100 - 200 pgs

Australopithecus and Early Hominins

Species Overview

  • Australopithecus afarensis: A well-known early hominin species.

  • Ardipithecus ramidus: Another early hominin, appearing later than Ar. kadabba.

  • Ardipithecus kadabba: The oldest known member of the genus Ardipithecus.

  • Chimpanzee: Closest extant relative to early hominins.

Dental Anatomy Comparisons

Canine Structure

  • Figure 10.7: Illustrates the canines of Au. afarensis, Ar. ramidus, and Ar. kadabba.

  • In Ar. kadabba and chimpanzees, canines are sharpened against the first premolar, a feature not present in later australopithecines.

  • Ar. kadabba canines show similarity to those of Ar. ramidus and Au. afarensis.

Discovery of Ar. ramidus

  • Discovered in 1992 by Tim White's UC Berkeley-led expedition.

  • Initial finds included teeth, jaws, lower part of skull, and upper arm parts, dating to 4.4 million years ago (Ma).

  • Named from Afar words: "ardi" (meaning "ground/floor") and "ramis" (meaning "root").

  • Extensive collection: Over 150,000 fossils collected including 110 fossils from Ardipithecus.

Fossil Conservation Challenges

  • Fossils were in poor condition, often crumbled or crushed, requiring careful excavation.

  • Comprehensive description of Ar. ramidus published in 2009.

Ardipithecus ramidus (Ardi)

  • Nickname: ARA-VP-6/500, representing most of the skeleton of a single individual, known as "Ardi".

  • Physical Traits:

    • Weight: approximately 51 kg (112 lb).

    • Height: about 1.2 m (3.9 ft).

    • Sexual dimorphism suggested, being larger than a wild male chimpanzee yet smaller than a female gorilla.

    • Limb proportions similar to quadrupedal monkeys and Miocene ape Proconsul.

Habitat and Environment

  • Aramis Region was wetter 4.4 Ma, characterized by woodlands with patches of denser forest.

  • Diverse fauna including monkeys, kudus, etc.

Cranial and Facial Features of Ar. ramidus

  • Brain size: Ape-sized, with cranial capacity of approximately 300-350 cc.

  • Facial structure: Appears flatter than chimpanzee faces, with a slightly protruding middle section.

  • Upright posture indicated by spinal alignment.

Distinctive Dental Traits

  • Molar enamel is thicker and reduced in sexual dimorphism in canines.

  • U-shaped dental arcade resembling that of chimpanzees.

  • Smaller incisors suggest decreased frugivorous habits compared to modern chimpanzees.

Insights into Social Structure

  • Canines of Ar. ramidus display slight sexual dimorphism (10-15% difference between genders).

  • Sexual dimorphism in canines is not directly correlated to body size differences.

  • Overall reduction in canine size may imply different social structures compared to modern primates.

Locomotion and Posture

  • Evidence suggests bipedal locomotion in Ar. ramidus influenced by various anatomical structures.

  • Foot structure features: opposable big toe for grasping, while other toes modified for upright walking.

  • Pelvic Structure: A combination of ape-like and human-like features supports bipedalism but reflects adaptation to both climbing and walking.

  • Hands differ from knuckle-walking apes, indicating a change in locomotion method (

Specific Features of Pelvis and Legs

  • The pelvis structure is formed from ilium, pubis, and ischium, with adaptations allowing for upright walking.

  • Comparison between human and chimpanzee pelvises shows distinct adaptations in Ar. ramidus, suggesting a middle ground between climbing and walking.

Bipedalism: Its Advantages

  • Bipedal locomotion appears to confer several adaptive advantages:

    • Efficient harvesting of fruit from small trees.

    • Helps keep cool by reducing exposure to sunlight and facilitating air circulation.

    • Frees hands for tool use, food carrying, and other tasks.

The Hominy Community: Diversity from 4 Ma to 2 Ma

  • Hominins lived across Africa during this time period, divided into four genera: Australopithecus, Paranthropus, Kenyanthropus, and Homo.

  • A significant diversification in hominin species occurred with cohabitation observed among various species.

Characteristics of Genera

  1. Australopithecus (e.g., Au. anamensis, Au. afarensis): Features include small stature, adaptation to varied diets, and early signs of tool use.

  2. Paranthropus (e.g., P. aethiopicus, P. robustus): Specialized adaptations for heavy chewing.

  3. Kenyanthropus: Distinctive flat facial structure and smaller teeth.

  4. Homo: Emergence of larger brain sizes and more sophisticated tool use around 2 Ma.

Notable Fossils and Discoveries

  • Laetoli Footprints: Fossilized tracks showing evidence of bipedal locomotion and striding gait around 3.5 Ma.

  • Fossil discoveries reveal various dietary adaptations based on carbon isotope analyses.

Summary of Dental and Physical Characteristics

  • Australopiths generally shared characteristics such as bipedalism, generalized dentition for various diets, and adaptability between climbing and ground foraging.

  • Their dental morphology indicates dietary changes, particularly in relation to environmental shifts from forested to more open areas.

Evolutionary Context and Phylogenetics

  • Inferring relationships among early hominins remains complex due to fossil gaps and varying interpretations.

  • Significant traits include bipedalism, varied growth rates, and differing physical adaptations related to environmental changes.

  • Continued research highlights evolutionary pathways towards the genus Homo and our own species.

Key Terms

  • Hominins: Members of the group that includes modern humans and our extinct relatives.

  • Foramen Magnum: The hole in the skull where the spinal cord enters, indicative of bipedalism position.

  • Endocranial volume: Measurement of the volume of the brain cavity.