Thursday, November 6, 2014

From Bugs to Brains

Top: The head of Panthropus boisei - "Nutcracker Man", as recreated by anthropological sculptor Elisabeth Daynès Bottom: The skull of Paranthropus boisei, Nairobi National Museum in August 2012. Photo: Bjørn Christian Tørrissen
The human brain is a biologically expensive luxury in terms of caloric needs, mainly because glucose is critical for powering protein ion pumps that maintain the resting membrane potential - the difference in charges between the inside and outside of neuron cells. It is this built-up charge that allows neurons to fire and send signals to other neurons "downstream" in the brain's circuits.

But the human brain's rather sudden and enormous growth in complexity and bulk is something of a mystery. What led to this incredibly rapid (in terms of geological time) evolution? Previous studies have shown that the dietary addition of DHA from aquatic animals was a major factor, providing new essential fatty acids that spurred rapid evolution of hominid brains.

Now studies at Washington University in St. Louis point to a much less palatable source of additional proteins which drove primate brain growth - insects.

In times of scarcity, our primate ancestors turned to insects to survive - and learning to harvest hard-to-reach bugs like ants and slugs helped spur brain expansion and higher-level cognitive abilities.

Anthropologists have long recognized that the challenges of surviving in a rapidly changing environment - i.e. finding food and shelter - were important for brain and cognitive development in primates, according to anthropology professor Dr. Amanda D. Melin (although it may also have been due to mere happenstance in the form of a minor gene-duplicating glitch).

Digging for insects during times of scarcity likely helped spur hominid cognitive development, and may have helped prepare our ancestors' brains for tool use and manufacture. A number of human communities worldwide have long consumed burrowing insects, further bolstering theories that this played a major part in the evolution of the human mind.

By observing the effects of seasonal food supply changes upon foraging patterns among wild Costa Rican capuchin monkeys over the course of five years, Dr. Melin and her team found direct evidence linking the mental challenges of foraging for insects and other difficult-to-obtain foods with problem solving, sensorimotor development and tool use.

Seasonally, when more appetizing fruit isn't as readily available, some primates intensify their feeding upon embedded insects. Thus insects constitute an important "fallback food". Such fallback foods contribute to both cognitive and "morphological" evolution - the gradual development of primate body forms, such as teeth, jaws, and specialized digestive systems.

The influence of insects and other fallback foods upon primate brain evolution is most pronounced among primates found in habitats that experience wide seasonal weather swings, like the wet/dry cycles of some South American forests. Obtaining insects burrowed in tree branches or under tree bark is cognitively demanding, but with a high nutritive payoff, in the form of calorie-dense fats and proteins, which fuel the development of big brains.

However, some varieties of capuchin monkeys are more adept at tool use than others. Dr. Melin's work provides a possible explanation. For example, there is a significant difference in tool use between Sapaju (robust tufted) vs. Cebus (gracile untufted) capuchin monkeys.

Cebus monkeys are clever at tricks for harvesting food - banging fruit and snails against branches, but their Sapaju cousins are much better at the inventive use and modification of comparatively sophisticated tools.

Mitochondrial DNA analysis indicates the Sapaju-Cebus split happened during the late Miocene period, millions of years ago.

Cebus capuchins have long occupied tropical rainforests, while their Sapaju cousins spread from their original Atlantic rainforest habitat to drier, more temperate and seasonal climes.

Natural selection of course favors primates who are healthier - thus better able to eat, in this case by developing advanced sensorimotor skills gained from extracting hard-to-get fallback foods. Thus some capuchin groups have evolved to be better at using tools.

Among our ancient hominid ancestors, Paranthropus boisei (Nutcracker Man)left behind fossilized teeth - careful chemical analysis has detected traces of several extractable foods, including termites, roots, grass bulbs and tubers.

To this day, in many human cultures around the world, insects become seasonally important sources of protein when other animal foods are scarce. Dr. Melin's study indicates that surviving on a diet of such creatures insects was key to the development of human skills, and that insects may well have played a major role in human brain evolution.

Source: "Seasonality, extractive foraging and the evolution of primate sensorimotor intelligence", Amanda D. Melina, Hilary C. Young, Krisztina N. Mosdossy, Linda M. Fedigan, Journal of Human Evolution, June 2014

No comments:

Post a Comment