"why are proteins not considered to be a good candidate for the first living molecule?"

Proteins are not considered to be a good candidate for the first living molecule for several reasons:

1. Complexity: Proteins are made up of 20 different amino acids, and the formation of proteins requires specific sequences of these amino acids. The complexity of protein synthesis is not easily compatible with the conditions thought to be present on early Earth.

2. Catalysis and Information: While proteins are excellent catalysts (enzymes) for biochemical reactions, they do not inherently contain the information needed to replicate themselves. The first living molecule would need not only to catalyze reactions but also to store and transmit information required for self-replication.

3. Dependence on Nucleic Acids: Proteins are synthesized by the process of translation, which is mediated by RNA molecules and the genetic code stored in DNA. The synthesis of proteins requires a sophisticated cellular machinery that includes mRNA, ribosomes, tRNAs, and other components. All these components are necessary for protein synthesis, and it is unlikely they existed before the first living molecule.

4. The RNA World Hypothesis: The RNA world hypothesis suggests that RNA, which can store genetic information like DNA and can catalyze chemical reactions like proteins, could have been the first molecule to lead to life because of these dual properties. RNA is simpler and has the ability to self-replicate, making it a stronger candidate for the first living molecule.