Not so much ‘What would happen if one day we woke up and 40% of the world’s oceans suddenly became land?’, as much as ‘Would we even be here in the first place if most of the surface of the earth were land?’
Like, I’m thinking climate and rainfall - could the Earth have a comparable climate if it were mostly land-covered? Would it be much hotter/colder?
Has the 70/30 ratio changed much in the time that life has existed on the planet?
No need for an answer fast 
Thanks!
To tackle part of your questions, there is evidence for life on Earth in the range of 3.5 billion years ago, possibly even earlier. That is actually older than most (maybe all?) of the continental crust. So the earliest life may have been around when the Earth was 100% ocean, or darn close to it. (In the time of multicellular life, land area has fluctuated, but I don’t know how much off the top of my head. Nothing nearly as drastic as in the first couple of billion years, though.)
Well, there wouldn’t really be continents like we have now, there would be land with scattered unconnected oceans. And most of the land not near an ocean would be extreme desert, with pretty much zero precipitation. We’re talking deserts that would make the Sahara look like a rainforest.
Although there might be some extensive continental seas/great lakes, with lots and lots of endorheic basins. But if those basins never get any significant rainfall they’ll be completely dry. So near the margins of the oceans there will be a mixture of salt lakes and freshwater seas with no outflow, like the Caspian and the Great Salt Lake that over millennia fill with fresh water or evaporate depending on rainfall patterns. As you get farther away from oceans rivers mostly dry up or never form.
I’ll take a swing at the climate questions … starting with the second … yes, it would be hotter/colder … hotter during the day, colder at night … and overall far more temperature sensitive to any forcing going on … there would be more continentality, which is basically the measure of how far a location is from an ocean … this would have a direct impact on climate … typically continentality brings about larger temperature changes day-to-night and season-to-season … I suppose rainfall would be limited being so far from the oceans … but then again it rains in Winnipeg …
We’d have oceans the size of Eurasia, plus all the other continents … 30% ocean coverage is more than enough to keep rainfall coming down within 1,000 miles of any ocean …
Actually, let Randal Monroe tacklesome of this by proxy.
(Of course, that is if the water had been magically drained away. In the historical Earth, it would have just been redistributed, through freezing (as we have today, mostly deposited on Antarctica and Greenland) or through plate tectonics changing the size/shape/volume of the ocean basins and continents. And changes in temperature can cause the water to expand or contract some, changing ocean volume.)
Sure, 1000 miles from the ocean you’d get rain, and there’d be an ocean the size of Afro-Eurasia.
And there would be a vast Pacific continent that covered half the world. And that continent would have habitable margins for a thousand miles. But the interior of that continent would be much much farther away from any ocean than any point on Earth. In the interior of this continent it would never, ever rain.
Some random links for people with the same ideas:
Wouldn’t less ocean mean less water, therefore less water vapor (the most abundant greenhouse gas) and therefore mean the Earth would be colder, not hotter?
Take a lesson from central Asia, about as far from real water as you can get. Extensive moderately habitable quasi-desert, some good rivers, Aral ex-Sea and Caspian sea, Lake Baikal - A lot of very deep trench oceans along the continental collisions.
Basically, what you are asking is “what if the earth had a lot less water?”
An interesting speculation would be on climate - water moderates climate, the gulf stream for example keeps northern Europe from being more like Moscow or Mongolia. (or Siberia)
Or, you could keep the same amount of water, but dig out the ocean beds to about 2 or 3 times deeper
Changing the amount of water would also change the entire geological history of Earth.
Present day subduction appears to be lubricated by water. Water’s effect on minerals has significant effect on other tectonic activity Hydration lifts Earth's crust | Nature And of course there are all the minerals that are the result of biological activity in the oceans.
What naita said. Also, we can look at what a supercontinent was like on the interior, to get a sense of what it would be like in the middle of our new lands. Not pleasant. The coasts wouldn’t be a Garden of Eden, either. “Megamonsoon” doesn’t exactly sound pastoral, does it?
But did the mega monsoon of Pangaea require Panthallassa? You probably couldn’t generate a system like that when the biggest ocean on the planet is only the size of the North Atlantic.
No, it just needed the Tethys Sea, which was not that big…