Introduction

Primates originated once, long ago, in an unknown place, without leaving a fossil record of the event. At face value, learning anything about primate origins seems implausible. Can our curiosity about such a singular, unobserv-able, historical event be investigated within the realm of science?

The evolutionary history of any biological lineage is like a single experimental lab trial in one important way—both are unique events unfolding along a particular sequence of causes and effects. Once the sequence is over, it is history. Much has been made of the problem that unique historical events are inexplicable to science (e.g., Cartmill, 1990; Popper, 1957). This is why lab scientists never rely on a single experimental trial—they make several parallel runs and analyze the various outcomes comparatively. Lab scientists have learned to control as many variables as possible before launching a trial, so that the diverse outcomes of a set of trials can be interpreted more easily as a consequence of one theoretically interesting factor that is allowed to vary. The lab scientist is not attempting to explain a single experimental run per se, but rather to explain which variables influence the outcome of a set of runs.

The evolution of a single lineage is like a single lab trial in this way. Despite the philosophical similarities between a single experimental run and an evolutionary

D. Tab Rasmussen and Robert W. Sussman • Department of Anthropology, Washington University, St. Louis, MO 63130-4899

run, there are fundamental practical differences between the two. Evolutionary time exceeds the lifetime of the scientist, evolutionary lineages have much more complex cause-and-effect sequences, and parallel evolutionary runs were not designed with controls to highlight the effect of a single interesting variable. The evolutionist is in the position of a chemist who walks into a lab after several poorly controlled trials are over, with no lab notes, and maybe without even knowing what the question was.

But these problems are not insurmountable. First, an evolutionary biologist interested in processes of adaptation has the advantage of knowing what the questions are. Evolutionary theory is robust enough that many meaningful questions can be generated about the variable outcomes of a set of evolutionary runs, including questions about how organisms are morphologically adapted to their ways of life. Second, the evolutionist has no shortage of evolutionary runs—there are literally millions of them. Even within mammalogy there are thousands, and within primatology there are over 200 trials still living and many more extinct (but represented in the fossil record). The biggest problem confronting the biologist is that the evolutionary runs cannot be controlled in advance. Instead, the challenge is to identify after the fact which outcomes are relevant to a given question and then to control for confounding variables through proper comparisons.

To understand the singular origin of the order Primates, one must look for other animal lineages with parallel outcomes. By definition, no other evolutionary run yielded a primate, so the questions really center around the origin of key primate-like attributes that occur in other animals, either in combination or dismantled piece by piece. Which experimental runs on Earth have yielded grasping hands and feet? Which have yielded large brains? Primate-like visual systems? This kind of approach to primate origins is epitomized by Cartmill's (1972, 1974a) landmark studies on the mammalian visual system and the grasping extremities, which highlighted how the comparative study of completed evolutionary experiments could be used to test hypotheses about primate adaptations.

This comparative approach to primate origins requires that we find and examine as many parallel independent evolutionary runs as possible. The pha-langeroid marsupials of Australia and New Guinea are one such mammalian group that shows parallel development of primate-like traits. Smith (1984a) wrote that the phalangeroid diversification "has led to some remarkable convergences of form, function and behavior with the arboreal lemurs, bush babies, monkeys and squirrels of other continents." While New World marsupials have received some attention regarding questions of primate origins (Cartmill, 1972, 1974a, 1992; Hamrick, 2001; Larson et al., 2000; Lemelin, 1996, 1999; Rasmussen, 1990), few studies have drawn on information about the Australasian marsupial radiations (Cartmill, 1972, 1974a; Larson et al., 2000). In a recent analysis of the primate gait, the marsupial Phascolarctos (the koala) was also examined and found to be quite primate-like (Larson et al., 2000), but the study included no phalangeroids, several species of which could be expected to be even closer to primates in their adaptations. Other studies have compared phalangeroid and prosimian radiations from an ecological point of view, but not with a focus on the issue of primate origins (Smith and Ganzhorn, 1996; Winter, 1996). When morphological parallels were first identified between the grasping extremities and the convergent orbits of some phalangeroids and those of prosimian primates, there was inadequate ecological and behavioral data to correctly interpret what these features meant. With growth of knowledge about free-ranging phalangeroid behavior we know now, for example, that the one phalangeroid model (Cercartetus) held up to epitomize the visual predation hypothesis of primate origins (Cartmill, 1974a) is, in reality, a flower specialist (Lee and Cockburn, 1985; Turner, 1984).

In this paper we review what is known ecologically and behaviorally about the phalangeroid marsupials of Australia and New Guinea. The purpose of this paper is to highlight which phalangeroid species and behaviors may offer promise in researching primate origins. We believe that some of the hypotheses generated in this symposium can be examined further by investigating phalangeroid marsupials. Of course, the converse is also true, that the wealth of studies on the living primates may prove valuable in testing hypotheses about the origin and radiation of phalangeroids.

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