Before we go looking for signs of life on other planets, we might want to know what “biosignatures” can be detected on a world already known to be inhabited by organisms—our own Earth. This was one of the main themes of the Second Workshop of the German Astrobiological Society held in Potsdam, Germany, this week. This future-focused meeting started off with a reminder of our planet’s past: Some participants were delayed getting to Potsdam after the city’s main train station had to be evacuated due to the discovery of a 250-kilogram bomb from World War II that had to be defused.
On the first day of the workshop, Alessandro Airo from the Technical University Berlin set the tone for the following talks by delving into the types of biosignatures we find in different terrestrial environments, from desert areas to tidal flats. Henry Strasdeit from the University of Hohenheim then discussed the reliability of different biosignatures, particularly the danger of false positives. For example, bacterial cells (living) and silica spheres (not living) can look nearly identical under the microscope.
Fortunately, said Strasdeit, some reliable biosignatures can be preserved for a very long time. He gave three intriguing examples: Collagen peptides in a bone matrix can survive up to 80 million years; porphyrins, the breakdown products of chlorophyll found in green plants, has been recovered from 500 million-year-old rocks; and lipid (carotene)-derived compounds, common remnants of cell membranes, have been recovered from 1.6 billion-year-old rocks. Strasdeit and one of his postdoctoral students recently published a thorough review of the topic, with even more examples.
Other talks at the meeting dealt with organic compounds recovered from meteorites and comets, and how to decide whether they were of purely chemical or biological origin. Tetyana Milojevic and colleagues from the University of Vienna, Austria, talked about metal-encrusted microbes found on certain terrestrial and extraterrestrial materials. Their lab experiments provide new insights on how this surface biomineralization results in long-term preservation of microbial cells. Knowing which parts of different organisms survive over time is a critical piece of the puzzle if we want to understand the early evolution of life on Earth, and whether it ever existed or may still exist on other planetary bodies such as Mars.