Evolutionary Steps

New research involving a USF geologist shows the earliest humans moving from tropical forests to grasslands.

 

Video by Amy Mariani. Slideshow at the end of article.

 

By Vickie Chachere

USF News

 

TAMPA, Fla. (Aug 3, 2011) – Since the first finds of the early human fossils were recorded over the past century, scientists have debated how these ancient ancestors might have lived as they took their initial tentative steps and developed the attributes that have come to define modern man.

 

Now, new research to be published Thursday in the journal Nature by an international group of scientists including University of South Florida geologist Jonathan G. Wynn, provides scientific evidence of the landscape that was the stage for human evolution six million years ago. The new findings indicate that early man came to learn to walk on two feet, find new food sources and developed more advanced survival skills in the wooded grasslands of East Africa as they left their ancestral home in tropical forests behind.

 

Using chemical signatures recorded in ancient soils, the researchers show that some of man’s most important evolutionary steps were taken on the savannahs of Ethiopia and Kenya during the time that spans the separation between man and chimps. By analyzing the sediments where these fossils have been found, the scientists were able to recreate the landscape and answer some of the persistent questions on how a changing environment helped drive human evolution.

 

“It really is a big step because we know that chimps just can’t do it today,” said Wynn, who calls the research the culmination of decades of researchers working toward filling in the blanks on what early hominin environments were like.  “They can live at the margins of the savannah woodlands, but do not habitually inhabit the grasslands themselves.

 

“This is a whole new step in human evolution – even if it just means they (the early humans) were foraging for plants, they were learning new skills and new behaviors.”

 

The ‘Savannah Hypothesis’

 

The new research is a culmination of nearly two decades of findings tracing the earliest origins of man and spanning Ethiopia, Kenya and using sites as far afield as Australia as a point of comparison, Wynn said.

 

Wynn played a central role in the announcement of one of the earliest and best-preserved hominin fossil ever found:  “Selam,” a juvenile Australopithecus afarensis female whose bones were found by  Zeresenay Alemseged in Dikika, Ethiopia in 2000.

 

Selam’s remains are believed to be about 3.3 million years old, and her discovery opened the door for both a series of new understandings and new questions. Selam had both well-developed feet and a lower body adapted for walking and very human-like locomotion, but also the long arms that serve apes so well.

 

Since discoveries in the 1920s, the role of African savannahs in the evolution of early man has been a point of debate. Scientists knew that the last common ancestors of chimps and humans lived in East Africa’s rainforests, but had not been able to reconcile how and when early humans moved into more open environments where they adapted by being able to walk, run and regulate their body temperatures in the hotter and less shaded environments.

 

Despite the hypothesis, though, there had not been a common scientific understanding of what those “savannahs” might have been when the lineage of man and chimps diverged, the new paper said. Climate, tectonic and geological changes over time have rendered an African landscape vastly different from those times, and scientists had not developed a precise, working understanding of the degree of “woodiness” of the environment until now.

 

The Awash Valley (where famed fossil, “Lucy” was discovered in 1974) and Omo-Turkana basins in Ethiopia and Kenya are now barren and unforgiving places where deep gorges are carved into the earth and only a few scrub bushes and sparse trees manage to survive. The two basins have been the sites of most of the early human fossils recovered.

 

Working with scientists from the University of Utah, the Kenya Wildlife Service, James Cook University in Australia, Johns Hopkins University and the University of Arizona, the researchers sought to document how wooded the environment in which Selam’s and other early humans lived.

 

Ancient wooded areas leave a distinct and very different stable carbon isotopic signal from former grasslands, Wynn said. By comparing those chemical signatures to the areas where the fossils were found, the scientists were able to reconstruct that the fraction of woody cover in the tropical ecosystems.

 

They found that wooded cover of those areas was less than 40 percent at most sites, showing evidence that those early humans were living in open environments. Closed forests were actually a very small percentage of the area where early human ancestors have been found. Their research shows that even before Selam and Lucy lived on lands covered by shrubs and bushes, “Ardi” – one of the oldest hominid fossil every found at some 4.4 million years old – lived in areas of wooded grassland that then covered the Awash basin.

 

And as climate change brought about the development of the savannahs and the retreating of forested areas, the researchers have found that the early humans continued to develop their survival skills to fit the new environment. That too is lending new insight into what likely is a myriad of different species of early humans, some of which died out as their environment changed, Wynn said.

 

By establishing scientific evidence of their habitats, researchers can now move on to other significant questions of how these creatures continued to evolve, including understanding how they adapted to the environment as the tropical forests gave way to savannahs, learned to walk on two feet for extended distances and how their teeth developed. It will also help scientists understand why some species of early humans did not survive.

 

“The human family tree has a lot of branches on it, some of them dead branches,” he said “As environments continue to change, the lineage of humans that seem to survive are those that are adept at finding new foods or adapting to different conditions. It’s the change in the environment that creates the new species.”

 

This month, Wynn will return to Ethiopia for the next phase of the research, understanding what these early humans ate. He has been granted permission from the government of Ethiopia to collect tiny amounts of tooth enamel from the fossils and analyze them for their diet. Tooth enamel, he said, would bear the chemical signatures of what would have been consumed, be it grasses, tubers, insects or even meat.

 

The study has been funded by the National Science Foundation and the Leakey Foundation.

 

Vickie Chachere can be reached at 813-974-6251.