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

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

This week, Zeray and his colleague David Green, of Midwestern University, have published a study in Science (the cover story, no less!) thanks to Selam and her shoulder blades. The researchers find that even though A. afarensis was bipedal, Lucy, Selam, and their kin also climbed trees.

When the Lucy skeleton was discovered in 1978, “there was no question that this creature was an upright walking species,” says Zeray. “The foot, the knee and pelvis are all very human-like.”

However, the upper body told a different story. The features were much more ape-like, including long, curved fingers.

At the time, says Zeray, scientists split into two groups. “One group said, ‘Yes, A. afarensis has ape-like features, but the species doesn’t need them for survival, it’s just retention, evolutionary baggage. They can’t be interpreted for function.” A function like tree climbing found in earlier human ancestors.

The second group interpreted the ape-like characters for their function: “they saw the lower body for bipedalism and the upper body for climbing,” Zeray explains. “This discussion went on for the past 35 years.”

Now Zeray and Green’s paper may help the second group’s debate.

When Zeray published the findings of Selam in 2006, he noted that Selam’s scapulae, or shoulder blades, were gorilla-like, but he didn’t attempt to interpret that finding. Mainly because even though the two shoulder blades were intact, they were completely buried in compacted sand. Zeray and Kenyan lab technician Christopher Kiarie had to remove the super-thin fossils from the rest of the skeleton. Sand grain by sand grain.

That sort of work doesn’t happen overnight. It took them 11 years! Once the scapulae were extracted, Zeray could analyze the fossil in a detailed fashion. He confirmed they were gorilla-like, but there still wasn’t enough information to interpret the findings. Then, he and Green began their collaboration.

Green works on living species, Zeray on fossil species. The team began to collect data on many living primates—humans, chimpanzees, gorillas and orangutans—juveniles and adults. They also looked at the Turkana Boy’s shoulder blades. This Homo erectus fossil provides the second earliest and most complete scapulae of our ancestors—1.5 million years-old. Selam, at 3.3 million years, provides the oldest.

After the researchers analyzed the data, they concluded that A. afarensis climbed trees. Their evidence? The shoulder blades were indeed ape-like anatomically. In addition, they found that Selam’s shoulder blades developed like modern apes, not like humans. In apes, the shoulder anatomy of juveniles and adults are similar, but in humans, the anatomy changes quite a bit between the young and old. Finally, Zeray and Green demonstrated that the way the bone and muscles were oriented—in an upward direction—allowed more flexibility for climbing.

Zeray offers an explanation for this adaptation. “This species was small—chimpanzee-sized—in a wild environment with just a few stone tools, surrounded by scavengers and predators. They needed to nest at night, provision themselves and evade predators and carnivores. That’s why they maintained climbing, even though they were bipedal.”

This finding offers a bigger picture of human evolution. “When we became humans, we did not just jump from tree and start running. It was progressive,” says Zeray.

“The Selam skeleton is a mine of information for years to come,” he continues. And not just for his research, but many others. He can’t predict what we might find with this fossil, but he can imagine the possibilities, “how the species behaved, moved, looked, developed, voiced.”

For his own research into Selam, he’s not looking for anything specific and has no expectations. Zeray lets the data dictate the results—as interesting things pop up, he’ll give them the attention they need. He’ll let the data Selam provides lead to the science.

Image: Zeray Alemseged/Dikika Research Project