Ulam's work on non- Euclidean distance metrics in the context of molecular biology made a significant contribution to sequence analysis and his contributions in theoretical biology are considered watersheds in the development of cellular automata theory, population biology, pattern recognition, and biometrics generally (David Sankoff, however, challenged conclusions of Walter by writing that Ulam had only modest influence on early development of sequence alignment methods.). Colleagues noted that some of his greatest contributions were in clearly identifying problems to be solved and general techniques for solving them.