According to researchers at Penn State and Rutgers University, environmental changes in East Africa that occurred approximately 2 million years ago might be what is responsible for human evolution.
Penn State professor of Geosciences Katherine Freeman, believes that the leading hypothesis is that there were evolutionary changes among humans that relate to a steady environmental decline in climate. According to Clayton Magill, who is a graduate-level student in the faculty, “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 year. These changes happened very abruptly, with each transition occurring over hundreds to just a few thousand years.”
Freeman agrees: “There is a view this time in Africa was the ‘Great Drying,’ when the environment slowly dried out over 3 million years. But our data show that it was not a grand progression towards dry; the environment was highly variable.”
Many anthropologists seem to be under the assurance that it is the variability of experience can trigger cognitive development. Because early humans went from huge changes from vast amounts of trees to just grasses, their diets had to change in response to that. This resulted in changes in food type and availability, along with the way they could actually access and/or harvest their food. This means there had to be a corresponding increase in the size of the brain and the level of cognition, along with changes in movement and social connections, such as interacting with others. Magill explains, “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 removed organic matter than had been blown into the lake from Olduvai Gorge in northern Tanzania two million years ago. The team looked at biomarkers such as fossil molecules from ancient organisms from the coating on plant leaves. The waxy coating survives the rough nature of the sediment. To analyze them, they used a method calls gas chromatography, along with another labeled mass spectrometry, in order to determine any relative abundance of different leaf waxes and if there were changes or additions of carbon isotopes among these waxes. This helped them discover and rebuild any type of the vegetation that might be present in the lake’s area during specific times and eras.
Through their research, the team found the there was a quick back and forth transition in the environment, moving from a closed woodland to an open grassland. To further investigate this, the researchers used various mathematical models, including statistics, to find the changes and connect the changes that they saw in the environment to correspond with other things that might have happened simultaneously in the Earth or sea.
Freeman explains they found a connection between the movement of Earth and the changes in the environment. She describes this as saying, “The orbit of the Earth around the sun slowly changes with time. 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.”
In addition to the connection they found with the Earth, the researchers also discovered changes in the sea-surface and temperature levels. Both discoveries are important, showing that food security–or lack thereof–is tied to water found in arid landscape. “We find complementary forcing mechanisms: one is the way Earth orbits, and the other is variation in ocean temperatures surrounding Africa,” Freeman explained.
The researchers published these recent findings in the Proceedings of the National Academy of Sciences, along with a second paper in the same issue that expands on their discoveries, which shows that rainfall was greater when there were an abundance of trees (and less if trees were lacking and, instead, the area was filled with a grassland).
The combination of these papers effectively connects human evolution to the environment, and how changes in food and water were linked to major evolutionary changes. The research was funded by The National Science Foundation.