February 1, 2013
Questions About Human Evolution Raised In New Geological Study
Alan McStravick for redOrbit.com - Your Universe Online
Long before anyone pondered whether the chicken or the egg came first, there was another mystery that plagues us still to this day. Well, at least, until earlier this month. There has been some light shone on the question, “What came first: the bipedal human ancestor or the grasslands which spread across the African continent, reclaiming territory from the shrinking forests?"
An ambitious analysis of the past 12 million years of vegetation change in northeastern Africa is proving to be a bit of a game change in the understanding of the world as it was when our ancestors first walked on two feet. With the release of the study, on January 17 in the online journal Geology, came a stark challenge to all previous theories that have enjoyed a widespread acceptance in the scientific community.
The research team utilized a combination of data collection methods to arrive at their findings. Sediment core studies of the waxy molecules from plant leaves were extracted. The team also employed pollen analysis which presented them with data of unprecedented scope and detail. The pollen analysis was able to provide clear evidence to the researchers on exactly which types of vegetation dominated the landscape that immediately surrounded the African Rift Valley. The African Rift Valley includes what is present-day Kenya, Somalia and Ethiopia. Their research focused on this area as it is the location where early hominin fossils exist that have aided us in mapping the history of human evolution, thus far.
The use of the term “hominin” has become more en vogue, scientifically speaking, in the past few years. Hominin is a taxonomic subset off of hominid, relating more specifically to humans and our direct ancestors. Basically, it directly reflects an evolutionary change in the understanding of what it means to be human.
Referring to the team's data collection models, Sarah J. Feakins, assistant professor of Earth sciences at the USC Dornsife College of Letters, Arts and Sciences and lead author of the study said, "It is the combination of evidence both molecular and pollen evidence that allows us to say just how long we've seen Serengeti-type open grasslands."
Feakins worked with USC graduate student Hannah M. Liddy, USC undergraduate student Alexa Sieracki, Naomi E. Levin of Johns Hopkins University, Timothy I. Eglinton of the EidgenÃ¶ssische Technische Hochschule and Raymonde Bonnefille of the UniversitÃ© d'Aix-Marseille.
For more than a century, there has been a debate regarding the role the environment played in the rise of the hominins — the tribe of both human and ape ancestors. About 6 million years ago, the hominins made a split from the ancestors of what we know today as chimpanzees and bonobos.
One of the older theories, first proposed in 1925, hypothesizes: the reasoning for our early ancestors adopting bipedalism for their primary mode of transportation was as a direct result of the grassy savannas rapidly spreading, moving with seeming impunity into the already shrinking forests of northeast Africa. The theory claims that as there were fewer trees to swing from, our ancestors found the use of walking was a more prodigious method for getting around.
The theory sounds viable. Well, it sounded viable. It is generally accepted that our ancestors transition to bipedalism occurred between 6 and 4 million years ago. Feakins´ study, however, has quite clearly ruled out the possibility that there were any thick rainforests at that time. In fact, they had disappeared completely by that point. They were replaced an estimated 12 million years ago by a combination of grasslands and seasonally dry forests.
But Feakins´ study was not done establishing itself as the new paradigm in understanding, in relation to our earlier ancestors. The team found tropical C4 grasses and shrubs that are familiar to the modern African savannah had introduced themselves to the landscape earlier than was previously thought. Tropical C4 vegetation replaced C3-type grasses. The C3 and C4 designations for vegetation refer specifically to the method of photosynthesis employed by each type. As it turns out, C3 vegetation is more suited to thrive in a wetter environment.
Previous studies have attempted to explain the evolution of vegetation in the African Rift Valley. However, each of the previous studies focused on smaller, individual sites. Their results were, at best, narrow snapshots, unable to show a comprehensive view of the region. Feakins´, by extracting a sediment core from the Gulf of Aden and cross-referencing it with data that was compiled from ancient soil samples that were collected throughout eastern Africa, was able to present findings that are more far-reaching and all-encompassing than any study previously.
"The combination of marine and terrestrial data enable us to link the environmental record at specific fossil sites to regional ecological and climate change," Levin said.
While the earliest days of our ancestors are certainly a subject many are interested in, other science disciplines are also excited by Feakins´ study results. For instance, also of note in the study is how the landscape was affected by herbivores, such as the horse and hippo, which grazed these lands. Another interesting discovery for the team was learning how plants across the landscape reacted to periods of global and regional environmental change.
According to Liddy, "The types of grasses appear to be sensitive to global carbon dioxide levels."
Liddy wants to observe the data with a focus mainly on the Pliocene, an epoch represented by an extended period of global cooling. It was as a result of cooling and drying during this period that we may have seen the enormous spread of grasslands and savannahs.
Liddy explains that her goal is to provide an even clearer presentation of the Pliocene, especially as it is believed it experienced similar atmospheric carbon dioxide levels to present day.
"There might be lessons in here for the future viability of our C4-grain crops," says Feakins.