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Eukaryotes
and the First Multicellular Life Forms
A fundamental
biological change occurred with the appearance of eukaryotes.
Eukaryotes differ from prokaryotes in that their cells contain membranous
sacs called organelles, including mitochondria, chloroplasts, and
the nucleus. Many scientists think these organelles are descended
from formerly free-living prokaryotic organisms. Thus, many important
functions of eukaryotic cells, such as photosynthesis, and respiration
(the process by which organisms use oxygen to metabolize organic compounds
to produce energy, giving off carbon dioxide) were acquired through
a symbiosis of independent forms of life. Eukaryotes
flourished as the environment became richer in oxygen, perhaps in
part because of their more complex intracellular function.
Some of the earliest known single-celled eukaryote fossils are acritarchs,
which become conspicuous at about 2.1 billion years ago. In fact,
acritarchs are the most common fossils of the late Proterozoic. Some
are thought to have been the resting stages, or cysts, of dinoflagellates,
which are one of the most prominent groups of planktonic algae
today. Whatever their origin, the large size of many acritarchs (typically
60 to 200 microns or larger) indicates they were eukaryotes.
An important evolutionary innovation was multicellularity. The oldest
known possible multicellular eukaryote is Grypania spiralis,
a coiled, ribbon-like fossil two millimeters wide and over ten centimeters
long. It looks very much like a coiled multicellular alga and has
been described from banded iron formations in Michigan 2.1 billion
years old. Grypania may not be a eukaryote, but another,
unrelated colonial eukaryote, Horodyskia, is known from sedimentary
rocks dated at 1.5 billion years in western North America and from
rocks more than 1 billion years old in Western Australia.
The earliest known occurrence of multicellular animals
is the Ediacaran fauna, named for the Ediacaran hills
of South Australia. Some of these Ediacaran animals resemble modern
jellyfish and segmented worms, found in great numbers in the seas
today. Others are unlike any known organisms and cannot be classified
with certainty. All these early creatures lack the rigid, supporting
skeletons and protective shells that characterize the first fossils
of the Cambrian Period. |
Eon Overview |
Earth's Crust as a Platform for Prokaryotic Life |
Eukaryotes and the First Multicellular Life Forms |
Changes in the Atmosphere |
Proterozoic Mountains and Glaciers
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