the idea was subsequently largely discredited , admittedly by the americans , who had a vested interest in seeing it quashed since they had been responsible for developing the vaccine in the first place , but nevertheless quite convincingly , when computer extrapolations back of the mutations that would have had to occur in the dna sequences between the siv virus and the hiv virus , indicated that the crossover must have occurred significantly further back in time , perhaps in the 1930s .