Iron key dating
Fish are the original vertebrates (animals with a backbone) and the ancestors of all other vertebrate groups.
Vertebrates made the transition to land-based life when fish evolved into amphibians: cold-blooded tetrapods that lay aquatic eggs.
The Lower Paleolithic was the age of human evolution.
Various species of (aka "modern behaviour"), which emerged ca. Humans of the Lower and Middle Paleolithic exhibited primitive behaviour; though they could make simple stone tools and eventually came to control fire, these humans did not behave much differently than the rest of the animal kingdom.
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.
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).
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.
The Paleolithic, which spans over ninety percent of human existence, can be further divided into three parts.
Consequently, reptiles were the first vertebrates that could live their entire lives on dry land.
The final two major vertebrate groups, birds and mammals, evolved from reptiles.