Paleoanthropology and Hominin Evolution

Paleoanthropology and Hominins

What is Paleoanthropology?

Paleoanthropology is the scientific study dedicated to the excavation and analysis of fossils pertinent to human and nonhuman primate evolution.

Research in Paleoanthropology

Site Selection

Geological Surveys: Involve systematic investigation of geological formations to identify potential fossil-rich areas.
Local Knowledge: Often critical, as indigenous or local communities may have historical or experiential understanding of certain sites.
Luck: Unpredictable element that can lead to significant discoveries.

Systematic Excavations

Excavation Units: Defined areas within a site delineated for meticulous digging.
Digging by Layers: Excavation proceeds by carefully removing soil in stratigraphic layers.
Starting Layer: Typically begins with removing layers of about $10-20$ cm in thickness to ensure careful documentation.
Sifting Dirt: All excavated soil is sifted through screens to recover smaller fossils, artifacts, or fragments that might otherwise be missed.

Fossilization

Taphonomy: This is the specialized study of how plant and animal remains are preserved and the environmental conditions they undergo from death to discovery. It encompasses processes like decay, transport, burial, and fossilization.
Process of Fossilization: Involves the gradual replacement of organic minerals, such as calcium and phosphorous (found in bones and teeth), by rock-forming minerals like iron and silica over vast periods of time. This mineral replacement creates a stony replica of the original organic material.

Dating Techniques

There are two main categories of dating techniques used in paleoanthropology:
- Absolute Dating: Provides a specific numerical age or a very narrow age range for an object or event.
- Relative Dating: Determines the age of objects or events in relation to other objects or events, without providing a precise numerical age.

Absolute Dating: Radiocarbon Dating

Purpose: Primarily used to provide a direct date for organic remains themselves.
Mechanism: Based on the known and constant rate at which the radioactive isotope carbon- $14$ ( $C^{14}$ ) decays into nitrogen- $14$ ( $N^{14}$ ).
Methodology: The amount of remaining carbon- $14$ in a sample is compared to the quantity of stable carbon- $12$ ( $C^{12}$ ), which does not decay. The ratio indicates how long ago the organism died.
Age Range: Effective for dating remains up to approximately $75,000 - 50,000$ years ago. Beyond this, too little carbon- $14$ remains to be accurately measured.

Absolute Dating: Radiopotassium Dating

Purpose: Provides a date for the volcanic substrate (e.g., ash layers, rock) surrounding the fossilized remains, rather than the remains themselves.
Mechanism: Relies on the known decay rate of the radioactive isotope potassium- $40$ ( $K^{40}$ ) into stable argon- $40$ ( $Ar^{40}$ ) gas.
Methodology: When volcanic rock forms, it traps potassium- $40$ . As it decays, argon- $40$ gas accumulates within the rock. By measuring the quantity of trapped argon gas and comparing it to the potassium- $40$ content, the age of the volcanic substrate can be determined.
Age Range: Effective for dating materials starting from around $200$ thousand years ago and extending much further back into geological time, making it suitable for very old sites.

Relative Dating: Stratigraphic Correlation

Stratigraphic Correlation: This technique involves correlating a fossil's age with the time period represented by the geological strata (layers of rock and soil) in which it is found.
Law of Superposition: A fundamental principle in geology and archaeology stating that in an undisturbed sequence of sedimentary strata, the oldest layers are at the bottom, and the youngest layers are at the top.
Difficulties with Stratigraphic Correlation: Exceptions to the Law of Superposition can occur due to geological disturbances such as:
- Tectonic Activity: Folding, faulting, or uplifting of rock layers can invert the sequence.
- Erosion and Redeposition: Older layers might be eroded and their sediments redeposited on top of younger layers in a new context.
- Human/Animal Activity: Burials or excavation by animals can introduce younger material into older layers or vice versa.

Paleoanthropology and Anatomy

Studying the anatomy of fossilized remains provides crucial insights into:
- Individuals: Information about the age at death, overall health, and sex of the individual.
- Adaptations: Understanding how past hominins were adapted to their environments, including locomotion, diet, and behavior.
- Evolutionary History: Tracing the changes and developments in human and primate lineages over time.
- Evolutionary Relationships: Determining how different species are related to each other and understanding the branching pattern of the evolutionary tree.

Bones of Interest

Skull

Cranium: The part of the skull enclosing the brain.
Maxilla: The upper jawbone.
Mandible: The lower jawbone.
Foramen Magnum: The large opening at the base of the skull through which the spinal cord passes. Its position is indicative of posture and locomotion.
Occiput: The back part of the skull.
Dental Arcade: The shape of the jawline formed by the teeth.
Glenoid Fossa: A shallow depression in the scapula (shoulder blade) that articulates with the head of the humerus (upper arm bone), forming the shoulder joint.
Zygomatic Arch: The cheekbone.
Palate: The roof of the mouth.
Occipital Condyle: Bony protrusions on either side of the foramen magnum that articulate with the first vertebra of the spinal column.

Axial Skeleton

Vertebral Column: In humans, it has a distinctive S-shape, which is critical for upright posture and bipedalism.
Pelvis: In humans, it is characteristically short and broad, forming a basin that supports internal organs and provides muscle attachment for bipedal locomotion, contrasting with the longer, narrower pelvis of quadrupeds.

Shoulder and Arms

Scapula: The shoulder blade.
Humerus: The bone of the upper arm.
Radius: One of the two bones of the forearm (thumb side).
Ulna: The other bone of the forearm (pinky side).
Shoulder Joint: Formed by the articulation of the humerus with the scapula.

Legs

Femur: The thigh bone, the longest and strongest bone in the body.
Tibia: The shin bone, the larger of the two lower leg bones.
Fibula: The smaller bone of the lower leg, parallel to the tibia.

Hands and Feet

Hands:
- Carpals: Wrist bones.
- Metacarpals: Bones of the palm.
- Phalanges: Finger bones.
Feet:
- Tarsals: Ankle bones.
- Metatarsals: Bones of the sole of the foot.
- Phalanges: Toe bones.

What is a Hominin?

Hominins are defined by three key evolutionary shifts that distinguish them from other primates:
- $1$ ) Changes in the chewing apparatus.
- $2$ ) A fundamental shift to bipedalism (walking on two legs).
- $3$ ) Increased brain size over evolutionary time.

Hominin Chewing Apparatus

Characterized by:
- Smaller Canine Teeth: Reduced in size compared to other primates.
- Loss of Diastema: The gap between the canine and premolar teeth, typically present in apes to accommodate large canines, is absent in hominins.
- Thick Enamel: Dental enamel is notably thicker, which is an adaptation for processing tougher, more abrasive foods.

Hominin Bipedalism

Identified through several anatomical features:
- Centrally Placed Foramen Magnum: In bipedal hominins, the foramen magnum is located more centrally at the base of the skull, allowing the skull to balance directly atop the vertebral column. In quadrupeds, it is positioned more posteriorly.
- Short, Broad Pelvis: The pelvis is reconfigured to be shorter and broader, providing better support for the upper body and efficient muscle attachment for bipedal locomotion, creating a stable platform.
- Femoral Angle Inward (Valgus Knee): The femur (thigh bone) angles inward from the hip to the knee, bringing the knees and feet closer to the body's midline. This allows the weight to be directly over the feet during single-leg support phase of walking, enhancing balance.
- Long Femoral Neck: A longer neck on the femur helps to position the leg muscles for bipedal walking.

Why Bipedalism?: The Benefits

Thermoregulation: Standing upright reduces the body surface area directly exposed to the sun, especially during the hottest parts of the day. It also increases exposure to cooler breezes at higher elevations, leading to less solar radiation and more wind, thus aiding in heat dissipation.
Energy Efficiency: For certain activities, bipedalism can be more energetically efficient than quadrupedalism, especially for covering long distances at a walking pace.
Hands-Free: Frees the hands for carrying objects (food, tools, infants), making and using tools, and gathering food. This was a critical advantage in environmental shifts.

Hominin Brain Size

Average Cranial Capacity (Volume of the braincase):
- Chimpanzee: ~$400 cc (cubic centimeters)
- Gorilla: ~$500 cc
- Human: ~$1350 $cc</li></ul></li><li>The fossil record shows a gradual increase in brain size throughout hominin evolution, with significant expansion occurring within the genus Homo.</li></ul><h4 id="088d624a-9778-4ac5-89ec-79109c750e96" data-toc-id="088d624a-9778-4ac5-89ec-79109c750e96" collapsed="false" seolevelmigrated="true">How to Classify “Paleospecies”</h4><ul><li>There are two primary schools of thought regarding the classification of fossil species:<ul><li>Lumpers:<ul><li>Prefer to group various fossil forms or varieties under a single species designation.</li><li>Tend to recognize fewer overall species, emphasizing variation within a species.</li></ul></li><li>Splitters:<ul><li>Prefer to separate varieties into distinct species.</li><li>Tend to recognize more species, emphasizing subtle morphological differences as indicative of separate evolutionary lineages.</li></ul></li></ul></li></ul><h4 id="183fec3d-093b-41fa-a6f5-1f6f9fd5da4a" data-toc-id="183fec3d-093b-41fa-a6f5-1f6f9fd5da4a" collapsed="false" seolevelmigrated="true">Early Hominins</h4><ul><li>These earliest hominins belong to a group collectively referred to as the “pre-australopithecines.”</li><li>They lived during the period from approximately$ 7-4.4 $million years ago (mya) and their fossil remains have been discovered in Central and East Africa.</li><li>Key Genera and Species:<ul><li>Sahelanthropus tchadensis: Lived approximately$ 7-6 $mya, found in Chad.</li><li>Orrorin tugenensis: Lived around$ 6 $mya, discovered in Kenya.</li><li>Ardipithecus (including kadabba and ramidus) : Lived from$ 5.8-4.4$$ mya, primarily found in Ethiopia.