 Those Tweenies who are on Earth were presumably brought there as infants by human spacemen returning from Mars.Their human parents (probably the crew of Earth ships who have traveled to Mars) have abandoned them, and they have become wards of the state, being raised in state-run orphanages.When Scanlon goes searching for a female Tweenie companion for Max, he only finds one of a suitable age, which implies that the number of Tweenies on Earth at that time is small.