Hi, this is a "what if" question so the answers to this would only be theory...and it's kind of an odd question. And multiple part.
I'm asking it because I'm a science fiction writer. I need some help on the science part of my idea.

One of the species, physically very similar to humans, is unisex. And by that I mean they are all hermaphrodites and there is no male/female sex. Gender has played a big part in shaping human evolution (as far as I know), so how would humans have evolved differently if we were unisex as well, but still reproduced together (not asexually), so we still have good genetic variation?
Would we have remained simple organisms, or would we still have eventually used tools and fire and language? Why/why not? How differently do you think young would be raised?

And last question, is there any species that exhibits these characteristics (the ability to impregnate and become impregnated)? Because if so they would be exemplary for me to study and help me in building the culture and biology of my fictional species. I've only heard of sexual (male/female) and asexual organisms.

Hey Nathan

Before going straight into the question of how would human evolution be different if we were not under the "laws of sex" as we know them, it is important to clarify some terms you are using to avoid confusion:

Unisex, which can also be referred to as gonochorism (http://en.wikipedia.org/wiki/Unisexual), means that each individual of a species only displays characteristics typical of one sex. As such, we (humans) and a vast majority of familiar animals, are unisex, because sex organs are normally present in different individuals (except in cases in which mutations cause hermaphroditism).

Hermaphrodite  applies when the same individual is able to produce gametes (reproductive cells) from two different sexes. Considering this, if humans were naturally hermaphrodites, we would be simultaneously female and male rather than unisex.

The scenario that you seem to be arguing for would be simultaneous hermaphroditism, which happens when both female and male gonads (the organs that produce the gametes) are functional at the same time and in the same individual. This condition is known in the banana slug and some earthworms (check out the wikipedia page on the subject http://en.wikipedia.org/wiki/Hermaphrodite). However, there are cases in which you can have hermaphrodites that are born with one sex and later in life acquire the opposite (e.g. some fishes and amphibians can be born male and then change to female or viceversa).

Now to the point. I am afraid I will have to be very anticlimactic and say that it would be very difficult to envisage human evolution as a whole based on this drastically different strategy. You could expect to have rather different human population dynamics because instead of having one gravid individual each time there is mating, you would end up with two, and consequently a similar number of births. It becomes even more difficult to imagine such a scenario because there are no "higher vertebrates" where simultaneous hermaphroditism is functional. If the creatures in your story are able to reproduce between them, thus ensuring genetic variation, there is no reason why you could not argue that they could become complex organisms. The utilization of tools and development of complex cognition is a whole different issue and making speculations (even more) on the matter would start to get uncomfortably unscientific. Finally, regarding parental care, again it is very difficult to say something relevant when there are no higher vertebrates with such strategy for comparison. In the case of the slugs and worms, they get impregnated, lay the eggs and continue their lives without much concern for their offspring.

Hope this helps

An
interesting extra in copulations between hermaphrodites is the observation of
acts such as ‘penis fencing’ in flatworms or even the act of attempting to eat
the partners penis in banana slugs. Both situations seek to have the victor
investing cheap to produce sperm, while the ‘loser’ is forced into the role of
mother and will have their eggs fertilized. In these scenarios while both
organisms are capable of acting as both male and female, it is often
advantageous to try and avoid being the ‘female’ in any given pairing as it
substantially increases the amount of investment you are putting in for the
same amount of genetic yield (offspring). So in these hermaphroditic pairings
we still see often see a male and female role being assumed in the pair, but it
is determined by fighting and inducing subordination in the partner, and
obviously a ‘male’ in one pairing might well end up being the ‘female’ in
another.
Hope this helps