Not much is known about the earliest development of life. However, all existing organisms share

certain traits, including the cellular structure, and the genetic code. Most scientists interpret this to mean all existing organisms share a common ancestor, which had already developed the most fundamental cellular processes, but there is no scientific consensus on the relationship of the three domains of life (Archea, Bacteria, Eukaryota) or the origin of life. Attempts to shed light on the earliest history of life generally focus on the behavior of macromolecules, particularly RNA, and the behavior of complex systems.

Though the origins of life are murky, other milestones in the evolutionary history of life are well-

known. The emergence of oxygenic photosynthesis (around 3 billion years ago) and the subsequent emergence of an oxygen-rich, non-reducing atmosphere can be traced through the formation of banded iron deposits, and later red beds of iron oxides. This was a necessary prerequisite for the development of aerobic cellular respiration, believed to have emerged around 2 billion years ago. In the last billion years, simple multicellular plants and animals began to appear in the oceans. Soon after the emergence of the first animals the Cambrian explosion (a period of unrivaled and remarkable, but brief, organismal Persity documented in the fossils found at the Burgess Shale) saw the creation of all the major body plans, or phyla, of modern animals. About 500 million years ago, plants and fungi colonized the land, and were soon followed by arthropods and other animals, leading to the development of land ecosystems with which we are familiar.

Most scientists assume that all life evolved through a succession of stages from a common ancestor,

generally thought to be a single-celled simple organism, like a bacterium, or a blue-green alga, that lived over 3500 million years ago. There is a great deal of evidence to relate all present-day organisms to each other, and that they all arose from this single ancestor.

Many scientists are interested in what these relationships would be like and they reconstruct them to look very much like a branching tree. The pattern of these branching relationships is called a phylogeny. Because the pattern of a phylogeny is essentially branching, it is possible to arrange all species of plants and animals into a hierarchical arrangement, where species fit into genera, genera into families, and so on, up to kingdoms.