Men and animals, specially primates, share many communication systems. Some animal species have even originated specific social rituals that we thought were characteristic of men. And rituals happen to be one of the first signs of equality among intelligent species.
Some scientists have said throughout history that rituals, in the animal world, are plain instinctive, inbred, natural - whilst they are completely rational in the human world. Why, it is true that we set certain rationality in some ritual processes, but in the vast majority of them we are still acting in an unconscious way? Because of an innate behavior.
An example of this is how we move our head from side to side (or backwards, in some parts of Greece) to say "no," or how we shake hands as a friendly gesture. The origins of these signs are also animal, since one is related to the refusal of nourishment or repulsion of a displeasing smell, and the other has to do with establishing convenience bonds (a chimpanzee who has seen a member of his species in possession of food will reach him with an outstretched arm and an open hand).
Another example of human behavior inherited from primates is territorial organization. For both man and primate, a defined space exists as a personal territory. That's where they both live, eat, hunt, and feel safe. Whilst animals mark this space as their own with their urine, screams and shouts, and visual signs, men do this via boundaries, such as walls, clothes, and personal tools.
This private space can be invaded by an individual. If this happens, both species (man and primate) adopt a defensive attitude that can become aggressive. Just as animals show their teeth or claws, man stands up with the (subconscious) intention of looking physically bigger. It's also common to see his hair - from arms and legs - standing up; a physical reaction that answers the same intention.
In spite of these signs that show a somewhat violent condition, the ultimate goal is to scare the invader. Fighting is the last thing wanted here, for even the most powerful individual or animal can get hurt. It's for this reason that it's quite frequent to see people who, in a tense situation, look for rational excuses to not attack ("if I wasn't so tired...", "if I wasn't with my girlfriend, you'd see..."). The real translation to these excuses is nothing else than "I want to keep my power over my territory, but I do not want to be hurt."
Another thing that makes us close to animals is physical contact. This is a sign of affectivity and trust. We easily assume that two individuals who walk very close to each other, and one has an arm over the other one, are friends. This same picture has been seen in chimpanzees.
Another example is the simple act of kissing - something that Darwin considers not an inherited expression, but a learnt one, since in different cultures love is expressed in different ways. However, many theoreticians today believe that kissing is an innate expression, since animals lick their pups and calves (even those humans who they feel affection for!)