There can be no serious doubt that intelligence represents a spectrum with little separation between different species when arranged in an order of ascending complexity. Even humankind, which traditionally has been viewed as high above the rest, comes closer than one might imagine to being a near relative of higher primates. And even today some primitive human cultures are not so far removed intellectually from the level of certain primates. The homo sapiens of 50,000 years ago must surely have been even more like the other primates of that era.

A great deal of research performed on primates over the past two decades has forced us to reappraise our relationship to them. Higher primates are extraordinarily intelligent creatures.

Primates as well as many other animals are frequently challenged with new situations definitely not in their experience-behavior repertoire, situations which demand unique responses and the synthesis of new behaviors.⁵...It is becoming increasingly apparent following close observation of chimps that they display many behavioral patterns similar to humans, including discovery of tool usages, learning of artificial languages, observance of incest taboos, and invention of new behaviors.⁶

Not only do certain animals, particularly primates, display many behaviors similar to those of humans, but if one compares their genetic structure one finds only minor differences. The chimpanzee has a DNA molecular structure which is 99% the same as that of humans.

Molecular biologists have also developed a new technique, referred to as the DNA clock, which is a way of measuring genetic distances and dating evolutionary branching points. The conclusions reached support the concept that the higher primates, and particularly the chimpanzee, are very close relatives to humankind.⁷...This closeness has also been observed by Judith Hooper and Dick Teresi, who believe that non-human primates have a "mental landscape resembling our own." They "are capable of elementary logic, jokes, banter, deliberate misinformation, cajoling, deep sorrow, rich communication.".⁸

As indicated before, chimpanzees appear to be our closest relative. They display many behaviors which are similar to those seen in humans. In addition to those intelligent behaviors already mentioned, there are many more which might be listed.

Chimpanzees have learned to converse in sign language as well as to use a computer typewriter to do all the primary things with their specialized iconic speech that a young child does with theirs.⁹...Beatrice and Robert Gardner have been teaching American sign language to chimpanzees with more than modest success. Some chimps have vocabularies of up to 200 words which they can use to communicate intelligently in simple, easily understood sentences. They can communicate with humans as well as with each other. They have even been inventive with their new language by combining two symbols to make complex words, as in using the signs for "water" and "bird" to produce "duck.".¹⁰...The chimps have steadily broadened their intelligent world. They have demonstrated the ability to absorb, generalize, combine and invent an extensive system of names for objects in their environment.¹¹...They can use their words to distinguish agents, actions and concepts. One chimp that had learned to understand spoken English words for several objects quickly learned the sign language equivalence without the objects being anywhere in sight. This represents an impressive cross-modal transfer. They can understand, construct and modify formal language semantically.¹²

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