Distinguishing biotic compounds from abiotic ones is important in resource geology, biogeochemistry, and the search for life in the universe. Stable isotopes have traditionally been used to discriminate the origins of organic materials, with particular focus on hydrocarbons. However, despite extensive efforts, unequivocal distinction of abiotic hydrocarbons remains challenging. Recent development of clumped-isotope analysis provides more robust information because it is independent of the stable isotopic composition of the starting material. Here, we report data from a C-13-C-13 clumped-isotope analysis of ethane and demonstrate that the abiotically-synthesized ethane shows distinctively low C-13-C-13 abundances compared to thermogenic ethane. A collision frequency model predicts the observed low C-13-C-13 abundances (anti-clumping) in ethane produced from methyl radical recombination. In contrast, thermogenic ethane presumably exhibits near stochastic C-13-C-13 distribution inherited from the biological precursor, which undergoes C-C bond cleavage/recombination during metabolism. Further, we find an exceptionally high C-13-C-13 signature in ethane remaining after microbial oxidation. In summary, the approach distinguishes between thermogenic, microbially altered, and abiotic hydrocarbons. The C-13-C-13 signature can provide an important step forward for discrimination of the origin of organic molecules on Earth and in extra-terrestrial environments.