I am wondering what allows us to have a personality, how is it made, what makes it, etc? Also, how would a personality be useful enough to be evolved, do single celled organisms have a personality (I'm assuming not as they don't have a brain), and when did personality's 'arive' in the evolutionary chain?

Personality is a pretty complex topic. You will find lots of disagreement even among experts as to how exactly to define it. How it is formed is even less well understood. Genetics certainly play a role because genes control the wiring of the brain during development and many personality traits are inherited. But experience is also important. It is also hard to say how or when personality evolved. It is not a trait unique to humans. Most vertebrates and even some invertebrates (octopus for example) exhibit some behavioral variation that can be considered a form of personality. I would speculate that personality is a useful adaptation for any social species. To be successful, the group would need a mix of personalities: aggressive, fearless individuals to guard against predators and explore new territory; docile, cooperative individuals to care for the young, etc.

Agree with all you say Brent.

I would suspect that personality may not be a "selected for" trait, rather it goes along with a host of other higher brain function - reasoning, self awareness, memory etc. That said other large mammals appear to have varying personalities eg dogs ("...dogs got personality, personality goes a long way" Vincent Vega), cats and horses.

Personality could be defined simply as variation in behaviour patterns within a species. If Irene the Iguana is consistently a little more aggressive than Igor the Iguana, then the two of them have different personalities, at least at a rudimentary level. I can say from personal experience that variations like this exist in reptiles, and I wouldn't be surprised if they occur in fish and amphibians. I agree with Brent that octopuses belong on the list as well.

In principle, it seems possible that variations in behaviour might exist even among very simple organisms. Are some individual amoebas quicker than others to approach and engulf a particle that might be edible, and therefore more "adventurous" (or greedy)? I don't know of any evidence for such "personality" differences among amoebas or other single-celled creatures, but I wouldn't rule out the idea. The genes that influence their behaviour, through whatever comparatively crude mechanisms, should be able to vary in a population just like any other genes.

In animal behaviour - and  parts of psychology with which it overlaps - "personality" is increasingly being defined as among-individual variation in behaviour that is repeatable across time and context.

While being a bit of a mouthful, what this arguably and simplistically amounts to  is the fact that there are consistent differences among individuals in the way they behave.  At the moment there is a lot of research going on into what causes this variation in vertebrate animal species but also in some invertebrates. certainly such variation in aspects of personality such as aggressiveness and boldness is well documented in fish species, as well as birds and mammals, but you can show "personality" as defined above in inverts too.

This article in the NY times is a bit old but still quite an interesting read

http://www.nytimes.com/2006/01/22/magaz … l&_r=0