Multiregional or Replacement?

There are two main models which attempt to explain the emergence of the modern human, how they colonised the world and outlived the archaic humans. These are the multiregional model and the replacement model.

Homo erectus 

The Multiregional Model

The multiregional model suggests that Homo sapiens didn't evolve at a specific time or place, instead they slowly evolved from Homo erectus (right) an ancestor of the modern human, that lived between 800,000 and 1.8million years ago, all over the world. Because they lived all over the world, gene flow between the geographical regions would have been constant. The multiregional model also suggests that interbreeding occurred between the Homo erectus and different types of archaic humans. And eventually, over many hundred thousands of years, all over the world, the Homo erectus became the modern human.

But how can this be possible? The earth is a large place, the chances that several different populations from several parts of the world, evolved in ‘exactly’ the same way at ‘exactly’ the same time is about as close to impossible as you can get. This unlikely type of evolution is known as ‘convergent evolution’, one of the main reasons the multiregional model is opposed by the replacement model.

The Replacement Model

The replacement model agrees that the Homo sapiens evolved from the Homo erectus, however, the replacement model believes that some populations of Homo erectus, migrated out of Africa and spread throughout the world. Those that migrated out evolved in a different way to those that stayed behind in Africa. The Homo erectus that stayed behind in Africa evolved into the modern humans, the Homo erectus in Europe evolved into the Neanderthals and the Homo erectus in Asia evolved into Denisovans. Another migration then occurred around 50,000 years ago, where the modern humans from Africa spread to Asia and Europe, replacing the archaic humans there, without interbreeding. The replacement model believes that at some point modern humans, Neanderthals and Denisovans cohabited in some parts of the world.

Although the replacement model is the leading and most widely accepted theory of the origin of the modern human, more recent genetic evidence shows that modern humans DID in fact interbreed with the European Neanderthal and the Asian Denisovans. So which is it?