What is the meaning of the ending of "A Darkling Sea" by James Cambias?

Attention! Spoilers below!

So I just finished reading this book. It was amazing. But my mind goes totally blank trying to interpret the ending.

The following excerpts from the ending explain what the object is:

“Broadtail gave me this,” said Rob. “He got it from one of the bandits; where the bandit got it from is anyone’s guess but the surface erosion suggests it’s pretty damn old.”

...and...

“It was scratched and chipped, but not yet opaque. It was a lens.”

But what is the meaning of it? Does that mean there have been humans on Ilmatar previously? Or did the Sholens give a lens to the bandits? Or did the lens belong to one of the humans killed? Perhaps Isabel?

Please help me understand so I can have peace of mind :)

Thanks!