So here’s the thing: we don’t know that, really. If we find a spear that has one side sharpened and it’s dated 40,000 bc, then we find another spear and both sides are sharpened dated 30,000 bc, it may appear that way… But we literally have a vague guess at best. If 1 million years from now an archeologist finds a kitchen knife and concludes that in the 2000’s we were still figuring out double sided knives, then they would be hilariously wrong.