As we saw in Chapter 2, a planet's average temperature is greatly affected by the volumes of greenhouse gases in its atmosphere. Much of this gas enters a planet's atmosphere from actively erupting volcanoes. Although there are abundant volcanic eruptions in the sea as well, most of the carbon dioxide from these events does not make its way into the atmosphere. Cold seawater can hold large amounts of dissolved carbon dioxide, and below 700 meters, CO2 will settle to the bottom of the ocean as it reaches saturation in the water. At the time of Snowball Earth, enough CO2 would eventually reach the atmosphere to melt back the sea ice and, in so doing, expose the metal-rich waters of the sea to the atmosphere. The time necessary for this "melt-back" has been estimated by Hoffman and his group to be between 4 and 30 million years. With the ice melted back from the sea, and temperatures again warming, Earth would have undergone spectacular changes. Here is how Kirschvink has described these events:

Escape from this "icehouse" condition was only accomplished by the buildup of volcanic gases, particularly carbon dioxide, mostly from undersea volcanic activity. Deglaciation during the end of these glacial events must have been spectacular, with nearly 30 million years of carbon dioxide, ferrous iron, and long buried nutrients suddenly being exposed to fresh air and sunlight. Hundreds of meters of carbonate rock are preserved capping the glacial sediments, at all latitudes, on all continents, as a direct result of wild photosynthetic activity. For a brief time, the Earth's oceans would have been as green as Irish clover, and the sudden oxygen spikes may have sparked early animal evolution.

The most important source of biological productivity in the oceans of today derives from the growth of phytoplankton, the single-celled plants that are the pastures of the sea. The growth of these plants, so important for producing oxygen, is limited by the availability of nutrients and iron. If iron is dropped into the oceans of today, a great bloom of phytoplankton results. Such was probably the case soon after the end of the first Snowball Earth event. As the ice-covered seas began to melt, the fine iron- and magnesium-rich dust coating the surface of the sea ice would have acted as a fertilizer, tremendously stimulating growth of the blue-green "algae" (really photo-synthesizing bacteria known as cyanobacteria). Enormous populations of cyanobacteria would have clotted the surface regions of the liberated seas, releasing huge volumes of oxygen as a consequence of their photosynthetic activity. This sudden appearance of so much life, after the millions of years of cold and dearth of life, would have been a great revolution, and it probably stimulated new evolutionary changes.

These events would have had profound geological as well as biological ramifications. The sudden rush of oxygen into the sea and air would have caused the iron- and manganese-rich oceans to precipitate out iron and manganese oxides. In a previous chapter we saw how banded-iron deposits began to accumulate about 2.5 billion years ago. Kirschvink and his group argue that the appearance of banded-iron deposition occurred soon after the first Snowball Earth ended. Not only iron deposits but magnesium-rich deposits as well were immediate results of the end of the first Snowball Earth event. Evidence of this is seen in South Africa, where the world's largest land-based deposit of manganese minerals has been dated at 2.4 billion years of age and sits just above sedimentary deposits that were laid down during the 2.5-billion-year-old Snowball Earth episode. Like the banded-iron formations, these manganese-rich deposits appear to be a direct consequence of the oxygen bloom that occurred when the planetary snowball melted.

The cessation of the 2.5-billion-year-old Snowball Earth thus appears to have resulted in a rise in the amount of oxygen both dissolved in the sea and free in the atmosphere. Probably for the first time in Earth's history, the sunlit portions of the sea became too oxygen-rich to allow iron to exist in solution in seawater. Kirschvink and his colleagues argue that this dramatic change in the chemistry of the sea would have exerted intense evolutionary pressure on life on Earth, then no more advanced than prokaryotic bacteria. Oxygen, indispensable to the survival of animals, was at that time a poison to perhaps the majority of life forms. Having evolved in environments with little or no oxygen, most life experienced the sudden appearance of the chemically reactive element as a global disaster—but for the rest it was a powerful evolutionary spur. There were but two choices facing life on Earth in that long-ago time: Adapt through evolution, or die.

All organisms in the sea had to adapt in two major ways. First, they had to evolve enzymes capable of mitigating the ravages of dissolved molecular oxygen and chemicals called hydroxyl radicals. (We humans are still trying to do this. Our ingestion of antioxidants such as vitamin E and vitamin C is an attempt to reduce the ravaging effects that dissolved oxygen and "free radicals" have on living cells.) Second, with the banded-iron formations' precipitation out from seawater, living cells no longer inhabited a solution rich in iron. After having been surrounded by high-iron solution since the first formation of life, proteins within cells had to be reengineered for life in an environment low in iron.

Recent DNA sequencing has shown that several enzymes found in ar-chaeans and eukaryotes are left over from this event of 2.5 billion years ago. No such enzymes occurred in the older bacteria. The implications of this are profound: Kirschvink and his colleagues are proposing no less than complete rejection of the Tree of Life models we examined at the end of Chapter 3, which suggest that the three great domains (Archaea, Bacteria, and Eucarya) all arose soon after life's first evolution at least 3.8 billion years ago. The new study has not only uprooted this tree; it has burned it. If the Kirschvink group is correct, two of the three domains—Archaea and Eucarya—arose only after the 2.5-billion-year-old Snowball Earth and are thus much younger than the bacteria. Soon after this, in rocks about 2.1 billion years of age, we find a record of the oldest organelle-bearing eucaryan—the creatures known as Grypania, which we mentioned in Chapter 3.

This new version of the Tree of Life is a revolutionary scientific discovery, and if true, it will utterly reshape our understanding of life's evolutionary path. The Snowball Earth events can be seen as biologically important in two ways. First, the inception of the Snowball produced what may have been the largest "mass extinction" (the subject of Chapter 8) in our planet's history. The persistence of globally freezing temperatures, the isolation of the ocean from sunlight, the change in the precipitation patterns on Earth, and the removal of all water from the surfaces of continents would have removed the majority of surface habitats then available for microorganisms. In only a few places could microorganisms have survived: in the deep earth, around hot springs, and in hydrothermal deposits. Second, Earth's release from this icy prison after 30 million years brought about a new catastrophe: from cold to hot, from oxygen-free to oxygen-rich. Again, organisms had to adapt rapidly. It is this legacy that we may be seeing in the DNA of all living organisms; those that survived all bear witness in their DNA to this dual catastrophe— first cold, then warmth and oxygen. Life on the early Earth went through an icy bottleneck, and it came out the other side radically changed.

The Snowball Earth of 2.5 billion years ago may have given our planet eucaryans and the eukaryotic cell necessary for animal life. The second series of Snowballs (there were several in rapid succession) may have bequeathed our planet an even more interesting biological legacy—animal life as we know it.

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