Contemporary humans are no longer marked by an identifying set of social security numbers, but by biometrics. Biometrics marks each individual by certain biological and behavioral characteristics. These characteristics make each person unique digitally and include palm, fingerprint, iris, facial and voice recognition.
Social security numbers identify each American by area, group, then a sequence of four digits within that group. They are never recycled, so they are useful tools for identity verification. The first three digits indicated state, prior to 1972; after that the numbers became issued by centralized social security offices using zip codes as basis for assignment. The two middle digits represent your group, the last four digits are group serial numbers ranging from 0001-9999.
Social security numbers are less identifying than biometrics as they can be stolen. Identity thieves hijacking your identifying biometric patterns is unlikely as sensors gather your personal data, convert it into a unique template which is stored and compared against future scans. Authorities may use social security numbers and biometrical data to identify a person, making social security identification stronger.