All life on Earth is based on DNA. If we ever find life on other worlds, are there viable alternatives for coding for life that it may be based on?
Mike Follows
Sutton Coldfield, West Midlands, UK
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Life on our planet relies on carbon-based DNA. The panspermia hypothesis suggests that life could have originated in a different part of the universe and then been transported to Earth. In that case, the coding we see on Earth might be universal. However, while molecules like amino acids have been discovered in space, the extraterrestrial formation of complex molecules like DNA and their survival in transit is entirely speculative.
Although carbon-based DNA could have formed independently in different parts of the universe, it is also possible that any alien life could be based on different building blocks, like silicon, or could rely on a different biochemistry. Extraterrestrial life could even use quantum mechanics to process information, transfer energy and communicate.
In his 1944 book What is Life?, Erwin Schr枚dinger introduced the possible role of quantum mechanics in biological processes. Since then, it has been suggested that quantum phenomena 鈥� such as superposition (where particles exist in multiple states) and entanglement (where particles affect each other across distances) 鈥� could enable life to function more efficiently, with faster energy transfer and information exchange, which could allow alien life to survive in extreme environments.
Quantum phenomena might enable alien life forms to function more efficiently and so survive in extreme environments
It is possible that many of the plausible life forms imagined in science fiction, as well as any artificial intelligence or artificial life that we might develop or initiate in the future, already exist somewhere else in the universe.
Garry Marley
Stillwater, Oklahoma, US
First, let鈥檚 assume that this question is aimed towards life as we know it in the 鈥�Goldilocks zone鈥� of planetary temperature, where water is mostly in liquid form. Also, let鈥檚 assume that matter there consists of the atomic elements we know. Those criteria favour a biochemistry heavily dominated by carbon, which has exceptional affinity for itself. That is why the discipline of organic chemistry 鈥� the chemistry of carbon compounds 鈥� is so vast.
In the 1952 Miller-Urey experiment, the presumed gases (ammonia, methane and hydrogen) of Earth鈥檚 primordial atmosphere were mixed with water vapour and exposed to electrical sparks (simulating lightning). The reaction products were a biochemical feast that included amino acids and nitrogenous bases, the organic building blocks of proteins and nucleic acids, respectively.
When made non-biologically, amino acids are produced in a random mixture of 鈥渓eft-handed鈥� and 鈥渞ight-handed鈥� forms. All amino acids in terrestrial life are 鈥渓eft-handed鈥�. Some meteorites have yielded fascinating data on amino acids, starting with the Murchison meteorite that hit Australia in 1969. Samples from its fragments revealed a mixture of amino acids that were skewed towards 鈥渓eft-handed鈥� forms.
DNA is a unique, self-replicating, polymeric molecule that carries genetic information encoded as three-letter sequences. In contrast, crystals, for example, can replicate with precision, but don鈥檛 convey any coded information.
However, many molecular biologists hypothesise that DNA had chemical precursors at the dawn of terrestrial life. In our time, some RNA sequences, known as ribozymes, can catalyse processes such as the formation of short chains of amino acids called peptides.
DNA is only subtly different from RNA, with one of its nitrogenous bases being different and having a deoxyribose sugar instead of ribose. Yet these changes impart a greater chemical stability to DNA and thus to the life it encodes.
The incredible variety of enzymes present in life also facilitates complex biochemical pathways far beyond the catalytic properties of RNA. Therefore, given our current environment, DNA seems to be at the spearhead of a long and complex chemical evolution that highly fit extraterrestrial life would be likely to emulate.
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