Thank you for your insightful comment. I did ponder that, and it does make sense if the Arc gene evolved right after the evolution or birth of tetrapods before they ventured onto land. The Arc gene was absent before tetrapods and in all fish species known, so I doubt the Arc gene had evolved any earlier.

I assume (yes, I admit it's an assumption) that tetrapods evolved the Arc gene in response to environmental pressure on new terrestrial habitats partially because of Dr. Shepherd's (who directed the Arc study) hypothesis:

"One explanation is that when animals moved onto land, there were all these new environments they had to adapt to – so you needed a smarter brain, and a more plastic brain. Perhaps that is why Arc is so useful: It can give that extra plasticity or some sort of advantage."

But that said, I could be wrong in my reasoning as I'm no evolutionary biologist expert. So, please feel free to share any insights you may have.