A while ago I wrote “the children of climate change,” a post discussing research about how climate variability may have caused the evolution of Homo sapiens’ large brain. The crux of this “environmental complexity thesis” is essentially:
- A variable environment will favour a versatile species able to survive well in a range of environments.
- Increased brain size (and associated increases in cognitive ability) can make a species more versatile by allowing them to change their behaviour to suite new environments.
- Around the time the hominin brain began to significantly grow the environment was variable.
Therefore it would seem that environmental variability was one of the driving forces behind the evolution of our impressive intellect. The logic behind this idea is quite appealing and there’s plenty of evidence to support it as well. Animals with larger brains, for example, are consistently found to innovate more than their similar, but small brained, counterparts. Birds which don’t migrate over the winter (i.e. have to deal with a more variable environment) have larger brains than their migratory relatives.
Of course the ECT is only one of many other explanations about how our brain became so peculiarly large. Nearly everything from “remembering where resources are” to “lying” has been suggested as the reason why our brains got big. Whilst many of these lack evidentiary support there are still some that have a solid case. In particular the “social brain hypothesis,” a popular explanation which suggests our brain got bigger to facilitate more social skills.
Although this may seem in conflict with ECT in reality they are just different sides to the same coin, one explaining the proximate causes while the other explains the more “ultimate” reason for brain growth. Our brain got bigger to facilitate more social skills (SBH) which was beneficial as it made our ancestors more versatile (ECT). For example, fission-fusion behaviour (splitting up a group to search a larger area for food) is a social skill that would’ve enabled our ancestors to survive in a new environment where resources are more scarce.
However, not being content with “explaining the ultimate cause of brain evolution” Dr Grove – the author of the other ECT article I wrote about – has pushed back the chain of causality further and attempted to identify the cause of the climate chance which caused a social brain to be favoured which caused brain evolution. Surprisingly the answer is not “turtles all the way down” but orbital cycles.
Orbital cycles (also known as Milankovitch cycles, after their discoverer) are changes to the Earth’s orbit. They stem from the fact the orbit of our planet is not perfect and it gradually changes, shifting from one extreme and back again over the course of thousands of years. This influences how much sunlight the Earth receives and whereabouts it gets it, which can have some pretty big implications for the climate (you can probably see where this is going). Orbital cycles come in three flavours: precession, axial tilt and eccentricity.
Eccentricity refers to the shape of our orbit around the sun. It shifts from not particularly elliptical to quite elliptical (“not particularly” and “quite” are real technical terms…in my head) and back again over the course of over 100,000 years making it the longest of the orbital cycles. It’s also the one that has the most effect since an elliptical orbit means the Earth travels further away from the sun for a good part of its orbit. Moving a planet away from a star is going to have a rather noticeable effect.
Axial tilt refers to how much the Earth is tilted. As many of you may know the northern hemisphere of our planet is “leaning” so that it is closer to the sun during some parts of the year and further away during others. This is one of the primary sources of seasonality with winter stemming from the fact the northern hemisphere gets a bit further away from the sun during parts of the year. This tilt becomes more/less severe over the course of 41,000 years influencing the severity of the seasons.
Precession is the shortest cycle, changing every 21,000 years or so, and the one I least understand. So if anyone with a better of understanding of orbital mechanics can clarify, please speak up. At any rate it seems to be the rate at which the earth “wobbles” on its axis which does something or other. This basically changes how much light the north pole gets which can influence the climate by making it melt/freeze.
Since these various cycles take differing lengths of time sometimes they will overlap and interact. Further, the severity of these cycles is not a constant and one eccentric cycle may not move the Earth as far away from the sun as another. These two factors combine and mean that the amplitude of these orbital effects can vary, sometimes producing more climate change than others. As such they can contribute to a variable climate, the kind involved with the evolution of the brain.
So Dr Grove input-ed all these orbital variables into his model and ran various statistical analyses to see if any of these climate influences also influenced human evolution. The results the computer spit out (with great disdain) showed that there was indeed a correlation between minimum amplitude of the eccentric cycle and increased hominin brain size, accounting for 43% of the change in our brains. This certainly answers some questions but, as with all good science, asks a lot more. Why is it the eccentric cycle, not the others, which influenced our evolution? Why the minimum amplitude?
The first question is relatively easy to answer since the eccentric cycle is the one that has the biggest influence on the other two. They simply change where the sunlight hits the Earth, the eccentric cycle actually alters how much sunlight the Earth gets. As such it is understandable why the biggest orbital influence is the biggest influence on our evolution.
Why it is the minimum amplitude influencing brain evolution is a trickier question to answer. Dr Grove suggests that during the more severe changes they may have simply migrated to a new area rather than adapt to particularly harsh conditions. It’s only the smaller changes which they wouldn’t have had to run away from that they adapted to. Certainly this explanation seems plausible, although he can offer little data to support it. This isn’t a slight against the research since answering the question falls outside his goals, it’s just an area for future study: Can migration deal with the other orbital changes?
Overall we’re left with an interesting piece of research about a plausible model well supported by the evidence. Whilst it doesn’t answer every question it certainly answers several and so it would seem that we’ve identified the “ultimate” cause of 43% of hominin’s increase in brain size. Fascinating stuff.
|Grove M (2012). Orbital dynamics, environmental heterogeneity, and the evolution of the human brain Intelligence DOI: 10.1016/j.intell.2012.06.003|