«MIOCENE APES David R. Begun PRELUDE: A CASE HISTORY J uly in Catalonia can be ideal if you are on the beach, or it can be unbearable if you are in a ...»
David R. Begun
PRELUDE: A CASE HISTORY
uly in Catalonia can be ideal if you are on the beach, or it can be
unbearable if you are in a building with no air conditioning,
you have nothing to do, and you are seven. That was my son
André’s problem one summer day in 1991. He was patiently waiting for me to finish planning for our 1991 field season at the
Miocene hominoid locality of Can Llobateres, about twenty kilometers northeast of Barcelona. When I suggested that we take a trip to the site late that afternoon, André was thrilled. We drove to the site to look over the area. I wanted André to run around a bit, and I gave him a pick to poke around with. I was also looking for the type of sediment we knew from previous work to be most likely to contain fossils. The hard green clays of Can Llobateres are the richest in ape fossils, and I wanted to find a new layer of this sediment. André and I had a great time chopping dirt and I did find some green sediment that looked very promising. The next day the field season began officially. My Spanish colleague and I, along with André and a team of excavators, arrived the next morning, and after setting up I showed my colleague the area I considered most promising. Our picks rose together as we prepared to clear away the layer of overburden covering the fossiliferous sediment. As they struck on both sides of the mark I had made the previous day, a tooth popped out. My colleague’s pick had hit an upper jaw, or maxilla, dislodging the first premolar tooth. As we watched the tooth roll down the slope, we realized we had something significant. Looking at the spot from which the tooth had come, we saw broken bone. When we finished three days later, we had a nearly complete face, by far the most intact specimen of Dryopithecus (discussed later) ever recovered from Spain.
Discoveries in paleoanthropology are often a combination of luck and homework. In the case described above, we were lucky to clean a section right where a beautiful specimen was buried. But we also knew that apes had been found at Can Llobateres, and we knew from the nature of the sediments where our chances of finding good fossils were greatest. Sediments reveal details of the environments in which they were deposited. We knew that fossil apes are associated most often with fine grained sediments indicative of very slow moving water, such as that of a river delta, floodplain, or lake margin. In fact, the goals of our project at Can Llobateres were not only to find fossil apes, but also to collect as much information as possible about the environment in which they lived and died, and their geologic age. This information is combined with data on
4 RESEARCH FRONTIERSthe anatomy of the fossils to tell us how those organisms lived, what they ate, and how they moved around in their environment.
Our research at Can Llobateres is just one example of many projects on Miocene apes in the last few years. This chapter summarizes research on Miocene apes, its implications for our understanding of ape and human evolution, and the prospects for future work in Miocene ape paleobiology. I will focus on those Miocene apes that are relatively well known and whose general relations to other apes and to humans are reasonably clear.
BACKGROUNDBefore discussing Miocene apes, a few terms must be defined.
Hominoids, or the Hominoidea, is a superfamily in the Order Primates that includes all living apes and humans. The Hominoidea is divided into families, the exact number of which is controversial.
Most researchers studying Miocene hominoids recognize two families. One is the Hylobatidae (hylobatids), including the gibbons and siamangs (genus Hylobates) of Southeast Asia. The other is the Hominidae (hominids), including the great apes and humans. The great apes include the orang-utan (Pongo pygmaeus) from Indonesia, and the African apes—the chimpanzee (Pan troglodytes), the bonobo, sometimes called the pygmy chimpanzee (Pan paniscus), and the gorilla (Gorilla gorilla). Many researchers, and most text books, continue to separate the great apes and humans taxonomically by recognizing a third family, the Pongidae (pongids) for the great apes.
This reflects tradition and a bit of anthropocentrism that often prevents anthropologists from seeing the remarkable similarities between humans and great apes. In fact the overwhelming majority of evidence indicates that African apes and humans are more closely related to one another than either are to orangs. To many paleoanthropologists, this means that African apes should not be placed in a separate family from humans. However, since orangs, African apes, and even the earliest members of the human lineage, Australopithecus, all look very similar, at least from the neck up, and since all are so different from hylobatids, two hominoid families separating the lesser apes and the great apes and humans is most practical, and most in agreement with current interpretations of hominoid relations (see later discussion).
Living hominoids share a set of characteristics that distinguish
MIOCENE APES 5them from other living anthropoids, or higher primates. Their cheek teeth, or molars, have a distinctive arrangement of cusps.
Their brains are also somewhat larger than expected for an anthropoid of their size range. But most dramatically, hominoids can be distinguished from other living anthropoids by their postcranial skeleton. All hominoids have skeletons bearing the hallmarks of a suspensory arboreal animal, even those who, like humans, no longer frequent the trees. Hominoids have rather loose but powerful, outwardly facing shoulders, highly specialized elbows to maximize stability in a wide range of positions, mobile wrists capable of adopting a wide range of positions, and long and powerful fingers.
All hominoids lack an external tail, and all have specific attributes of the vertebral column, pelvic basin, hip joint, ankle, and foot related to arboreality and more vertical postures.
Apes use these characteristic features to grasp branches and support their body weight from above, unlike most arboreal primates, which move about on top of branches. Humans retain these features because they allow the wide range of arm and hand positions that are crucial to the human way of life, one that is dependent on intensive and elaborate manipulation of the environment.
Hominids (great apes and humans) share many additional characteristics that set them apart from other hominoids (hylobatids). Hominids are all large in body size and relative brain size compared to other hominoids. They have very enlarged front teeth, or incisors, and most have a greatly elongated front part of the palates or upper jaws. They share many other more subtle traits in the dentition and skull, and a large number of features of the postcranium not found in the hylobatids or other primates.
The early history of research on Miocene hominoids has been described in detail elsewhere. 1 Through the 1960s the story of hominoid evolution seemed relatively straightforward. Early Miocene hominoids such as Proconsul were thought to be directly related to the great apes (Table 1). Earlier researchers had recognized a closer evolutionary relationship between African apes and humans than between African and Asian great apes.2 But this view was later abandoned, prematurely as we shall see, and the great apes were lumped together as the descendants of Proconsul.3 At this time most fossil apes were placed in the genus Dryopithecus, a taxon named in 1856 for a lower jaw from France.4 Many other names had been used for a wide variety of great apelike Miocene specimens from Europe, Africa, and South Asia, including for example Sivapithecus from India and Pakistan, and
6 RESEARCH FRONTIERSProconsul from Kenya and Uganda.5 These and many other names were later subsumed under Dryopithecus.6 One group of fossil apes was excluded from Dryopithecus. These specimens were most often referred to the genus Ramapithecus, and looked more human, mostly by virtue of the thick covering of enamel on their cheek teeth (molars). Figure 1 shows the consensus classification and phylogeny (evolutionary tree) of hominoids as of 1969. It reflects the then accepted division of great apes and humans into “pongids” and “hominids,” with the “dryopithecines” as ancestors of the former, and the “ramapithecines” ancestral to the latter.
Table 1 Fossil Hominoid Taxa and Chronology Taxa are listed opposite their localities. First occurrences are listed in their entirety; subsequent occurrences are abbreviated. Localities within a row are contemporaneous if separated by a comma or placed in relative stratigraphic position if separated by a period. This table illustrates the incredible diversity of Miocene hominoid taxa.
Many remain unnamed (Hominoidea indet.), but when these are combined with named taxa approximately thirty genera of hominoid can be identified. Given that this probably represents a small percentage of the total number of forms that actually existed (known fossils greatly underrepresent taxonomic diversity in the past) and considering the fact that only five genera of hominoid exist today, the Miocene can truly be considered the golden age of apes.
Three more recent developments in paleoanthropology have completely undermined this view of hominoid evolutionary history. Interpretations or hypotheses in paleoanthropology, as in other sciences, are subject to testing made possible by new discoveries and new techniques for generating data. A field in which ideas are always changing in the face of new discoveries is exciting and dynamic; method is more important than interpretation. A field in which interpretations never change, hypotheses are never falsified, and theories are unaffected by new data, is not much of a science at all.
Of the three recent developments, two fall into the category of new techniques, while the third is categorized as new discoveries.
The first development, relatively new to paleoanthropology but with a long history in biology, is molecular systematics. Since the turn of the century, researchers have been attempting to reconstruct evolutionary history using molecules rather than morphology.7 In the 1960s a number of researchers had concluded on the basis of work on proteins that humans were more closely related to African apes than to orangs,8 as Huxley had suggested a century earlier on the basis of anatomical comparisons. The antiquity of the split between apes and humans was also dramatically different according to the molecular evidence. Sarich and Wilson estimated that the split could not have occurred more than three to five million years ago,9 whereas most paleoanthropologists at the time, recognizing Proconsul as a great ape ancestor, placed the split someMIOCENE APES 9 Figure 1 Evolutionary Relations among Fossil and Living Hominoids, as Interpreted circa 1968 Aegyptopithecus, now known to be a primitive anthropoid, was then thought to be an early hominoid, broadly ancestral to all later forms. Most authors placed Sivapithecus, Dryopithecus, and Proconsul in the same genus, Dryopithecus, and thought that these were ancestral to living great apes. Ramapithecus, now recognized to be a synonym of Sivapithecus, was thought by most to be the first “hominid.” See text for details.
time before the evolution of this form, at least twenty million years ago. Though Sarich’s molecular clock is not widely cited today, his estimate is probably much closer to the truth than the estimate based on Proconsul. Modern research in molecular systematics now shows, based mostly on DNA sequencing, how very closely related we are to the African apes and maybe more specifically to the chimpanzee.10 The second development, again new to paleoanthropology but with a longer history in paleobiology, is cladistics. Cladistics or
10 RESEARCH FRONTIERSphylogenetic systematics is an approach to biological classification that is explicitly evolutionary.11 All organisms must be classified according to their evolutionary interrelationships. Organisms placed together in a group, or taxon, must be more closely related to the other forms in that taxon than to any organism in another taxon. So, for example, African apes and orangs cannot be grouped to the exclusion of humans, because African apes are more closely related to humans than they are to orangs. This is the main reason the taxon Pongidae is no longer used by many. Taxa like the Pongidae are paraphyletic, meaning they fail to include some lineages, like humans. Including humans with the great apes changes the name of the taxon to Hominidae, because Hominidae was named first. Cladistic methodology provides an explicit protocol for determining ancestor-descendant relationships.12 The main effect of the application of this method in the analysis of Miocene hominoids has been a thorough re-evaluation of the evolutionary significance of the characteristics used to reconstruct evolutionary relations.
Characteristics once thought to indicate a close evolutionary relationship among the great apes, such as large, elongated faces, large canine teeth, very elongated forelimbs, short hindlimbs, and others, are now recognized as primitive characteristics that were also present in the ancestors of humans. Therefore they do not distinguish the human lineage from that of the apes. In contrast, other characteristics, such as well-developed brow ridges, elongated crania, a reduced number of wrist bones, and other details of cranial and postcranial anatomy found only in African apes and humans, suggest that these forms share a period of common ancestry not shared by the orang. Cladistic methods allow researchers to distinguish characteristics indicative of a close evolutionary relationship, or derived characteristics, from those that are more primitive. This type of revision has lead to substantial changes in interpretation of the pattern of relationships among Miocene apes.
The third development of importance to Miocene hominoid research has been new discoveries. The old view of a dichotomy between the “ramapithecines” and the “dryopithecines” has been completely rejected on the basis of new discoveries of both groups.