Modern human divergence and evolutionary pathways

¶ … divergence between humans beings was once commonly considered by scholars to have happened no later than the early Pleistocene, or over 1.5 million years ago. Why did 19th and even late 20th century evolutionary theorists believe in an early divergence of the human species? Why did most scholars after World War 2 believe that Neandertals were our ancestors? In anthropology and paleontology, recent work has focused on evidence that the earliest African modern human fossils and their recent ancestors were from a single human ancestor, poetically named mitochondrial Eve. Since 1988, DNA evidence has come to light that shows that all living humans have a common ancestor only 160,000 years ago while neandertals diverged at least 500,000 years ago. The evidence was revealed in popular magazines such as Newsweek (Tierney 46-52.). Given the popular and widespread access to the evidence why was the molecular genetic evidence for this late divergence considered controversial and unacceptable, even though the same scholars accepted genetic evidence for the late divergence of humans and African apes? How does our recent African origin affect concepts of race and racism? How has the new reports of hybridization between neandertals and non-African populations reignited the debate?

In this author's opinion, what is very startling about this early date not only upsets religious fundamentalists, but it humorously challenges many evolutionists' equally fundamental and orthodox conviction that the human family tree began much earlier., that is, millions of years earlier than actually happened.

Ironically both are based upon a dogma that has been set up and a community's agenda, existence (and usually careers) have been based upon an established orthodoxy whose overthrow will jeopardize an established order. The only difference between the two is that the early evolutionary orthodoxy which overthrew the creationist orthodoxy is now paradoxically in the same position battling the forces of analysis in evolutionary change. In addition, this most recent biological evolution is not propelled as much by an agenda other than the one to gather data. Largely, it has been allowing the data itself to frame the debate and to define the agenda by not defining it at all. It is truly a pursuit of scientific truth via the scientific method at its best.

Traditionally, most scientists support the multiregional hypothesis maintain that Homo sapiens evolved as a series of geographically separate but interbreeding populations stemmed from a worldwide migration of Homo Erectus out of the continent of Africa nearly 2.5 million years ago. More recently uncovered evidence suggests that Neanderthal genomes may have contributed about 4% of non-African heredity (Johanson).

Broadly speaking, two themes unify the present search for biological Adams and Eves. First, there is an interest in identifying the biological mechanisms by which human diversity was generated, with a particular focus on the role of population extinction as an engine of change as evolutionary bottlenecks become paradigmatic catalysts. Secondly, there is the integration of morphological, behavioral and genetic patterns that investigators have identified among recent groups of people with the fossil and archaeological data. All of these investigations are leading to a better understanding of the role of history and geography in the process of differentiation. In addition, the search for genetic sequencing and genome mapping is taking the search exclusively from the rift valley of Africa into the lab. Lab scientists and assistants are providing the genetic data to link a jumble of archaeological finds into a coherent picture. As DNA analysis becomes more sophisticated and allows us to sequence older and older DNA to match with the record as taken from human placentas, this picture can only become more distinct and more accepted. What is becoming very clear is that evolutionary "bottlenecks" caused by catastrophic climatic change have propelled and sped human evolution. A 1400-year-old volcanic or other induced "winter" likely spurred the divergence possibly even later than 150,000 years ago, brought about by an explosion of Toba in Sumatra. The elimination of this bottleneck 10, 000 years later allowed another wave of emigration from Africa. Volcanic winter may have succeeded in the reductions of populations to levels low enough for founder effects, genetic drift and local adaptations to produce rapid population differentiation (Ambrose 623 -- 651) .

This new research posits new assumptions about evolutionary rates, anagenesis, gene flow and population stability. Most biological evolution consists of the following two processes: anagenesis and cladogenesis. Anagenesis describes the transformations that occur within a single lineage, that is, as a population develops new characteristics. Cladogenesis, describes the splitting of a single species into two or more groups that later subsequently diverge in their individual traits through the anagenetic process. Gene flow and population stability are now seen as being much more relative. Also, the perceptions of the genetic basis of anatomical differences between human populations has also changed dramatically. The assumptions about gene flow for interpreting fossil species as grades and clades has been radically altered (including assumptions of relative and absolute ages of fossils, and the implications of revised ages). The fact that the human evolution was influenced by bottlenecks and rapid evolution has changed the entire perception of the continuity of the human clade as the Australopithecines, Homo Erectus and Neanderthal are not seen as part of the human lineage, but rather as possible marginal contributors of DNA if the other species interbred with Homo Sapiens

(Washburn 76).