Tuesday, October 31, 2017

Grandma, what a big brain you have!

All the better to survive with, my dear

 

Used with permission from Nicola L. Robinson.

Editor’s Note: SciColl held our first community workshop on Emerging Infectious Diseases in October 2014. This October we're posting several pieces that highlight the important work where collections continue to play an integral role.

Editor’s Note: SciColl intern, Ebubechi Okpalugo from Pembroke College, contributed this article as part of her time in the SciColl office during Summer 2017.

Sit down with a prehistoric relative of ours from the genus Homo - their extinction making it a lot easier said than done - and you’ll notice something significantly different: their heads, and therefore brains, are a fraction of the size of a modern human’s. Why exactly did brain size increase? Quite simply, large and complex brains can process and store more information, which was hugely beneficial to humans as they developed skills and social interaction.

The Smithsonian NMNH Human Origins exhibit houses a collection of fossil skull remnants dating back as far as 3 million years ago, as well as silicon reconstructions of early humans. One can see a gradual change in facial features, body hair and most significantly, head shape and size. Over the course of human evolution, it’s found that brain size tripled, with the modern human brain being the largest and most complex of any living primate. But, size alone doesn't tell us much about thinking ability.

Here’s a bit of what we know. 

From 800,000–200,000 years ago, there was a dramatic increase in brain size (Figure 1). Smithsonian anthropologist and director of the Human Origins program, Richard Potts, noticed this sudden growth occurred during a period of dramatic climate change. At this point, an arid grassland environment was thought to be the main driving force behind human specializations such as bipedal locomotion, known as the savannah theory. Carrying out field research in southern Kenya, sediment layers revealed the periodic occurrence and disappearance of a great lake, as well as periodic distributions of volcanic ash dated to 1 million years ago. This evidence of dramatic changes in the environment led Potts to question how these frequent changes could support the theory of adaptations towards one specific environment. Rather, perhaps the climate fluctuations resulted in organisms having to increase their adaptability to survive. This concept came to be known as the variability selection hypothesis.


Fig. 1. Top graph depicts fluctuation of the Earth's climate over the past 2 million years, highlighting an increase in fluctuations between 800,000 and 200,000 years ago. This was found throughout the study of fossils with tiny organisms found in ocean sediment cores, measuring oxygen within their skeletons to record oxygen stable isotopes.
Bottom graph shows brain size increasing over the past 3 million years - particularly between 800,000 and 200,000 year ago. This graph was constructed through measuring the brain cavities of more than 160 early human skulls.
Karen Carr Studio, 2016.

The brain is our organ of plasticity, allowing us to be flexible, an incredibly important characteristic in the existence of our species. Potts suggests that ‘evolution favored larger brains that helped humans cope with challenges posed by these dramatic swings of climate’. As the climate and environment became more unpredictable, a better memory, communication skills, and creativity enabled big-brained humans to identify and remember new food sources and strengthen other methods of survival. 

‘Survival of the fittest’ becomes ‘survival of the most flexible’.

When asked what parts of the brain were most important in this rapid increase in brain size, Potts replied that it’s less to do with specific parts of the brain, but more to do with the versatility of the connections. In chimpanzees and humans, the neocortex grey matter is pretty much the same. However, the difference is in the white matter (Figure 2), creating in human beings the potential of all types of connections which allow us to be defined by so many different expressions. Using neuroanatomical samples from the Yakovlev-Haleem slide collection at the National Museum of Health and Medicine, a study compares myelin in the human brain to chimpanzee brain samples. It was found, that in chimpanzees, the density of myelinated axons increased steadily until sexual maturity. In contrast, humans showed slower myelination during childhood, but development continues beyond late adolescence. This may provide more opportunities for social learning to influence connectivity, contributing to our species-specific cognitive abilities. Neocortical myelin development disruption has also been found in some disorders which affect cognitive ability.

Fig. 2. Image showing the neocortex, seen as the thin dark purple/grey (stained grey matter) layer on the surface of the brain. The white areas below are connecting fiber pathways (white matter).

Linking brain size with intelligence, Potts discussed the controversial nature of a study of 109 fossilized hominid crania over the past 2 million years. Using the locations where the specimens were found, the paper claims that ‘as seasonal variation in temperature due to distance from the equator increased, there was a corresponding increase in brain size’. However, there is no mention of comparison with lean body mass, with equatorial individuals tending to be leaner and people in higher latitude environments having squatter body types. It then goes on to correlate displacement from the equator with performance in IQ tests, despite it being known that IQ tests are based on Westerns ways of thinking and standards of intelligence. In the search for a reliable anatomical measure for intelligence, there is a danger in trying to pin point one part or characteristic of the brain to such a multi-faceted and abstract concept. Once asked ‘what is intelligence’, Potts responded by saying intelligence is doubting everything you think may be intelligent’. Certain parts of the brain definitely have their specialties, but with so many ways of being intelligent or creative, we can only expect neuroanatomy to reflect its intricacy.

References:
Gallup, G. and Ash, J. (2016). Global warming: implications for human brain evolution. Atlas of Science, [online] p.1. Available at: https://atlasofscience.org/global-warming-implications-for-human-brain-evolution/ [Accessed 31 Aug. 2017].

Humanorigins.si.edu. (2016). Brains | The Smithsonian Institution's Human Origins Program. [online] Available at: http://humanorigins.si.edu/human-characteristics/brains [Accessed 31 Aug. 2017].

Humanorigins.si.edu. (2016). Climate Effects on Human Evolution | The Smithsonian Institution's Human Origins Program. [online] Available at: http://humanorigins.si.edu/research/climate-and-human-evolution/climate-effects-human-evolution [Accessed 31 Aug. 2017].

Scott, M. (2016). Climate and human evolution. Climate.gov, [online] p.1. Available at: https://www.climate.gov/news-features/climate-and/climate-and-human-evolution [Accessed 31 Aug. 2017].

Miller, D., Duka, T., Stimpson, C., Schapiro, S., Baze, W., McArthur, M., Fobbs, A., Sousa, A., Sestan, N., Wildman, D., Lipovich, L., Kuzawa, C., Hof, P. and Sherwood, C. (2012). Prolonged myelination in human neocortical evolution. Proceedings of the National Academy of Sciences, [online] 109(41), pp.16480-16485. Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3478650/ [Accessed 31 Aug. 2017].


Glossary

Grey matter
The cell bodies of neurons, which are grey in appearance.
Neocortex
Often known as the "thinking brain," involved in functions such as sensory perception, cognition, generation of motor commands, spatial reasoning, and language.
Variability selection
The idea that structures and behaviors which enable species to cope with a larger variety of different environments are favored, and therefore selected for.
White matter
Axions extending from neuronal cell bodies connect neuron together, and are wrapped in myelin, which is white in appearance, providing insulation for the electrochemical signal.


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