Life’s cold start

The hot spot for life on early Earth may have been a very cold place. Tiny pockets and channels that form inside ice can contain and protect replicating molecules, researchers report September 21 in Nature Communications.

The paper suggests that life could have sprung from icy slush covering a freshwater lake, rather than a broiling deep-sea hydrothermal vent or the “warm little pond” proposed by Charles Darwin. And perhaps the frigid, icy surfaces of other planets are not as barren as they appear, proposes the research team from the MRC Laboratory of Molecular Biology in Cambridge, England.

Scientists studying the origin of life have long been vexed by the problem of protecting and containing life’s starter molecules before the advent of the tidy compartments known as cells. In present-day organisms, cells concentrate molecules, keeping ingredients and machinery within each others’ reach. Cells also protect the molecules that encode genetic information: RNA, the molecule of heredity that presumably got the whole life thing going, is a very fragile affair.

Previous work had shown that nooks and tiny crevices within ice could provide a cozy, safe place for the construction of an RNA molecule. As ice forms, pure water becomes crystallized, while salts and other bits of debris accumulate in watery pockets. These impurities lower the water’s freezing point, and the little pockets may remain unfrozen within an otherwise solid chunk.

To see if RNA could replicate within these liquid pockets, a team led by chemical biologist Philipp Holliger took test tubes of water and added salts and some of life’s presumed starter ingredients — an RNA molecule that can make reactions go, known as a ribozyme — and the building blocks this molecule would need to make a full copy of itself. Then they cooled the tubes to a range of temperatures.

Not only did the ribozymes go about their business of building RNA strands, but the reactions continued for much longer in the icy test tubes than at ambient temperatures, the team reports.

“It’s a little like the tortoise and the hare,” says Holliger. “At ambient temperatures it goes quite quickly, but then it just stops. But at cold temperatures, the reaction proceeds, it slowly catches up to that of the ambient temperature and then it overtakes it and keeps going.”

While the experiments suggest the reactions necessary for life can proceed in ice, “we are some distance from self-replication,” cautions Holliger. The team got strands that were 32 building blocks long, while the full-length ribozyme is 190.

Nonetheless, the results show serious RNA activity in ice, a real feat, says Pierre-Alain Monnard of the University of Southern Denmark. “That ice can be a quasi-compartment — this is essential,” he says. “It’s beautiful work, the way it has been done, really beautiful.”

It remains to be seen whether full cycles of replication, which involve reactions that typically occur at higher temperatures, could also proceed, says Howard Hughes Medical Institute investigator Jack Szostak of Harvard Medical School.

The team also examined the microstructure of ice containing varying concentrations of salts. When the solution was more dilute, the channels threading through the ice fractured and disconnected. This suggests better compartmentalization occurs in a more dilute solution, perhaps pointing to a freshwater origin, not a salty one. Such a primordial slush might even exist on ice-encrusted bodies such as Jupiter’s moon Europa, Saturn’s moon’s Enceladus and any number of comets.

“There are caveats, of course,” says Holliger. “But it is very striking that if you look at our solar system you find ice pretty much everywhere.”