WASHINGTON — A 2 million-year-old flat-faced skull pulled from sandstones of East Africa has shored up claims that at least three species of early humans coexisted in an ‘‘evolutionary experiment’’ that saw an explosive brain-size increase paired with radically different faces, teeth, and jaws.

While the partial skull and two newly found jawbones look very different from modern humans, they match an enigmatic, nearly complete skull found 40 years ago that paleoanthropologists have struggled to fit into the human family tree.