What types of geographic barriers did humans have to overcome in order to populate the earth? Which barrier do you think would be the hardest to overcome?

There are many geographical barriers humans overcame in the process to populate the planet. These include factors related to the landscape like deserts, mountains and oceans and also factors like the climate, rarity of resources, beliefs that the earth was flat etc. I think the barrier that was the hardest to overcome was the water in rivers, lakes, seas and oceans (especially oceans, glaciers and straits). Making three quarters of the Earth's surface area, these endless borders separates most of Earth's landforms, especially the North and South America from the rest of the world.

If you have any further questions or comments, feel free to ask me.