The closest thing to a scientific explanation to "passion" would be that at some point, probable homo erectus, started using it's early language to describe thair instinctive impulses in a way they could rationalize, also helping reinforce their own tendencies by doing so, because any vague concept can become a life defining notion once you back it up with words and this with time became the concept we call passion, now during the neolithic passion started getting positive selective pressure as they help with work-force distribution

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1. an intense, driving, or overwhelming feeling or conviction. Passion is often contrasted with emotion, in that passion affects a person unwillingly. 2. intense sexual desire. [APA Dictionary of Psycology]

instinct, an inborn impulse or motivation to action typically performed in response to specific external stimuli [Britannica web page]

Researchers have long debated when humans starting talking to each other. Estimates range wildly, from as late as 50,000 years ago to as early as the beginning of the human genus more than 2 million years ago [www.science.org\]