Earth was a frozen Snowball when animals first evolved
Canada Goose Online The ice brought Earth to a standstill. Where there were once waves lapping onto a tropical shore and warm waters teeming with life, there was just canada goose black friday sale the whistling of the wind and a cold barren landscape, covered in canada goose outlet ice as far as the eye could see. Even at the equator the warmest place on Earth the average temperature was a frigid 20 equivalent to modern day Antarctica. Most life was wiped Canada Goose Outlet out, and the creatures that did survive huddled in small pockets of open water, where hot springs continued to bubble up. Canada Goose Online
This was "Snowball Earth" a deep freeze that began around 715 million years ago and held Earth in its icy grip for a good 120 million years. "There buy canada goose jacket are no other comparable glacial periods on Earth. This one was really quite catastrophic," says Graham Shields of University College London in the UK.
However, some canada goose coats on sale scientists now believe that this crushing catastrophe drove one of the most incredible steps in evolution: the development of the first animals, and a dramatic flourishing of life known as the Cambrian explosion.
canada goose View image of Trilobites like these first evolved during the Cambrian (Credit: Sinclair Stammers / NPL) canada goose
Canada Goose Jackets Around 540 million years ago, a host of exotic creatures suddenly appeared. They included giant woodlouse like creatures known as trilobites, the five eyed Opabinia, and the spiny slug like Wiwaxia. Suddenly, Earth leapt from being dominated by single celled bacteria to a world teeming with exotic multicellular creatures, all in a geological blink of an eye. Canada Goose Jackets
The Cambrian explosion remains a puzzle
For Charles Darwin, trying to demonstrate his theory of natural selection, this sudden burst of evolution was a major problem. "The case must at present remain inexplicable; and may be truly urged as a valid argument against the views here entertained," he wrote in On the Origin of Species in 1859.
canada goose factory sale To this day the Cambrian explosion remains a puzzle. But maybe a planet encasing icy catastrophe could help explain canada goose it. canada goose factory sale
canada goose clearance sale View image of When Earth was canada goose clearance a frozen snowball (Credit: Stocktrek Images, Inc. / Alamy) canada goose clearance sale
Canada Goose online The evidence for a Snowball Earth first emerged in the early 1990s. Unexpectedly, geologists discovered evidence of glaciers such as stones that had clearly been carried on ice rafts and then dropped in the tropics. Since then, a growing body of evidence has shown that the global deep freeze began around 715 million years ago, and lasted nearly 120 million years. Canada Goose online
Canada Goose Outlet Most scientists agree that the Snowball formed suddenly Canada Goose Outlet
uk canada goose Exactly how far the ice extended is still debated. Some argue that the entire Earth was encased in ice, with just a few small pockets of open water where hot springs bubbled Canada Goose sale up. Others believe that a belt of open water remained around Earth's equator. uk canada goose
Regardless of how far the cheap Canada Goose ice stretched, most scientists agree that the Snowball formed suddenly. It was probably caused by rapid weathering of Earth's continents, which sucked carbon dioxide a planet warming greenhouse gas out of the atmosphere and caused temperatures to plummet. There were two distinct pulses of extreme glaciation, interspersed with a 20 million year warm period. Finally, around 660 million years ago, Earth's volcanoes topped up the atmospheric carbon dioxide enough to haul the climate out of its frozen state.
buy canada goose jacket cheap So why on Earth would this period of extreme cold canada goose store cause life to switch gear so rapidly? Maybe, say many geologists, because it pumped lots of life giving oxygen into the air. buy canada goose jacket cheap
View image of There may have been a few ice free patches (Credit: NASA, CC by 2.0)
canada goose coats on sale The idea is that the ice gave a boost to microscopic plants, which released oxygen as a waste product. During the Snowball, the glaciers would have worn weezer-online huge amounts of phosphorus rich dust away from the underlying rocks. Then, when the ice retreated at the end of the Snowball, rivers washed this dust into the oceans, where it fed the microbes. canada goose coats on sale
"High phosphorus levels would have increased biological productivity and organic carbon burial in the ocean, leading to a build up of atmospheric oxygen," says Noah Planavsky of Yale University in New Haven, Connecticut. In 2010 he identified a massive spike Canada Goose Parka in phosphorus levels in sediments from around the world, just as Snowball Earth was ending.
Some animals can survive with much less oxygen than previously thought
That was suggestive, but in 2014 Planavsky found more direct evidence. His team estimated oxygen levels prior to Snowball Earth, by studying chromium which exists in different states depending on the amount of oxygen in the air in ancient rocks. Until Canada Goose online 800 million years ago, atmospheric oxygen levels were just one Canada Goose Jackets hundredth of today's levels.
canada goose uk outlet Planavsky thinks that is far too low to support complex animal life. "In modern low oxygen environments there is less ecosystem canada goose deals complexity and a more limited range of animal behaviours," says Planavsky. "So it is reasonable to expect that an oxygen rise would pave the way for animal and ecosystem diversification." canada goose uk outlet
canadian goose jacket But there's a problem with that idea. Experiments published in 2014 showed that some animals can survive with much less oxygen than previously thought. Sponges, one of the oldest kinds of animal, need just 0.5% of modern oxygen levels. That suggests oxygen wasn't enough of a trigger. canadian goose jacket
View image of Sponges may have been the first complex animals (Credit: Matt Kleffer, CC by 2.0)
In recent years another idea has come to prominence. Maybe it was the ice itself that drove the evolutionary leap, says Richard Boyle of the University of Southern Denmark in Odense. "There are no animals more complex than a sponge prior to the last of the Snowball glaciation events, and in my opinion this is not coincidence," says Boyle.
Snowball Earth may have pushed animal cells to specialise
For Boyle the real puzzle isn't the appearance of multicellular animals. Instead, it's the rise of cellular differentiation cells with specific roles like liver, muscle and blood. These specialised cells allowed animals to become much more intricate. "What sets animals apart from plants and fungi is this irreversible cellular differentiation, which, for instance, is what allows animals to have more cell types," canada goose coats says Boyle.
Canada Goose sale It's hard to see how this could have evolved, because specialised cells lose the ability to reproduce on their own. Instead they have to be distinctly self sacrificing, cooperating with other cells in the body for the greater good of the animal. Only the specialised reproductive cells, the sperm and eggs, get to create a new generation. Canada Goose sale
canada goose coats By contrast, plants don't just rely on specialist sex cells to reproduce. They can also reproduce themselves from cuttings taken from their stems or roots. "You can't canadian goose jacket take a cutting from an animal," says Boyle. He thinks the severity of Snowball Earth may have pushed animal cells to abandon this flexibility, and specialise. canada goose coats
View image of Cells have to work together to make animals (Credit: icelight, CC by 2.0)
canada goose uk black friday "During the Snowball period, life will have been confined to small geothermally heated areas, and will have experienced frequent extinctions and population crashes," says Boyle. The populations that did survive were often reduced to just a handful of organisms. Boyle suggests that these little groups of survivors were often closely related, encouraging them to cooperate more than usual canada goose uk black friday.