Human Evolution and Biological Theories of Adaptation

Hominid Cranial Evolution and Physical Traits

The Australopithecus possessed a larger cranium than its predecessors, featuring an accentuated undershot jaw and a marked supraorbital rim; its teeth were human-like. Homo ergaster adapted to hard foods and maintained a large habilis-like cranium with a developed supraorbital arch and small teeth. Homo erectus consumed softer foods, utilized fire, and featured a low oblique border with a prominent face. Homo antecessor displayed a solid jaw and a combination of modern and archaic characters.

Neanderthals, associated with cannibalistic practices, had a highly developed musculature, standing approximately 1.65m tall, and a long cranium. The modern hominid features a brain-based bipedal structure with the Foramen Magnum (FM) positioned for upright posture. The vertebral column has three curves, with an elongation of the legs and an integrated thumb. The palate is U-shaped, and the cranium contains a voluminous brain.

Theories of Biological Origin

Fixism and Creationism

Fixism-creationism posits that species have remained identical since their origin. Linnaeus developed a system of natural classifications, suggesting there are as many different species as the supreme creator formed.

Evolutionary Theory

The theory of evolution suggests that species originated from other species. Each species has an ancestor, and all organisms are related to each other. This hypothesis includes components from common species and common ancestors, where evolutionary remains show similar characteristics between fossil ancestors and modern species.

Scientific Evidence of Evolution

• Paleontology: The study of fossil remains between different species.
• Biochemistry: Comparison of chemical components; identical metabolic processes suggest kinship between species.
• Comparative Anatomy: Similar DNA and homologous organs (same origin) or vestigial organs (residual).
• Domestication: Changes produced by artificial selection over a short period of time.
• Geographical Distribution: Locomotion and geographical barriers affecting species distribution.
• Embryology: Similarities in early embryonic stages show ancestral characters.
• Systematics: Genealogical trees and phylogenetics show evolutionary relationships between related bodies.

Lamarckism: The Theory of Acquired Characteristics

Lamarck explained species changes resulting in new species through several basic ideas:

• Simple forms arise through spontaneous generation.
• Organisms possess an inner drive toward perfection.
• Environmental changes cause new needs, leading to the use and disuse of organs.
• Alterations are transmitted to offspring.

Rejection: This theory is rejected because it cannot demonstrate a trend toward complexity in all living beings, nor can it prove the inheritance of acquired characters.

Darwinism and Natural Selection

Darwin's theory describes a changing world where organisms undergo a gradual and continuous process of change. Descendant organisms are similar to their common ancestors, a result of years of evolutionary mechanisms and natural selection.

In natural selection, variants within populations exist among individuals. These differences are heritable and come in two types: those conditioned by the medium and spontaneous modifications. While the number of individuals in a population remains constant, evolutionary change occurs through successful reproduction.