Published by the American Geological Institute
of the Earth Sciences
Summertime is prime time for beach season — when
the seashore beckons much of the human race to hit the waves. But when
did humans first venture toward the oceans? New research suggests humans
have been adapted to seaside living for at least 125,000 years.
|Researchers combing the Pleistocene-age Abdur Reef Limestone of Eritrea found not only edible mollusks and land mammal remains, but also flat, teardrop-shaped hand axes alongside more technologically advanced obsidian flake and blade tools. In the midst of a country at war with Ethiopia, the scientists found the first clear evidence of shoreline occupation. Previous findings of seashell-littered inland rock shelters only hinted at human activity near the coast. “We found stone tools in excellent, albeit unusual, geological context that is eminently datable,” says Robert C. Walter of the Departmento de Geologica, Centro de Investigaciôn Cientifica de Educación Superior in Ensenada, Mexico, who led the international team of researchers through two field seasons in 1998 and 1999. Their discoveries suggest more than the origins of beach fever. Early coastal adaptation may add support to the “Out of Africa” theory of early human migration.
(a) An inset map of the Red Sea basin shows red circles depicting areas of last interglacial
coral terraces. (b) A Landsat Thematic Mapper image of the study area shows the Abdur
Reef Limestone and the archaeological sites near the village of Abdur (arrow).
M. Abdelsalam, University of Texas at Dallas.
“We were of course interested and fascinated by the human origins potential of Eritrea, but we recognized that we have an opportunity and obligation as geologists to bring more to the story than the fossils and artifacts alone.” As a result, he and his colleagues investigated the geological history of the Danakil Rift Valley of Eritrea, including the origin and evolution of Cenozoic rifting, sedimentary basin formation and local rift volcanism, and the effects of climate change as recorded in the region’s sediments, fossils and artifacts. “We are, in short, after the big picture,” Walter says.
The breakthrough for the Eritrean discovery is the relatively precise geologic context for the artifacts. The team applied thermal ionization mass spectrometry (TIMS) to Uranium-Thorium dating, a method pioneered in part by team member R. Lawrence Edwards of the University of Minnesota. This method for dating corals relies on the solubility of uranium-234 in sea water, and the insolubility of thorium-230. From a set of five aragonitic coral specimens, the researchers dated the Middle Stone Age (African Middle Palaeolithic) artifacts found on the raised fossil reef to the last interglacial: 125,000 (plus or minus 7,000) years before the present.
According to Walter and his team, shoreline occupation dating back to 125,000 years ago supports the “Out of Africa” theory of human evolution. While scientific consensus dictates that our ancestors originated on the African continent, the timing and method of dispersal into other continents remain controversial. The “Out of Africa” theory asserts that early humans evolved within the continent 100-200 thousand years ago and then spread outwards, colonizing the rest of the planet and displacing other hominid species. Walter and his team suggest that human coastal exploitation over 100,000 years ago allowed the more advanced humans to travel along the coasts from Africa to the rest of the Old World. Later isolations may have led to the genetic variation we see today.
In contrast, the “Multiregional evolution” hypothesis asserts that our ancestors left Africa much earlier and interbred for millennia, implying that the last major human evolutionary bottleneck occurred as much as 2 million years ago (Geotimes, March 2000). Under this view, European, Asian and African groups resulted not from recent genetic isolation, but from a variety of environmental factors that impacted a larger, intermingling population.
John Hawks of the University of Utah, a proponent of the multiregional evolution hypothesis, is not convinced that the Eritrea finds strongly support the “Out of Africa” view. “While ‘Out of Africa’ requires that [coastal use] innovations occurred exclusively in Africa, the Multiregional model makes no prediction about where new behaviors should emerge,” Hawks says. “Africa is just as likely a place as any.”
Walter strongly disagrees. “One must conclude,” he says, “that positive evidence on one hand far outweighs a lack of evidence on the other.”
But Hawks also suggests that genetic evidence used by the “Out of Africa” theory’s proponents imply that Africa’s population size 50,000 years ago was small. “Genetics and archaeology are not telling us one picture about what happened during this time,” Hawks says, “and I think that’s very interesting.”
While the debate continues, the recent discoveries of Walter and co-authors prove that previously under-explored coastal sites are now a high priority for scientific research. These sites may provide the key archaeological finds from the late Pleistocene that eluded researchers at inland sites.
“Our discovery now opens the door for other coastal
sites to be explored,” Walter says, “and for the beginning of interesting
debates on [their] significance.”
Geotimes contributing writer