Dating practices through history
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.
An aquatic egg is laid in water, and thus lacks a hard shell. 49 - "Native Americans of Middle and South America", Encarta 2004.
(Animals that walk upright are referred to as bipedal.) When these bipedal mammals started to develop larger brains, they evolved into a remarkable new creature: human.
The stone age can be divided into the Paleolithic ("Old Stone Age"), during which all humans were hunter-gatherers; the Mesolithic ("Middle Stone Age"), the transitional phase to agricultural life; and the Neolithic ("New Stone Age"), when humans subsisted through agriculture.
All would eventually go extinct, however, leaving the world to be dominated by a single human species: .
Modern humans colonized most of the inhabitable world during the Middle and Upper Paleolithic periods.
The Paleolithic, which spans over ninety percent of human existence, can be further divided into three parts.
The Lower Paleolithic was the age of human evolution.
The term cold-blooded denotes that a creature's internal temperature is determined by its environment (in contrast to warm-blooded creatures, whose bodies maintain an internal temperature independent of their environment). One day, a species of Sub-Saharan tree-dwelling mammal returned to a ground-based life, leaving the forests behind for open grassland. This animal subsequently evolved to walk upright (rather than on all fours), allowing it to see much farther across the plains. 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). Sub-Saharan Africa was one region where tree-dwelling mammals thrived.