Dec. 24, 2012 — A series of rapid environmental changes in East Africa roughly 2 million years ago may be responsible for driving human evolution, according to researchers at Penn State and Rutgers University.
“The landscape early humans were inhabiting transitioned rapidly back and forth between a closed woodland and an open grassland about five to six times during a period of 200,000 years,” said Clayton Magill, graduate student in geosciences at Penn State. “These changes happened very abruptly, with each transition occurring over hundreds to just a few thousand years.”
According to Katherine Freeman, professor of geosciences, Penn State, the current leading hypothesis suggests that evolutionary changes among humans during the period the team investigated were related to a long, steady environmental change or even one big change in climate.
“There is a view this time in Africa was the ‘Great Drying,’ when the environment slowly dried out over 3 million years,” she said. “But our data show that it was not a grand progression towards dry; the environment was highly variable.”
According to Magill, many anthropologists believe that variability of experience can trigger cognitive development.
“Early humans went from having trees available to having only grasses available in just 10 to 100 generations, and their diets would have had to change in response,” he said. “Changes in food availability, food type, or the way you get food can trigger evolutionary mechanisms to deal with those changes. The result can be increased brain size and cognition, changes in locomotion and even social changes — how you interact with others in a group. Our data are consistent with these hypotheses. We show that the environment changed dramatically over a short time, and this variability coincides with an important period in our human evolution when the genus Homo was first established and when there was first evidence of tool use.”
The researchers — including Gail Ashley, professor of earth and planetary sciences, Rutgers University — examined lake sediments from Olduvai Gorge in northern Tanzania. They removed the organic matter that had either washed or was blown into the lake from the surrounding vegetation, microbes and other organisms 2 million years ago from the sediments. In particular, they looked at biomarkers — fossil molecules from ancient organisms — from the waxy coating on plant leaves.
“We looked at leaf waxes because they’re tough, they survive well in the sediment,” said Freeman.
The team used gas chromatography and mass spectrometry to determine the relative abundances of different leaf waxes and the abundance of carbon isotopes for different leaf waxes. The data enabled them to reconstruct the types of vegetation present in the Olduvai Gorge area at very specific time intervals.
The results showed that the environment transitioned rapidly back and forth between a closed woodland and an open grassland.
To find out what caused this rapid transitioning, the researchers used statistical and mathematical models to correlate the changes they saw in the environment with other things that may have been happening at the time, including changes in the Earth’s movement and changes in sea-surface temperatures.
“The orbit of the Earth around the sun slowly changes with time,” said Freeman. “These changes were tied to the local climate at Olduvai Gorge through changes in the monsoon system in Africa. Slight changes in the amount of sunshine changed the intensity of atmospheric circulation and the supply of water. The rain patterns that drive the plant patterns follow this monsoon circulation. We found a correlation between changes in the environment and planetary movement.”
The team also found a correlation between changes in the environment and sea-surface temperature in the tropics.
“We find complementary forcing mechanisms: one is the way Earth orbits, and the other is variation in ocean temperatures surrounding Africa,” Freeman said. The researchers recently published their results in the Proceedings of the National Academy of Sciences along with another paper in the same issue that builds on these findings. The second paper shows that rainfall was greater when there were trees around and less when there was a grassland.
“The research points to the importance of water in an arid landscape like Africa,” said Magill. “The plants are so intimately tied to the water that if you have water shortages, they usually lead to food insecurity.
“Together, these two papers shine light on human evolution because we now have an adaptive perspective. We understand, at least to a first approximation, what kinds of conditions were prevalent in that area and we show that changes in food and water were linked to major evolutionary changes.”
The National Science Foundation funded this research.
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How climate shifts in Africa sparked human evolution (MSNBC)
Scientists say landscape transitions may have forced early humans to think on their feet
Nicolle Rager Fuller / NSF. The first specimen of Paranthropus boisei, also called Nutcracker Man, was reported by Mary and Louis Leakey in 1959 from a site in Olduvai Gorge, Tanzania.
By Charles Choi – LiveScience Contributor
updated 12/26/2012 2:16:27 PM ET
At Olduvai Gorge, where excavations helped to confirm Africa was the cradle of humanity, scientists now find the landscape once fluctuated rapidly, likely guiding early human evolution.
These findings suggest that key mental developments within the human lineage may have been linked with a highly variable environment, researchers added.
Olduvai Gorge is a ravine cut into the eastern margin of the Serengeti Plain in northern Tanzania that holds fossils of hominins — members of the human lineage. Excavations at Olduvai Gorge by Louis and Mary Leakey in the mid-1950s helped to establish the African origin of humanity.
The Great Drying?
To learn more about the roots of humanity, scientists analyzed samples of leaf waxes preserved in lake sediments at Olduvai Gorge, identifying which plants dominated the local environment around 2 million years ago. This was about when Homo erectus, a direct ancestor of modern humans who used relatively advanced stone tools, appeared.
“We looked at leaf waxes, because they’re tough, they survive well in the sediment,” researcher Katherine Freeman, a biogeochemist at Pennsylvania State University, said in a statement.
After four years of work, the researchers focused on carbon isotopes — atoms of the same element with different numbers of neutrons — in the samples, which can reveal what plants reigned over an area. The grasses that dominate savannas engage in a kind of photosynthesis that involves both normal carbon-12 and heavier carbon-13, while trees and shrubs rely on a kind of photosynthesis that prefers carbon-12. (Atoms of carbon-12 each possess six neutrons, while atoms of carbon-13 have seven.)
Scientists had long thought Africa went through a period of gradually increasing dryness — called the Great Drying — over 3 million years, or perhaps one big change in climate that favored the expansion of grasslands across the continent, influencing human evolution. However, the new research instead revealed “strong evidence for dramatic ecosystem changes across the African savanna, in which open grassland landscapes transitioned to closed forests over just hundreds to several thousands of years,” researcher Clayton Magill, a biogeochemist at Pennsylvania State University, told LiveScience. [Know Your Roots? Take Our Human Evolution Quiz]
The researchers discovered that Olduvai Gorge abruptly and routinely fluctuated between dry grasslands and damp forests about five or six times during a period of 200,000 years.
“I was surprised by the magnitude of changes and the rapid pace of the changes we found,” Freeman told LiveScience. “There was a complete restructuring of the ecosystem from grassland to forest and back again, at least based on how we interpret the data. I’ve worked on carbon isotopes my whole career, and I’ve never seen anything like this before.”
The investigators also constructed a highly detailed record of water history in Olduvai Gorge by analyzing hydrogen isotope ratios in plant waxes and other compounds in nearby lake sediments. These findings support the carbon isotope data, suggesting the region experienced fluctuations in aridity, with dry periods dominated by grasslands and wet periods characterized by expanses of woody cover.
“The research points to the importance of water in an arid landscape like Africa,” Magill said in a statement. “The plants are so intimately tied to the water that if you have water shortages, they usually lead to food insecurity.”
The research team’s statistical and mathematical models link the changes they see with other events at the time, such as alterations in the planet’s movement. [50 Amazing Facts About Earth]
“The orbit of the Earth around the sun slowly changes with time,” Freeman said in statement. “These changes were tied to the local climate at Olduvai Gorge through changes in the monsoon system in Africa.”
Earth’s orbit around the sun can vary over time in a number of ways — for instance,Earth’s orbit around the sun can grow more or less circular over time, and Earth’s axis of spin relative to the sun’s equatorial plane can also tilt back and forth. This alters the amount of sunlight Earth receives, energy that drives Earth’s atmosphere.
“Slight changes in the amount of sunshine changed the intensity of atmospheric circulation and the supply of water,” Freeman said. “The rain patterns that drive the plant patterns follow this monsoon circulation. We found a correlation between changes in the environment and planetary movement.”
The team also found links between changes at Olduvai Gorge and sea-surface temperatures in the tropics.
“We find complementary forcing mechanisms — one is the way Earth orbits, and the other is variation in ocean temperatures surrounding Africa,” Freeman said.
These findings now shed light on the environmental shifts the ancestors of modern humans might have had to adapt to in order to survive and thrive.
“Early humans went from having trees available to having only grasses available in just 10 to 100 generations, and their diets would have had to change in response,” Magill said in a statement. “Changes in food availability, food type, or the way you get food can trigger evolutionary mechanisms to deal with those changes. The result can be increased brain size and cognition, changes in locomotion and even social changes — how you interact with others in a group.”
This variability in the environment coincided with a key period in human evolution, “when the genus Homo was first established and when there was first evidence of tool use,” Magill said.
The researchers now hope to examine changes at Olduvai Gorge not just across time but space, which could help shed light on aspects of early human evolution such as foraging patterns.
Magill, Freeman and their colleague Gail Ashley detailed their findings online Dec. 24 in two papers in the Proceedings of the National Academy of Sciences.