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Dinosaur Home  A-Z Dinosaurs List Dinosaur Principles
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Not much is known about the earliest development
of life. However, all existing organisms share
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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.
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Though the origins of life
are murky, other milestones in the evolutionary
history of life are well- |
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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 diversity 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.
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Most scientists assume that
all life evolved through a succession of stages
from a common ancestor, |
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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.
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Many scientists are interested
in what these relationships would be like and
they reconstruct them to |
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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. |
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