A verdant expanse of grass rippling in the wind, interspersed by a few trees casting umbrellas of shade with their branches is the likely landscape over which humans and their ancestors learned to walk.
Open woodlands and savanna dominated the East African homeland of the human species as we diverged from other primates, said researchers in a recent edition of the journal Nature.
“Wherever we find human ancestors, we find evidence for open habitats similar to savannas – much more open and savanna-like than forested,” said Thure Cerling, a University of Utah professor of geology, geophysics and biology, and lead author of the study, in a press release.
The researchers found that large areas of grassland and open woodland were consistently present for the past 7.4 million years. Understanding ancient vegetation patterns could help scientists resolve some questions about what kind of environment humans evolved in and the pressures that influenced development of features like our upright stance and dexterous hands.
“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,” Cerling said. “This study shows that during the development of bipedalism [about 4 million years ago] open conditions were present.”
“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,” which is when the first accepted ancestors of humans branched off from other primates, said Cerling.
“In some periods, it was more bushy, and other times it was less bushy,” Cerling said. “Hardly anything could have been called a dense forest, but we can show some periods where certain environments were consistently more wooded than others. We find hominins (early humans, pre-humans and chimp and gorilla relatives) in both places. How early hominins partitioned their time between ‘more open’ and ‘more closed’ habitats is still an open question.”
The researchers used a recently developed technique for analyzing the isotopes locked in fossilized soil to determine what kind of plants had been taking root over the years. They found that the large parts of East Africa had less than 40 percent tree cover over the past 7.4 million years.
Isotopes such as carbon-12 and carbon-13 are variations of a chemical element, in this case carbon. Because tropical grasses and sedges tend to absorb the rare isotope carbon-13, while trees, bushes, and herbs, which dominate woodlands and forests, tend to stick with the standard carbon-12 the ecologists were able to map the past landscapes based on the isotopes in the fossilized soil and current botanical knowledge.
“This study is based on the geological axiom that the present is the key to the past,” said Cerling. “We assume soils in the past had similar relationships to vegetation as what we observe today.”
This technique of estimating the types of plants growing in the ancient past based on the ratios of the isotopes in the soil provides “a new way to quantify the openness of tropical landscapes,” Cerling added. “This is the first method to actually quantify the amount of canopy cover, which is the basis for deciding if something is savanna.”
To understand what kind of landscape correlated to what carbon isotope ratio, the scientists looked at the carbon isotope ratios in 3,000 modern soil samples. The study looked at 75 tropical sites, half of which were in Africa. Satellite photos then helped the researchers determine how to classify the areas by their amount of tree cover.
The researchers defined grasslands as having less than 40 percent tree cover; woodlands as having between 40 and 80 percent; and anything with more than that as forests.
The modern soil samples were then compared to 1,300 fossil soil samples from areas near where human ancestors were found. Seventy percent of the sites had less than 40 percent trees and other woody plants. That puts them within the definition of grassland or wooded grassland. Only one percent of the samples exceeded 70 percent forest cover, meaning dense forests seem to have been rare.
The researchers note that this does contradict finds that show human ancestors lived in some wooded or forested areas, such as the Middle Awash, nor does it refute previous observations that savannas expanded about 2 million years ago. But it does show that for the entire time humans were evolving from other primates, East Africa had extensive grasslands in the same places human ancestors were found.
IMAGE 1: African Elephant trying to reach leaves in Kenya. (Wikimedia Commons)
IMAGE 2: A map with the fossil sites of the earliest hominids and hominoids. (Wikimedia Commons)
IMAGE 3: Savanna near Kuruman, South Africa. (Wikimedia Commons)