Pitt stars as Ian Gray, a fervent atheist and proponent of Darwinian evolutionary theory who decides to prove, once and for all, that the human eye developed through random selection and was not created by God. Brit Marling plays his lab assistant, Karen, a brilliant student who carries a torch for the bow-tie-wearing professor.
For the first year of their relationship, Ian hardly notices Karen. His heart is stolen by the beautiful Sofi (Astrid Berges-Frisbey), a runway model who has a deep commitment to a loosey-goosey form of New Age spiritualism.
She's a firm believer in reincarnation - which Ian mocks.
Despite its science-related plot, I Origins progresses through the ebb and flow of this love triangle. And it is love that eventually inspires in Ian a profound spiritual experience and a huge shift in his belief system. (Suffice it to say, it has to do with reincarnation.)
"Despite all the science in the movie, it seems to me the root of what [Cahill] was trying to get to was about love," said Pitt, who made a publicity stop in Philadelphia on Tuesday with his director.
Some critics took Cahill's first film to task for its fuzzy science. This time, the filmmaker said, he decided to stick close to fact, including some parts of Ian's grand anti-theological eye experiments.
To prove the human eye evolved, Ian decides to find the least complex organism that possesses the gene in charge of regulating development of the eye. He finds it first occurs in a small worm with no sight - the gene hasn't been activated. He decides to turn on the gene, giving the worm a primitive form of vision.
His goal: To follow the development of the eye up the chain of complexity until he reaches humans.
"We did a lot of research with biologists at Johns Hopkins who had done this worm experiment," said Cahill, 35.
Philosopher Michael Ruse, who teaches the philosophy of science and biology at Florida State University, said the argument over the human eye was a big feature of the 19th-century debate about evolution.
"The Rev. William Paley in 1802 published Natural Theology, a textbook that tried to prove the existence of God using the argument by design," said Ruse. "And he took the eye as the paradigmatic example of something that is so complex it couldn't come about by chance."
That was the argument Darwin addressed in his 1859 masterwork, On the Origin of Species.
"He had little experimental evidence to go by, but Darwin said . . . there exists a whole range of organisms, from ones whose eyes are nothing more than a few light-sensitive cells all the way to the human eye," said Ruse. "For Darwin, the fact that this range exists is a strong indication that it developed over time."
Pitt said Ian's ambition - or his hubris - is to provide empirical proof for Darwin's theoretical musing. Despite his brilliance, Ian refuses to consider an idea that seems natural to Sofi.
Ian's worm, Sofi points out, once had no sight, which means an entire dimension of existence that humans know intimately didn't exist for the worm. She asks whether it isn't the same for people who have perception of the spiritual realm. Perhaps their senses are further developed than those who don't see this realm.
"It's so easy for him to dismiss Sofi as a wild, silly hippie, but she says something very profound and it gets to him," said Pitt. "And she says it in his language, in the language of science."
Sofi seems to voice Cahill's own belief.
"I wanted to show that they are exactly the same thing, science and religion," he said. "That they can live together very comfortably."
Opens Friday at Ritz Five and Ritz Center 16.