Humans have a complex history regarding genetics and gene flow. Evidence suggest

Question

Humans have a complex history regarding genetics and gene flow. Evidence suggests that not only are we all related to a single group/individual in Africa, but that our species has interbred with at least two other species of hominid (and evidence points toa third) 1. What does this mean as far as our cladistic "family tree," and what does it mean to say we belong to the species Homo sapiens? 2. How do we classify species, when such interspecies gene flow occurs? 3. Are species distinct categories, or do they lie on a continuum?

Explanation / Answer

There are two branching hypotheses on the origin of the human species. The most widely accepted is the “out of Africa” (OOA) theory, which holds that archaic Homo sapiens evolved into anatomically modern humans solely in Africa between 200,000 and 60,000 years ago. This hypothesis further proposes that members of one branch of H. sapiens left Africa at some point between 125,000 and 60,000 years ago, and that over a long period, these H. sapiens replaced more “primitive” populations of other hominins in Asia or Europe, such as Homo neanderthalensis and Homo erectus.

The competing theory is the multiregional evolution hypothesis, which argues that some or all of the genetic variation among contemporary human races is attributable to genetic inheritance from either other H. sapiens subspecies or from other hominid species. In the multiregional model, all archaic human forms worldwide, such as H. erectus and Neanderthals, as well as modern forms, subsequently evolved together into the diverse populations of modern H. sapiens, which are considered to make up a single, continuously gradient (as distinct from categorically separate) human species.

DNA analysis demonstrating the existence of “Mitochondrial Eve” has strongly corroborated the recent African origin model of OOA by providing crucial support to the theory that H. sapiens moved from Africa to replace residing hominin populations elsewhere. Mitochondrial Eve is the most recent matrilineal common ancestor of all humans currently alive. Women pass along mitochondrial DNA unchanged during sexual reproduction, and the DNA of this most recent woman from whom all currently living humans descend through an unbroken line on their mother’s side proves that modern humans only evolved once, most likely in East Africa, sometime between 150,000 and 200,000 years ago.

At the same time, however, studies suggest that Neanderthals, our closest-known evolutionary relatives, coexisted with H. sapiens on Earth for more than 5000 years and frequently interbred with modern humans. According to researchers, at least one-fifth of the Neanderthal genome may lurk within modern humans, influencing traits including the appearance of the skin and hair people have today and the diseases they get. This finding indicates that a true “extinction” of Neanderthals may not have occurred, but that they may have been absorbed into H. sapiens. Genetic evidence shows that other archaic hominins, such as the Denisovans, also interbred with H. sapiens ,The most current version of the OOA hypothesis emphasizes the African origin of most human populations but allows for the possibility of local contributions/interbreeding between humans and other hominins. Consequently, this article mainly discusses Neanderthal–human interbreeding, while also explaining other admixtures of archaic humans with hominins who were their contemporaries, such as Denisovans and H. heidelbergensis

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