Core questions about human evolution

by PETE ZRIOKA

Lucy, the famous fossil skeleton discovered in 1974, was an Australopithecus afarensis, a species of human ancestors that lived 3 million years ago. Illustration by Michael HagelbergLucy, the famous fossil skeleton discovered in 1974, was an Australopithecus afarensis, a species of human ancestors that lived 3 million years ago. Illustration by Michael Hagelberg
A multinational research team is taking a look back in time to study the relationship between climate and human evolution. Like all living things, humans have adapted to their environments over time. So understanding changes in environmental conditions, such as climate, can help us understand why and how our distant ancestors evolved.

To peer into the past, researchers from Arizona State University and other institutions will drill into dry lakebeds in Africa’s Rift Valley, which stretches along northeastern Africa from Ethiopia to Mozambique. The ancient lakebeds lie near archaeological sites that have produced fossils of hominins, the group of organisms that includes humans and our ancient ancestors.

This work, led by Andrew Cohen of the University of Arizona and including researchers from around the globe, is based on previous drilling projects that sampled ocean sediment cores off Africa’s eastern coast. Those cores indicate periods of aridity and varying climate that are widely thought to have played a major role in the evolution of early hominins.

But the ocean cores paint the environmental history of the Rift Valley in very broad strokes, says Chris Campisano, an assistant professor in ASU’s School of Human Evolution and Social Change (SHESC) and a research associate at the Institute of Human Origins (IHO).

“The ocean cores are great, they have a lot of different proxy data, but it’s homogenizing a huge area of east Africa,” says Campisano. “Things didn’t evolve in one great swoop across east Africa. They evolved in specific spots. So we want to know what’s going on in those specific spots at those specific times.”

To find that information, the project team will drill at five different sites in Kenya and Ethiopia that are important to the fossil record of early hominins. The core samples will cover a range of time periods critical to human evolution over the last 4 million years. Some cores will represent the same period as sediments near the Hadar archaeological site, where Lucy, the famous three-million-year-old Australopithecus afarensis fossil, was discovered by IHO’s Don Johanson in 1974.

Exposed lake sediments, approximately 2.95 million years old, near one of the drill sites. The white layers contain microscopic lake organisms, and the dark portions are volcanic ash. Analysis of the microscopic organisms allows researchers to piece together information on the lakes like pH content and water levels. Photo by Chris CampisanoExposed lake sediments, approximately 2.95 million years old, near one of the drill sites. The white layers contain microscopic lake organisms, and the dark portions are volcanic ash. Analysis of the microscopic organisms allows researchers to piece together information on the lakes like pH content and water levels. Photo by Chris Campisano

Thanks to erosion, researchers won’t have to drill too far to access these ancient sediments. But at the same time, the sediments have been well preserved, protected by subsequent depositional layers.

“The reason we want to do the lake cores is that there’s no exposure. They’re not oxidized,” says Kaye Reed, a professor in SHESC and a research associate of IHO who studies evolutionary paleoecology.

By matching layers of rock and sediments from the cores with the layers from important fossil sites, Campisano, Reed and the rest of the team will be able to determine the environments and climates hominins like Lucy lived in.

“What we’re going to get is a high-resolution paleoclimate record, which you can’t get unless you do something like this,” adds Reed, who will be collecting fossils from the same time periods represented by the cores.

Fossils discovered at the Hadar site in 2007. Nearby drilling of lake cores will help determine the type of climate these specimens lived in. Photo by Chris CampisanoFossils discovered at the Hadar site in 2007. Nearby drilling of lake cores will help determine the type of climate these specimens lived in. Photo by Chris Campisano

The climate characteristics represented in the cores will be uncovered using a host of techniques. The samples will be sent to the National Lacustrine Core Facility at the University of Minnesota, where they’ll be split in half. One half will be archived. The other will be run through various scanners and analyzed for geochemical changes and microscopic fossils.

What can this tell us about the climate millions of years ago? Quite a lot. For example, microscopic fossils of pollen will yield information about the vegetation surrounding the lakes. Microfossils of algae and tiny crustaceans called ostracods will piece together lake levels and pH content. Isotopic changes in organic remains, such as preserved leaf waxes, can tell researchers about the temperatures of air and water as well as changes in precipitation.

The scanners will also look for things like magnetic susceptibility. This would show where there was an influx of sands or dusty conditions, because of the large amounts of iron in dust, according to Campisano, who is the scientific project manager.

Ramon Arrowsmith, a professor of geology in ASU’s School of Earth and Space Exploration, will analyze the history of the watersheds that drained into the lakes. He and his colleagues use a method called cosmogenic nuclide dating. The technique measures isotopes such as beryllium-10 and aluminum-26, which are only formed when neutrons from cosmic radiation strike materials at the Earth’s surface.

“If we can see these elements in a rock, we know that it’s been near the Earth’s surface for some time,” says Arrowsmith. “And we know what the production rate of these nuclides should be, so if we can count how many there are we can see how long the rock has been near the Earth’s surface.”

With this data, Arrowsmith can piece together the erosion that has happened in a watershed, based on the elements’ decay and the overall age of the core. An abundance of beryllium-10 and aluminum-26 means low erosion in the watershed, because the rocks were near the surface long enough for nuclides to form. Low amounts of the elements signify high erosion rates, where rocks were stripped from the watershed rapidly, limiting nuclide production. Combining the nuclide dating with core dating methods allows researchers to assemble an erosion rate history of the lakes, which helps construct a clearer overall climate picture.

An overview of one of the drilling locations in the dry lake bed. Beneath the modern surface, millennia's worth of lake sediments are buried, containing information vital to the reconstruction of the climate and environments of early hominins. Photo by Chris CampisanoAn overview of one of the drilling locations in the dry lake bed. Beneath the modern surface, millennia's worth of lake sediments are buried, containing information vital to the reconstruction of the climate and environments of early hominins. Photo by Chris Campisano

“It’s part of the toolkit that we look at altogether to paint this picture of the changing landscape in and around the lake,” says Arrowsmith. “That then lets us pursue the questions we’re after about the importance of changing environments on human evolution.”

Multiple theories link dry climates to hominin evolution. One idea suggests a drying event led to open grasslands, which forced hominins to walk upright on two feet. Another theory posits that arid conditions at a different time caused food scarcity, shifting our early ancestors to a more carnivorous diet. This would give rise to the use of tools to cleave meat off bones or break them open for marrow. But Reed says it’s unlikely that humans would have evolved one behavior in one dry period, and the other at a different time.

“You can’t invoke the same process over and over for all these different features,” she says. “You would think they would sort of happen all at once if it was necessary.”

Drilling is slated to start in the summer of 2013, and the team is eager to get underway.

“The potential of what we might learn from this is huge,” says Campisano. “This is really a paradigm shift in how we go about reconstructing environments for paleoanthropology.”

The project is funded in part by the National Science Foundation and the International Continental Scientific Drilling Program. Other principal investigators come from the University of Addis Ababa, the Smithsonian Institution, Rutgers University, Emory University, Mekelle University, the University of Wales at Aberystwyth, the National Museums of Kenya, the University of Nairobi, Hong Kong Baptist University, the University of Saskatchewan, the University of Cologne, the Université de Rennes 1, the University of Potsda, and Los Alamos National Laboratory.

The School of Human Evolution and Social Change, the School of Earth and Space Exploration and the Institute of Human Origins are all units of the College of Liberal Arts and Sciences.