🦕Intro to Paleoanthropology Unit 3 – Miocene Hominoids: Origins of Bipedalism

Key Concepts and Terminology

• Miocene a geological epoch spanning from ~23 to 5 million years ago, crucial period for understanding the origins of bipedalism in hominoids
• Hominoids a taxonomic group including apes and humans, characterized by lack of tail, upright posture, and larger brains compared to other primates
• Bipedalism habitual locomotion on two legs, a defining characteristic of the human lineage
• Proconsul an early Miocene hominoid genus, considered a stem hominoid and potential ancestor to later apes and humans
• Oreopithecus a late Miocene hominoid from Italy, exhibiting some bipedal adaptations but with debated phylogenetic position
• Ardipithecus early hominin genus from the late Miocene and early Pliocene, showing a mosaic of bipedal and arboreal adaptations
• Orthogrady upright posture and locomotion, as opposed to the more horizontal orientation of quadrupedal primates
• Positional behavior the ways in which an animal moves and orients its body in its environment, including locomotion and posture

Geological Context of the Miocene

• Miocene epoch part of the Neogene period, following the Oligocene and preceding the Pliocene
• Began with the Miocene Climate Optimum a warm and humid period with higher sea levels and expanded tropical forests
• Marked by tectonic events the uplift of the Tibetan Plateau, the Andes, and the East African Rift Valley, altering global climate patterns
• Grasslands expanded particularly in the late Miocene, replacing forests in many areas and creating new ecological niches
• Fossil sites spanning the Miocene found in Africa (East Africa and Chad), Europe (Germany, France, Italy, Spain), and Asia (Pakistan, India, China)
• Volcanic activity in East Africa led to the preservation of numerous hominoid fossils in ash layers
• Sahara Desert did not exist for most of the Miocene, allowing for a more continuous biogeographic distribution of fauna between Africa and Eurasia

Miocene Climate and Environment

• Early Miocene characterized by the Miocene Climate Optimum, with global temperatures 3-4°C higher than present
• Warm and humid conditions supported the expansion of tropical and subtropical forests
• Middle Miocene saw a gradual cooling trend and increasing seasonality
• Late Miocene marked by further cooling, aridification, and the expansion of grasslands and open woodlands
• C4 grasses evolved and spread during the late Miocene, adapted to lower CO2 levels and more seasonal rainfall
• Faunal turnover occurred in many regions, with the diversification of grazing mammals and the decline of forest-adapted species
• East African Rift Valley began to form, creating a mosaic of habitats including forests, woodlands, and grasslands
• Himalayan uplift intensified the Asian monsoon system, leading to increased seasonality and the development of the Sahara Desert in the late Miocene

Major Miocene Hominoid Fossils

• Proconsul early Miocene genus from East Africa (Kenya, Uganda), with multiple species showing a mix of primitive and derived hominoid features
• Afropithecus early Miocene genus from Kenya, with a more robust build and some dental similarities to later hominids
• Kenyapithecus middle Miocene genus from Kenya, showing some dental and postcranial adaptations towards a more terrestrial lifestyle
• Dryopithecus widespread middle to late Miocene genus from Europe and Asia, with a mix of primitive and derived hominoid features
• Oreopithecus late Miocene genus from Italy, with unique adaptations suggesting a semi-terrestrial and possibly bipedal locomotion
• Sivapithecus late Miocene genus from South Asia, closely related to orangutans and showing adaptations for a more terrestrial lifestyle
• Ardipithecus late Miocene to early Pliocene genus from Ethiopia, with a mosaic of bipedal and arboreal adaptations, considered an early hominin

Anatomical Changes Towards Bipedalism

• Foramen magnum the opening at the base of the skull for the spinal cord, shifted to a more anterior position in bipeds
• Lumbar lordosis the inward curvature of the lower spine, more pronounced in bipeds to support upright posture
• Shorter and broader iliac blades of the pelvis, providing attachment for gluteal muscles and stabilizing the trunk during bipedal locomotion
• Larger acetabulum the socket of the hip joint, accommodating the increased load bearing during bipedal locomotion
• Valgus knee the inward angle of the knee joint, aligning the lower leg with the body's center of mass in bipeds
• Longer and more robust lower limbs relative to upper limbs, a characteristic of bipedal hominins
• Arched foot with a non-opposable big toe, providing a stable platform for push-off during bipedal walking
• Reduced curvature of the phalanges (finger and toe bones), indicating less time spent grasping branches and more time on the ground

Theories on the Origins of Bipedalism

• Savannah hypothesis proposes that bipedalism evolved as an adaptation to more open, grassland environments for improved visibility and efficient travel
• Postural feeding hypothesis suggests that bipedalism originated to facilitate foraging from low branches and bushes while freeing the hands
• Provisioning hypothesis argues that bipedalism allowed for more efficient carrying of food and other resources to provision offspring and mates
• Thermoregulatory hypothesis posits that upright posture and bipedal locomotion helped early hominins regulate body temperature in hot, open environments
• Threat display hypothesis proposes that bipedal posture evolved as a means of intimidation and display in male-male competition
• Endurance running hypothesis suggests that bipedalism, combined with other adaptations, allowed early hominins to engage in persistence hunting
• Wading hypothesis argues that bipedalism may have originated in coastal or wetland environments, with water providing support for upright posture

Comparative Analysis with Modern Primates

• African apes (chimpanzees, bonobos, gorillas) are the closest living relatives of humans, sharing a common ancestor around 5-7 million years ago
• Chimpanzees and bonobos exhibit occasional bipedalism, particularly in the context of carrying objects or threat displays
• Gorillas have a more upright posture compared to chimpanzees and bonobos, but still primarily use quadrupedal knuckle-walking locomotion
• Orangutans are more arboreal than African apes, using a form of quadrumanous climbing and suspension, but also exhibit occasional bipedalism on branches
• Gibbons and siamangs are highly specialized for brachiation (arm-swinging), but also engage in bipedal walking along branches
• Baboons and other terrestrial monkeys provide insights into the biomechanics and energy costs of quadrupedal locomotion on the ground
• Bipedal locomotion is rare among non-human primates, highlighting the unique evolutionary trajectory of the human lineage

Debates and Controversies

• Exact timing of the origin of bipedalism, with some researchers proposing a gradual transition starting in the Miocene, while others argue for a more punctuated event in the Pliocene
• Relative importance of different selective pressures (environmental, social, dietary) in driving the evolution of bipedalism
• Degree of arboreality and bipedalism in different Miocene hominoids, particularly in genera such as Oreopithecus and Ardipithecus
• Phylogenetic relationships among Miocene hominoids and their connections to later hominins, with ongoing debates about the position of various genera
• Role of climate change and tectonic events in shaping the evolution of Miocene hominoids and the origins of bipedalism
• Interpretation of fragmentary fossil evidence and the limitations of inferring locomotor behavior from isolated anatomical features
• Integration of new methodologies, such as 3D morphometrics and biomechanical modeling, to better understand the functional implications of Miocene hominoid morphology