Sivapithecus

Patel (2005) examines the morphology of the proximal radius in different species of apes. He sets the work into the context of earlier work on hominid positional behavior and locomotion based on the radius; in particular, the work by Richmond and Strait (2000) suggesting knuckle-walking adaptations for early hominid distal radii. A question arising from this work is whether radial morphology specifically indicates functional correlates such as knuckle-walking or climbing, or whether instead it is more generalized in its anatomy. However, the proximal radius reflects not the wrist joint but the elbow, and may not be expected to reflect the same locomotor or positional constraints as the distal end.

The analyses are admirably complex. My favorite is the one where a ball bearing is allowed to roll freely in the proximal fovea in order to measure its shape.

But the results show that the proximal radius is not a strong indicator of function:

[W]hen hylobatids were included in the comparative analysis, they were not clearly distinguishable from African apes, and early hominins resembled both African apes and hylobatids. Because African apes and hylobatids have different locomotor behaviors (see Tuttle 1986; Fleagle 1999), the results of this study suggest that determining specific locomotor behaviors from the proximal radius may not be possible -- it cannot be determined whether the bony morphology is indicative of terrestrial quadrupedal locomotion or acrobatic suspensory behaviors (i.e. brachiation). Thus, with reference to the proximal radius, it is difficult to determine whether early hominins may have had the ability to utilize any form of terrestrial locomotion similar to extant African apes, a conclusion that is similar to those of previous studies of the distal humerus (Feldesman 1982; Senut and Tardieu 1985) and proximal ulna (Aiello et al. 1999) (Patel 2005:426, references therein).

Patel does draw a contrast between monkey-like quadrupedalism and potential suspensory locomotion for the elbow joint:

Although most early Miocene hominoids [or proconsuloids (e.g. Harrison 2002)], such as Proconsul, Afropithecus, and Turkanapithecus, were quadrupedal, with a monkeylike elbow morphology (Napier and Davis 1959; Fleagle 1983; Rose 1988; 1993b; 1994; 1997; Richmond et al. 1998), the elbow region of later hominoid taxa, such as Oreopithecus, Dryopithecus, and Sivapithecus, resemble both African apes and hylobatids. This suggests that these taxa may have utilized suspensory behaviors (e.g., Begun 1992; Rose 1993b; 1997; Richmond et al. 1998) (Patel 2005:428, references therein).

This places emphasis on the question of the evolution of suspensory posture. It is quite clear that early hominoids that are probable relatives of both Asian apes and the European and African clade of apes were suspensory, such as Dryopithecus and Pierolapithecus. Gibbons are obviously also suspensory. Were the common ancestors of all living hominoids suspensory, or were they independently derived from Proconsul-like quadrupeds?

What is unsatisfying about this study is the lack of biomechanics. Consider the following:

African apes and hylobatids have relatively small radial foveae resulting from an expanded proximal articular surface. A smaller fovea results in a smaller area of contact between the radius and the capitulum, indicating an emphasis on stability (e.g. Godfrey et al. 1991).... Although the depth and the curvature of the radial fovea were not measured in this study, it would be expected that the fovea in African apes and hylobatids would be deeper and more curved to promote increased stability in the elbow joint (e.g., Hamrick 1996) (Patel 2005:429, references therein).

Why? How does the specific form of this joint promote stability? What is the contrary force making stability less desirable in species with less extensive bony articular surfaces? Patel notes that it is a bit of a mystery why orangutans should not have proximal radii more like the other apes and speculates that they accentuate mobility rather than stability. Is this true? Are chimpanzee elbow joints less mobile than humans? Are gibbons really like the African apes in function, or is there an allometric difference that leads to the appearance of similarity between them? Gorillas, chimpanzees, and orangutans are different on average, but there is overlap in some features. Do individuals in this overlap region have similar biomechanical properties, and if so, why does this degree of variability persist?

Answering these questions requires proper biomechanical modeling. One must explore the relationship between radius shape and forces acting on the elbow joint. Statistical comparisons of similarity among hominoid species are interesting, but they do not replace this process.

References:

Patel BA. 2005. The hominoid proximal radius: re-interpreting locomotor behaviors in early hominins. J Hum Evol 48:415-432.

Lufengpithecus lufengensis is a fossil ape from China, dating to the latest Miocene and Pliocene. A single mandible from the site of Longgupo argues that Lufengpithecus may have survived until as recently as a million years ago, possibly overlapping with both Gigantopithecus and ancient Pongo in the region (Crummett et al. 2000). Like Sivapithecus, Lufengpithecus has thick molar enamel and it also has relatively low canine teeth, especially in females. The lower third premolars sometimes have a slight second cusp, denoting a shift from their principal role as cutting teeth in other ape species.

A related species from Thailand, Lufengpithecus chiangmuanensis, has recently been uncovered. This species is known only from teeth, but these appear to be intermediate in morphology between Sivapithecus and recent orangutans. At 10 million years old, the fossils may be ancestral to later Pongo (Chaimanee et al. 2003).

More on Lufengpithecus

The hominoids--the group including humans and living and fossil apes--originated sometime during the Oligocene period, between 34 and 24 million years ago. But it was during the Early Miocene that ancient hominoids began the impressive differentiation that created many diverse ape lineages that came to inhabit most of the tropical and subtropical Old World. During most of the Miocene, climatic fluctuations appear to have been much less marked than during the subsequent Pliocene and Pleistocene. Tropical and subtropical forests covered large parts of the Old World. The East African Rift Valley and the mountains of South Asia, including the great Himalayas, had only begun to form, so that the climatic patterns of today, dominated by strong dry and monsoonal wet seasons, had not yet emerged. Early apes adapted to exploit the large forested environment, becoming established in Africa, across South and East Asia, and in large parts of Europe and West Asia.

Although fossils representing many Miocene ape lineages have been found, paleontologists do not have a clear idea of their relationship to the living species of hominoids. The common ancestors of orangutans, chimpanzees, gorillas, and humans all existed during the later parts of the Miocene, between 15 million and 6 million years ago. Thus, the earliest hominoid fossils, which date from earlier than 25 million years ago, lived long before the divergences of the living great apes. Some of these ancient apes may be our ancestors, but most of the great hominoid diversity of the Early Miocene ultimately became extinct.

Our own lineage, the hominids, arose during the latest part of the Miocene in Africa. In some senses, fossil apes are a sidebar to the story of our own evolution. Paleontologists do not know which, if any, of the fossil apes of the Miocene may have given rise to the hominids, but the impressive diversity of fossil apes has yielded insight into the evolutionary pathways taken by our ancestors. Most important, the Miocene apes show us the primitive conditions of many of the skeletal features that were later to undergo great changes during human evolution.

Fossils from the Oligocene that may be hominoids are rare. The most notable, Aegyptopithecus, may represent the ancestral catarrhines that later gave rise to both hominoids and Old World monkeys. But by the early Miocene, a great diversity of apes had arisen with a range of body sizes and dietary adaptations. Fossil genera such as Proconsul, for example had monkey-like locomotor adaptations and ape-like jaws and teeth. Others, like Morotopithecus, may show the development of the suspensory locomotor pattern of later great apes, while retaining dental anatomy somewhat distinct from the later apes.

Great ape fossils and relatives

By 13 million years ago, a series of fossil apes from Europe and Asia show clear signs that they belong to the group including living great apes and humans. Some of these apes had a full adaptation to suspensory locomotion of the form found in living chimpanzees and orangutans. Others--especially the larger apes--may have had a mixture of suspensory and quadrupedal adaptations. All these apes appear to have matured relatively slowly, like the living apes, and may have had brains essentially the same relative size as chimpanzees (Begun, 2003). Thus it is likely that these fossil apes represent the origin of the great ape adaptive pattern.

Shortly after the first of these apes arose, the early great apes divided into two lineages. One of these invaded South Asia, and ultimately its descendants colonized the tropics of China, Southeast Asia,and Indonesia. The other lineage spread across Europe and diversified quickly into several species of a single genus, Dryopithecus, and its larger descendant, Ouranopithecus. These European apes share many features with the living chimpanzees and gorillas, which may indicate that they are the closest fossil relatives of the living African apes and humans.

Asia has produced a great quantity of great ape fossils from the Miocene up through the Pleistocene. Ultimately, these apes were a distinct clade from those found in Europe and Africa, and they had no close relationship to the hominids. The most recent members of this clade are the living and fossil orangutans, of the genus Pongo. Today, orangutans survive as remnant populations only on the islands of Sumatra and Borneo, though fossil Pongo is found as far north as China. From China in the east to Turkey in the west, the fossil apes of Asia were a large and diverse group.

The common ancestors of humans, chimpanzees, and gorillas belong to a clade separate from the Asian fossil apes. The earliest representative of this clade is probably Dryopithecus, which itself is likely to have been very similar to the common ancestor of both clades. Dryopithecus and later European genera, called dryopithecines, belong to an adaptive radiation that resulted in a substantial diversity in body size and possibly in locomotor patterns. This radiation almost entirely known from European sites, since African fossil apes have not yet been found from the important time span from 10 million years ago to 7 million years ago, during which the African apes and hominids diverged. Nevertheless, the known dryopithecines present evidence of the anatomy of close relatives to later hominids, much closer in time to the beginnings of human evolution than are living species. It is likely that when African great ape fossils from this time period are discovered they will share many features with the European ape radiation. Thus, the anatomy of these ancient apes may partially represent the starting point from which human evolution began.

Fossil ape genera

Aegyptopithecus (likely ancestral catarrhine)

Proconsul

Afropithecus

Otavipithecus

Turkanapithecus

Dryopithecus

Ouranopithecus

Sivapithecus

Gigantopithecus

Ankarapithecus

Lufengpithecus

Oreopithecus