The old saying "You are what you eat" takes on new significance from
Africa in the most comprehensive analysis to date of early human teeth.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 the week of June 3 in the Proceedings of the National Academy of Sciences, by a group of international scientists spread over three continents. "These papers present the most exhaustive isotope-based studies on early
human diets to date," said Dr. Zeresenay Alemseged, Senior Curator and
Chair of Anthropology at the California Academy of Sciences, and
co-author on two of the papers. "Because feeding is the most important
factor determining an organism's physiology, behavior, and its
interaction with the environment, these finds 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," said 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," said 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.
Source: Proceedings of the National Academy of Sciences
Source: Proceedings of the National Academy of Sciences
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