In the 2 million years that humans first came into existence in Africa, they have populated every continent (Antarctica is arguable) and every island they consider viable. However to think about this as a steady, onward march is to get a false picture about how this was achieved.

When plotting the journeys made, genetic studies provide a picture of remote population centres being established between which there would be occasional traffic  resulting in a mixing of the genes. These populations would spawn further centres but journeys might be a return to old lands rather than new ground. The timescales are long and the environment would have alternated between hot and cold. Sea levels would risen to create islands and then fallen to create land bridges. Populations would have left Africa only to return hundreds or thousands of years later.

Homo Erectus was the first to migrate out of Africa, reaching initially into Eurasia and then spreading quickly eastwards into what is now China. There (currently) no evidence that they made it to Australia, probably because of the existence of the deep seafloor and dangerous currents  of the 'Wallace Line' , which separates Indonesia and Australia. Nor is there evidence that they make the crossing into North America. The earliest peoples to cross what is now the Baring Straits but 20kya, during the last glaciation, was land called the Beringia Land Bridge, were Homo Sapiens called the Clovis people.