Just my two cents:
Ego, as the term gets used here, refers specifically to ones idea (model) of one's 'self'.
This model comes to seem more real than the actual self, and typically gets defended with vigor equal to that which one would defend the 'real' self with. We "mistake the map for the territory." These maps, in modern man at least, have all sorts of distortions, typically, which adds another layer of complexity.
Notice that if you have no model about yourself, you will not then have any ego.
As model-skills develop (in a life or across evolution) and one can begin to model the self, 'ego-ness' can creep in. In general, the more sophisticated the modeling system (mind), and the more pixilated (higher definition) the sensation systems, the more can ego, with all its problems, manifest. The invention of the mirror made the situation worse: The image of the self is more clear than any other, and so by that virtue replaces the 'real' self in our modeling. Would we still have ego problems here in this current human world if we were all to wake up blind tomorrow? Yes, but they would quickly attenuate quite a bit. Now imagine a dolphin ... see if you can enter his mind (their intelligence is said to be comparable to humans') ... there you are swimming around with your small group ... there are no mirrors in your culture though. No clothes. No money. No careers. Division of labor is barely emergent, sporadic and situational, for the most part. They may or may not be particular about a regular location, as human are with their houses. There is no writing, though there are languages of unknown sophistication. The modeling system is certainly there ... but which kinds of ego-properties would be likely to occur (or not) in this situation?
Ideas of self vary quite a bit: many dogs and humans both tend to identify with the leaders of 'their group', whatever that group may be, and take great exception to anyone's tampering with that image. (Just make a pope-joke in the presence of a devout catholic, or share a candid assessment of musical talent in the presence of an elvis-lover, or dis someone's football team, and see!) In the orient, the idea of 'self' is much more group oriented. My understanding is that commonly when one thinks of 'self' one more likely thinks of the whole family of which he is a part ... that is, the family is conceived of as the primary unit.
There are whole classes of beings that are totally without ego in the sense we describe here ... some not as advanced as man, others more so; some not as evolved as man, others more so.
There is a whole string of threads about ego... you will find lots to think about there. Not everything will make immediate sense, but you will find lots more thoughts to think about the issue with ... and really that's the best thing: 'answers' are not very effective, except with concrete questions like "Honey, do you know where I left my truck keys?" But if you have lots of tools to grapple with questions yourself, well, you're way better off that way.