Why is there a comma in this sentence?
Jack has been studying zebras since 1972, when he started the famous Animal Center.
Isn’t the first clause independent, and isn’t when a subordinating conjunction making the second clause dependent? In this case, my basic grammar book says there’s no need for comma. Where am I wrong?
The explanation in my book says “the comma is appropriate between the noun ‘1972’ and the dependent, relative clause that begins with ‘when’.”