Final answer:
Proteins were once thought to be carriers of genetic information because their structural diversity, owing to 20 different amino acids, matched the diversity needed to explain inheritable traits. DNA was underestimated due to its simpler structure of four nucleotides, which made it seem insufficient for complex genetic encoding.
Step-by-step explanation:
Proteins seemed like a likely candidate for carrying genetic information primarily because of their diverse types, shapes, and functions, which provided a structural diversity believed necessary to account for the myriad of inheritable traits found in organisms. Initially, DNA was viewed as too simple due to its composition of only four nucleotides, thus lacking the complexity to correspond to the wide variety of traits that needed explanation.
Proteins, on the other hand, are composed of up to 20 different amino acids, allowing for a multitude of combinations and consequently the potential to account for the growing number of heritable traits observed. This structural variability in proteins aligned with the observable diversity in biological characteristics, making them prime candidates for the molecules of inheritance before the discovery of DNA's role.