Scientists didn't find the human gene that *makes* insulin. They found the gene that *codes for* insulin, meaning it contains the instructions for building the insulin protein.

Here's a breakdown of how they did it:

* Understanding the role of insulin: Scientists knew insulin was a key hormone in regulating blood sugar.

* Identifying insulin's structure: They figured out the structure of insulin and how it interacted with cells.

* Mapping the genome: The Human Genome Project, a massive international effort, mapped the entire human genome, identifying the sequence of all our genes.

* Searching for the insulin gene: Scientists used their knowledge of insulin's structure and the sequenced genome to pinpoint the gene that coded for the insulin protein.

So, the process involved a combination of:

* Understanding the function of insulin.

* Determining insulin's chemical structure.

* Mapping the entire human genome.

* Using this knowledge to identify the gene responsible for building insulin.