We know the first single-celled organisms were swarming across the planet over 3.5 billion years ago. The big question is how the precursors of those cells made the jump from collections of chemicals to multicellular organisms. Two recent theories explain that it's all about ice.
Many geologists believe that the early Earth underwent several phases called snowball Earth, where the planet was entirely covered in ice. The last snowball would have ended before the Cambrian explosion over half a billion years ago. These super-ice ages were caused in part by the relative dimness of the Sun — these days, our star is too hot for a snowball to happen. It's also possible that a spike in oxygen could have cooled the planet even further. 3.5 billion years ago, cyanobacteria, or bluegreen algae, started releasing oxygen during photosynthesis. This could have cooled the planet down enough for ice to form at the poles. The more ice formed, the more sunlight was reflected back into space, making the planet even cooler. Eventually, there was a runaway icehouse effect, turning our blue marble into a cue ball.
How did anything survive in these snowball conditions? One geologist, Adam Campbell, believes that life endured by clustering in small seas that remained liquid even in the deepest icehouse conditions.
Campbell believes that a long, narrow body of water connected to the ocean could have sheltered the algae, while the rest of the Earth looked like Hoth from The Empire Strikes Back, only without the Wampa ice creatures.
The Red Sea is a good modern example of this type of body of water. The Red Sea is about 6.5 times longer than it is wide, and it connects to the Indian Ocean. A long sea like that during the “snowball Earth” period would have provided enough resistance to glaciers to remain ice-free.
"The initial results have shown pretty well that these kinds of channels could remain relatively free of thick glacial ice during a 'snowball Earth' event," Campbell said.
After the most recent snowball, life on Earth underwent the massive diversification in evolutionary development that's often called the Cambrian Explosion. Multicellular plants and animals filled the seas with life unlike anything seen before on this planet.
Another theory is that the earliest forms of life evolved in strange tubes of ice called brinicles. In a recent paper, Bruno Escribano and colleagues argue that long before the snowball seas that Campbell describes, single-celled organisms may have assembled out of chemical traces in these brinicles. In the journal Langmuir, the researchers write:
Brinicles are hollow tubes of ice from centimeters to meters in length that form under floating sea ice in the polar oceans when dense, cold brine drains downward from sea ice to seawater close to its freezing point. When this extremely cold brine leaves the ice, it freezes the water it comes into contact with: a hollow tube of ice—a brinicle—growing downward around the plume of descending brine.
It's very possible that these hollow tubes beneath glaciers provided the ideal conditions for life to evolve in a world of water and ice. According to a release about the paper:
The analysis concluded that brinicles provide an environment that could well have fostered the emergence of life on Earth billions of years ago, and could have done so on other planets. “Beyond Earth, the brinicle formation mechanism may be important in the context of planets and moons with ice-covered oceans,” the report states, citing in particular two moons of Jupiter named Ganymede and Callisto.
Both theories rely on the idea that portions of the planet remained ice-free, allowing life to find a tiny niche where it could continue to evolve. Plenty of life still thrives in the ice today, and recently scientists discovered bacteria in the Arctic living in the ice at -15 C.
Adam J. Campbell, "Refugium for surface life on Snowball Earth in a nearly-enclosed sea? A rst simple model for sea-glacier invasion" [PDF], Geophysical Letters
Annalee Newitz is the author of the book, Scatter, Adapt and Remember: How Humans Will Survive a Mass Extinction. Follow her on Twitter.