Current theories about the origin of life include the two competing
ideas (1) that the primitive Earth environment led to the spontaneous
organization of organic chemicals into primitive cells and (2) that
primitive forms of life arrived on Earth from space. Basic units of life
were probably similar to present-day prokaryotes. These primitive
cells could have changed through time as a result of mutation and in response to a changing environment. The presence of living things has affected
the nature of Earth's atmosphere. The first organisms would
have been anaerobic since there was no oxygen in the atmosphere.
Photosynthesis by organisms such as cyanobacteria would have resulted
in oxygen being added to the atmosphere. The presence of oxygen
allowed for the development of aerobic respiration. Three major
kinds of organisms are referred to as domains: Eubacteria, Archaea,
and Eucarya. It appears that Eubacteria are the oldest group, followed
by Archaea and Eucarya. The Eucarya appear to have arisen as a result
of the process of endosymbiosis.
All sexually reproducing organisms naturally exhibit genetic diversity
among the individuals in the population as a result of mutations
and the genetic recombination resulting from sexual reproduction. The
genetic differences are reflected in physical differences among individuals.
These genetic differences are important for the survival of the
species because natural selection must have genetic diversity to select
from. Natural selection by the environment results in better-suited individual
organisms that have greater numbers of offspring than those that are less well-off genetically.
Organisms with wide geographic distribution often show genetic differences
in separate parts of their range. A species is a group of organisms
that can interbreed to produce fertile offspring. This definition of a
species is also known as the biological species concept. People often distinguish
species from one another based on structural differences. This is
known as a morphological species concept. The process of speciation usually
involves the geographic separation of the species into two or more isolated populations. While they are separated, natural selection operates to adapt each population to its environment.
At one time, people thought that all organisms had remained unchanged
from the time of their creation. Lamarck suggested that change did occur and thought that acquired characteristics could be passed from generation to generation. Darwin and Wallace proposed the theory of natural selection as the mechanism that drives evolution.
