Table of Contents
Human Evolution and Geographic Lineages
David Reich explains that while the modern human lineage, leading to the majority of today’s human population, was likely in sub-Saharan Africa for the last 500,000 years, the situation is less clear for the period between 2 million and 500,000 years ago. During this time, human ancestors were present in many regions of both Africa and Eurasia, with parallel increases in brain size. There is no clear evidence suggesting one region had closer ancestral ties to modern humans than the other.
Assumptions About Africa’s Role
Reich addresses a common assumption that Africa has always been the primary location for human evolution. While Africa has been central at various periods, it is uncertain where key developments in human evolution took place during the period when species like Homo habilis, Homo erectus, and Homo heidelbergensis evolved. This uncertainty extends to the shared ancestor of Neanderthals, Denisovans, and modern humans. Both archaeology and genetics provide limited clarity about whether Africa or Eurasia was the central location for this evolutionary phase.
Genetic and Archaeological Evidence
The belief that human evolution predominantly took place in Africa is partly based on inertia, or long-held assumptions. However, archaeological evidence is not definitive, and genetic evidence reveals many early branches from Eurasia and only one from Africa. DNA samples from archaic human lineages in Eurasia, such as Neanderthals and Denisovans, offer more complexity in the evolutionary record than those from Africa. This could be due to the better preservation of ancient DNA in Eurasia, but it also raises the possibility that human ancestors resided in Eurasia during certain periods.
Hypotheses on Human Ancestry
Reich presents a hypothesis in which an extension of the modern human substructure existed in both Africa and Eurasia. According to this model, early human populations entered the Near East hundreds of thousands of years ago, mixing with Neanderthals. Evidence of modern human incursions into Neanderthal territories in Western Eurasia is supported by skeletal remains and DNA data, indicating significant gene flow during this period.
Human Mobility and Global Gene Flow
Reich suggests that human populations, even hundreds of thousands of years ago, experienced gene flow on a global scale. Africa and Eurasia were not isolated by barriers that would have prevented population movement over long periods. This challenges the notion that geographic separation played a significant role in limiting the interaction between African and Eurasian populations during this era.
Need for Ancient DNA from Africa
To better understand human evolution, Reich emphasizes the need for more ancient DNA from Africa, particularly samples from 50,000 to 200,000 years ago. The archaeological record indicates a complex history within Africa, with multiple archaic human forms contributing to modern populations. Obtaining this data could provide a clearer picture of how modern human lineages combined and how they relate to other archaic groups.
