Life on Earth
In three and a half year, life on Earth has transformed from single cell to complex multi-cellular organisms. Life permeates all environments on Earth and is a defining characteristic of our planet. It occurs in the biosphere, a thin layer between the upper part of Earth troposphere and the topmost layers of porous rocks and sediments. The size and nature of the biosphere has grown and changed over time, as has the relationship between organic elements in the biosphere.
In the beginning, the conditions on Earth were limiting, and the first cells formed molecular building blocks reflects this. Early organisms were single-celled prokaryotes, such as bacteria, that lacked a defined nucleus. A billion and a half years later, eukaryotes, such as amoebas, appeared: single-celled organisms that possess a bounded nucleus and rely on oxygen to function. These two categories of cell have persisted through time in an unbroken sequence, while also evolving into the myriad and astonishingly diversified life-forms found on the planet today.
Humans should not be viewed as the culmination of life on Earth. Like all situations involving natural selection, the pathway that led to Homo sapiens was not predetermined, but variable. A sequence of many favorable circumstances was necessary for the human species to evolve. Had not cataclysmic event occurred some 65 million years ago, dinosaurs still might be the dominant vertebrates on the planet, as they had been for the 100 million years during which they coexisted early mammals. If one group of organisms is to be singled out for its longevity and adaptability, it is bacteria. Bacteria have been since the beginning and in all likelihood will be here at the end.