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Answer:
There are two main theories The Out-Of-Africa theory and The Multiregional, I'll try to explain both of them as best as I can.
They both agree that Homo erectus originated in Africa and then expanded to Eurasia about one million years ago, but then differ on where the modern human originated from
The Out-Of-Africa theory:
It states that a second immigration out of Africa happened about 100,000 years ago, through this immigration it is said that anatomically modern humans of Africa overtook and conquered the world completely replacing archaic human populations.
The Multiregional theory:
It states that there were multiple independent origins OR there was a multiregional evolution with gene flow between different continental populations. This happening over the million years after Homo erectus came out of Africa.
bonus
The Trellis theory:
It is basically a compromised version of the Out-of-Africa theory but, emphasizes the African origin of most human populations but allows for the possibility of minor local contributions.
Hope this helps! :}