Hominin Evolution Hypotheses

Savannah Hypothesis (or Aridity Hypothesis)

  • Proposed by Darwin (1871) and supported by anthropologists like Raymond Dart (1925).
  • Suggests that the expansion of the savannah forced early hominins from an arboreal lifestyle to a terrestrial one where bipedalism was more efficient.
  • Later refined as the Aridity Hypothesis.
  • States that the long-term aridification and expansion of savannah biomes drove diversification in early hominin evolution.
  • Advocates for periods of accelerated aridification leading to early hominin speciation events.
  • Evidence:
    • Early bipedal hominins are often associated with wetter, more closed environments.
    • Marine and terrestrial records support general cooling, drying conditions.
    • Isotopic records indicate an increase in grasslands between 8 mya and 6 mya across Africa (Cerling et al. 2011).
    • Aeolian dust records show increases in seasonal rainfall between 3 mya and 2.6 mya, 1.8 mya and 1.6 mya, and 1.2 mya and 0.8 mya (deMenocal 2004; deMenocal and Bloemendal 1995).
  • Interpretation(s):
    • Two important factors occur around the time of increasing aridity:
      • Diversification of taxa, leading to the naming of multiple hominin genera and species.
      • Earliest hominin fossils appear to have traits associated with bipedalism and are dated to around the drying period.
    • Some argue for a combination of bipedalism and arboreal locomotion.
    • Local environments of early specimens do not appear to have been dry.

Turnover Pulse Hypothesis

  • Proposed in 1985 by paleontologist Elisabeth Vrba.
  • In periods of extreme and rapid climate change, ungulates with generalized diets fared better than those with specialized diets (Vrba 1988, 1998).
  • Specialist eaters faced extinction at greater rates than generalists because they were unable to adapt to new environments (Vrba 2000).
  • Periods with extreme climate change would be associated with high faunal turnover.
  • Extinction of many species and the speciation, diversification, and migration of many others to occupy various niches.
  • Evidence:
    • Onset of the Quaternary Ice Age, between 2.5 mya and 3 mya, brought extreme global, cyclical interglacial and glacial periods.
    • Faunal evidence from the Turkana basin in East Africa indicates multiple instances of faunal turnover and extinction events.
    • Global climatic change resulted in changes from closed/forested to open/grassier habitats at single sites (Behrensmeyer et al. 1997; Bobe and Behrensmeyer 2004).
    • Work in the Cape Floristic Belt of South Africa shows that extreme changes in climate play a role in extinction and migration in ungulates.
    • Theory was originally developed for ungulates but proponents argue that it can be applied to hominins as well.
    • The link between climate and speciation is only vaguely understood (Faith and Behrensmeyer 2013).
  • Interpretation(s):
    • Evidence of rapid faunal turnover among ungulates during this time period appears clear.
    • Debate around its usefulness as applied to the paleoanthropological record.
    • Specialist hominin species do appear to exist for long periods of time during this time period.
    • HomoHomo, a generalist genus with a varied and adaptable diet, ultimately survives the majority of these fluctuations, and the specialists appear to go extinct.

Variability Selection Hypothesis

  • First articulated by paleoanthropologist Richard Potts (1998).
  • Links the high amount of climatic variability over the last 7 million years to both behavioral and morphological changes.
  • Hominin evolution does not respond to habitat-specific changes or to specific aridity or moisture trends.
  • Long-term environmental unpredictability over time and space influenced morphological and behavioral adaptations that would help hominins survive, regardless of environmental context (Potts 1998, 2013).
  • Hominin groups would experience varying degrees of natural selection due to continually changing environments and potential group isolation.
  • Allows certain groups to develop genetic combinations that would increase their ability to survive in shifting environments.
  • These populations would then have a genetic advantage over others that were forced into habitat-specific adaptations (Potts 2013).
  • Evidence:
    • Large climatic variability and higher survivability of generalists versus specialists.
    • Accommodates for larger time-scales of extinction and survival events.
  • Interpretation(s):
    • Allows for a more flexible interpretation of the evolution of bipedalism in hominins and a more fluid interpretation of the Turnover Pulse Hypothesis, where species turnover is meant to be more rapid.
    • Accommodates both environmental data and our interpretations of an evolution toward greater variability among species and the survivability of generalists.