It sounds like science fiction, but Mr. Sitchin is sure this is how it all went down hundreds of thousands of years ago in Mesopotamia. Humans were genetically engineered by extraterrestrials, he said, pointing to ancient texts to prove it.

In Mr. Sitchin’s Upper West Side kitchen, evolution and creationism collide. He is an apparently sane, sharp, University of London-educated 89-year-old who has spent his life arguing that people evolved with a little genetic intervention from ancient astronauts who came to Earth and needed laborers to mine gold to bring back to Nibiru, a planet we have yet to recognize.

Outlandish, yes, but also somehow intriguing from this cute, distinguished old man whom you may have seen shuffling slowly down Broadway with his cane, and thought, “Is Art Carney still alive?”

So you bring your laptop to his kitchen table, as if to take dictation, and ask what to write about him. He pads slowly to the stove and puts on the kettle.

“Well, you could start by calling me the most controversial 89-year-old man in New York,” Mr. Sitchin says. “Or you could just say I write books. I understand you’ve got to have an opening sentence, but describing my theories in a sentence, or even something like a newspaper article, is impossible. It will make me look silly.”

Mr. Sitchin has been called silly before — by scientists, historians and archaeologists who dismiss his theories as pseudoscience and fault their underpinnings: his translations of ancient texts and his understanding of physics. And yet, he has a devoted following of readers.

His 13 books, with names like “Genesis Revisited” and “The Earth Chronicles,” have sold millions of copies and been translated into 25 languages…