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
If the citation link isn’t working, read why here
September 21st, 2010 at 12:29 pm
Great article.
I cringed a little while reading this:
“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.”
September 21st, 2010 at 1:59 pm
Fascinating. My favourite hypothesis for origin of life is Gold’s “deep-hot biosphere” one, but the ice one sounds promising indeed
What an article, you sure have a gift for clarity. And of course I loved Darwin’s quote. The only thing I didn’t like, the metaphor of blueprint for DNA. I know everybody in the English-speaking world use it, but as none less than Richard Dawkins pointed out, it’s not a very brilliant metaphor, recipe is a better one
***
““The actual events of the origin of life are unknown and probably unknowable. What can be tested is the plausibility and consistency of theories””. And, as – again – Richard Dawkins points out in one of his books, one should not necessarily think of a very likely scenario: origin of life needed to happen just once for all we know, so it could well have been a very improbable event. After all, the improbable *can* happen – otherwise we would call it impossible
September 21st, 2010 at 3:09 pm
The icy start hypothesis is an interesting one., but it would seem, at first blush, like the hot vent starting point may be preferential for establishing the dominance of right-handed chirality currently observed in life.
September 21st, 2010 at 4:22 pm
Very Interesting. This is good news for the possibility of life under the ice sheets on Europa, right ?
September 21st, 2010 at 4:29 pm
It seems counterproductive to oppose ice origins to hot-vent origins. You could have RNA life starting in ice, and spreading from there to less hospitable environments, and then DNA life starting as an adaptation to conditions in hot vents, and then exploding out to supplant its progenitor.
Do we know for certain there is no RNA-only life any more? If it were found in ice that would be pretty persuasive.
September 22nd, 2010 at 3:46 am
Very interesting, would never have considered that life could start in cold conditions. I would love to hear your take on the chicken and egg paradox that still remains: how did the first ribozymes form when, presumably, you need a catalyst to form a catalyst? I know there are several ideas about this, including the one found in this recent paper: http://www.ncbi.nlm.nih.gov/pubmed/20813983. Would love to hear if anyone has come across other interesting hypotheses!
September 22nd, 2010 at 9:32 am
In addition to the hot “black smoker” vents, there are also cooler “white smokers”. Nick Lane has made a case for these as the ideal sites for the origin of life:
http://www.nick-lane.net/LAM%20BioEssays.pdf
September 22nd, 2010 at 10:52 am
Sarah – There is no paradox re ribozymes; ribozymes have been isolated from pools of random RNA sequences (which is what, effectively, would have occurred naturally on the early earth). In other words, if eggs are being assembled at random, given enough time one will hatch a chicken. Bear in mind ribozyme activity can be chemically pretty simple, so there are no great leaps really.
See Bartel & Szostak’s stuff.
September 22nd, 2010 at 4:28 pm
The article is a bit confusing, since we know that there was an RNA world before DNA-protein synthesis: it was successfully tested when we observed that the DNA to protein translator ribosome is a ribozyme. What we don’t know is if the RNA world constituted the first life.
Unlikely, since it doesn’t solve the chicken-and-egg problem of metabolism-enzymes, where metabolism depends on enzymes not only to speed up but to stop side reactions, while enzymes depends on metabolism. The “catalysts need catalysts” problem Sarah mentions (related to the “chirality needs chirality” problem), and that needs to be solved to understand the likeliest pathway where RNA nucleotides come from in the first place.
Hear, hear! Since most of development and so its learned information lies not in the genome but in its interaction with the environment “recipe” is a brilliant analogy.
No one would think of calling camera drawings that are based on photoshops of a city map (say) “blueprints”, or browser software that asks for chosen parts of the latest game to work “self-contained”. But try to describe the genome and everyone goes Lady Gaga!
That is all well and good until you look at the timing, life was existing in the first rocks that survived plate tectonics to be analyzed today. The speed with which life come to be makes a slowly mapped “improbable” pathway unlikely, and a fast mapped likely one the likeliest. (Well, duh!)
September 22nd, 2010 at 8:17 pm
Alex, thanks for the info! I guess what I am really interested in, as Torbjörn Larsson pointed out, is the (fascinating) problem of how monomers formed before there were catalytic polymers to synthesize them. There are various chemical hypotheses for how that might have happened in primordial conditions, and investigating them (as scientists are doing) could help narrow down which origin of life ideas are more plausible.
September 23rd, 2010 at 2:02 am
I find this quite interesting! It sounds like life could start even in a large ‘enough’ icy meteoroid – and there’s plenty of those around.
We just need to send someone to sample a bit…
September 24th, 2010 at 12:41 pm
The easiest way is to use inorganic catalysts. Mineral crystals can form spontaneously and can be catalytic. Indeed, there are many enzymes in which the catalytic core is a mineral ion or a small mineral crystal (and interestingly, many of these enzymes are ones involved in the very core processes of life) – the inorganic mineral is actually doing all the catalytic work, while the surrounding protein is just a carrier, or tweaks the conditions in which the catalysis is most optimal, or regulates the catalysis (ie by folding into configurations that either hide or expose the catalytic site, depending on conditions).
As I understand it, most abiogenesis research/ideas are focused on looking at potential prebiotic conditions/environments where the biologic monomers and the initial steps of their polymerization either form spontaneously, or are catalyzed by inorganic catalysts that form spontaneously in those environments.