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ResearchBlogging.orgIf you were asked what it was that makes Homo sapiens so successful, how might you respond? That it is our technology, which enables  us to conquer any environment? But what allowed the development of such technology? Our big brains of course! But why did our brains get so big? To facilitate larger group sizes! But how can we support big populations? Efficient foraging.

Ultimately the foundation of our success is our ability to gather sufficient food from the land. This naturally prompts the follow-up question, how are we able to do so?Again, one might be tempted to respond with tools, but then it should be remembered there are hunter-gatherer groups out there with vastly different levels of tool complexity.

I mean, some people are still using internet explorer 6!

Clearly, having a highly specialised toolkit isn’t mandatory for efficient foraging, so what is?

The answer is frustratingly simple: our ability to split into smaller groups when searching for food so as to cover more land. “Fissioning” into such subgroups to gather food, then “fusing” into a larger group for protection, food sharing and all the other benefits of group life lies at the heart of our success.

But we aren’t the only species to engage in such “fission-fusion” behaviour, with several species of primate – including, of course Pan troglodytes (chimpanzees) – also splitting up to more efficiently cover their territory. Indeed, studies suggest that if they didn’t chimps would only be able to live in groups of around 16, a fraction of the 30-70 actually witnessed.

So why are we the only species living all over the world? If chimps also engage in this behaviour, how come they aren’t our furry overlords (yet)?

I cannot stress the “yet” enough.

Again, the answer is rather simple: we split up into more groups than they do and so can gather more food from the same area.

This, somewhat soberingly, means that one of the main factors responsible for our success over chimps isn’t some distinct difference, but simply a matter of degree. We aren’t especially unique, we just do what they do but more of it.

It also means tracking the development of our increasingly complex “fission-fusion” system is important for understanding our evolution and a new paper purports to have done just that.

First, they used the correlation between brain size and group size to calculate how large a group our ancestors would’ve lived in. Then, by looking at the population density of existing hunter-gatherer groups they were able to calculate how much land our ancestors would’ve required, which produced this chart.

The most scientific poka-dots you ever did see

At certain points, these circles get just too big for the group to be able to cover as a single unit and so they would’ve had to split up.

When that would’ve happened is tracked by this following chart. It took the location fossils have been found at to identify how big the area required would have been, enabling them to calculate the number of groups a population would have to split into (or “fission points”) of various fossils.

Probalby the most colourful chart ever posted at EvoAnth

Now, that chart is rather intimidating, so thankfully the scientists summarised the overall trend with nice and simple (well, simple for a journal article) words.

Analyses of the fossil record suggest latitudinally-driven differences in the area requirements of 1) australopithecines from East and South Africa, and 2) African and Asian H. erectus, whilst…differences [due to group size] appear between 3) H. erectus as a whole and H. heidelbergensis, and 4) between H. heidelbergensis and the Neanderthals.

Of course, there are a few flaws with the data behind this conclusion, notably that they haven’t included all the variables which might influence required territory size (such as diet, tool use, body size etc.) into their statistics and the latitude data comes from modern environments – it was likely different in the past.

Whilst these factors might influence the specific trend spelled out in the colourful graph, I (and the researchers) doubt that it would be drastic enough to alter the general trend presented in the quote. Still, before saying that for sure it would be nice to have some follow-up data confirming it. In the mean time, I’m happy to draw vague conclusions from preliminary work.

There is another issue, however, namely what they want to conclude from this trend.

The results suggest a model of human social evolution whereby new grouping levels…are added as the existing maximum group size hits each of a series of critical fission points.

Now, I don’t see how this can be inferred from the data since it all it does is identify where these “critical fission points” (where the group would’ve have had to have split into more subgroups) are. This places a limit on the time/location by which more sub-groups would be necessary. It doesn’t say when they actually appeared.

Given that, what can you really conclude from this research? Well, if you just take the trend they’ve described then you have a pretty good understanding of the territory required by our ancestors which is a pretty important conclusion as is.

This allows you to place a limit on when various fission levels must have evolved by, although it doesn’t allow you to say when it actually did. It could also be used in further research, say to identify if new tools developed as territory requirements increased, perhaps allowing you to spot a pattern.

So, it’s a good foundation for further research that provides a good preliminary trend on our ancestors’ territory requirements. But take their ultimate conclusion with a pinch of salt.

Grove, M., Pearce, E., & Dunbar, R. (2011). Fission-fusion and the evolution of hominin social systems Journal of Human Evolution DOI: 10.1016/j.jhevol.2011.10.012

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4 Comments

Wyrd Smythe · 23rd January 2013 at 4:55 am

“We aren’t especially unique, we just do what they do but more of it.”

That prompts the question, do you think humans are indeed at one end of a fairly continuous spectrum, or is there a significant discontinuity between humans and all other animal life? Maybe the way to frame the question is, just how far away from the Bonobo and other close ancestors are we really?

    Adam Benton · 28th January 2013 at 3:57 am

    There’s something of a running joke amongst scientists that as soon as one of them decides to write a book that makes reference to a “unique human trait” it will be found in a species of animal before the end of the year.

    I think it’s pretty clear that humans and animals do lie on a continuum, but one should remember just how far along we’ve moved. Yes, the seeds of many human behaviours are present in animals but that fact should not be oversold (as many do with chimp sign language, for example); nor should we forget just how tall the “tree” of humanity has become.

      Wyrd Smythe · 29th January 2013 at 4:41 pm

      That’s a little bit like the old philosopher’s joke about the professor who said that, “Every time I think I’ve come up with an original idea, I find out some damned Greek thought of it first.”

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