MOST people agree that for something to be alive, it must be able to make copies of itself. But by that rationale, a crystal growing in a solution is alive. So biologists studying how life began 4 billion years ago look for characteristics shared by all living things and absent in minerals.
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This approach yields three distinct features: all organisms on Earth have a code that, like a builder’s blueprint, allows copies to be made (see “How to think about… Genes”); they can generate energy to power the copying process; and they have the machinery to build the copies. Crystals have none of these, so are firmly dispatched to the realm of minerals.
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This list kicks up other sticking points, however – notably certain parasites. Viruses, the ultimate example, have a code in the shape of DNA or RNA, but rely entirely on the cells they have invaded for energy and copying machinery. The debate over whether they are alive is decades old.
Synthetic biology raises even more basic questions. at the Scripps Research Institute in San Diego has spent 20 years trying to hack life’s code. His team has created two “unnatural” genetic letters similar in molecular structure to the five used in all living organisms on Earth: four in DNA and an additional one in RNA. Last year, they used this unnatural code to coax a cell to produce proteins not found in nature.
For Romesberg, this calls into question whether the chemistry of life is any different from inert chemistry. “All you have is parts. All you have is carbon, nitrogen, phosphorus, oxygen,” he says. Yet some of these parts end up being living and some don’t. “In a really weird way, maybe there’s just not that much difference between living and non-living things.”
NASA, in its hunt for life on other planets, has its own definition. Extraterrestrial life may look very different but will, it says, be a “self-sustaining chemical system capable of Darwinian evolution”. It is a definition that humans might be about to bust. The pinpoint gene-editing method known as CRISPR could soon allow us to place DNA sequences into our eggs and sperm, so creating children that are fitter by design, rather than evolution. If we ever succeed, are we still living things – or have we transcended that state?
This article appeared in print under the headline “How to think about… Life”
