Bronze age dating
Some mammals came to abandon a ground-based life, evolving to swing among the trees.
This lifestyle encouraged the development of hands with dextrous fingers and opposable thumbs (for grasping branches), as well as sharp colour eyesight (for navigating the complex, multicoloured, shadowy environment of the forest).
An aquatic egg is laid in water, and thus lacks a hard shell. 49 - "Native Americans of Middle and South America", Encarta 2004.
Amphibians evolved into reptiles, which lay hard-shelled eggs. 50 - "History of technology", Encyclopedia Britannica. 51 - "Guns, Germs, and Steel: The Fates of Human Societies", Jared Diamond.
Consequently, reptiles were the first vertebrates that could live their entire lives on dry land.
A tetrapod is a creature with four limbs; a limb is a jointed appendage that extends from an animal's body. 52 - "Hand tool: smelting", Encyclopedia Britannica. The contrast between humans and animals only became sharp with the onset of modern behaviour. As noted above, various species of human evolved in Sub-Saharan Africa prior to the evolution of modern humans. Some of these species migrated out of Africa, settling regions of Eurasia. All would eventually go extinct, however, leaving the world to be dominated by a single human species: .