The question of how nucleic acids assembled into DNA and RNA polymers, without enzymes, still roils researchers who think about life’s origins on Earth. Some think a clue lies in a 40-year-old idea from Francis Crick—that Watson-Crick base pairs, made with one purine and one pyrimidine base, might have evolved from purine-purine pairs. Evidence exists to support that idea: Purines are products of model prebiotic reactions, and purine-purine pairs can be as stable as Watson-Crick pairs. Now, a study with small paired strands of nucleotides throws more support Crick’s way (ChemBioChem, DOI: 10.1002/cbic.201200601). With polyacrylamide gels and a chemical activating agent, Elizabeth Kuruvilla, Gary B. Schuster, and Nicholas V. Hud of Georgia Institute of Technology measured how well different strands polymerized. Sure enough, all-purine strands performed better than counterparts containing both purines and pyrimidines. Chemists know that stacking interactions between bases help nucleotide polymerizations along and that purines’ bigger surface area makes them better stackers. The team thinks these concepts explain their results. They propose that a primitive information polymer, perhaps an ancestor of RNA, would have had optimal base-stacking properties.