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Intro | Precambrian Eon | Paleozoic Era | Mesozoic Era | Cenozoic Era

Precambrian Eon: (4,550-543 mya)

Hadean | Archaean | Proterozoic

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Proterozoic Era (2,500-543 mya)

During the Proterozoic era, oxygen produced by photosynthesizing bacteria collects in the oceans and then in the atmosphere. Iron present in exposed rocks reacts with this oxygen, and rust colors the planet red. While oxygen-loving organisms evolve in these changing conditions, oxygen-intolerant anaerobes survive only in airless pockets.

Soft-bodied animals, large enough to be seen by the naked eye, evolve around 900 mya and are numerous by 650 mya. By roughly 550 mya, however, they are scarce. Because so little fossil evidence of animals of this age has been discovered, scientists are not certain whether these earliest animals were wiped out through an extinction event at the end of the era or whether they evolved into the hard-shelled life that follows.

2,500 mya: Banded iron formations

Banded iron formations (BIFs) are sedimentary rocks deposited extensively on the ocean floor over hundreds of millions of years. Their alternating rust- and gray-colored bands are evidence that oxygen is being produced in the oceans at least as early as 2,500 mya.

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Air apparent: The oxygen revolution (2,500-1,900 mya)

As more and more cyanobacteria (blue-green bacteria) spread across Earth, the oxygen waste they produce through photosynthesis proves toxic to most other microbes. In fact, only those sheltered in oxygen-poor habitats like the murky depths and those with genetic mutations that somehow enable them to tolerate oxygen survive.

How can we be sure that oxygen was present so long ago? Evidence lies in banded iron formations -- the primary source of today's global iron ore supply. From the 600 trillion tons of iron ore present today, we know there was plenty of iron in Precambrian waters.

Iron normally stays dissolved in seawater; it falls out of solution when it comes in contact with oxygen. The fine, bright orange particles that settle on the ocean floor are the product of a chemical reaction: rust. The alternating layers of rust-colored and gray deposits suggest oxygen production fluctuated over time.

As underwater chimneys called deep-sea vents release dissolved iron into Precambrian waters, oxygen is used up as quickly as it is produced. Once the iron supply is exhausted, however, oxygen begins escaping the seas into the open air. Evidence of a buildup of atmospheric oxygen first appears in rock layers 2,200-1,900 million years old, during which time most of the planet's exposed surface rusts.

By 1,900 mya, oxygen composes about three percent of the atmosphere's vital gases. This level will later rise to present-day levels of 21 percent. As the supply of oxygen increases and organisms increasingly tap energy from it, cells grow larger and divide more quickly.

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2,000 mya: Oxidation produces "red beds"

The reaction of oxygen with iron in exposed rocks begins producing "red beds" of rust.

1,900 mya: Oxygen levels reach three percent

Oxygen now composes three percent of the atmosphere, still less than one-fifth of the present-day level.

1,800 mya: Oldest eukaryote fossils

Though eukaryotes are thought to have evolved much earlier, the oldest definitive fossils date to 1,800 mya. Acritarchs are probably single-celled algae in a resting stage of their life cycle that have grown tough outer coats. They are spherical in shape and float at or near the surface of the water.

1,200 mya: True algae

The oldest multicellular algae fossil dates to 1,200 mya. At this time, biological diversity increases greatly, with algae, fungi, animals, and plants diverging from their common eukaryote ancestors.

1,100 mya: Rodinia supercontinent forms

The late-Precambrian supercontinent Rodinia forms when smaller continents converge.

900 mya: Soft-bodied animals

The oldest fossil evidence of multicellular animals, or metazoans, are burrows that suggest smooth, wormlike creatures live 900 mya or more. Found in rocks in China, Canada, India, and elsewhere, the imprints of these soft-bodied creatures reveal little else besides their basic shapes.

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The riddle of Ediacara (900 mya)

Trace fossils of wormlike creatures suggest complex multicellular life evolved as far back as 900 mya. More extensive Precambrian finds, particularly those discovered in Australia's Ediacara Gorge in rocks 565 million years old, include fossil imprints of creatures that resemble everything from leafy fronds to jellyfish. So what exactly were these highly diverse Precambrian creatures? Plants, animals, or of another kingdom entirely?

Nearly all scientists today agree that they were soft-bodied marine-dwelling animals. Their discovery confirmed what many scientists had long hypothesized but could not prove: that skeletonized creatures of the Cambrian period, long thought by scientists to be the first multicellular animals, had predecessors.

Debate continues, however, as to what became of the first animals. While many scientists think that they subsequently evolved into new forms of animals that continued into the Cambrian period, others say that they were wiped out by extinction.

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800 mya: Major glaciation period begins

The first of four global ice ages sets in. While cold-loving microbes evolve and thrive in the frigid waters and snow, repeated glaciation factors heavily into future extinctions of other organisms.

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Did you know?

An ice age is characterized by intervals of repeated glacial advances and retreats, not a single and prolonged deep freeze. Because glaciers reflect solar light rather than absorb it, temperatures at the planet's surface continue to drop, forming even larger glaciers. What causes glaciers to form in the first place? Scientists say changes in atmospheric composition or fluctuations in Earth's orbit and spin axis are the most likely triggers.

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700 mya: Breakup of Rodinia supercontinent

As tectonic plates shift, the Rodinia supercontinent begins to break apart into the smaller continents of Laurentia, Baltica, Siberia, and Gondwana.

600 mya: Protective ozone layer in place

With the exception of bacteria, life on Earth has been restricted to the oceans, where the risk of exposure to ultraviolet radiation is greatly reduced. But now, atmospheric oxygen levels are high enough to form a protective ozone layer over the planet. This shield is a critical factor in the emergence and survival of complex life on land.

Vendian extinction

Date:

543 mya

Intensity:

3

Affected:

Some single-celled algae and soft-bodied animals go extinct

Hypotheses:

Sea-level changes, oxygen depletion

Summary:

Single-celled algae called acritarchs and the Ediacaran animals vanish from the fossil record at the end of the Vendian, the period that closes the Proterozoic era. If they were wiped out by an extinction event, changes in sea level or lowered oxygen levels are likely to blame. But if, as most scientists think, the soft-bodied Ediacarans are actually ancestors of Cambrian faunas, their disappearance in the fossil record may simply demonstrate the incompleteness of this record.

An estimated 250,000 fossil species have been recorded to date, which is only about one percent of the 4 billion animal and plant species thought to have existed over the past 600 million years or so. A high percentage of these -- perhaps 95 percent -- are hard-shelled marine creatures. For all that we have learned about evolution from the fossil record, we know very little, relatively speaking, especially of soft-bodied animals, which generally do not preserve as well.

-> Learn more about the Paleozoic Era

Intro | Precambrian Eon | Paleozoic Era | Mesozoic Era | Cenozoic Era

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