Earth's mantle has an extra ocean in it. Back in the 3000s Mya, the lowest third was still Basal Magma Ocean, in effect more core. But the outer two thirds had gone rockier. If still very hot. Too hot to hold water in fact.
Conclusion: after the Moon broke off and the various bombardments leveled out, which they're positing 4 billion years ago, Earth was a water-world and the water would scald you. As the [outer] mantle was left to cool, it sucked more water down, leaving about the volume of ocean we got today. SEQUEL 5/16: And come 3.6 Bya, the plates moved.
The Sun was cooler then, tho' maybe more prone to flare. I think they assume Earth had some magnetic-field, and an ozone layer - not from plants of course but from oxygen ions direct from (at least) atmospheric water vapour. So the vapour wasn't boiled so far as into space.
Although: maybe some excess was. Before 541 Mya (they say) it's hard to tell. They seem to have pulled that date out of the Cambrian; maybe because there aren't trilobites before then, for dating proxies. I'd honestly turn that dial to 565 Mya when we know we had magnets. I don't know this for 4-3.2 Bya, liquid lower mantle or no.
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