Early Humans May Have Evolved Bigger Brains By Eating Insects
April Flowers for redOrbit.com – Your Universe Online
A new study, led by Washington University in St. Louis, suggests that seasonal diet changes may have played a role in the development of bigger brains and higher-level cognitive functions in human ancestors and other primates. The findings, published in the Journal of Human Evolution, show that figuring out how to survive on a lean-season diet of hard-to-reach ants, slugs and other bugs might have been the catalyst for early tool use.
“Challenges associated with finding food have long been recognized as important in shaping evolution of the brain and cognition in primates, including humans,” said Amanda D. Melin, PhD, assistant professor of anthropology in Arts & Sciences. “Our work suggests that digging for insects when food was scarce may have contributed to hominid cognitive evolution and set the stage for advanced tool use.”
Melin and her colleagues, biologist Hilary C. Young and anthropologists Krisztina N. Mosdossy and Linda M. Fedigan, all from the University of Calgary, studied Capuchin monkeys in Costa Rica for five years. The evidence from their findings support the evolutionary theory that links the development of sensorimotor (SMI) skills to the creative challenges of foraging for insects and other buried, embedded or hard-to-obtain foods. SMI skills are those needed for increased manual dexterity, tool use, and innovative problem solving.
The team believes that their study is the first to provide detailed field evidence showing how seasonal food changes influence the foraging patterns of Capuchin monkeys in the wild. Many human populations consume embedded insects on a seasonal basis, leading the team to suggest that this practice played a vital role in human evolution.
“We find that capuchin monkeys eat embedded insects year-round but intensify their feeding seasonally, during the time that their preferred food – ripe fruit – is less abundant,” Melin said. “These results suggest embedded insects are an important fallback food.”
A 2009 study published in the American Journal of Physical Anthropology defined fallback foods as a term “to denote resources of relatively poor nutritional quality that become particularly important dietary components during periods when preferred foods are scarce.”
Other studies have shown that these foods help to shape the evolution of primate body forms. In primate species whose fallback foods are mainly vegetation, these forms include strong jaws, thick teeth and specialized digestive systems.
The current study presents evidence indicating that fallback foods can also play an important role in shaping brain evolution in primates that fall back on insect-based diets. The strongest influence of this sort can be observed in primates that have evolved in habitats with wide seasonal variations — the wet-dry cycles in some South American forest, for example.
“Capuchin monkeys are excellent models for examining evolution of brain size and intelligence for their small body size, they have impressively large brains,” Melin said. “Accessing hidden and well-protected insects living in tree branches and under bark is a cognitively demanding task, but provides a high-quality reward: fat and protein, which is needed to fuel big brains.”
Not all Capuchin monkeys have the same abilities with tools, however, and Melin believes that her research reveals why.
There are two major groups of Capuchin monkeys: the gracile (untufted, genus Cebus) and the robust (tufted, genus Sapajus). Scientists say that the split between the two groups occurred millions of years ago during the late Miocene epoch, according to genetic analysis of mitochondrial chromosomes. One of the most observable differences between the two lineages is their variation in tool use. While the Cebus lineage is known for clever food foraging tricks like banging snails or fruits against branches, they are not adept tool-users like the Sapajus cousins.
Melin said that the explanation could be found in habitat differentiation. Cebus capuchins have historically and consistently occupied tropical rainforests. In contrast, the Sapajus capuchins spread from their original Atlantic rainforest habitat into drier, more temperate and seasonal habitats.
“Primates who extract foods in the most seasonal environments are expected to experience the strongest selection in the ‘sensorimotor intelligence’ domain, which includes cognition related to object handling,” Melin said. “This may explain the occurrence of tool use in some capuchin lineages, but not in others.”
“We predict that the last common ancestor of Cebus and Sapajus had a level of SMI more closely resembling extant Cebus monkeys, and that further expansion of SMI evolved in the robust lineage to facilitate increased access to varied embedded fallback foods, necessitated by more intense periods of fruit shortage,” she said.
Modern examples of this behavior exist, most notably the seasonal consumption of termites by chimpanzees. The chimps’ use of tools to extract the protein-rich termites is an important survival technique in a harsh environment.
How does this research affect our understanding of hominids?
Using the fossil record to decode the extent of the seasonal dietary variations is challenging. However, seasonsal variation in diet for at least one South African hominin (Parathropus robustus) has been suggested by stable isotope analysis. Such research has also suggested that early human diets may have included a wide range of extractable foods, including termites, plant roots and tubers.
Even today, humans frequently consume insects as a seasonally important diet when other animals are scarce.