Snowball Earth

Snowball Earth is the name given to a series of extensive glaciations that took place around 600 million years ago that might have triggered the appearance of the first complex animals on earth. When life developed the ability to make creatures out of many cells rather than just one that led to the possibility of eyes, teeth, legs, arms, shells, claws—all the organs and appendages that distinguish complex animals from primordial slime. Hence, whatever caused this development in the history of life was responsible for most of the biodiversity that we see on earth today. Some researchers believe that the global "Snowball" glaciations were the trigger. Most researchers agree that this was the coldest period the earth has ever experienced. Although opinion is divided about whether the glaciations triggered the development of complex life, and if so, by what mechanism, the coincidence in timing is widely believed to be significant.

The origin of complex animals has long been a mystery. Earth first formed more than 4.5 billion years ago, and for most of its his tory the only living things on its surface were simple, single-celled creatures like bacteria and algae. Traces of complex animals date to around 580 million years ago. Tiny preserved embryos have been found in Guizhou Province, South China, dating to some time after 590 million years ago. Evidence of large, complex, jellylike creatures called Ediacarans has been found in South Australia, Namibia, the White Sea area, and Newfoundland (Xiao et al., 1998). The oldest of these have not been accurately dated, but they appear to be older than 575 million years. After the early embryos and Ediacarans the so-called Cambrian explosion occurred, which set the foundations for every family of complex animal living on earth today—including humans. The question that remains unanswered is this: when single-celled creatures ruled the earth for most of its history, what caused complex animals to emerge suddenly and unexpectedly from the primordial slime, and why did it happen at that particular moment in time?

Some researchers believe that the birth of complexity was triggered by the largest and most severe ice age ever to strike the earth. Rocks dating from around 700 to 600 million years ago bear the indisputable signs of ice. Ancient icebergs carried boulders far off land and then dropped them onto the silty seafloor when they melted. These so-called dropstones are now preserved in siltstones around the world. Also, ice on land slipped and slid over the surface, dragging pebbles, stones, and rocks along with it. The melange of rocks bulldozed together by the ice is also preserved in many places around the world, often bearing scrape marks where the rocks were dragged along the ground. The glaciations were extremely widespread. Evidence for them is found on every continent (apart from Antarctica, where evidence of any ancient processes is hard to come by, since most of the surface is buried under modern-day ice).

How cold was it? Over geological timescales, earth's continents shifted on its surface, and hundreds of millions of years ago the continents were in different positions. Research measuring the magnetic fields trapped in the ice deposits from the Flinders Ranges of South Australia shows that the continent was close to the equator when the ice formed (Sohl et al., 1999). Since the equator receives the highest concentration of sunlight in the world, it is the hardest part of the planet to freeze. Thus, the Snowball Earth must have been at least very heavily ice covered, if not totally frozen. There is some controversy about whether the continents were completely covered in ice, and whether the oceans froze over all the way to the equator. However, some researchers (Hoffman et al., 1998) estimate that global average temperatures then were around minus 40 degrees centigrade, compared with an average today of around plus 15 degrees centigrade. That was almost certainly the coldest the earth has ever been. This period must have delivered a severe shock to the earth's climate system, and many scientists find it at least highly suggestive that all the evidence for the emergence of complex creatures appears later than the rock horizons bearing signs of ice. There are no undisputed signs of complex animals before the ice signs.

If the ice did trigger the emergence of complex life, what mechanism was involved? Nobody really knows. One possibility is that the severity of the ice age wiped out single-celled creatures from many of the world's environmental niches. Thus, when the ice finally melted, the niches were available to be occupied by more complex animals. Another possibility is that larger, complex animals were more energy-hungry than their predecessors. Since burning oxygen is a very efficient way of providing energy, complex animals may have required a pulse of oxygen to trigger their development. Some researchers speculate that just such a pulse of oxygen may have accompanied the melting of the ice.

—Gabrielle Walker

See also: Evolution; Evolutionary Biodiversity; Geological Time Scale; Mass Extinction; Paleontology

Bibliography

Hoffman, P. F., et al. "A Neoproterozoic Snowball Earth." Science 281:1342-1346; Hoffman, P., and D. Schrag. 2000. "Snowball Earth." Scientific American (January), www.scientificamerican.com; Sohl, L. E., N. Christie-Blick, and D. V. Kent. 1999. "Paleo-magnetic Polarity Reversals in Marinoan (ca. 600 Ma) Glacial Deposits of Australia: Implications for the Duration of Low-latitude Glaciation in Neoproterozoic Time." Geological Society of America Bulletin 111: 1120-1139; Walker, Gabrielle. 2003. Snowball Earth. New York: Crown; Xiao, S., Y. Zhang, and A. H. Knoll. 1998. "Three-dimensional Preservation of Algae and Animal Embryos in a Neoproterozoic Phosphorite." Nature 391:553-558.

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