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How changes in the jaw and teeth illuminate human evolution

Researchers say changes in human teeth and jaws may have driven evolution of the skull, brain, and body. Science magazine published online June 20, 2014 a new study, "Neandertal defining skull features illuminate human evolution," on new fossil human skulls from a Spanish site that looks at our Neanderthal roots: Cranial and chronological evidence from Sima de los Huesos (Pit of the Bones).

How changes in the jaw and teeth illuminate human evolution.
Photo by Amy Sussman/Getty Images

The study focuses on a collection of seventeen fossil human skulls excavated from an archaeological site known as the Sima de los Huesos (Pit of the Bones) in the Atapuerca hill in northern Spain and comes 21 years after the announcement of the first three skulls from the site was published in Nature in 1993. What the study found suggested that facial modification was the first step in the evolution of the Neanderthal lineage, pointing to a mosaic pattern of evolution, with different anatomical and functional modules evolving at different rates, according to the study's abstract.

During the last four decades, the Atapuerca team has carried out painstaking excavations at the site, which is particularly difficult to access, and patiently reconstructed the skulls in the laboratory, often from large numbers of tiny cranial fragments recovered from the site, according to a June 19, 2014 news release, "Science magazine publishes study on new fossil human skulls from Spanish site." These efforts are still ongoing on both fronts, and to date it is estimated that around 30 individuals, comprised of complete skeletons, were accumulated at the site, with the bones being found broken and jumbled together in the sediments.

Why did so many human bodies end up thrown, dumped, or placed in one cave 430,000 years ago?

Did the people of those times think the cave was an underworld of sorts that people were sent to when they died? Or was the cave used to toss in the unwanted among the living or only the recently passed on people to cross over some dimension from above the earth to below? In addition, considerable advances have been made in understanding the geology and dating of the site, which are important to approach the question of how so many cadavers ended up there in the first place. The results of multiple independent dating methods indicate an age of around 430,000 years ago for the fossil assemblage, placing them within the Middle Pleistocene time period.

No other site in the world has yielded so many skulls of an extinct human species. Since the 1980s, the Atapuerca team has maintained that the Sima population is evolutionarily closely related to the Neanderthals, and the Sima fossils are the oldest to show clear Neanderthal features in the skull. One of the central debates among anthropologists is how the Neanderthal skull evolved through time.

Soft, cooked food may have changed the teeth and jaws as the earliest changes appeared in the teeth, jaw and face, suggesting that these features together represent a single functional complex related with a specialization in the chewing apparatus

The study of the Sima fossils confirms the idea, proposed by other researchers, that the evolutionary pattern is "mosaic" in nature. Rather than gradual, steady changes occurring in the entire skull, the earliest changes appeared in the teeth, jaw and face, suggesting that these features together represent a single functional complex related with a specialization in the chewing apparatus. Other regions of the skull, such as the cranial vault, or neurocranium, and the brain housed inside it, underwent changes later in time.

One of the important findings of the study was the homogeneity of the sample of fossils from the Sima

All of the individuals recovered at the site represent the same biological population which makes it possible for anthropologists to study individual variation as well as sexual differences in the skeleton and patterns of growth and development, among other aspects. But how many were relatives of the same families? It would require DNA testing rather than solely archaeology analysis to find out how they were related, if they were. What the researchers do know is that the skulls came from the same population.

While considerable differences in size are apparent within the collection, with some larger skulls and some smaller ones, the anatomical features that anthropologists study to examine evolutionary relationships do not vary much within the Sima population. This combination of mosaic evolution and anatomical homogeneity led the authors to favor a branching pattern of evolution, known as cladogenesis in evolutionary studies, in the European Middle Pleistocene.

What species do the Sima fossils represent?

This question too is addressed in the recent study, although the authors do not assign the fossils to a particular species. However, they argue that genetic differences from the Neandertals, as seen in the mitochondrial DNA recently recovered from one of the Sima fossils, suggests they are not simply 'early' Neanderthals. Nor should they be placed in the species known as Homo heidelbergensis since the Sima jawbones (mandibles) are distinct from the type specimen of this species, a mandible from the site of Mauer in Germany.

This is one of the outstanding questions and one that is sure to inspire considerable debate and controversy within the field. With excavations continuing and new fossils being discovered each field season, there is certainly reason to believe that the Sima de los Huesos will yield more surprising findings in the future. You also may wish to check out the website, "Centro Mixto UCM-ISCIII de Evolución y Comportamiento Humano."

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