Origins of Man

Previous


Hominin evolution includes all species which have evolved since the human lineage split off from the one that gave rise to the great apes

Around 8 million years ago (in the late Miocene epoch), a simultaneous global cooling and drying trend created a climate change in Africa, which split the hominin sub-family into two distinct populations

  • One population remained in the wet rainforests of western Africa and gave rise to modern chimpanzees (genus = Pan)
  • The other population adapted to the increasingly open, dry habitats of east and north central Africa, and eventually gave rise to modern humans (genus = Homo)


Origins of Man

The following elements provide a framework of the broad patterns of hominin evolution:

  • Bipedalism developed in late Miocene to early Pliocene hominins (~6 - 4 m.y.a), possibly in response to more open habitats
  • An adaptive radiation of African hominins took place between 4 to roughly 1.7 million years ago in response to further climate changes
  • In the period between 1.7 million years ago and the present, there has been an explosive geographic expansion and rapid divergence of the genus Homo (including a dramatic increase in cranial capacity)
  • This expansion has been followed by a subsequent reduction in species richness until only one lineage of the Homo genus remains - Homo sapiens sapiens (modern man)
  • Anthropologists are divided as to whether current human populations evolved in Africa before migrating around the globe ('Out of Africa' model), or evolved as one interconnected global population ('Multiregional' Hypothesis)


Multiregional Hypothesis

  • According to this theory, early hominins (Homo erectus) expanded to Eurasia roughly 1 million years ago
  • Human evolution since then (from H. erectus to modernĀ H. sapiens) has been a continuous process across all regions, with gene flow between the different continental populations


Out of Africa Model

  • The 'Out of Africa' model also suggests that Homo erectus migrated to Eurasia approximately 1 million years ago
  • However, according to this model, Homo sapiens evolved in Africa approximately 200,000 years ago
  • H. sapiens began migrating out of Africa around 70,000 - 50,000 years ago and eventually replaced existing hominin species in Europe and Asia
  • Recent mtDNA evidence supports the 'Out of Africa' model, however debate still exists as to whether this expansion occurred as a single exodus or several


Multiregional versus Out of Africa


Hominin Evolution

By comparing the different types of hominin fossils, we can identify key evolutionary trends:

  • More downward facing foramen magnum (caused by a transition to bipedalism)
  • S-shaped curvature of the spine (an artifact of an increasingly erect posture)
  • Lower and broader pelvis (bipedalism has changed the hominin birthing patterns and behaviours)
  • Change in relative lengths of arm and leg bones (arms have become relatively shorter and legs longer due to walking upright)
  • Increased size of heel bone and alignment of big toe (changes in feet to become greater weight-bearing structures)
  • Flatter faces, with reduced brow ridges and jaw protrusion (head is no longer the most anterior part of the body)
  • Larger cranial capacity with increased brain size and greater encephalisation (greater intellectual prowess)
  • Smaller teeth and jaws more V-shaped (reflecting changing dietary requirements with less emphasis on tough vegetation)
  • Marked reduction in body hair (improved hunting and cultural practices have lead to the development of warm clothing)
  • Shift in muscle groups (particularly the gluteal and hamstring groups, in order to accommodate new mode of locomotion)


Comparison of Early and Recent Hominin Skeletons


History of Hominin Evolution

Across the course of hominin evolution, a number of key species have emerged as a result of different environments and habitats

Not all hominin species are direct ancestors of modern man, many arose from separate lineages now extinct


Early - Late Hominins

  • Early hominins first appear in the fossil record approximately 4 million years ago
  • Collectively, they were very ape-like in structure - with a prognathic profile and longer arms, they were likely facultative bipeds (arms used for support)
  • They had large jaws, broad molars and thicker enamel, indicating a diet that was heavily dependent on nuts, grains and hard fruits
  • They had a relatively small cranial capacity (roughly 300 - 450 cm3), indicating smaller brains
    • Ardipithecus ramidus (~4.4 m.y.a) is one of the oldest fossils and was very ape-like in appearance, with wider zygomatic arches and a sagittal crest
    • Australopithecus afarensis (~4.0 m.y.a) and A. africanus (~2.5 m.y.a) had non-opposable big toes and were likely the first bipeds (facultative)


Early Homo

  • Early Homo species first appear in the fossil record approximately 2 million years ago
  • Compared to Australopithecines, they had a marked increase in brain size (cranial capacity ~ 700 - 1,000 cm3) and reduced sexual dimorphism
  • They had a reduction in the size of their teeth, indicating a change in diet and further skeletal changes to support a more erect posture
    • H. habilis (~2.0 m.y.a) are thought to be among the first to use stone (Oldowan) tools, with shortened digits suggesting the use of precision grip
    • H. erectus (~1.6 m.y.a) was the first to widely distributed thoughout the Old World, may have used fire and possessed rudimentary language


Late Homo

  • Late Homo species first appear in the fossil record under 1 million years ago (~800,000 y.a)
  • These species have a significantly increased cranial capacity (~1,300 - 1,500 cm3) and demonstrate advanced cultural and technological practises
    • H. heidelbergensis (~600,000 y.a) were among the first to bury their dead and are thought to be a direct ancestor of H. sapiens
    • H. neanderthalensis (~200,000 y.a) used Mousterian (flint-flake) tools and likely co-existed at the same time as H. sapiens
    • H. floresiensis (~80,000 y.a) has been nicknamed 'hobbit' for its small size; debate exists as to whether it is a separate species or a primitve human with major genetic deformities
  • At some point between 200,000 and 100,000 years ago, a population of early humans crossed the morphological threshold to become modern humans: Homo sapiens sapiens


Proposed Timeline of Hominin Evolution


  • It is important to note that there is significant, continuous scientific debate regarding the timeline for human evolution
  • Models are continually being proposed, rejected and refined as more fossil evidence comes to light