Genesis 38:8

And Jehuda said unto Onan, Enter thou to thy brother's wife, and marry her, and raise up seed unto the name of thy brother.

AND MARRY HER AS BROTHER-IN-LAW, AND RAISE SEED TO THY BROTHER. The son will be called by the name of the deceased. This is Rashi’s language.
But this is not true, for in the same commandment of the Torah it likewise says, And it shall be, that the firstborn that she beareth shall succeed in the name of his brother that is dead, that his name be not blotted out of Israel, and yet the brother-in-law is not commanded to call his son by the name of his dead brother. In the case of Boaz it says, Moreover Ruth the Moabitess, the wife of Machlon, have I acquired to be my wife, to raise up the name of the dead upon his inheritance, that the name of the dead be not cut off from among his brethren, and from the gate of his place, and yet she called him Obed,, Verse 21. not Machlon. Moreover, it says here, And Onan knew that the seed would not be his. Now what misfortune would have befallen him — to the point that he wasted his seed from before her — if his son was to be called by the name of his dead brother? Most people even desire to do so. Again, Scripture does not say, “And Onan said,” but instead it says, And Onan ‘knew’ that the seed would not be his. This would indicate that Onan had some definite kind of knowledge in this matter which made him certain that the seed would not be his.
The subject is indeed one of the great secrets of the Torah, concerning human reproduction, and it is evident to those observers who have eyes to see, and ears to hear. The ancient wise men who were prior to the Torah knew of the great benefit in marrying a childless dead brother’s wife, and that it was proper for the brother to take precedence in the matter, and upon his failure to do so, his next of kin would come after him, for any kinsman who was related to him, who would inherit his legacy, would derive a benefit from such a marriage. And it was customary for the dead man’s wife to be wed by the brother or father or the next of kin in the family. We do not know whether this was an ancient custom preceding Judah’s era. In Bereshith Rabbah they say that Judah was the one who inaugurated the commandment of marrying a childless person’s widow, for since he had received the secret from his ancestors he was quick to fulfill it. Now when the Torah came and prohibited marrying former wives of certain relatives, it was the will of the Holy One, blessed be He, to abrogate the prohibition against marrying a brother’s wife in case he …

ויבם אותה, it appears that the practice of the levirate marriage, yibbum, was something accepted long before the Torah was given to the Jewish people.