To explain how life on Earth began, the big challenge is to identify the molecules and processes that enable non-living chemical systems to become more complex.
“IT IS one of these big questions that we truly are clueless about at this point,” says Lena Vincent at the NASA Jet Propulsion Laboratory in Pasadena, California. “Anyone who says otherwise, I would be sceptical of.”
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Which isn’t to say we are short of ideas for how life on Earth began. On the contrary, all manner of hypotheses have been put forward to explain how non-living chemicals might self-assemble into a living organism. Some rely on hypothetical self-replicating molecules, some on blob-like structures that could have been the predecessors of cells, while others focus on complex cycles of chemical reactions.
Yet none of these ideas has gained widespread acceptance, never mind led to a definitive experiment. As a result, thinking about life’s origins is an exercise in reasoning under uncertainty. It is about how to tell the difference between a truly promising idea and one that merely has a veneer of plausibility.
The truth is that we can’t know for sure exactly how life began on Earth, says Vincent. “We just don’t have access to the part of history on this planet that would allow us to verify that.” The best we can do is to demonstrate processes that produce life and show that they are compatible with what we know about the early Earth.
For that reason, Vincent prefers to talk about “origins of life”, to avoid implying that we are studying a single, specified event. It “may have happened many times”, she says, and “may be continuing to happen” somewhere in the universe.
What is important is whether an experiment seeking to demonstrate how life began is plausible; is it reasonable to suppose that something like it happened somewhere on Earth billions of years ago? Here, again, we face uncertainty: little is known about what the planet was like when it was young, so almost anything can be argued to be “plausible”.
The best we can do is attempt to recreate essential processes of life, such as metabolism or replication. “An interest in the processes, regardless of what they are, to me is more interesting than just finding a molecule,” says Joseph Moran at the University of Strasbourg in France.
The problem is that we don’t know which of life’s processes came first, and therefore which we should be seeking to demonstrate in the lab. For example, by comparing the DNA of distantly related organisms, biologists have partially reconstructed the Last Universal Common Ancestor (LUCA): the species from which all living organisms today are descended. Unfortunately, it turns out LUCA was complicated, with hundreds of genes and proteins. There must be a long prehistory of life prior to LUCA, about which we know virtually nothing, so we can’t use LUCA to figure out which of life’s processes came first. “Simply saying that something is universal does not constitute an argument [that it came first],” says Eric Smith at the Earth-Life Science Institute in Tokyo, Japan.
Experiments have produced increasingly complex chemical systems that aren’t alive but have some of the features of life. However, these chemical systems are still a far cry from even the simplest known forms of life. Smith says that one of our biggest issues is that we have no idea what the intermediate stages looked like.
A key challenge will be to identify the bottlenecks that prevent non-living chemical systems from becoming more complex. The most promising molecules and processes will be those that offer ways through those bottlenecks, says Smith. “This is why I think the origin of life will turn out to remain an interesting and fun problem for quite a long time.”