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Early Human Ancestors were at Home on the African Savanna

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Josie Linde
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« on: August 06, 2011, 12:15:40 am »

Early Human Ancestors were at Home on the African Savanna

By Dan McLerran   Wed, Aug 03, 2011

Viewing the African landscape that our distant human ancestors saw millions of years ago may not require much imagination, if a recent University of Utah study has anything to do with it. The study, involving a new method of measuring and analyzing chemical isotopes in comparative samples of modern and fossil-bearing (paleosol) soils, allowed a team of scientists to determine the canopy density or tree cover in East African prehistoric environments going back more than 7 million years, the time when human ancestors and their ape cousins were evolving.

The result: Wherever the fossils of early human (hominin) and related ape species were discovered, there was strong evidence that the prehistoric environments in which they lived consisted predominantly of open habitats much like the savannas we see today -- wide-open grasslands spotted with trees.

"Currently, many scientists think that before 2 million years ago, things were forested [in East Africa] and savanna conditions have been present only for the past 2 million years," says University of Utah geochemist Thure Cerling (photo right), who led the study. "This study shows that during the development of bipedalism [about 4 million years ago] open conditions were present."

"This is the first method to actually quantify the amount of canopy cover, which is the basis for deciding if something is savanna," Cerling says.

The new method involved correlating data from 3,000 modern soil samples and satellite images of vegetation cover over 75 different tropical sites across the globe, with data from 1,300 soil samples of paleosols taken from key fossil-bearing areas, particularly in East Africa. The scientists measured the ratio of the Carbon 13 (C-13) isotope to Carbon 12 (C-12) in the samples. The higher the ratio of C-13 to C-12, the more savanna-like the environment. The C-13 quantity is larger in decayed plant material from tropical grasses and sedges, such as those typically found in the open African savannas today. The isotope C-12 occurs in much higher quantities in decayed material from plants like trees, shrubs, herbs, and vegetables. The method was designed to search for savanna as far back as 7.4 million years ago, around the time when many scientists hypothesize that ancestral humans or hominins, and apes, split away from a common ancestor.

Significant to the study of human evolution and its environmental context, the researchers developed vegetation chronologies of the Awash Valley in Ethiopia, and the Omo-Turkana Basin area common to both Ethiopia and Kenya. These are key areas where many hominin fossils have been found, including Homo, the genus that includes our own species, Homo sapiens. The data will be valuable for use by future scientists conducting excavations and research in these areas. 

In sum, the analysis showed that more than 70 percent of the sites occupied anciently by hominins had less than 40 percent canopy or tree cover (characteristic of a savanna), and less than 1 percent indicated a canopy cover of more than 70 percent (characteristic of a dense forest or jungle).  "Therefore", conclude the researchers, "'closed' forests (more than 80 percent canopy cover) represent a very small fraction of the environments represented by these paleosols."

"We conclude there have been open savannas all the time for which we have hominin fossils in the environments where the fossils were found during the past 4.3 million years", Cerling says. Hominin fossils as old as 4.3 million years are considered to be the oldest fossils generally accepted by paleoanthropologists as possible human ancestors. 

Cerling conducted the study with biologists Samuel Andanje and David Kimutai Korir of the Kenya Wildlife Service; geologist Michael Bird of James Cook University, Cairns, Australia; University of Utah graduate students William Mace of geology, Anthony Macharia of geography and Christopher Remien of mathematics; and former Utah geology graduate students Jonathan Wynn of the University of South Florida, Naomi Levin of Johns Hopkins University and Jay Quade of the University of Arizona.

The National Science Foundation and the Leakey Foundation funded the study.

Cover Photo, Top Left: An East African savanna landscape of tree-dotted grassland is shown in this image from Samburu National Reserve in Kenya. The more heavily vegetated area in the middle distance is the corridor of the Ewaso Ngiro River. A new University of Utah study concludes that savanna was the predominant ecosystem during the evolution of human ancestors and their chimp and gorilla relatives in East Africa. Credit: Thure Cerling, University of Utah.

Photo, Second from Top, Right: University of Utah geochemist Thure Cerling developed a new way to measure ancient tree cover using carbon isotopes in fossil soils. In a study in the journal Nature, he used the method to show that grassy, tree-dotted savannas prevailed at most East African sites where human ancestors and their ape relatives evolved during the last 6 million years. Credit: Lee J. Siegel, University of Utah.

http://popular-archaeology.com/issue/june-2011/article/early-human-ancestors-were-at-home-on-the-african-savanna
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Josie Linde
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« Reply #1 on: August 06, 2011, 12:16:45 am »

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« Reply #2 on: August 06, 2011, 12:17:13 am »

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« Reply #3 on: August 06, 2011, 12:17:39 am »

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« Reply #4 on: August 06, 2011, 12:17:58 am »

By Dan McLerran

Dan McLerran

As Managing Editor of Popular Archaeology Magazine, Dan is a freelance writer and journalist specializing in archaeology.  He studied anthropology and archaeology in undergraduate and graduate school and has been an active participant on archaeological excavations in the U.S. and abroad.  He is the creator and administrator of Archaeological Digs, a popular weblog about archaeological excavation and field school opportunities.  He is also the creator and administrator of ArchaeologyNet, a business-oriented social network for archaeologists, students, volunteers, and educators.
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