If humans hadn't invented agriculture the world would be extremely different.
There would be no cities, and virtually no towns. The human population would be MUCH smaller than it is today. No more than a billion people worldwide, and probably much smaller than that.
In many areas humans would be extremely nomadic, following the weather/seasons, and herds of wild game in a constant quest for food.
The only place "towns" would exist would be places like the Pacific Northwest. There would be enough game available year round to allow humans to build perminant villages.
Virtually all humans would live close to rivers and lakes, and other sources of clean surface water. Very few humans would be able to survive in the deepest deserts (Australian Aboriginies, and Bushmen of Africa being some notable exceptions).
With no agriculture there would be no domestic livestock. The notable exception would be dogs. Humans would still have dog packs that would live near, and be hunting companions, beasts of burden, and food in lean times. Very probable that people who lived in areas where wild horses exsisted would learn to domesticate and ride horses. Nomadic cultures can domesticate horses, and follow the herds to fresh grass, never needing to develop agriculture to feed the horses.
The horses would be much smaller, more the size of ponies. With no agriculture, horses would never reach the Americas. After horses died out in North America, they would never be here again, unless the land bridge again formed between Russia and Alaska.
People that lived where coconuts are, would be some of the greatest of the seafaring people in the world. Without agriculture humans cannot venture very far out to sea, safely. Coconuts would give them water, and an extremely valuable food source, adding any fish they could catch along the way.
Humans who were living as hunter gatherers would be about as big as modern humans, because of the high protien, meat diets they would eat. They would have very good bones, and very good teeth. Disease like diabetis, blood pressure, clogged arteries, anorexia, and ostioperosis would be almost completely unknown. Humans would live shorter lives.
Cooking over fires would spell the doom to many women, filling their lungs with soot from the campfires over the years. Men would die early from the wounds they get from hunting big game animals.
Life would completely revolve around the family, and few extended people of your tribe. Story tellers, or people who could make primitive forms of music would be held in the highest estem.
Life would be very, very different than it is today.
