Probably not what everyone was expecting given what Big Mom mentioned, but after the latest chapter, I think that humans descended from whatever race the Yeti Cool Brothers are.
Chopper possesses the Human-Human fruit, and probably has the genetic lineage of whatever humans used to be. His monster point has some pretty strong design similarities to the Yeti Cool Brothers.
Extra lanky fingers that start at the knuckle. Super long arms. The same shorts style! And the creepy eyes (which in the case of Scotch, looks to be a weird mask thing).
Given that they also have straw hats and there’s a giant frozen straw hat in Mary Geoise, my guess is that the first owner of the actual straw hat was part of the Ancient Human Race that were similar to the Yeti Cool Brothers. These might be the ancient giants, but they could be a completely different race of Ancient Humans entirely.
There’s a possibility that there are a lot of different types of giants, and the Yeti Cool Brothers are a subset of that race, or that Chopper’s devil fruit was derived directly from the Yeti Cool Brothers.
The fruit has existed for a long time, potentially, and might have been a “snapshot” of the Human race when it was created. This might also be why awakened Zoans in Impel Down are so weird, that’s what the original version of those animals looked like a very long time ago.
*Theory by the_idiotlord