According to your textbook, homo erectus was originally thought to have been the first homo species to leave africa, however, new evidence suggests that it may have instead been that migrated out of africa first.

Homo Habilis .

Several scientific studies have indicated that this hominid species was the first to travel outside of Africa, it was not homo erectus as was previously believed, but homo habilis since it lived 800,000 years before homo erectus.

this null effect is caused by a weak manipulation and ineffective internal validity. in a scientific study, as the one dr. morimoto is trying to perfom, internal validity refers to the confidence placed in the cause-effect relationship and to how well an experiment is done and able to eliminate confounding variables.        

answer; lead-based paint;