Smithsonian National Museum of Natural History Geologic Time The Story of a Changing Earth
Presented by the Department of Paleobiology.
The Proterozoic
Eon Overview
Earth's Crust as a Platform for Prokaryotic Life
Eukaryotes and the First Multicellular Life Forms
Changes in the Earth's Atmosphere
Proterozoic Mountains and Glaciers
Rangea from the Ediacaran Fauna, Namibia.
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First eukaryotes
Ediacaran Fauna 1
Ediacaran Fauna 2
Ediacaran Fauna 3
Ediacaran Fauna 4
Banded Iron Formations
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Foundational Concepts
Dating Methods
Earth Processes
Life Processes
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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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