The Americas are two very important continents, there's no doubt about that. In addition to being the home of just over a billion of the Earth's people, the Americas have given us tobacco, tomatoes, potatoes, chocolate, rubber and the fifty United States of America (God bless them).
How would human history have been altered if they didn't exist? It sounds dumb as shit but seriously, let's ignore the effects removing two continents would have on the climate for a second and assume that in 1491 the Americas are replaced by a vast area of submerged continental shelf that stretches from the Arctic to the former Drake Passage - no land exists there anymore whatsoever, Atlantic islands like Bermuda and St Helena are for all intents and purposes on the western end of the world with non plus ultra.
Poor Columbus sails the ocean blue in 1492 but, because the Greeks didn't calculate the circumference of the Earth properly and tricked him into embarking on a much longer voyage than he anticipated, he doesn't make it to India alive. In the short term it's no great loss, Vasco De Gama finds an actual route there a few years later by going around the Cape of Good Hope, but as time goes by things start to diverge from reality.
There's no Mexican and Peruvian gold to supercharge the colonial empire of the Spanish Hapsburgs, Britain is on the edge of the world and probably not in as good a position to control shipping lanes as it was in reality, the immense wealth that came from the cash crop plantations of the Americas (and by extension the slave trade) is never a thing, and no potatoes (highly nutritious while being a staple carbohydrate source) along with the aforementioned things probably means no industrial revolution as we know it occurs. Even if industrialisation does occur, you'll need to find a substitute for rubber. Europe almost certainly isn't as able to kick the rest of the world's ass as it was in real life.
Population growth is different, although perhaps a better word to describe it in comparison to real life's would be lackluster. Without the Columbian Exchange bringing a new staple crop to the cold, damp northern half of Europe, it's less populous compared to the Mediterranean. I have a feeling France, which had several times the population of England in 1500 and which enjoys a more oceanic climate than its neighbours to the north and east, will come out more powerful relative to all of its neighbours (including Spain) than in real life.
What do you think would have happened if the Americas didn't exist and why? Basically...
Do the scientific revolution and the enlightenment occur?
Would there have been industrialisation at all? If not, does the world just get stuck in a perpetual high-level equilibrium trap 18th century?
If the answers to the above are no and yes, what forms of social and political organisation are attainable to Europeans in this scenario?
Will there be any Malthusian issues in Europe with no Columbian Exchange, or will the lack of it just mean the population keeps itself at a lower level instead of exploding like in Ireland and nothing bad happens? Does Europe inevitably suffer from some sort of overcrowding anyway, and if so what happens as a result?
How much would a lack of the potato impact the world in general and Europe in particular? Does a lack of cassava impact Africa similarly?
How is geopolitics and the age of discovery in general affected by the fact that going to and around the Cape of Good Hope is now the sole way for Europeans to go anywhere useful as far as spices and valuable commodities are concerned?
Does Northwest European-style settler colonialism still occur in places suitable for it (Australasia, South Africa, possibly the Maghreb given it's not malarial and the pieds-noirs settled there quite easily)?
How big is Europe's advantage over Asia and Africa with no initial conquest of the Americas?
Is this good or bad for the Turks?
