Life is a continuum extending from the earliest organisms to the great variety of species that exist today. It is difficult for us to appreciate exactly how far back life has existed on Earth.
In this unit, we will survey the diversity of life and trace the evolution of this diversity. Throughout this unit, we will see examples of the connection between biological history and biological evolution.
If we calculate Earth to be about 4 billion years old and we know that there were stromatolite–forming bacteria by 3.5 billion years ago, we must assume that living organisms arose sometime in between.
Where did these living organisms come from?The conditions on Earth are very different than they were 4 billion years ago. In Earth's early years the atmosphere was essentially devoid of oxygen and the planet was subjected to more lightning, UV radiation, and falling meteorites (A stony or metallic mass of matter that has fallen to the earth's surface from outer space ). Somehow life managed to arise under these conditions, and various hypotheses attempt to explain how this might have happened.