The old saying “You are what you eat” takes on new significance in the most comprehensive analysis to date of early human teeth from Africa.

Prior to about 3.5 million years ago, early humans dined almost exclusively on leaves and fruits from trees, shrubs, and herbs—similar to modern-day gorillas and chimpanzees.   However, about 3.5 million years ago, early human species like Australopithecus afarensis and Kenyanthropus platyops began to also nosh on grasses, sedges, and succulents—or on animals that ate those plants.

Evidence of this significant dietary expansion is written in the chemical make-up of our ancestors’ teeth.  These findings are reported in a series of four papers published this week in the Proceedings of the National Academy of Sciences, by an international group of scientists spread over three continents.

“These papers present the most exhaustive isotope-based studies on early human diets to date,” says the Academy’s own Zeresenay Alemseged, Senior Curator and Chair of Anthropology, and co-author on two of the papers (available here and here). “Because feeding is the most important factor determining an organism’s physiology, behavior, and its interaction with the environment, these findings will give us new insight into the evolutionary mechanisms that shaped our evolution.”

Plants can be divided into three categories based on their method of photosynthesis: C3, C4, and CAM.  C3 plants (trees, shrubs, and herbs) can be chemically distinguished from C4/CAM plants (grasses, sedges, and succulents) because the latter incorporate higher amounts of the heavier isotope carbon-13 into their tissues.  When the plants are consumed, the isotopes become incorporated into the animal’s own tissues—including the enamel of developing teeth.  Even after millions of years, scientists can measure the relative amounts of carbon-13 in teeth enamel and infer the amount of C3 vs. C4/CAM plants in an animal’s diet.

“What we have is chemical information on what our ancestors ate, which in simpler terms is like a piece of food item stuck between their teeth and preserved for millions of years,” says Alemseged.

These papers represent the first time that scientists have analyzed carbon isotope data from all early human species for which significant samples exist: 175 specimens representing 11 species, ranging from 4.4 to 1.3 million years in age.  The results show that prior to 3.5 million years ago, early humans ate almost exclusively C3 plants.  But starting about 3.5 million years ago, early humans acquired the taste for C4/CAM plants as well, even though their environments seemed to be broadly similar to their ancestors’.  The later genus Homo, including modern-day Homo sapiens, continues the trend of eating a mixture of C3 and C4/CAM plants—in fact, people who enjoy mashed potatoes with corn are practicing a 3.5 million-year-old habit.

What the studies cannot reveal is the exact identity of the food, and whether it also included animals that ate C4/CAM plants (an equally valid way to acquire carbon-13).  Possible C4/CAM-derived meals include grass seeds and roots, sedge underground stems, termites, succulents, or even small game and scavenged carcasses.  In 2010, Alemseged and his research team published the earliest evidence for meat consumption using tools, dating back to 3.4 million years ago—an additional line of evidence showing a dietary shift in human evolution.

“The change in isotopic signal documented by the new studies, coupled with the evidence for meat-eating in Australopithecus afarensis from Dikika around 3.5 million years ago, suggests an expansion in the dietary adaptation of the species,” says Alemseged.

The authors of this week’s papers also sampled fossils of giraffes, horses, and monkeys from the same environments and saw no significant change in their carbon isotope values over time—suggesting that the unique dietary transformation of early humans did not apply to other mammals on the African savanna.  The question of what drove the transformation, however, remains unresolved.

Andrew Ng is Communications Manager at the California Academy of Sciences.

Images: National Museums of Kenya. Photos by Mike Hettwer, Yang Deming

Imagine losing and regrowing teeth every year. You might feel less guilty when you forget to floss. Indeed, most vertebrates experience such guilt-free existence because, unlike humans, they can replace teeth throughout their lives. (No wonder there’s no call for dentists in the animal world!)

“Humans naturally only have two sets of teeth—baby teeth and adult teeth,” says Dr. Cheng-Ming Chuong, director of the Laboratory of Tissue Development and Regeneration at the University of Southern California (USC). “Ultimately, we want to identify stem cells that can be used as a resource to stimulate tooth renewal in adult humans who have lost teeth. But, to do that, we must first understand how they renew in other animals and why they stop in people.”

To understand how teeth regenerate in other animals, Chuong and his colleagues looked to a very toothy vertebrate—the American alligator. Alligators have well-organized teeth with similar form and structure as mammalian teeth and are capable of lifelong tooth renewal. “They have 80 teeth, each of which can be replaced up to 50 times over their lifetime, making them the ideal model for comparison to human teeth,” explains Dr. Ping Wu, who collaborates with Dr. Chuong at USC.

Using microscopic imaging techniques, the researchers found that each alligator tooth is a complex unit of three components—a functional tooth, a replacement tooth, and the dental lamina (the band of tissue crucial to tooth development). The tooth units are structured to enable a smooth transition from dislodgement of the old tooth to replacement with the new tooth. The researchers identified that new replacement teeth develop from stem cells within the alligator dental laminae.

“In the future, we hope to isolate those cells from the dental lamina to see whether we can use them to regenerate teeth in the lab,” says another USC researcher, Randall B. Widelitz. The scientists hope to use this research to stimulate tooth regeneration in people with missing choppers.

The study is published this week in the Proceedings of the National Academy of Sciences.

Image: David R. Tribble/Wikipedia

With six research papers in the current issue of Science, and numerous articles and blog posts surrounding those papers, Australopithecus sediba is the hominin du jour.

The papers reveal different anatomical features of Au. sediba and discuss their similarities to, and differences from, early human features. One news article calls them a “hodgepodge,” while another describes them as a “mosaic.”

“Amalgam” is how Zeray Alemseged, the Academy’s curator of anthropology, describes Au. sediba’s combination of human-like and more primitive features. Take the species’ heel. You and I walk by putting our broad and robust heel down and rolling to our toes, but Au. sediba’s heel was so narrow, these hominins couldn’t land on their heel, and likely walked on the sides of their feet and then pronated.

Similarly, Au. sediba’s torso had a conical and quite primitive shape, and their shoulders were “shrugged.” Alemseged explains, “With short necks and a narrow clavicle, they appeared to be ape-like with a substantial adaptation for climbing.”

However, the lower ribs were slightly human-like and the teeth were a mixture “of primitive and human traits,” according to an accompanying article in Science.

The findings are based on fossils found in South Africa by Lee Berger’s team in 2008, and include three skeletons.  The recent studies pinpoint Au. sediba’s existence to around 1.98 million years ago and make a few proposals on how to place the species in our lineage. In fact, Berger suggests that Au. sediba could be the direct ancestor to our genus, Homo.

“Lee is a good colleague, but I happen to disagree with him about that,” Alemseged says. “It’s a fascinating discovery and the quality of preservation of the fossils and number of skeletons are great,” but Alemseged sees no evidence that Homo descended from Au. sediba. “The fossil record indicates that by 2.33 million years ago, Homo already exists,” predating Au. sediba, Alemseged explains.

In addition, the findings (especially in regards to the dental study) suggest that Au. sediba was closely related to Australopithecus africanus, but not Australopithecus afarensis, the species Alemseged studies. He finds the evidence linking Au. sediba and Au. africanus solid, but that doesn’t leave Au. afarensis out. Given the timing, Au. afarensis, which lived between 3.8 and 2.9 million years ago, was likely the ancestor of Au. africanus, which lived between 3.3 and 2.1 million years ago and in turn was the ancestor of Au. sediba.

Alemseged notes that the studies underscore the diversity of our lineage. “It’s not surprising, in the natural world, to find multiple species of any given group,” so why should our family tree be any different?

Despite his scientific disagreement with his colleague, Alemseged lauds Berger’s generous sharing of the fossils and studies related to Au. sediba. “He’s introduced a new culture in paleontology of being very open.”

Finally, the image accompanying many of the articles (above right) is very similar to an animation comparing Au. afarensis, humans and chimpanzees in the Academy’s current exhibit, Human Odyssey that Alemseged curated. If you haven’t visited it yet, it explores Australopithecus, Homo, and more!

Image: Lee R. Berger And The University of the Witwatersrand

Catching glimpses into our fossil ancestors’ daily lives is a tricky business. Fossil remains of our ancestors can only tell us so much concrete information, and tracing our DNA backwards in time can only get us so far.

But bones and teeth hold more clues than you’d think, if you just know how to extract them. In a new research paper published in the journal Nature, evolutionary anthropologists harnessed cutting-edge chemical tools and analyses to uncover the social patterns of our earliest ancestors and in so doing, discovered that males weren’t too keen on leaving their childhood homes.

The study, led by Sandi Copeland of the Max Planck Institute for Evolutionary Anthropology, looked at fossilized teeth from South Africa: eight Australopithecus africanus (2.2 million years ago) individuals and 11 individuals belonging to the Paranthropus robustus (1.8 million years ago) species. Using a laser, the team extracted a key element from the tooth enamel called strontium.

The strontium found in tooth enamel is like a snapshot into where the person lived during childhood, when permanent teeth developed. The various types of strontium, called isotopes, can be connected with specific geographical regions. “The strontium isotope ratios are a direct reflection of the foods these hominids ate, which in turn are a reflection of the local geology,” Copeland explains.

The research team divided sets of teeth for both species into male and female based on size (male teeth are generally larger). They then performed strontium isotope analysis on each, looking for clues into the each specimen’s childhood geographical landscape. They found that a large majority of male specimens – nearly 90% – grew up in the same general area where the fossilized teeth were uncovered. They were born, grew up, and died in pretty much the same place: the prehistoric equivalent of their hometown.

But analysis of female strontium isotopes revealed a different history. Over 50% of female remains trace to further afield, away from the dolomite cave systems that so many males grew up near. It seems that many females spent their formative years elsewhere, only arriving in the area once they reached adulthood.

Chimpanzees, our closest living primate relatives, exhibit a similar social structure. Male chimps are highly territorial, and will not leave their home base, even upon reaching adulthood. To prevent inbreeding, females are often forced to leave their childhood groups in search of new mating partners in other groups. Copeland’s strontium-isotope analysis lends support to the idea that early hominids might have done the same. If this structure exists in both chimpanzees and early hominids, perhaps its origins extend much further back in time.

“One of our goals was to try to find out something about early hominin landscape use. Here we have the first direct glimpse into the geographic movements of early hominids,” says Copeland.

The study not only provides insight into previously unknown aspects of ancient hominin social structure, it also highlights exactly how much new information can be squeezed out of a fossil specimen. As Julia Lee-Thorp, one of the study’s co-authors, explains, “Studies like these really bring home that finding and describing fossils is not the end of the story. Thoughtful application of these new analytical methods can tell us such a lot more about the details and lives of the distant past.”

Anne Holden, a docent at the California Academy of Sciences, is a PhD trained genetic anthropologist and science writer living in San Francisco.

Image: Darryl de Ruiter

To survive in a tumultuous environment, sea urchins literally eat through stone, using their teeth to carve out nooks where the spiny creatures hide from predators and protect themselves from the crashing surf on the rocky shores and tide pools where they live.

Scientists agree that the rock-boring behavior is astonishing, but what is truly remarkable is that despite constant grinding and scraping on stone, urchin teeth never, ever get dull. The secret of their ever-sharp qualities has puzzled scientists for decades, but now a new report in the journal Advanced Functional Materials has peeled back the toothy mystery.

Pupa Gilbert, of the University of Wisconsin-Madison, and her colleagues studied the self-sharpening mechanism used by the California purple sea urchin to keep a razor-sharp edge on its choppers.

“The sea urchin tooth is complicated in its design. It is one of the very few structures in nature that self-sharpen,” says Gilbert, explaining that the sea urchin tooth, which is about .8 of an inch long, is a biomineral mosaic composed of calcite crystals with two forms — plates and fibers — arranged crosswise and cemented together with super-hard calcite nanocement.

From National Geographic Daily News:

Between the crystals are layers of weaker organic material. By striking the teeth with microscopic, diamond-tipped probes, the scientists found that the teeth break along these organic layers.

The scientists think the organics are predetermined weak spots in the teeth that allow parts of the material to “tear” away, similar to perforations in a sheet of paper. This means the teeth, which grow continuously, can regularly shed damaged areas to keep a well-honed edge.

Knowing the secret of the ever-sharp sea urchin tooth, says Gilbert, could one day have practical applications for human toolmakers. “Now that we know how it works, the knowledge could be used to develop methods to fabricate tools that could actually sharpen themselves with use,” notes Gilbert. “The mechanism used by the urchin is the key. By shaping the object appropriately and using the same strategy the urchin employs, a tool with a self-sharpening edge could, in theory, be created.”

Photographs courtesy of Pupa Gilbert, University of Wisconsin

Three large, extinct swimming reptiles, the ichthyosaurs, plesiosaurs and mosasaurs, were the top ocean predators during the Mesozoic era, about 251 to 65 million years ago.

Most reptiles today are cold-blooded, meaning their body temperature is determined by how warm or cold their surroundings are. But, some of the modern ocean’s top predators, tuna and swordfish, are “homeothermic” (aka warm-blooded), or able to keep their body temperatures at a constant temperature despite changing environmental conditions.

To see whether the three lineages of Mesozoic marine reptiles were also homeothermic, Aurélien Bernard and colleagues analyzed various types of oxygen in the teeth of these reptiles. They compared the oxygen in the reptile teeth with the oxygen in the teeth of fish from the same environments.

This tooth oxygen is a clue to an animal’s body temperature, because it reflects the composition of oxygen in the blood.

The researchers knew that the fish whose teeth they were studying were cold-blooded. So, when they found reptile teeth with different oxygen signatures, it probably meant that those reptiles had warmer body temperatures than the fish did.

The results suggested that ichthyosaurs and plesiosaurs, which chased their prey, probably controlled their own temperatures. The data for mesosaurs, which are thought to have hunted by ambush, were less clear, but it’s possible that these reptiles could control their body temperature to some degree.

In the Nature blog The Great Beyond, Robert Eagle, PhD, of the California Institute of Technology, challenges the methods used in the study:

Isotope ratios in teeth are dependent not only on body temperature, but on the levels of the oxygen isotopes in the animals’ bloodstreams when their teeth were growing—something that might vary among species due to dietary and physiological differences. In a study published last month in the Proceedings of the National Academy of Sciences (see our previous article on this study), Eagle and colleagues presented an alternative analysis, based on a combination of oxygen and carbon isotopes, which they believe to be less affected by factors other than temperature. “It would be interesting to see if we get the same answer,” he says.

Stay tuned to find out…