Horseshoe crabs of today are a tiny fraction of the sizes they once were, as per their fossil records. Might the size-change adaptation been brought about by the reduction in oxygen percentages, as evidenced by ancient glacial ice-bubble analysis?
Add to that, internal changes- like resisting new diseases.
So how do you think it all started? Or did it just “evolve” from nothing? How old is the earth? What is “pre-history”? Please show me your wisdom... so I, too, can be enlightened. I must be a rube.