Neither slavery nor involuntary servitude, except as a punishment for crime whereof the party shall have been duly convicted, shall exist within the United States, or any place subject to their jurisdiction.
That except. Though their wording about free people seems misguided in the context.
Well, they've been selectively bred over thousands of years, but mass farming has really sped it up.
https://www.zmescience.com/feature-post/how-chickens-tripled-in-size/