Chordate Evolution and Human Ancestry Explained

Chordate Characteristics

Chordates are defined by a dorsal notochord, a supporting rod present in most during embryonic stages. Key features include:

• Dorsal hollow nerve cord: Branches at regular intervals to connect to organs, muscles, and sensory systems.
• Pharyngeal pouches: Paired structures in the throat. In fishes and amphibians, these develop into gills. In humans, they develop into the Eustachian tubes, tonsils, thymus, and parathyroids.
• Post-anal tail: Present at some point during development.

Chordate Classifications

• Chondrichthyes: Sharks and rays.
• Actinopterygii: Ray-finned fish.
• Sarcopterygii: Lobe-finned and lungfish.
• Amphibia: Frogs and salamanders.
• Reptilia: Snakes and lizards.
• Aves: Birds.
• Mammalia: Quadrupeds.

Mammalian Biology

Mammals are homeothermic (warm-blooded), regulating internal body temperature. Key traits include hair for metabolic heat insulation and mammary glands.

Reproductive Strategies

• Monotremes: Lay eggs; the female incubates them in a burrow.
• Marsupials: Young begin development in the female's body but are born prematurely.
• Placental Mammals: Most mammals utilize extraembryonic membranes for internal development. The chorion forms the fetal portion of the placenta, while the uterine wall forms the maternal portion, facilitating the exchange of nutrients, oxygen, and waste.

Mammals exhibit expanded cerebral hemispheres and long gestation periods, allowing for extended learning.

Primate Evolution

Primates are characterized by opposable thumbs and fingers, nails instead of claws, and forward-facing eyes for stereoscopic vision. There is a selection against a strong sense of smell, and they typically produce single offspring.

Hominid Timeline

• Ardipithecus ramidus (4.4 mya): Not erect.
• Australopithecines: Characterized by smaller teeth and skulls; they were erect.
• Australopithecus anamensis (4 mya): Possessed human-like legs.
• Australopithecus afarensis (Lucy, 3.4 mya): Bipedal and erect. Canine teeth were smaller than those of apes, and brain size was half that of modern humans.
• Australopithecus africanus: Slender; found in Southern Africa.
• Robustus and boisei: Ate meat but lacked tool usage.
• Aethiopicus (2.5 mya): Likely ancestral to robustus and boisei.

The Genus Homo

• Homo habilis (1.6–2 mya): 45% larger brain, smaller teeth, division of labor, speech, and culture.
• Homo erectus (1.9–0.5 mya): Larger brains, prominent brow, and flatter faces. Used advanced tools and fire; a striding gait allowed for migration.
• Modern Humans: Archaic Homo sapiens evolved approximately 300,000 years before present (ybp).

Theories of Human Migration

• Multiregional Continuity: Each population evolved into modern humans, deriving from H. erectus ancestors in each specific location.
• Out-of-Africa: Advanced H. sapiens migrated to Europe and Asia (100,000 ybp), interbreeding with or supplanting other populations.

Archaic Humans

• Neanderthalensis (200,000 ybp): Characterized by larger musculature.
• Homo sapiens (Cro-Magnon): Resembled modern humans in face and brain size. They practiced societal living, used language, created drawings, and utilized Aurignacian tools.