In this Yukon mountain, a maroon-coloured, iron-rich layer of glacial deposits 716.5 million years old can be seen on top of older, grey carbonate reef. (Francis A. Macdonald/Harvard University)

Geologists working in the Canadian North have found evidence that the Earth's oceans were covered in ice all the way to the tropics more than 700 million years ago.

The researchers' findings give strength to the "snowball Earth" idea, that the planet was virtually covered in ice around the same time the first animals evolved.

The American and Canadian geologists analyzed rocks from the Yukon and Northwest Territories and found glacial deposits 716.5 million years old, based on precision uranium dating techniques.

The purplish glacial deposits were found on top of old reef material composed of carbonate, shifted north by the movement of tectonic plates.

The magnetic and chemical composition of the rocks indicate that they were located at sea level in the tropics — about 10 degrees latitude — when the deposits took place.

The study published this week in Science is the first evidence, the geologists say, that this glacier, called the Sturtian glaciation, reached the tropical latitudes.

Francis Macdonald of Harvard University, the study's lead author, said that the research provides "direct evidence that this particular glaciation was a 'snowball Earth' event."

"Our data also suggests that the Sturtian glaciation lasted a minimum of five million years," said Macdonald, in a statement.

Other evidence, from fossils and mineral isotopes, suggests the Sturtian glaciation could have lasted as long as 20 million years, from 720 million to 700 million years ago.

Geologists don't know what caused the Earth to freeze over or what ended the glacier event.

Fossils also show that early complex life forms, called eukaryotes, emerged before the Earth froze over and somehow survived the glaciation.

"The fossil record suggests that all of the major eukaryotic groups, with the possible exception of animals, existed before the Sturtian glaciation," Macdonald said.

"The questions that arise from this are: If a snowball Earth existed, how did these eukaryotes survive? Moreover, did the Sturtian snowball Earth stimulate evolution and the origin of animals?" he said.

MacDonald said that even a snowball Earth would have warm spots of open water that would allow sunlight through, allowing life to survive. The deep freeze could have even spurred the evolution of animals from more primitive eukaryotic life.

"From an evolutionary perspective, it's not always a bad thing for life on Earth to face severe stress," he said.