In the talk show titled “How Dogs Evolved Into 'Our Best Friends'” on NPR’s “Fresh Air” aired on November 8, naturalist Mark Derr offered an intriguing story about how humans and wolves developed a friendly relationship, and wolves evolved into today’s dogs.
[Mark Derr] tells Fresh Air's Dave Davies that he believes humans and wolves developed a close relationship after recognizing themselves in each other while hunting on the trail of big game. "[That's when] they started traveling together, and they've been at it ever since," he says. "The dog is a creation of wolves and humans — of two equal beings that came together at a certain point in history and have been together ever since."
I thought “They’ve been at it ever since” means “they stayed in the same situation / status, or maintained the same (close) relationship,” from the context of the narration.
As I wasn’t sure whether “be at it” is an idiom or not, I consulted both Cambridge and Merriam-Webster online dictionaries, neither of which registers “be at it,” as an idiom, though Cambridge carries “hard at it” as a U.K idiom meaning “putting a lot of effort into what you’re doing."
On the other hand, Google Ngram provides a graph of “be at it” usage, which can be traced back to circa 1840. The incidence of usage started to decline once around 1940, and picked up again coming into 2000.
What does “be at it” mean in the above sentence? Is “be at it” an idiom, or simple combination of words, i.e. “They’ve been at it (the same place, status, position, level, relationship)”?
P.S. Wisdom English Japanese Dictionary at hand registers ‘be at it (again)’ as an idiom with definition ‘be involved aggressively in work,. quarrel, wicked deed, and “Yesterday, they were at it again” as an example of usage. However, I don’t think this definition applies to the above context at all.