Deep into teeth

Modern humans are the only species still alive that fall within the genus Homo. Our relatives, such as Homo erectus and Homo habilis, emerged, mated, migrated and died out all within the last 2.4 million years, and we have little more than excavation sites and fossilized remains to piece together what happened.

One burning question is how diverse the Homo genus was. Anthropologists estimate that at least ten other species once existed, but their interpretations sometimes rest with a single excavation site. Therefore, the uniqueness of each species remains under debate;the Homo genus could be less, or more, diverse. It all depends upon current species classification systems and anthropologists' continuous hunt to improve them.

One such system, based on the size and shape of teeth, has been the focus of Clément Zanolli's graduate thesis work at the National Museum of Natural History of Paris, in France and now his postdoctoral work at ICTP. The visible part of a tooth, called the enamel, is what remains long after the roots, gums and surrounding tissue decompose. Enamel can, therefore, fossilize for archeologists to unearth millennia later.
 
"Teeth are very sensitive to genetic changes", Zanolli says. "This means that each species has its own tooth morphology. However, it's sometimes difficult to be sure and precise just by looking at the enamel of a tooth because at times it is not well preserved."

Digging deeper for a solution, Zanolli and his academic advisors in France applied a high-resolution CT (computed tomography) scanner to a series of teeth from various species including Neanderthals and Homo erectus. The CT scanner x-rays the tooth, providing a complete 3-dimensional reconstruction of its inner structure where the hard inner structure, called dentine, and enamel meet.

"If you look at the dentine underneath the enamel cap, it's preserved very well the characteristic grooves and indentations and these reliefs are sharper than the enamel, allowing you to have more precise elements to determine the species," Zanolli says.

Zanolli's graduate thesis was the first publication to include an analysis of the 3-D inner structure of teeth from the species, Homo erectus. He analyzed fifteen teeth, mostly permanent molars, uncovered in Central Java, Indonesia. The size of the teeth varied greatly from small, similar to modern humans, to large, similar to modern orangutans.

At first, Zanolli was unsure which species to which the teeth belonged, but the detailed 3D reconstruction gave him confidence as to the samples' Homo erectus origins. The broad variation in size, he argues, parallels some of the global climate changes that affected Southeast Asia between 1.5 and 0.1 million years ago, altering entire ecological systems including different Homo species.

Zanolli discovered that some of the teeth were similar to Homo erectus species found further north in Southeast Asia and to some extent in Northeast Africa, suggesting that the species migrated great distances and survived in different climates.

Zanolli continues to study the inner structure of different Homo species at ICTP's Multidisciplinary Laboratory. Using the Mlab's advanced X-ray microCT system, Zanolli is currently reconstructing 3D images of fossilized teeth from Homo erectus to Neanderthals and modern humans. Ultimately, he hopes to construct a clear picture of the migratory patterns of the various human species.

"There are a lot of ways to make a fossil, but it's a very rare and arduous process," Zanolli says. "So fossils are like a treasure that gives us just a little piece of the overall puzzle. I'm always excited to work with new and different fossils."