Dunbar’s Number

Robin Dunbar is an anthropologist/primatologist who discovered a correlation between the size of a primate’s brain (or really its neocortex) and the size of that animal’s social network. For example, clever chimpanzees tend to live in groups of 60 or so individuals, who maintain complex intimate social relations (yet chimpanzees don’t really care about outsiders without elaborate introductions). Howler monkeys tend to live in groups of 6 or 7. Dunbar studied primate brains until he believed he found the correlation index… then he applied it to human beings based on our own neocortices (is that the right word?). The number he arrived at was around 150. He posited that this is the average number of stable meaningful social relationships we can have at once. Here is a humorous (yet oddly serious) article which explains the concept elegantly (albeit with some fairly salty language and preachy talk).

When one starts looking for the number 150, it crops up all over the place. Hunter gatherer tribes were (and still are) limited to about that number. Military companies of all sorts of different armies throughout history have been that size. Business consultants say that this is an ideal size for companies (come to think of it there are 150 people at the company where I work) or for departments of companies.

But of course the 150 people I work with are not the entirety of my social interactions. I have 500 or so Facebook friends and not a one of them is from work….and the people I am closest to are not always on Facebook. And there are people I know about but have never (and will never) meet (like Susanna Hoffs, the emir of Qatar, and…Robin Dunbar). High functioning individuals like Presidents, CEOs, and world famous artists probably know many thousands of people—or at least know the one or two key pieces of information which makes each contact useful.

So there are lots of troubles and quibbles with Dunbar’s number…yet if you really write out everyone you have a true worthwhile meaningful relationship with you will probably come up with about 150 (if you are a gregarious adult with a full life in a big city—you can have many fewer close relations and there is nothing wrong with that…it doesn’t mean you are a capuchin monkey or something).

(Not that there is anything wrong with that either)

There is a line we draw around our tribe. Within this line are people we care about and need, outside it are… others—people we may care about in the abstract, or because they share a language, or a characteristic, or a nationality with us…but who are not dear to our heart in the same way as our intimate associates. The writer I linked to in the first paragraph up there asks us to imagine having a beloved pet…or two beloved pets…or six, or 23. How long would it be before our love and our attention were so diluted that we only cared about them in the most general abstract terms (or just outright despised them as a furry horde)? Whether you accept the premise of Dunbar’s number or not, it is a worthwhile question. If our brains are built by evolution in such a way as to make an “us” and a “them” what does it mean for all of us?

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The Long Summer of the Eocene

The Eocene (Illustration by Bob Hynes for the Smithsonian Institution)

The Eocene epoch (which lasted from 56 million ago to 34 million years ago) was hot!  Temperate forests ran all the way to the poles.  Steamy tropical jungles grew in the latitude where Maine is now and the equatorial regions of earth were (probably) sweltering. Tropical reefs formed in the coastal waters around a heavily forested and ice-free Antarctica. Since there was not year-round ice at each pole, the sea levels were much higher.

A Global Map of the Early Eocene (map by Dr. Ron Blakey)

The Eocene was a time when most of the contemporary mammalian orders first appeared.  The earliest artiodactyls, perissodactyls, rodents, bats, probiscideans, sirenians, and primates all originated during this time.  Of course mammals were not the only story: the Eocene was also a time of great diversification for birds and many familiar orders of avians developed then.  Reptiles begin to put the setbacks which marked the end of the Cretaceous behind them and several giant new species emerged including an immense tropical ur-python and a host of crocodiles and turtles. It is harrowing to think that the first wee dawn horses and cute little early atiodactyls were forced to contend with a 13 meter long super snakes and giant crocodilians (which flourished in the great hot swamps of Alaska), but such is the case.

Titanboa with Ancient Crocodilian (painting by Jason Bourque)

The high temperatures of the Eocene are perplexing to scientists.  By contrast, the temperatures of the Paleocene (which was the first era of the Cenezoic and had directly preceded the Eocene) were much more temperate. In fact the temperature spike of 56 million years ago seems to have ended the Paleocene and brought about the diversification of Eocene life.  The rapid warming is known as the Palaeocene-Eocene thermal maximum and scientists have been vigorously debating what caused the climate change.  An immense amount of carbon seems to have entered the atmosphere at this time, which in turn led to greenhouse warming.  It remains controversial as to how such a large quantity of carbon got into the atmosphere.  Comet/meteorite impact, massive peat fires, and volcanic activity have been suggested as triggers, however supporting evidence is lacking.  The release of globally significant quantities of hydrocarbons–which had been trapped in undersea clathrates seems like a more feasible hypothesis, as does the idea that the earth’s orbit brought the planet closer to the sun for a time.

Phenacodus, a goat-sized grazer of the Eocene era (painting by Heinrich Harder)

The end of the Eocene was also linked to the carbon cycle.  Reduced carbon dioxide in the atmosphere seems to have led to global cooling and newly evolved varieties of grasses began to invade large swaths of the world.  Additionally two massive meteor strikes in Siberia and Maryland combined with substantial volcanic activity to finish off the long hot summer. But during the Oligocene, the era which followed the Eocene, the world was a much more familiar place inhabited by orders of animals which are still here with us today (or are us–since primates first evolved during the Eocene).