Abstract: As the interdigital patterns of primates are different from that of human being, the studies and the comparisons of them will result in the clarification of the course and factors of interdigital patterns evolution.
1 Origin of Interdigital Patterns
General opinion about how dermatoglyphy comes into being is that primates need to enhance the frictional force and sense of the palms and soles for tree-inhabiting lives. Interdigital pattern is one of the earliest dermatoglyphies. When interdigital patterns have just taken shape, those in different area did not contact each other.There were still large fur areas between dermatoglyphy areas.On palms of Tupaia, there are always 3 to 4 muscle pads, fairly close to each other, opposite to each finger, with simple arch patterns on each pad. Ridge trends are different from one pad to another. During latter primates' development, some of these pads expand, and others get narrowed.Neighboring pads contact each other. Dermatoglyphies are distributed all over the palm.There is a tripartite point left at the demarcation of each three pads.On the pads, appear more complicated patterns.
2 Evolutionary course of Interdigital Patterns
Commonly speaking, there is only one biggest pad in each interdigital area of those species higher to Tupaia. Analyzing the ridge trends and the positions of tripartite points, we find that primates have reserved different pads according to Tupaia, which suggests some big categories split up quite more earlier than we think.So the evolutionary pattern of primates is bush-like.
There are mainly 5 evolution branches out of Tupaia. ①Tarsidae.There is only one pad in each interdigital area, and fairly big space between pads.On the pads, there is only simple arch pattern. ②Lemur and Lorisidae, one pad in each area, too, small gap between pads.On the pads, equally develop out primitive pattern, loop or small whorl. ③New World monkey. Side interdigital areas are developed while central areas are retrograde.In Cebidae interdigital area II is a big whorl, and in area IV is a loop.In Hapalidae area I is a big whorl, area IV also loop.Interdigital areas have completely connected. ④Old World monkey. The interdigital patterns of Old World monkey are the most developed.There are loops in different sizes in Colobidae.In Macaca area II, III and IV are all big obvious whorls. ⑤Human-like ape. Their interdigital patterns are quite retrograde. Gibbons have no interdigital patterns. Chimpanzees occasionally have some little loops. Humankind has developed out of ape. So the original interdigital patterns are also degenerate. But opposite to the third finger, appears a new various pattern.
3 Evolution Factors of Interdigital Patterns
Comparing the categories of primates, we find interdigital areas of tree-inhabiting primates developed rather unequally, while ground-inhabiting ones are on the contrary. Equality of interdigital patterns is directly proportional to the ground-inhabited tendency. It is possibly because palms of whom that is ground-inhabiting need to touch ground equally, but while living on tree, palms must press the branch from both sides. So interdigital areas of tree-inhabiting primates are side developed and central retrograde.Pattern intensity is also directly proportional to the rate of utilization. This is reflected very well in human-like apes.Gibbons use fingers to seize hold branches, and seldom use interdigital area ( so that to be nimble when seize or release). Then there are no patterns in their interdigital areas. When chimpanzees are walking, they let fingers back to touch ground. Only when they hold something occasionally, they will use interdigital patterns. So their interdigital patterns are very degenenate. Only few individuals have small loop in their interdigital areas.
Interdigital patterns of humankind, corresponding to those of human-like apes, are also very degenerate. They are all inperfectible dominant or recessive small loops. It does not fit the high frequency and intensive use of interdigital areas when we are laboring. So the force focus of human palm when seizing something, opposite to the third finger, appears a new pattern. This kind of new pattern is various and inheriting distinctively.

Key words: Interdigital patterns; Primates; Evolution