I have found an interesting observation in Josephus Flavius' Antiquities of the Jews account of the creation:
"And on the fifth day he produced the living creatures, both those that swim, and those that fly; the former in the sea, the latter in the air: he also sorted them as to society and mixture, for procreation, and that their kinds might increase and multiply.
On the sixth day, he created the four-footed beasts, and made them male and female: on the same day he also formed man."
As we can see here, there's a clear distinction between "sexless" lower animals, that merely multiply "by society and mixture" and higher animals that, having distinctive sexes, procreate by having sex.
So, because the 6th day is associated with the male/female sin, I speculate that the mammals(?) were grouped with humans because they possess that distinction.