Hominid Species Time Line

Page 29

Alternative Theories of How Humans Developed.

Historically, many of the more recent H. heidelbergensis skulls and others remains were formerly classified as early or “Archaic” Homo sapiens.

The skull shown above left is approximately 300,000 years old and was found near Petralona in northeastern Greece.  Classification of this skull initially proved difficult, because it exhibits features of late Homo erectus/ergaster. The same skull is shown in profile in the center image. The partial, distorted skull above right is also over 300,000 years old; it was found in the cave of Arago in Tautavel in southern France.

These specimens are now classified as H. heidelbergensis.  This change in classification represents an important shift in understanding the emergence of modern humans, Homo sapiens sapiens.  From skulls such as these, many observers had earlier concluded that the transition from Homo erectus or H. antecessor to modern humans was a gradual progression that occurred in many parts of Eurasia and Africa.  That is, modern humans were thought to have evolved out of H. erectus or related species roughly in parallel in several regions of the African-Eurasian super continent.

If the transition from H. erectus to our own species was gradual and occurred over a wide area geographically, these intermediate forms are indeed what we would expect to find.  However, many scientists objected to the statistical unlikelihood of such an evolutionary development occurring "in parallel" over wide areas.  Such a simultaneous development would imply that somehow a single, active breeding community had been maintained over those immense distances for about a million years.

If Homo sapiens developed gradually and in parallel out of widely dispersed Homo erectus populations in Africa, Europe, and Asia (as the physical evidence here suggests), the “races” are quite old, emerging from the original dispersals of Homo erectus or closely related populations 700,00 to l million years ago.  As these assumptions were articulated more clearly, they became known as the “Parallel Evolution Theory.”

The “parallel evolution” theory holds that several dispersed populations of Homo erectus (or closely related species) evolved into archaic forms of Homo sapiens at roughly the same time within the last several hundred thousand years. The arrows suggest that intermingling of these regional populations was going on.  This theory finds some support in the fossil evidence, which shows many intermediate forms and continuities of characteristics of earlier Homo species into modern populations.  The best example of such evidence is probably in east Asia, in the continuity of shovel-shaped incisors from Homo erectus to modern humans.

The Radiation Theory:  a recent migration of modern Homo Sapiens out of Africa

The theory of a gradual parallel evolution in many regions is opposed by another, more recent theory based on genetic evidence.  The opposing theory holds that modern human beings arose in Africa about 200,000 years ago and migrated out of Africa via the routes roughly indicated on the map below.  The theory is sometimes called the “Out of Africa” theory.

The genetic evidence appears very compelling.  First, the DNA of modern human populations from all races and regions of the earth is nearly identical, implying that our species has a fairly recent and common point of origin.  Second, the greatest amount of genetic diversity among modern humans is found in Africa (the origin point of the new species).  Europe and the Near East have a fraction of the genetic diversity of Africa, as if only a subset of that population migrated to these regions. Asia, in spite of its huge population, has only a subset of the diversity found in Europe and the Near East, with the same implications.  And the Americas, which were populated from northeast Asia, have only a tiny amount of the genetic diversity found on the Asian continent. 

In this controversy, some of the physical evidence suggests one hypothesis while genetic evidence suggests the other.  DNA evidence from various groups around the world indicates that our species, Homo sapiens, evolved once, in Africa, and radiated or migrated from that point to the Middle East, Europe, and Asia, replacing Homo erectus and related populations.  According to this view, the races or regional variations in human populations, are relatively recent—perhaps within 100,000 years or so.