May 24, 2000 -- The precursor of modern animals may have been able to survive a Snowball Earth era that occurred some 600 million years ago because of a belt of open water along the equator, suggests scientists from the University of Toronto and Texas A&M University. This was a time considered critical in the evolutionary development of multi-celled animals and therefore the most important interval for biological evolution in general.
In a paper to be published in the May 25 edition of Nature, U of T physics professor Richard Peltier and Texas A&M oceanographers William Hyde, Thomas Crowley and Steven Baum note that the late Proterozoic era (600-800 million years ago) was the most important period of evolution for multi-cellular creatures. However, this period was also a time in Earth's history that has come to be referred to as the Snowball Earth. At that time, the planet was thought to be completely ice-covered. Geological and paleomagnetic evidence indicates that for alternating periods, the Earth was completely covered by ice sheets over the continents and sea ice over the oceans, followed by sudden warming trends that melted the ice.
"If the suface of the planet was covered by ice, the question arises as to how early life managed to survive under such environmental stress," says Peltier. To find an answer, the scientists employed several different models of the climate systems and ran detailed computer simulations of the climate thought to have been characteristic of that time. To simulate the Snowball Earth, they reduced the amount of sunlight reaching the Earth -- to account for the fact that the sun was about six per cent less luminous than it is now -- and varied the concentration of atmospheric carbon dioxide within the range expected for that time.
In most of the simulations, their analysis revealed the presence of a belt of open water near the equator when the general circulation of the ocean was taken into account. "It is this open water that may have provided a refuge for multi-celled animals when the rest of the Earth was covered by ice and snow," Peltier explains.
The findings of this research are critical to understanding how early life evolved, he states. "This could help clarify how multi-celled animals managed not only stay alive, but to thrive given the Earth's harsh conditions. The extreme climates may even have exerted pressure on these animals to evolve and adapt, possibly leading to the rapid development of new forms of animals and their movement into new, unpopulated habitats when the Earth exited the snowball state. It was during the warm Cambrian era -- immediately following the late Proterozoic -- in which life proliferated."
The late Proterozoic period was also a time when the supercontinents Rodinia and Pannotia formed and subsequently rifted and disassembled. Located over the south rotational pole in the position of present-day Antarctica, these supercontinents were made up of the current land masses of Africa, South America, Antarctica, Australia, Greenland, Laurentia and parts of Asia. According to Peltier, the entry of the Earth into the snowball state required not only the weak sun and atmospheric carbon dioxide levels not significantly higher than present-day, but also this high degree of polar continentality.
Funding for this research came from the Natural Sciences and Engineering Research Council of Canada and the National Science Foundation in the U.S.
February 14, 2000
But 2.4 billion years later, an unlikely winner has emerged from that first planetary deep-freeze, and it's none other than us modern industrial humans. New research from the California Institute of Technology reveals that the world's largest deposit of manganese (a component of steel) was formed by the cascade of chemical reactions caused when the planet got so cold that even the equators were icy -- a condition now known as "Snowball Earth."
In a special issue of the Proceedings of the National Academy of Sciences on global climatic change published February 14, Caltech geobiology professor Joe Kirschvink and his team show that the huge Kalahari Manganese Field in southern Africa was a consequence of a long Snowball Earth episode. Kirschvink, who originated the Snowball Earth concept more than a decade ago, says the new study explains how the drastic climatic changes in a Snowball Earth episode can alter the course of biological evolution, and can also account for a huge economic resource.
According to Kirschvink and his team, the planet froze over for tens of millions of years, but eventually thawed when a greenhouse-induced effect kicked in. This warming episode led to the deposit of iron formations and carbonates, providing nutrients to the blue-green algae that were waiting in the wings for a good feeding.
The algae bloom during the melting period resulted in an oxygen spike, which in turn led to a "rusting" of the iron and manganese. This caused the manganese to be laid down in a huge 45-meter-thick deposit in the Kalahari to await future human mining and metallurgy. Today, about 80 percent of the entire world's known manganese reserves are found in that one field, and it is a major economic resource for the Republic of South Africa.
The Snowball Earth's cascade of climatic chemical reactions also probably forced the living organisms of the time to mutate in such a way that they were protected from the excess oxygen. Because free radicals can cause DNA damage, the organisms adapted an enzyme known as the superoxide dismutase to compensate.
Kirschvink points out that the enzyme and its evolutionary history are well known to biologists, but that a global climate change apparently has never been suggested as a cause of the enzyme's diversification.
"To our knowledge, this is the first biochemical evidence for this adaptation," says Kirschvink, adding that the data shows that the adaptation can be traced back to the Snowball Earth episode 2.4 billion years ago.
Kirschvink, his former doctoral student Dave Evans (now at the University of Western Australia in Perth), and Nicolas J. Beukes of Rand Afrikaans University proposed the Snowball Earth episode in a 1997 paper in Nature. Their evidence for the freeze of 2.4 billion years ago was based on their finding evidence of glacial deposits in a place in southern Africa that in ancient times was within 11 degrees of the equator, according to magnetic samples also gathered there.
The other authors of the PNAS paper are Eric Gaidos of the Jet Propulsion Laboratory, who also holds an appointment in geobiology at Caltech; L. Elizabeth Bertani and Rachel E. Steinberger, both of the Division of Biology at Caltech; and Nicholas J. Beukes and Jans Gutzmer, both of Rand Afrikaans University in Johannesburg.
The work was supported by the NASA National Astrobiology Institute.
A detailed article on the Snowball Earth phenomenon was published in the January 2000 issue of Scientific American.