Human Origins

Common hospital gear opens up a new way of reconstructing Homo sapiens' ancestors.

By Carl Zimmer|Monday, October 24, 2005

In the past, most of the big news about human evolution came from remote dig sites in places like Africa or Indonesia. In the future, the big news will come from familiar sites closer to home: hospitals. That’s because hospitals are equipped with powerful new scanning machines primarily used to identify tumors, ballooning blood vessels, bone fractures, and a wide range of disorders in people. Those same scanners also make it possible for paleoanthropologists to look inside the fossils of ancient hominids and see things that until now have been shrouded in mystery.

Take brains, for example. The evolution of the human brain is one of the most important questions in the story of our origins. But when our ancestors died, their brains quickly rotted away. Fossilized skulls offer the only clues. Until recently, if a team of researchers found an intact braincase, they were limited in what they could learn unless they cut the fossil open. Because hominid skulls are rare, few would dare take such a radical step.

Now paleoanthropologists can put a hominid skull in a computed-tomography, or CT, scanner and create a virtual skull that they can split apart any way they want. If they remove that digital skull altogether, they leave behind the outlines of a virtual brain. In 2005 a virtual brain of the one known skull of Homo floresiensis—the three-foot-tall hominid discovered on the Indonesian island of Flores—provided evidence in the ongoing debate about whether the creature represents a separate species or was a human pygmy with a birth defect. The size and shape of the virtual brain lends credence to the separate species theory. Moreover, the brain was not just a simpler version of a human brain. Some regions were smaller than ours, but others were unusually large for such a small hominid, hinting that Homo floresiensis might have been capable of abstract thought and could make complicated plans.

Most hominid fossils are in much worse shape than the skull of Homo floresiensis. Over thousands of years, they have disintegrated. Reconstructing a skull from bone chips used to be like assembling a three-dimensional puzzle with most of the pieces missing. Debates flare up over reconstructions. Was this hominid tall or short? Was that fossil a single individual, or a mélange of several? When they try new reconstructions, paleoanthropologists often wind up damaging the fossils as they cut through the glue and varnish that held pieces together. And when fossils are particularly smashed up, paleoanthropologists simply don’t dare reconstruct them.

CT scans make it much easier to put these puzzles back together. Researchers can create virtual bone fragments and then use sophisticated mathematical software to find the best way to assemble them. In some cases, they can make the scans without even removing the fossils from the rock that encases them. This new method has already changed the way scientists think about Neanderthals. A Swiss research team has produced a virtual series of young Neanderthal skulls and compared their development with that of modern human children. It turns out that Neanderthal children are as different from modern humans as adult Neanderthals are—which suggests that Neanderthals did belong to a separate species and did not give rise to living Europeans.

As the use of CT scans expands, paleoanthropologists are developing new avenues for uncovering clues to our past. They are discovering signs of healed wounds, of toothless old hominids who must have been cared for by others. Some researchers are even producing full-length virtual skeletons to which they can attach virtual muscles and make the ancient hominids walk again. Most significantly, CT scans can liberate hominid fossils from museum drawers. Once a research team makes a scan, they can post the data on a Web site for other researchers to analyze, bringing a precious hominid fossil to new sets of eyes and new sets of questions.

Skull relevations
One of the biggest surprises in the attempt to figure out human evolution comes from a crushed skull dug up in the Sahara in 2001. It belongs to a species of prehumans called Sahelanthropus tchadensis, which lived between 6 million and 7 million years ago. Although the find was remarkable, it wasn’t until this year that a team led by French paleoanthropologist Michel Brunet used CT scans to create a virtual model of the skull, revealing precise measurements of the size of the brain cavity and information about the angle at which the spinal cord exits the brain. The results showed that even though this hominid’s brain was no larger than a chimpanzee’s, it most likely walked upright like modern humans. Bipedalism, one of the basic markers of humans, apparently developed long before other traits, such as stone-tool making.

Angle of attack: The chimpanzee (pan troglodytes) walks on all fours, a fact that is reflected in the anatomy of its head. The spinal cord enters high on the skull through a hole called the foramen magnum. A line drawn from this opening forms an acute angle with a line running through the eye socket (known as the orbital plane). In sahelanthropus tchadensis and homo sapiens, the spinal cord enters the brain on a nearly vertical line, creating a much larger angle. The larger angle corresponds to a head held in an upright position and may show that Sahelanthropus walked on two legs.

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