Originally Posted by Konnaha_yellow_flash

No it did not mention it... I agree... however, it literally showed it IMO.....

Madara uchiha with his EMS (the full power of the SG) had his body altered and infused completely with hashirama's cells to give him a hashirama body, thus created a madara/hashirama hybrid bringing together senju and uchiha which then created the RG out of the EMS...
It was due to the union of the elder son of the sage (uchiha ancestor) and the younger so of the sage (Senju/uzumaki ancestor) being reunited, after the sage himself split his power between his two sons...