[Rebel_Ace]

A plausible explanation might be that a genetic mutation causes multiple "two chamber" hearts to form, much in the same way that similar genetic "misfires" cause people to have six fingers, or three sets of teeth.

With a "double pumper", an organism might have better survival traits, and the genetic "defect" is preserved, and futher mutates into the 4 chambered hearts we see today.

[muawiyah]

Oh, you have to have lungs first ~ that’s what the atrial chambers do ~ pump blood through the lungs. No lungs, no need for four chambers.