The origin of life is surely one of the most important questions in biology. How did inanimate molecules give rise to the “endless forms most beautiful” that we see today, and where did this event happen? Some of the most popular theories suggest that life began in a hellish setting, in rocky undersea vents that churn out superheated water from deep within the earth. But a new paper suggests an alternative backdrop, and one that seems like the polar opposite (pun intended) of the hot vents –ice.
Like the vents, frozen fields of ice seem like counter-intuitive locations for the origin of life – they’re hardly a hospitable environment today. But according to James Attwater form the University of Cambridge, ice has the right properties to fuel the rise of “replicator” molecules, which can make copies of themselves, change and evolve.
When thinking about such replicators, DNA – the molecule that is virtually synonymous with life – springs readily to mind. But a world of independent DNA strands makes no sense, because this famous molecules accomplishes very little on its own. DNA needs special proteins in order to copy itself but it, in turn, provides the blueprints for making proteins. So neither DNA nor proteins should be able to evolve without the other.
The chicken-and-egg problem seems inescapably vexing, but it fades into irrelevance when you consider a related molecule called RNA. Today, RNA messages are transcribed from information coded within DNA and then translated into proteins. But RNA is much more than some unglamorous go-between; in fact, it probably deserves to take centre-stage.
Since the 1980s, it has become abundantly clear that RNA is more than capable of performing the roles of both of its partners. Like DNA, it stores information in the form of four ‘letters’ (nucleotides) arranged in specific sequences. But unlike the famous double helix of its relative, RNA is typically found as a single spiral, which can fold into complex shapes. Many of these can speed up chemical reactions in the same way that proteins do. RNA molecules that do this are called ribozymes, and they can even speed up the production of RNA itself.
So RNA can store information, speed up chemical reactions, and make copies of itself without any outside help. It evolves too – stick it in a test tube with the right raw materials and a source of energy and it eventually gets better and better at copying itself. This ability was first demonstrated in 1972 by Sol Spiegelman and the brutally efficient RNA strand that resulted was melodramatically known as Spiegelman’s monster.
In RNA, we have a plausible candidate for the original replicating molecule, from which all life is derived. This concept was deftly summarized by Nobel laureate Walter Gilbert when he coined the term “RNA world”. It’s a wonderfully evocative phrase that brings to mind a planet of evolving RNA molecules that predated the later DNA revolution.
But RNA’s unique physical properties aren’t enough. The molecule is also very fragile and it would rapidly degrade under all but the gentlest environmental conditions. It also needs to be concentrated in some way. A molecule that makes copies of itself needs to be kept in the same place as its constituent chemicals; if the parts are allowed to disperse, the whole will never come together. So RNA may have the right qualities, but it needs a stable and confined space to make the RNA world a reality. Attwater thinks that ice provides just such a space.
At first glance, this seems like a bizarre idea. For a start, cold temperatures can slow many chemical reactions to a crawl. Proteins that piece together RNA molecules stop working when they’re frozen. But remember, RNA in the form of ribozymes can speed up its own creation without any proteins. And Attwater found that one such ribozyme called R18 is still active at subzero temperatures. In fact, ice actually stabilised the ribozyme, preventing it from breaking down. On ice, the ribozyme was slower than at room temperature but it also carried on working for longer. As a result, it was actually more productive, creating longer lengths of RNA with no less accuracy.
That’s one problem down, but there’s also the fact that ice is solid. You might think that this would prevent molecules from meeting each other with ease, but ice isn’t completely solid. At a microscopic level, weaving their way between the crystals, there’s a complicated network of channels and spaces that haven’t frozen completely.
The water in these spaces is salty; as the surrounding molecules froze, any dissolved impurities were pushed away and became concentrated in the remaining liquid. Attwater found that this process boosts the concentration of ions, nucleotides and other chemicals in the liquid compartments by over 200 times. That accelerates the work of the ribozymes, and more than compensates for the slowing effects of the cold.
The liquid compartments provide everything that a RNA molecule needs to reproduce effectively. In these closed quarters, chemical reactions aren’t dependent on the vagaries of open space. Concentrated molecules have a high probability of bumping into one another and are slow to diffuse away.
Of course, this scenario only has a chance of being true if there was a lot of ice on primordial Earth. Attwater pictures frozen lakes and ponds but a decade ago, that would have sounded far-fetched. Scientists commonly assumed that, during our planet’s youth, temperatures on both land and ocean were scorchingly hot. But over the last decade, various studies have suggested that this early climate include more temperate conditions, which allows for the possibility of ice.
This isn’t to say that life began in ice. Attwater has simply demonstrated that ice provides the right conditions for a “cold RNA world” to take off. For now, there’s little evidence that it did so; we simply know that it might have.
There are other places that can provide similar conditions, including the undersea vents that I mentioned at the start of this piece. They too can concentrate molecules within rocky cells, and their high temperatures are a boon to many chemical reactions. Phil Holliger, who led Attwater’s study, points out that vents have high temperatures and high levels of heavy metals, both of which accelerate the breakdown of RNA. “It’s hard to imagine them as the places where RNA-based life could have arisen or thrived,” he says.
But the vent idea has the backing of decades of research. Bill Martin from the University of Duesseldorf certainly thinks that they are the more likely alternative. Of Attwater’s work on ice, he says, “Interesting experiments, I suppose, but holes in ice had as much to do with the origin of life as the electric toaster.”
Ultimately, as I wrote last week in a post about the origin of complex cells, it is predictable that these questions should generate debate. As Holliger concedes, “The actual events of the origin of life are unknown and probably unknowable. What can be tested is the plausibility and consistency of theories.”
Reference: Nature Communications http://dx.doi.org/10.1038/ncomms1076
More on origins: Tree or ring: the origin of complex cells
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