Chapter 9 - The first out of Africa migrants

Abstract

The hominid expansion out of Africa has important implications for understanding the complexity of modern human origins and diversity as well as the genetic and phenotypic structure of extant populations. The dates for the earliest dispersal of human and human-like ancestors from Africa continues to get moved back, as archeologists and paleontologists continue to find ancient artifacts and fossil evidence. Recent findings imply hominins left Africa around 2.1mya traveling east into Asia, during an initial migration and then much later into Europe during subsequent migrations. Later northerly expansion suggests that early hominins were able to adapt to somewhat colder regions. Before our ancient human-like ancestors could spread out of Africa, they needed to develop the physical and mental capabilities that would enable them to survive in Middle East and Asian environments where unique food sources and fresh water were scarce and seasonal. This would have required an increased level of intelligence to cope with unfamiliar environments, improve tool technology, and the ability to adapt to a diverse diet to aid subsistence. Bipedalism evolved in Africa in some of our ancient ancestors over 6mya, which allowed for easy movement on land over long distances. The fossil record reveals that 6–7mya, three Miocene candidates with potential hominin affinity were the first to show bipedal movement: Ardipithecus kadabba, Orrorin tugenensis, and Sahelanthropus tchadensis. Hominid brain size increased slowly until the arrival of H. habilis, upon which a trend of tremendous brain growth ensued. Hominin brain expansion tracks strongly with refinements in tool technology along with social complexity, foraging strategies, communication, and cultural behaviors that may have contributed to increasing brain size. Tool technology was an important attribute for hominins ability to move into different environments. Movements throughout the Stone Age occurred both within and outside of Africa, with tools meeting varying demands for adapting to the new environments. The origin of the genus Homo in Africa is indicated by the shift from bipedal apes to primitive, large-brained, tool-making meat-eaters that moved across Africa and beyond. The evolution of the genus Homo in Africa is represented by three species: Homo habilis, Homo rudolfensis, and Homo erectus, listed roughly in order of their appearance in the fossil record. Based on fossil evidence, most investigators have concluded that Homo erectus was the first Homo species to leave Africa. H. erectus evolved in Africa and then migrated outside Africa, possibly giving rise to other Homo species. This theory suggests H. erectus led to evolutionary lineages H. heidelbergensis, and H. sapiens in Africa, and Neanderthals in Europe and Denisovans in Russia. Current evidence suggests that the lineage of Homo sapiens probably originated in Africa at least 500kya, with the earliest observed anatomically modern human morphology appearing about 300kya. Nuclear DNA and mtDNA studies all indicate the highest level of genome diversity, when compared to populations outside Africa, supporting an African origin of modern humans. Although most researchers believe an African origin is well established, one area of early human evolution still in question is where in Africa Homo sapiens emerged. Developments across multiple disciplines indicate new data are no longer consistent with the single region or single population view. New data have challenged the view that H. sapiens was endemic to a single region or habitat, and implying a more complex African origin. New data suggest no specific point in time can be identified where modern human ancestry was confined to a single birthplace and instead suggest that humans originated from several diverse populations that lived across Africa, separated by geographical barriers, with different populations repeatedly drawn together exchanging genes. This African multiregionalism theory (evolution within one continent) has gained popularity, especially since Homo sapiens fossils were found in Northwestern Africa, at Jebel Irhoud in Morocco, and remains were found in both Eastern and Southern Africa. There is clear evidence that early humans migrated on a number of occasions out of Africa during the Upper Pleistocene (126 to 11.7kya). DNA evidence supports the skeletal and archeological evidence and confirms the early migrations of AMH outside of Africa, but results suggest that modern humans, outside Africa, are almost completely descended from a later migration of a single founding population 50–70kya. However, debate still exists regarding the times and routes utilized by early migrants. Two passageways have been delineated in connection with dispersals out of Africa: one is in Northeast Africa, the Levant, into what is today the Sinai Peninsula; and the other is the southern corridor from the Horn of Africa into modern-day Yemen. Early migrants (100–125kya) that most likely passed through the southern route died out and contributed essentially nothing genetically to modern-day humans. The timing and extent of environmental and climatic conditions in northeast Africa, the Levant, and the Arabian Peninsula during the Middle and Late Pleistocene are essential to determining how and why dispersals of hominins out of Africa took place. Researchers have found evidence that during the Middle and Late Pleistocene, episodes of climatic change may have allowed passage between Northern Africa and Western Asia, and several models indicate the continued existence of an arid barrier between northern Arabia and the Levant during the Middle and Late Pleistocene. A radical shift in body form and brain functionality and an increase in technology permitted hominid movement. These movements allowed AMHs to interbreed with Neanderthals, Denisovans, and possibly other archaic human groups outside Africa. Evidence for Neanderthal and Denisovan ancestry is found in modern-day genomes from individuals outside Africa and most likely originated from a single admixture event. AMHs eventually replaced Neanderthal and Denisovan populations between 42,000 and 30,000 years ago.