Another thing that no previous hominid had done was migrating into the Americas.
This actually sounds like another thing itself was migrating into the Americas. Not the hominids. The phrase another thing would be the subject of the sentence and was migrating would be the verb to migrate in the past continuous tense. In other words, changing the grammar of the sentence changes the meaning completely.
Another thing that no previous hominid had done was to migrate into the Americas.
Although this would sound closer to the truth and the sentence would work, it still has a similar issue as the meaning is again a little bit ambiguous. Was it the another thing itself that had to migrate into the Americas or the hominid? At least from a grammatical perspective, it's not entirely clear.
So, I can reassure you that the original grammar is perfectly fine. The reason it looks unusual to you has to do with the fact that you most likely have never encountered this type of grammar before. Well, maybe you have, it's just that you didn't notice that there was something strange about the grammar. Anyway, the grammar is fine and here are some more examples:
All I did was delete the old records from the database.
All he had to do to get the money he needed was sell his house.
What the water crisis did was bring to light all the other issues.