Evidence apparently shows Nolan Wells was still on Horn Island when his friends’ boat left … TMZ has learned.

Sources with direct knowledge of the investigation claim authorities have evidence of Nolan being on the Mississippi barrier island after the boat departed on the Fourth of July … which would be a huge development.

We’re told at least two witnesses who were interviewed by law enforcement say they saw Nolan on the island after the boat headed back to the mainland.

That’s significant for many reasons.

TMZ enhanced the audio of Nolan’s friends’ frantic call for help when their boat was sinking … and people believe you can hear someone saying, “Is he dead?”

But our sources tell us people on the boat have heard the enhanced audio and are telling people around them the audio is only panic from the boat sinking.

They say the call is from when one of the guys on the boat — Wyatt Pyron — called Sea Tow, a private tow service.

However, the sources wouldn’t elaborate on the specifics of what was said during the emergency call.

Our sources also explain that even though Sea Tow was sending a boat to tow Nolan’s buddies, a friend in another boat saw the commotion and towed them instead.

Nolan’s self-described best friend Warren Hudson recently told Brandon Tatum on “The Officer Tatum Show” that he last saw Nolan at 3 PM that day … but claims he “talked to somebody personally” who saw the teen at 6 PM on the island.

Hudson also insists he and his friends weren’t even the last people to see Nolan … alleging there were still “dozens on dozens on dozens” of boats around the island when they left.

He’s said his friend group told Nolan to get on the boat when they were leaving, but he decided to hang back.

Nolan was reported missing that night … and tragically turned up dead in the water a couple days later.

His parents are conducting their own investigation … and the independent medical examiner was sent Nolan’s body without his throat, lungs, and the original autopsy photos.