Wednesday July 27 2011
Are our brains shaped by how much daylight our ancestors had?
Scientists have revealed that “people have bigger brains the further north they hail from”, The Sun reported. However, it added, this is not because they are more intelligent than southerners, but because they have to cope with less light.
This research investigated whether there is an association between the size of eye sockets and the brainpan in the skulls of people from around the world and the distance to the equator. Those who came from places further from the equator seemed to have larger eyes and brains. The researchers thought that this physical difference would help people to detect the lower levels of light of these areas.
The finding needs confirmation in further research, as it only looked at 73 skulls, and it is not known if these skulls are representative of the native peoples of the areas that they were reported to come from.
As most newspapers reported, if this difference does exist, it doesn’t mean that people from more northerly areas are more intelligent. Furthermore, this study compared the skulls of people from countries around the world, and averaged measurements and latitudes within countries. As such, it is not clear whether there are any differences between people in the south of the UK and the north. While the study is interesting and may add to our knowledge of human evolution, it is unlikely to have any practical application to human health.
Where did the story come from?
The study was carried out by researchers from the University of Oxford. Funding was provided by the British Academy and the Boise Fund at the University of Oxford.
The study was published in the (peer-reviewed) medical journal Biology Letters.
Despite slightly misleading headlines, the media generally reported the story accurately. Most clarified that any increase in brain size was probably confined to areas responsible for processing visual information and not due to increased intelligence.
What kind of research was this?
This was a cross-sectional study that aimed to investigate whether there are any evolutionary associations between eye socket size, brain size and the person’s distance from the equator. The study was carried out using skulls housed in the Oxford University Museum of Natural History and the Duckworth Collection at the University of Cambridge.
Previous research in animals has shown that low light conditions (such as those found in regions further from the equator) are associated with larger eyes. Larger eyes have also been found to be associated with larger areas of the brain responsible for processing visual information in both animals and humans.
Cross-sectional studies are able to describe associations, but cannot provide causative explanations on their own.
What did the research involve?
Based on previous findings in both animal and human studies, the researchers had a theory that the further away from the equator a population lived, the larger their eyes and brains would be. This theory is based on the premise that these populations would have less daylight than those who lived closer to the equator, and so natural selection would result in a better ability of seeing and detecting detail in low light. This process would take place over tens of thousands of years, and result in larger eyes. Similarly, the regions of the brain that process visual information would be bigger.
To test this theory, the researchers compared the size of various parts of skulls from different countries around the world and the distance of that country to the equator. They measured the cranial capacity (volume of the space inside the skull), orbital volume (volume of the space inside the skull’s eye socket) and foramen magnum (size of the large hole at the base of the skull through which the brain connects to the spinal cord) in the skulls of 73 healthy adults. These skulls were drawn from 12 different native populations found at various distances from the equator. Cranial capacity was used as an approximation of brain size, orbital volume as an approximation of eye size, and foramen magnum dimensions as an approximation for overall body size. The researchers also analysed the genetic background of the skulls, and controlled for genetics in their analysis.
The researchers then found the average brain, eye and body size for each population, and analysed this data according to the distance from the equator the population lived. They also estimated the level of light and average temperature in each country. Average temperature information was collected in order to test the possibility that the larger eye socket provided more room for insulating fat needed to protect the eye at lower temperatures. During the data analysis, the temperature and foramen magnum measurement were used to account for any role that climate or body size might play in eye and brain size.
What were the basic results?
The researchers found that living further from the equator was associated with having larger eye sockets and cranial capacity. Neither average environmental temperature nor average body size explained this relationship, as the association held when both measurements were taken into account.
How did the researchers interpret the results?
The researchers say their results suggest that: “that there has been selection for larger eyeballs under progressively lower light conditions”. In other words, larger eyeballs are found in populations that live in regions with less light and shorter days. They further say that previous research has shown that as human eyeball size increases, increases are also seen in the brain regions responsible for processing visual information, which is likely to explain the associated increase in cranial capacity.
Based on these results, the researchers intend to conduct further research to examine whether ability to detect details is different in populations at varying latitudes.
This study found that the skulls of people who lived further from the equator had structural differences that suggested they had bigger brains and eyes than people living nearer the equator. However, brain size as described in this study does not equate to intelligence. The researchers suggest that increases seen may be due to increases in visual processing areas of the brain. The researchers also propose that this may have come about through evolutionary processes, which are evident only on a population level, not an individual level. This means that differences between groups may only be seen if we look at population as a whole, and that these changes have occurred very gradually over time.
This research describes average differences across populations, which are likely to have taken tens of thousands of years to develop. It also looks at averages across the globe, not within any given country. Although The Sun and many of the newspapers reported that these differences would be evident between northerners and southerners, this research looked at people in different countries at different latitudes around the world, and did not compare people within this country. Therefore it does not tell us whether there are any differences between people living in the north and south of the UK.
There are a number of other points to note:
- The study measured total skull volume and eye socket volume. The assumption is that larger skull and eye socket volume is associated with larger brain and eye size, although it is not possible to say for sure whether this was the case. There are also hypotheses made about the cause of these differences being the lower light in northerly areas. With evolutionary biology, it is often difficult to prove why changes in the past occurred, and researchers make hypotheses to explain their observations, and adapt these as needed when new information becomes available.
- The study was relatively small – analysing data from 55 skulls. Ideally, larger studies would be needed to confirm the findings.
- A study of living individuals could use for example, brain scans and vision tests to obtain similar data for living individuals from different latitudes.
This study aimed to describe trends across populations, and to look at how different populations might adapt to different environmental conditions. While it adds to the knowledge of human evolution, it does not have much practical application to human health.