Modern humans colonized most of the inhabitable world during the Middle and Upper Paleolithic periods. The Americas were colonized by a great migration across the Bering land bridge, which connected Siberia and Alaska. 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.
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 beginning of the Mesolithic age is consequently placed at ca. As noted earlier, hunting/gathering-based life is also known as Paleolithic life. 10,000 BC, however; the continuation of hunting/gathering-based life beyond ca. As the above map illustrates, Neolithic life radiated from Mesopotamia, both across Eurasia and into North Africa. 33 - "Pre-Columbian Civilization", Encyclopedia Britannica. 10,000 BC (in the new, warmer climate) is known as Mesolithic life. 10,000 BC-0), most of the world transitioned to the Neolithic age. (It should be noted that while most of Eurasia adopted agriculture as it diffused from Mesopotamia, some regions may have developed agriculture independently.) The diffusion of agriculture to Sub-Saharan Africa, however, was frustrated by the vast Sahara Desert; across this region, agriculture-based life only emerged over the last two millennia BC. Sub-Saharan Africa was one region where tree-dwelling mammals thrived. One day, a species of Sub-Saharan tree-dwelling mammal returned to a ground-based life, leaving the forests behind for open grassland. Depending on food availability, these bands might be nomadic (constantly on the move) or semi-nomadic (migrating between temporary settlements). Paleolithic life denotes a lifestyle based mainly on hunting/gathering, while Neolithic life denotes a lifestyle based mainly on agriculture (the production of food by raising domesticated crops and animals). About 4.5 billion years ago, our solar system (the sun and its planets) formed via the accretion of dust into spheres; Earth is therefore roughly 4.5 billion years old. Life began about 3.5 billion years ago, with the appearance of single-celled marine organisms; since then, evolution has continuously given rise to new forms of life. Increasingly large and complex types of multicellular life emerged, with one branch of evolution leading to fish. During the Paleolithic age, all humans were hunter-gatherers. They typically lived in small bands that followed an annual migration pattern, timing their movements according to ripening plants and travelling herds of game.