In 1967 the Kibish Formation in southern Ethiopia yielded hominid cranial remains identified as early anatomically modern humans, assigned to Homo sapiens. However, the provenance and age of the fossils have been much debated. Here we confirm that the Omo I and Omo II hominid fossils are from similar stratigraphic levels in Member I of the Kibish Formation, despite the view that Omo I is more modern in appearance than Omo II. 40Ar/39Ar ages on feldspar crystals from pumice clasts within a tuff in Member I below the hominid levels place an older limit of 198 +/- 14 kyr (weighted mean age 196 +/- 2 kyr) on the hominids. A younger age limit of 104 +/- 7 kyr is provided by feldspars from pumice clasts in a Member III tuff. Geological evidence indicates rapid deposition of each member of the Kibish Formation. Isotopic ages on the Kibish Formation correspond to ages of Mediterranean sapropels, which reflect increased flow of the Nile River, and necessarily increased flow of the Omo River. Thus the 40Ar/39Ar age measurements, together with the sapropel correlations, indicate that the hominid fossils have an age close to the older limit. Our preferred estimate of the age of the Kibish hominids is 195 +/- 5 kyr, making them the earliest well-dated anatomically modern humans yet described.

The locomotor behavior of seven sympatric species of New World m o n k e y s 4 n g u i n u s m.idns midas, Snimiri sciureus, Pithecia pithecia, Ch.iropotes satanas chiropotes, Cehus apella apella, Alouatta seniculus, and Ateles paniscus paniscus-was studied at the Raleighvallen-Voltzberg Nature Reserve in Central Surinam. This paper examines the way in which locomotor behavior is related to body size and to ecological parameters such as forest stratification, forest type, and diet.Locomotor behavior is clearly related to the size of the species; with increasing size, the amount of climbing increases and the amount of leaping decreases. In general, larger monkeys use larger arboreal supports; however, Saguinus midas midas uses relatively larger, and Ateles paniscus paniscus relatively smaller supports than expected from body size alone.Leaping is associated with use of the forest understory and with use of Lane forest. Other types of locomotion are associated with main canopy use in a variety of forest types.There are no consistent associations between diet and either locomotor behavior or forest utilization; rather, monkeys with similar diets show locomotor and habitat differentiation.A major goal of physical anthropology has long been, and remains, documentation and interpretation of the extraordinary morphological diversity exhibited by the Order Primates. There is considerable evidence that much of this diversity in musculo-skeletal anatomy is causally related to differences i n locomotor behavior leg., Keith, 1891; Ashton and Oxnard, '63; Stern and Oxnard, '73; Fleagle, '77a,b, '78b, '79). A n u m b e r of authors h a v e f u r t h e r suggested t h a t there a r e also predictable, causal relationships between locomotor behavior and other aspects of an animal's biology such as body size, forest utilization, or diet. Although a variety of theories have been proposed which relate these variables, the field data necessary to test the theoretical predictions have remained lacking (Stern and Oxnard, '73). This dearth of' data to test general hypotheses about locomotor behavior is particularly obvious when compared with the available information for the analysis of dietary behavior (e.g., Kay, '75; Clutton-Brock, '77).The information necessary for a broad comparison of locomotor behavior among extant primates is unlikely to become available anytime soon. We have, therefore, adopted a n alternate strategy for understanding t h e relationships between locomotor behavior, body size, and habitat utilization among arboreal primates. Our approach has been to examine these factors in a diverse group of sympatric species in hopes that this "natural experiment" will yield results that are representative for understanding locomotor evolution throughout the Order. To this end we have examined the naturalistic locomotor behavior of seven species of New World m o n k e y s S a g u i n u s rnidas midns, S a i m i r i sciureus, Pithecin pithecia, Chiropotes santanns chiropotes, Cehus npella npella, Alouuttn seniculus, an...

Wild, adult siamang were observed for over 800 h in lowland dipterocarp forest in the Krau Game Reserve, Pahang, West Malaysia. Siamang use four patterns of locomotion: brachiation, climbing, bipedalism and leaping. The pattern of locomotion used by the siamang varies with the size of arboreal supports and with major behavioral activity. Travel is primarily by brachiation along large boughs. Locomotion during feeding is primarily climbing among small branches. In feeding, siamang use suspensory postures among small supports and seated postures on large supports. Comparison of siamang locomotion and posture with that of other apes suggests that quadrumanous climbing during feeding is the basic hominoid locomotor adaptation.

Sympatric Malaysian leaf-monkeys, Presbytis obscura and Presbytis melalophos show quantitative differences in their use of different locomotor patterns during daily activities. P. obscura primarily moves quadrupedally along large boughs; P. melalophos relies more on leaping between smaller supports. P. melalophos also uses forelimb suspension more than does P. obscura. Comparison of the muscular anatomy of the shoulder, arm, trunk, hip and thigh show numerous statistically significant differences between the two species in relative mass of individual muscles and in details of muscular origin and insertion. In almost all instances these differences accord with the types of differences in muscular anatomy predictable from either biomechanical analyses of locomotor behavior or comparative anatomical studies of other mammalian species.