The Oxygen Revolution

It is probably impossible for us to conceive how entirely alien to ours this world truly was. Yet the strange microbial world of 2 billion years ago may be the norm in the Universe for those planets that harbor life. Traces of it exist still, here on Earth, in the bacterial froths and pond scum that persist across our planet, and perhaps nowhere more prolifically than in the rotting garbage dumps and landfills created by our own species—places where huge, visible colonies of rapidly growing bacteria exist still. But the rainbow slick of the oozing swamp is the exception in a world where the eucaryans are so much more in evidence than the prokaryotic forms. What would that 2-billion-year-old world look like? The best description we know of was penned by two scientists who have journeyed back to this world, in their imaginations, many times. We owe the following image of the ancient Proterozoic era (the formal name for the time interval of 2.5 to 0.5 billion years ago) to Lynn Margulis and Dorion Sagan, in their 1986 book Microcosmos:

To a casual observer, the early Proterozoic world would have looked largely flat and damp, an alien yet familiar landscape, with volcanoes smoking in the background and shallow, brilliantly colored pools abounding and mysterious greenish and brownish patches of scum floating on the waters, stuck to the banks of rivers, tainting the damp soils like fine molds. A ruddy sheen would coat the stench-filled waters. Shrunk to microscopic perspective, a fantastic landscape of bobbing purple, aquamarine, red, and yellow spheres would come into view. Inside the violet spheres of Thio-capsa, suspended yellow globules of sulfur would emit bubbles of skunky gas. Colonies of ensheathed viscous organisms would stretch to the horizon. One end stuck to rocks, the other ends of some bacteria would insinuate themselves inside tiny cracks and begin to penetrate the rock itself. Long skinny filaments would leave the pack of their brethren, gliding by slowly, searching for a better place in the sun. Squiggling bacterial whips shaped like corkscrews or fusilli pasta would dart by. Multicellular filaments and tacky, textilelike crowds of bacterial cells would wave with the currents, coating pebbles with brilliant shades of red, pink, yellow and green. Showers of spheres, blown by breezes, would splash and crash against the vast frontier of low-lying mud and waters.

This prokaryotic world was creating what has been called the Oxygen Revolution. The initiation of an oxygenated atmosphere was one of the most significant of all biologically mediated events on Earth. Prokaryotic bacteria, using only sunlight, water, and carbon dioxide, ultimately transformed the planet by generating an ever-increasing volume of atmospheric oxygen. This outpouring of oxygen created both biotic opportunity and biotic crises. Many of Earth's primitive organisms were metabolically incapable of dealing with abundant oxygen. For most of the archaeans, the oxygen boom of about 2 billion years ago was an environmental disaster, driving some species into airless habitats, such as lake and stagnant ocean bottoms, sediments, and dead organisms. Others were incapable of such migration and simply died out. For yet other creatures, however, the profound change in atmospheric conditions created new opportunities. Some prokaryotic cells began to exploit the enormous power of oxygen metabolism to break down food sources into carbon dioxide and water. This new metabolic pathway yielded far more energy than any of the anaerobic pathways. Organisms that adopted it soon began to take over the world. The most efficient of these were members of the domain Eucarya, which, more than 2 billion years ago, evolved true eukaryotic cell machinery.

The oldest known fossils of an organism that appears to have attained the eukaryotic grade of organization have been found in banded-iron deposits located in Michigan. The fossils themselves are about 1 millimeter in diameter and are found in chains as much as 90 millimeters long. The organism, then, is far too large to be a single-celled prokaryote or even a single-

celled eukaryote. This creature, which has been named Grypania, is preserved as coiled films of carbon on smooth sedimentary rock bedding planes, the places where sedimentary beds split apart. Its 1992 discovery indicates that the evolution of the first eukaryotic cell occurred during the banded-iron formation process, when there was still little free oxygen in the sea and probably none in the atmosphere. These early eukaryotes may have been vanish-ingly rare, for other eukaryotes do not occur in the fossil record for 500 million years after this first appearance, but with this form, a beachhead in life's advance had been established.

For the period between 2 and 1 billion years ago (see Figure 5.2), few notable achievements of life are recorded as fossils in the rocks. The first common appearance of eukaryotes begins about 1.6 billion years ago, when microscopic fossils called acritarchs begin to appear in the geological record. These are spherical fossils with relatively thick, organic cell walls. They are interpreted to be the remains of planktonic algae, forms that used photosynthesis and lived in the shallow waters of the world's oceans. Other life forms evolved as well, but as is also true of most living protists, such as the amoeba and the paramecium, their lack of skeletons renders them invisible in the fossil record. With a proliferation of plant-like forms, new varieties of predatory protists surely evolved. Whole armadas of single-celled, floating pastures and the somewhat larger and more mobile grazers on these fields of plankton lived and died in this seemingly endless epoch of geological time. The open ocean would have had little life, but the coastal regions richer in nutrients would have been awash with life—microscopic life. It was the Age of Protists, the Age of the Small.

We have now reached 1 billion years ago, in our march through evolutionary time. Finally, the tempo of evolutionary development increased, if we are correctly interpreting the fossil record, for there is a burgeoning in the number of eukaryotic species found in the rock record at this time. Some of these new forms include the first red and green algae, forms still crucial and varied in marine ecosystems. This diversification of eukaryotic species, including protozoans and plants, set the stage for the evolution of larger, multicellular forms and may have been triggered by the evolution of important new morphologies within the eukaryotic cell.

Early

Multicellular Fossils

Archaean

Proterozoic

Phanerozoic

l—l—

Paleoproterozoic —1-1-1-1-1-1—

Mesoproterozoic Neoproterozoic i i 1 i i i i i 1 i i 1 1

n—i—i—i—i—

Grypania megafossils

Grypania megafossils

Undoubted multicellular algae

Chuaria-Tawuia assemblage

Longfengshania Worm-like megafossils

Simple trace fossils

Skeletal fossils

8 Complex trace fossils tu ^mm^m

Trilobites

Chengjiang fauna

Figure 5.2 Early multicellular fossils. Broken bars indicate uncertain time ranges.

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