400,000 is relatively recently… we’re talking about half a million years… and again, they do not mean humans like we know today
According to scientists in this field, generally speaking, the claim is that “[o]ur own ancestors migrated out of Africa some time roughly 50,000 years ago, before establishing ourselves across the globe” (Science Alert).
According to Chabad Rabbi Manis Friedman, as far as I understood his explanation in a lecture he had at a local Chabad back in 2016, Hashem put those bones there and made the Earth appear old because science can only tell us how “aged” something is, but just because it fits the definition of “aged” does not mean that the world or fossils have been sitting there for that long… in other words, the Earth is still under 6000 years old, with aged relics that are no older than stated in the Torah.