i heard somewhere that evolved humanity came from somewhere in africa.

if that's so, how did they spread out throughout the whole world in all 5 different continents, including places like australia, the american continent, or even the japanese archipelago, or even indonesia, where they ahve to cross oceans just to get there?

From Africa to Asia, you'll just walk. The Suez canal didn't exist then.

From Asia to Europe, there's no ocean in between.

From Asia to North America, you walk across the Bering land bridge. The sea level had been both lower and higher in the past. When the sea level is low, more land is exposed. Asia has been connected to America in the past. You walk from Asia to Australia as well.

South America is still connected to North America, even today.

More interesting is how you get to the islands in the middle of the ocean. Iceland, Greenland, Hawaii. We'll you take boats. People have been making boats for thousands of years.

lol ... make sense