Run off

The OA is not a run-on sentence.
OA: Researchers in Germany have unearthed 400,000-year-old wooden spears from what appears to be an ancient lakeshore hunting ground, stunning evidence that human ancestors systematically hunted big game much earlier than believed.

Core meaning:
Researchers in Germany have unearthed 400,000-year-old wooden spears.

Blue portion:
The blue portion serves as an adverb modifying have unearthed, expressing HOW the researchers HAVE UNEARTHED the spears.
How have researchers unearthed the spears?
They have unearthed the spears FROM WHAT APPEARS TO BE AN ANCIENT HUNTING GROUND.

Red portion:
The red portion is a SUMMATIVE MODIFIER.
A summative modifier is composed of COMMA + ABSTRACT NOUN + OTHER WORDS.
In a summative modifier, the abstract noun is not assigned a verb.
The purpose of a summative modifier is to SUM UP or REDEFINE the preceding clause.
Here, the preceding clause -- that researchers have unearthed wooden spears from an ancient hunting ground -- is redefined as STUNNING EVIDENCE THAT HUMAN ANCESTORS SYSTEMATICALLY HUNTED BIG GAME MUCH EARLIER THAN BELIEVED.

Note that stunning serves not as a verb but as a MODIFIER describing the evidence.
What KIND of evidence?
STUNNING evidence.