Dead Sun - the advena double arms

The advenas evolved with a second arm coming out from their wrist, enabling them to either run or two legs, or descend and run on four limbs, as thanks to the extended lenght of the arms, now they can reach the ground. As for why advenas prefer running on four limbs, is due to their extreme tallness preventing them from reaching higher speeds otherwise. Above is illustrated what the usage of the arms are, and below is the transformation's movements.