifestyle to a terrestrial one where bipedalism was a more efficient form of locomotion

• Early bipedal hominins are often associated with wetter, more closed environments - both marine and terrestrial records seem to support general cooling, drying conditions

• Two important factors - increasing aridity

  • The first factor is the diversification of taxa - high morphological variation between specimens

  • The second factor is the observation that the earliest hominin fossils appear to have traits associated with bipedalism - dating to around the drying period

Turnover Pulse Hypothesis

• 1985 - paleontologist Elisabeth Vbra noticed that in periods of extreme and rapid climate change - ungulates that had generalized diets fared better than those with specialized diets - periods with extreme climate change would be associated with high faunal turnover

• Quaternary Ice Age (2.5 mya - 3 mya) brought extreme global, cyclical interglacial and glacial periods - faunal turnover occured - extreme changes in climate play a role in extinction and migration in ungulates and hominins

Variability Selection Hypothesis

• First articulated by paleoanthropologist Richard Potts It links the high amount of climatic variability - over last 7 million years to both behavioral and morphological changes

Origin of Bipedalism - Two theories

• Early hominins initially lived in trees - then started living on the ground, so we were a product of an arboreal last common ancestor (LCA) - most research supports this theory

• Our LCA was a terrestrial quadrupedal knuckle-walking species - more similar to extant chimpanzees

Selective pressures for bipedalism

• Energy conservation - bipedal humans conserve more energy - allows us to travel longer distances and expending less energy

• Thermoregulation - Less surface area would be exposed to sunlight - body needs to employ less cooling methods

• Bipedalism - This method of locomotion freed our ancestor’s hands so they’d be able to easily gather food and carry infants

Skeletal Adaptations for Bipedalism

• The majority of these adaptations occur in the postcranium - allow for greater stability and strength in lower limbs - allowing more shock absorption

• Adaptation cost - weaker joints (hips)

• S-shaped curve of our spine instead of C-curve allows us to stand up straight

• Teeth - reduced in size and our faces are flatter (orthognathic)

• Most hominins have - relatively large, flat, vertically aligned incisors that occlude (touch) relatively well, forming a “bite” - thick enamel and low cusps

• In humans - canines are often a similar size to our incisors and therefore considered incisiform

• Hominins dental arch - a parabolic (V-shaped) arch