If in an alternate universe humans never discovered agriculture, what would modern-day humans lives be like? Would they still be in the Stone Age, like we were before the invention of agriculture?
As I understand it, agriculture was a key to the evolution of villages, cities and other large groupings of humans. Without agriculture too much time is spent hunting 'n gathering to be able to have time/energy for development of technology.
If you limit agriculture to farming on land, then possibly coastal/island people who can eat an abundance of fish + naturally grown (i.e., not orchards) fruit might be able to develop an advanced civilization. But the process of developing technology would be slower as even then more time would be needed (on average) for taking care of the necessities of life. Keep in mind that agriculture doesn't just include more efficient ways of producing food - it also includes textiles (whether plant based or from herds of animals). Agriculture is also often a driver of technology - no point in inventing a metal plow - and the metallurgical advances that go along with it - if you don't need to plow a field.
Agriculture is something that is needed for any advanced civilization. It is pretty hard to imagine the population growing dense enough to allow for even basic civilization without agriculture in some form. Permanent settlements are needed for people to better pass down knowledge and for specialized knowledge that does not directly translate to acquiring food from being forgotten when the person dies. The permanent settlements mean people don't have to carry everything they own with them around, giving people more room to make things and for ideas to take root. Although permanent settlements do not require farming, one of the first things a permanent settlement will discover would be agriculture because it seriously increases the stability of food supply and allows for much more food to be produced per person.
It actually doesn't matter if farming comes first or settlements come first as they are so synergistic that if both develop, life is changed forever for the people. And realistically both are very efficient processes and people will easily see the benefit of one when they have the other so long as both are actually physically possible. People can see that seeds they throw out can grow plants and eventually food, people can see that if they keep animals, they can feed the animals and eventually get more meat. This kind of knowledge is also very explosive in the sense of once 1 group of people in an area learns this, it is very easy for other peoples to learn how to do it and since the gains are very apparent in the amount of food that can be produced, the idea spreads to all groups that can communicate rapidly.
Anyways, "modern-day human lives" can not exist without agriculture so what you have won't be anything to call "modern day human life". Likely humans would live in much smaller groups/tribes they would have limited specialization and thus not allow for easy advancement in more technological areas. There won't be anything modern about it. Getting beyond the stone age requires forging metals which requires permanent settlements, as the forging of metals requires a long time investment, which incentivises strongly for agriculture. I guess some things like sciences can develop without agriculture but with limited specialization of society, it would be hard to imagine people getting far.
It seems from the question that you are thinking of agriculture as the result of a research process.
Hey guys, let's invent agriculture!
It's usually accepted that agriculture allowed humans to start growing cities, ruling class and so on. But it doesn't mean that agriculture preceded the beginning of people gatherings.
For instance, I remember this National Geographic article about alcohol. There is a wild statement in it that agriculture might have started in order to produce grain to make beer. More precisely that there was a temple, and that this temple provided beer for a ?yearly? celebration where many people from around attended. In order to have enough beer, they started to grow cereals. Boom! agriculture!
Also I think that if people start to stay at the same place for some time, they are going to spread seeds of what they eat around their campsite. I mean, like throwing away the leftovers of an apple. So if you stay there, an apple tree is likely to grow. Boom! Agriculture again.
So rather than thinking of agriculture as an invention that allowed humans to progress, I would think of it as a by-product of humans' social, religious and observational tendencies.
We gather, we collect things, we share knowledge, we adopt everything that might simplify our lives and we are pretty clever. My opinion is that agriculture was not really invented, but rather discovered, as something that is logical from a certain level of population density.
Hence, to answer your question, they wouldn't have agriculture because they are still in the stone age, not the opposite.