Earth’s orbit and human evolution

ResearchBlogging.orgA 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:

  1. A variable environment will favour a versatile species able to survive well in a range of environments.
  2. 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.
  3. 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.

Ironically some might consider hanging around in the cold stupid

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.

I wish this were the answer to more things

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.

The three orbital cycles

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

12 thoughts on “Earth’s orbit and human evolution

  1. Why is increasing happiness right and causing suffering wrong? What if my brain says that causing harm and suffering is right? You said that the way we tell right from wrong is with our brain. What if my brain says it’s right to attack you and you are wrong to resist? What is your basis for believing that your position is correct and mine is incorrect?

    See, without an absolute standard to compare the behavior to, then right and wrong is simply subjective.

    • It’s an essentially universal condition to avoid suffering and increase happiness. We all flinch from pain and seek to repeat experiences which bring us joy, so the default stance should be that someone does not want to suffer. Thus you should ensure your activities do not cause them to suffer. Of course, should they stand up and say they think suffering is good they’re welcome to go and pursue their own interests, provided it does not bring suffering to people who do not want to suffer.

    • As an aside, it’s worth noting that this idea isn’t necessarily the “morality” which evolved. Our species developed the moral “gut” instincts and a deference to authority which enables us to peacefully co-exist, a beneficial adaptions. Whilst this might be a good rule of thumb, ultimately they fall victim to some of the problems you’ve noted. What if the gut instinct is broken, or just self-serving? What if the authority we have deference to is making arbitrary, unhelpful remarks?

      We’ve moved to the point where our inherent “morality” is no longer sufficient to govern our interactions. We simply haven’t developed the capacity to behave on a global level. Thus we must clad ourself in new ideas, based on certain common truths in much the same way we have to clad ourselves in protective gear to enter other environments we did not evolve to deal with.

        • I’m not quite sure how you got that from what I wrote.

          Anyhoo, I think there are certain objective statements you can make about what course of action is better in any given situation. Feeding an African child is better than shooting them, to pick an obvious and extreme example. However, this rule isn’t universal. The next time you encounter a child, they could be a child soldier about to murder a village and killing them is the only viable course of action. Thus you can make objective statements about certain situations, but you can’t really extrapolate the specifics. Is that absoluteness? I dunno, I’ll leave that up to you to decide.

          At any rate, this malleability is a strength of secular morality. You can react differently in different circumstances to ensure the best possible outcome is always reached. This is a real benefit that many “absolute” moralities have also tried to adopt. For example, I’ve heard many Christians argue that the 10 commandments say “do not murder” rather than “do not kill,” leaving the door open for situations where killing would not be murder.

          Similarly I once had a school assembly where the head teacher conveyed the story of some Christians who turned Jews who were hiding over to Nazis in the second world war because they were not to “bare false witness” and had to “render unto Caeser.” The moral they tried to teach there (in a religious context, when I was growing up schools were a lot more Christian even though that was only a decade ago) was that the spirit of the law was sometimes more important than the letter.

          At this point there’s only a few details between the secular and absolute morality, one saying x is good because a book I place faith in says so; the other that x is good because I have some evidence it will bring happiness, decrease suffering etc. At that stage I know which idea I’d rather side with: the one with evidence.

          Now, I’m not trying to say that you either endorse or think like any of the views mentioned above. I’m just trying to note that the best morality is the one that can adapt to situations; and many “absolute ” moralities are also adaptive as a result. But they deny it, still claiming some absoluteness. So I’d just like to ask for some introspection, for you to look at your code of ideas and ask yourself how much wiggle room you’re permitted. Can you kill in the right situation? Does the Old Covenant no longer apply to you, freeing you from rules that might otherwise seem extreme? Is it right to break rules in some situations?

          I don’t expect an answer for any of these, I just hope to prompt some critical thinking.

  2. Pingback: The evolution of lie-detection « EvoAnth

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