After Samus defeated the Queen Metroid on SR388, she climbed over the fearsome beast’s fresh corpse to discover one lone Metroid egg. The egg hatched before her very eyes, revealing a baby Metroid that immediately imprinted Samus as its mother. Samus could not bring herself to destroy the creature, which followed her back to her starship where she sealed it inside a bio capsule. She called it the Baby. It was the last Metroid in the universe.
This hatchling was brought back to the Ceres Space Academy Research Colony where Galactic Federation scientists studied it, revealing the Metroid’s energy-producing qualities. After Samus left Ceres, the colony was attacked by Ridley, all personnel were killed, and the Metroid hatchling was stolen. After a desperate battle, Ridley fled with the capsule to Zebes, activating Ceres’ self-destruct sequence on the way out. Samus later discovered the remains of the capsule outside Ridley’s room in Lower Norfair, the Baby long gone.