Australopith

The Australopithecus (or Australopith for short in-game) is a stand-alone fossil that can be donated to the museum in.

In New Horizons
"The australopith, thought to be one of the links between humans and apes, emerged 4,000,000 years ago. They lived long ago, even before the ancestors of the modern humans, so there are profound differences... Even so, it seems to me that you can see the beginnings of greatness here!" –Blathers

In Real Life
Australopithecus is one of the most well-known fossils in the lineage that leads to modern humans. It is one of the first human ancestors known to have walked on two legs.