Lecture 14: Early Hominins I

What Makes a Hominin a Hominin?

• Definition: Hominins are species more closely related to Homo sapiens than to any living great ape.

Uniquely Human Features

• Brain Size: Extremely large brain relative to body size.
• Bipedalism: Anatomical features that support walking on two limbs.
• Dentition: Very small canines and relatively flat faces (orthognathic vs prognathic).
• Tool Use: Ability to modify raw materials into recognizable tool types.
• Chin Development: Presence of a prominent chin.
• Language: Capacity to learn and invent language.

Theories of Human Evolution

1. Brain First Hypothesis:

• Suggests the big brain is the most distinctive feature and evolved earliest.

1. Bipedalism First Hypothesis:

• Proposes that bipedal walking freed hands for tool use, driving brain expansion.

Early Hominins Overview

• Homininae Subfamily: Includes Homo sapiens and fossil ancestors.
• The hominin lineage diverged about 7-5 million years ago (Ma), with few fossils available from this early period.

Clusters of Early Hominins

1. Pre-australopiths (7-4.4 Ma):

• Example: Ardipithecus ramidus (~4.4 Ma)
• Debate about full bipedalism.

1. Earlier, Primitive Australopiths (4.2-3.0 Ma):

• Example: Australopithecus afarensis (3.9-3.0 Ma)
• Fully bipedal and gracile.

1. Later, Derived Australopiths (2.5-1.0 Ma):

• Example: Paranthropus (robust australopiths)
• Fully bipedal and robust.

Detailed Study of Hominins

Sahelanthropus tchadensis

• Age: Lived around 6-7 Ma, considered the oldest hominin.
• Brain Size: Small (320-380 cc), similar to ape.
• Derived Traits:
• Intermediate foramen magnum position.
• Smaller face and canines.
• Intermediate enamel thickness.

Ardipithecus ramidus

• Age: Approximately 4.4 Ma.
• Brain Size: Similar to chimpanzee (~300-350 cc).
• Anatomical Features:
• Foramen Magnum: More anteriorly positioned for balance during bipedalism.
• Pelvis: Short and broad with laterally oriented iliac blades, offering support for bipedal locomotion.
• Opposable big toe indicating climbing capability.

Australopithecus afarensis ("Lucy")

• Age: Approximately 3.9-3.0 Ma, with the famous specimen "Lucy" dated to 3.18 Ma.
• Brain Size: Chimp-sized average 420 cc.
• Footprints: Evidence of bipedalism found at Laetoli, Tanzania, dated 3.7 million years ago.
• Dental Characteristics:
• Canines intermediate in size between chimpanzees and humans.
• Rounded dental arcade.

Australopithecus africanus

• Age: 3.0-2.4 Ma, discovered as the Taung Child in 1924.
• Cranial Capacity: ~450 cc.
• Physical Traits:
• Less prognathic face.
• Reduced canines with parabolic dental arcade.

Robust Australopithecines (Paranthropus spp.)

• Characteristics:
• Adaptations for Heavy Chewing:
  • Large sagittal crest and massive post-canine teeth.
• Three notable species:
  1. P. aethiopicus: Earliest robust species, discovered in Lake Turkana, Kenya.
  2. P. boisei: Discovered at Olduvai Gorge, notable for larger cranial capacity.
  3. P. robustus: Found in South Africa, notable for the largest cranial capacity in the genus.

Summary of Key Concepts

• Early hominins provide important insights into human evolution leading up to modern humans.
• Through anatomical studies and fossil evidence, we can trace the adaptations and changes that occurred during this significant period in our lineage.

Upcoming Classes

• Lab 7: Scheduled for February 27/28.
• Next Class: Lecture 15 on Early Hominins II.