Blurring the lines between life and inanimate matter, biologists announced Friday that they'd created six different chemical alternatives to DNA and coaxed them to undergo evolution.
A description of these code-carrying molecules, called XNAs, was published in the journal Science. The work bolsters a prevailing hypothesis that life as we know it evolved from simpler life forms, no longer here, and those evolved from something simpler. There may be no moment when the first life emerged, but instead an evolutionary process by which chemicals that most of us would consider non-life gradually gave rise to living cells through natural selection.
The work on XNA molecules adds to a growing field of test-tube evolution, in which scientists are nudging code-carrying chemicals to evolve into drugs or other useful compounds.
Scientists don't have a universal definition for life, but they do agree that to qualify as life, an organism must be subject to natural selection. All life on Earth is related and uses the same basic building blocks, but life elsewhere might be put together differently, challenging scientists to recognize it. NASA defines life as a self-replicating system capable of Darwinian evolution. They're keeping the definition broad and their search for life as open as possible.
Biochemist John Chaput, one of the authors of the XNA paper, said evolution can happen in a molecule as long as it can carry a code and copy itself with a few errors. At that point you have something capable of heredity.
Such chemicals might be commonly thought of as primordial soup but on a molecular scale they are highly organized.
By most definitions, including NASA's, these XNAs aren't alive because they need help to replicate. Gerald Joyce, a researcher at the Scripps Research Institute in La Jolla, said that's why he saw no particular danger at this stage. If someone spilled a flask of XNA molecules on the floor or down the drain, the molecules couldn't multiply or infect living things. They can't feed off our biology.
Joyce, who wrote a commentary piece accompanying the scientific paper, said there's more danger in making novel organisms with traditional biochemistry, which could potentially interfere with living things. "This is more radical but less dangerous," he said of these XNA molecules. "It's outside our biology."
Like DNA, these XNA molecules string together four different chemical units into long chains — thus carrying a code in four-letter alphabets. In DNA, the four chemical units are called bases and are identified by the letters A, T, C and G. Those are attached to a backbone or scaffolding, which in DNA is made from sugars and chunks called phosphates. The XNAs use the same characters — A, T, C, and G - but the backbones that hold them together are different.
Scientists suspect that DNA evolved from a simpler cousin called RNA. DNA comes as a double strand of code twisted into a spiralling ladder, while RNA is a simpler single strand. But RNA is still a complicated molecule, and so scientists studying it suspect it came about from a simpler precursor that no longer exists in nature.
Chaput, who works at Arizona State University, was involved with one of the XNAs called TNA, which has a simpler structure than either RNA or DNA. While the TNA isn't alive, it can be induced to undergo evolution, he said. In his lab, he pushed TNA molecules to evolve the ability to do one simple task — to stick to a particular protein. That's something molecules called receptors do in our bodies.
Scripps' Joyce says one interesting property of the XNAs is they're more durable than ordinary DNA or RNA. XNA's don't biodegrade, since they are outside our biology and can't be eaten or broken up by enzymes associated with life as we know it. With RNA and DNA, researchers have to wear gloves to keep from destroying these fragile molecules, but with XNAs that's not necessary.
That ruggedness might make XNAs useful for fishing out specific DNA sequences in genetic tests or other diagnostics, or for detecting contaminants in the environment. Chaput said they could be particularly important in a growing field of medicine in which scientists prompt chemicals to evolve into new drugs. Scientists refer to such evolution-derived drugs as aptamers. One, called Macugen, has already received FDA approval for the treatment of an eye disease known as macular degeneration.
Joyce, who is a fan of the Arthur C. Clarke novels 2001 and 2010, sees Clarke's fictional planetary explorations as a metaphor for the power and danger that could come if scientists someday created XNA-based life.
In 2010, astronauts discover life on Jupiter and Europa, and while humans are tempted to explore and exploit Europa, the computer, Hal, warns humanity to attempt no landings there, presumably because our life would contaminate and destroy theirs.
In this case, Joyce worries about a similar danger to life on Earth. Scientists, Joyce wrote in his commentary piece, "are beginning to frolic on the worlds of alternative genetics, but must not tread into areas that have the potential to harm our biology."