Answer: The Tigris-Euphrates river system, where Mesopotamia started, was an area prone to frequent flooding. The flooding deposited valuable silt onto the land and enabled crops to be grown and harvested in surplus. This was the first time farmers had an excess of what they needed. The surrounding land was devoid of most other raw materials, such as wood, stone and metal. To get these materials, the early Mesopotamians resorted to trade. At these gathering hubs, people began constructing buildings, which formed the basis for the first cities to appear and helped civilizations continue to develop.
Pioneer species are the first organisms to live in an area Some examples of a pioneer are: •bunch grass •milkweed •queen Anne’s lace •Sphagnum moss •sedge •pitcher plant
Any atom at the ground state has its orbital levels fill from the bottom (close to the nucleus) – up. The inner orbitals are of lower energy state and fill first because the atoms ‘likes’ to exist in the low energy and more stable state. Only when the atom is in an excited state do some of the electrons in the lower orbital levels shift to higher unfilled orbitals.
