Abstract

Are the characteristics of life as we know it universal? This paper explores the universality of life characteristics as we know them on Earth and addresses the “N=1 problem” – the challenge of making inferences about extraterrestrial life based on a single sample of life. We discuss Carol Cleland's proposal of a “Shadow Biosphere”, a hypothetical second biosphere with different biochemical and/or molecular features with respect to our known biosphere. This work critically examines Cleland's proposal, considering criticisms and possible cosmic scenarios, such as the Impact-Bottleneck hypothesis, suggesting that catastrophic events, like the Late Heavy Bombardment, may have influenced the evolution of life on Earth and the potential existence of a shadow biosphere. We also explore the habitability of celestial bodies in our solar system, such as Europa, Enceladus, and Titan, as potential targets for testing the universality of life characteristics. The conclusion highlights the importance of ongoing research on these environments to provide insights into the existence of a shadow biosphere and the universality of life beyond Earth.