Proteins' great potential for harm, via misfolding and aggregation, does not prevent them from being born de novo from non-coding DNA. To investigate how newborn proteins might "first, do no harm", we estimate fitnesses from an experiment that competed Escherichia coli lineages that each expressed a unique random peptide. A variety of peptide metrics significantly predict lineage fitness, but almost all this predictive power stems from simple amino acid composition. Low fitness is predicted by hydrophobic and positively charged residues, positive net charge, aggregation prone regions, and underdispersed hydrophobic residues, while high fitness is predicted by disorder-promoting, negatively charged, small residues, and negative net charge. The same amino acids that predict high fitness in E. coli are enriched in young Pfams in animals, but not in plants. To modify Jacques Monod's famous quote, what makes peptides benign in E.coli also makes them benign in elephants, but not in eucalyptus.