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Animal is an early self-repeating program from 1975. Its primary purpose was to distribute a "20 Questions" game in which the user gives the program clues as to what animal he/she is thinking of and the program guesses it. Though it is predated by Creeper, Reaper and some Rabbit programs, the Animal program is the earliest self-replicator whose existence can be confirmed.
When Animal is executed, it asks twenty questions for clues and tries to guess an animal that the user is thinking of. Animal calls the "Pervade" subroutine, which finds the directories that the current user has access to. If the directory does not contain an up-to-date version of Animal, it will copy itself to that directory. It will overwrite any older version of Animal, but will take special care to not delete any other programs.
Animal's creator, John Walker let the program loose in San Francisco, and after around a week it was in Washington DC. Univac's development center in Roseville Minnesota had an Animal infection, and some Univac users claimed that they had received infected software distribution tapes. The program spread for about a year, until an update for the Exec 8 operating system was released, which changed the system tables that the Pervade subroutine checks.
It is unlikely that Animal ever caused any measurable damage. The program contained no deliberately malicious code and did not even exploit any system vunerabilities. It even took special care to not affect any of the user's programs.
John Walker's original Animal program was coded in 1974 April, but the Pervade routine that gave the program its self-replication ability was introduced to the program in 1975 January. Walker observed many years later that the program would be considered a malicious program, but at the time he saw it as a good idea.
Legend to History
The story of the Animal program gained the status of an urban legend. One variation of the legend tells of a program named "Hunter" that was used to destroy Animal. Another version, described by John Walker as "garbled", appeared in Scientific American in 1985 in March (the articles can be found in the book "The Armchair Universe" by A.K. Dewdney), prompting Walker to write a letter to Scientific American to explain what really happened.
Because Walker came forward with the story, Animal is the first self-replicating program whose existence we can be completely certain of, as its source codes can be found online. Creeper, Reaper and Rabbit are still semi-legendary, as no creator has come forward and no source codes or binaries of these programs can be found.
John Walker. The Animal Episode. 1996.08.21
Alexander Dewdney. Scientific American, (1985, v.251, N 3, pp. 14-23) A Core War bestiary of viruses, worms and other threats to computer memories
Corinne Cullen Hawkins. A Software Bestiary: Computer Parasites & Remedies