The Shadow Biosphere: Life Could Be Missed Because We Only Look for DNA

In 2009, Paul Davies and Charley Lineweaver asked whether a second branch of life could have existed on Earth with a different chemistry and gone unnoticed. Laboratories usually search for DNA, RNA, proteins, and a known genetic code. A sample without those familiar signs can be labelled contamination, dead material, or a failed test. A living community could therefore exist on Earth without fitting the usual tests for life.

Every test has limits. PCR primers amplify only the DNA sequences they recognise. Fluorescent stains usually bind to familiar membranes or proteins. Culture media provide nutrients chosen for known microbes. A cell could use another polymer, lack an ordinary lipid membrane, or use a food source absent from the laboratory medium. The test might then miss it even while it exchanges energy.