There's a Lanthanite on Voyager!
For those not keeping up with "Strange New Worlds" or "Starfleet Academy," the Lanthanites are a race of long-lived humanoids who lived undetected on Earth for thousands of years. They look identical to humans, but have insanely long lifespans. Basically, "Star Trek" wanted an excuse to have immortal characters who would specifically remember stuff from Earth's history.
This unnamed science officer is a background character on "Star Trek: Voyager." But before serving on Voyager, he served on the Enterprise-D, also as a science officer.
...and before that, he served on another Enterprise, as a command officer! His name was Elliott.
Nothing phases this guy.
These background characters are all played by John Copage. At the time he played these roles, the concept of the Lanthanites hadn't been introduced to the franchise yet. Previously, I might've speculated that this dude was an El-Aurian who somehow found the Federation before the rest of his species, or that the science officer was simply the grandson of Elliott. But with the introduction of Lanthanites, we have a clean and easy explanation.
Here he is in "Year of Hell," looking almost bored as Janeway orders the crew to abandon ship. As a centuries-old Lanthanite who served on the two most famous Enterprises, this guy has clearly seen some sh*t.
But he probably isn't be that old...
This guy was not part of the away team to 1990s LA in "Future's End," so he either isn't old enough to be helpful in that situation, or he was living on a part of Earth radically different from Los Angeles. Otherwise, we'd need a reason for why he didn't join the away team to guide them around 20th Century Earth.
I'm thinking he was born during that dark age of the Eugenics Wars, or WWIII. Which would go even further to explaining his chill reaction to everything Voyager goes through.