Unit 4: Evolution
The Evolutionary History of Life
Life first evolved in the
between 4 and 3.5
billion years ago
when Earth had very low levels of atmospheric oxygen. Over time, the activity of photosynthetic
into the atmosphere and set the stage for the emergence of
, and eventually for multicellular organisms.
marked a period of rapid growth in biological diversity including
Most of the modern animal phyla and other groups now extinct appeared at this time, including the
to colonize the land during the
began to diversify, and invertebrate animals such as
such as ferns diversified on land, and the first chordates to
which probably evolved from
dominated land with innovations such as the
transport and support
flourished due to the success of the
which later led to the development of other
began to break
became extinct at the
end of this period.
started to dominate the terrestrial landscape during the
started to evolve during the
have occurred over the history of the Earth, with a sixth
extinction possibly under way.
The rapid diversification of a group of organisms is known as
, which often occurs after mass extinction events. During the history of life, the
increased with time.
Feb 21, 2006