“What method then is open to us who wish to conduct a human experiment but who lack the power either to construct the experimental conditions or to find controlled examples of those conditions here and there throughout our own civilisation?”
-(Margaret Mead p. 7, 1928 “Coming of Age in Samoa”)
Pygmy mammoths, giant rats, kiwis, Darwin’s finches, lemurs… surprisingly, all of these animals have a lot in common. They all live (or lived) on islands, and exemplify the different types of evolution that occurs on islands. When animals colonize islands and island-like environments, they commonly undergo changes in body size and shape that scientists explain by the “Island Rule”. Over evolutionary time, large animals become small and small animals become large on islands. Additionally, birds often lose their ability to fly, and few species may diversify into many new ones. Charles Darwin and Alfred Russel Wallace made initial extensive observations of island effects in the early 19th century. Since then, many scientists have examined the role islands play in animal evolution, and anthropologists recognized the importance of islands for understanding humans. Anthropologists such as Mead, Malinowski, Radcliffe-Brown, and Firth conducted research on humans in relative physical isolation on islands. They discovered unique human behaviors that helped lay the foundation for many anthropological and sociological ideas we use today. From the Kula ring to suicide voyaging to aspects of kinship, religion, and personhood, anthropologists studied how humans use culture to survive as individuals and communities on islands.
Various explanations have been proposed for physical transformations islands. A widely cited yet simple hypothesis states that limited food resources on islands result in energetic and reproductive pressures that lead to evolution of smaller body size. Another hypothesis focuses on ecological release accompanying a decrease in species diversity with island size. Decreased species richness results in fewer predators, thus relieving natural selection for increased body size allowing avoidance of predation. Large animals will hence undergo body size reduction on islands. On the other hand, for small-bodied animals on islands less competition frees up more resources than available on the mainland, thus allowing evolution of larger body size. These hypothetical explanations, along with several others, are often presented as mutually independent, but this is certainly unrealistic. For example, adaptive radiation of the finches Darwin observed on the Galapagos was due to a combination of ecological release and exposure to new resources. A better understanding of the suite of factors that influence evolution on islands will allow us to shed light on the “Island Rule” as a whole.
Importantly, and increasingly relevant for today, anthropologists have combined archaeological, cultural, and biological data to better understand how humans impact their environments on islands. Islands serve as excellent laboratories to examine such questions because they are circumscribed ecosystems. (Ecologists suggest that islands are like environmental microscopes for scientists who study evolution and ecology.) Further, with over half of all primate species on earth inhabiting islands, primatologists can use island science (such as island biogeography) to better understand how primate diversity coexists with humans inside their ecosystems. Deeper investigation of primate survival in dwindling island environments is now vitally important: 75% of primate species have declining populations and 60% of primates are threatened with extinction. Indeed, lemurs in Madagascar and tarsiers in the Philippines live on islands and may have adjusted their body size and shape in response to the unique pressures of their island environments. Anthropologists play an important role in exploring the science of climate change and conservation, and it is crucial that we direct more efforts toward understanding how isolation affects evolution. For example, how do humans impact primates through ecological encroachment, either direct (e.g., deforestation) or indirect (e.g., climate change)? How do primates survive with limited resources and adjust their behaviors and biology in the face of change?
Studying the “Island Rule” will allow us to better understand the unique body sizes and shapes we observe in some modern humans and their fossil relatives on islands. For example, Andamanese and other ‘negritos’ have short stature compared to nearby mainland humans. Furthermore, an extensively debated hominin from the island of Flores, Indonesia displays extremely small body and brain size and unique body shape. Influenced by Lord of the Rings scientists understandably nicknamed the initial specimen “The Hobbit”. The fossil remains are dated to 60,000 to 100,000 thousand years ago, but the brain size and stature of the single well-preserved individual are extremely small. In fact, the last known hominins with a brain size as small as the single known skull of The Hobbit were australopithecines, which went extinct almost 3 million years ago. An initial explanation proposed for the tiny brain and body size of the Hobbit was island dwarfing. However, few studies have explored how brain size and body size evolves in animals that reside on islands. Therefore, to understand the unique features of the Hobbit and other hominins that reside(d) on islands, it is necessary to expand our knowledge about the evolution of morphological traits on islands.
Anthropologists need to be mindful of the present state of island studies and how to best model island theory for primates. Indeed, the methods by which we study evolution on islands need to be developed further, with wider, more comprehensive approaches. For example, initial observations of organisms on islands led to the formulation of the Island Rule in ecology, which was at first considered to be a universal phenomenon, with large-bodied organisms evolving smaller body size and small-bodied organisms evolving larger body size on islands. In the last couple of decades, a series of studies have debated the universality of this “rule.” Generally, studies that do not use phylogenetic analyses, i.e. controlling for relatedness between species or subspecies in comparative analyses, confirm this phenomenon as a biological rule amongst vertebrates, whereas studies that control for interspecific or intraspecific relatedness indicate that the “Island Rule” is far from being a consistent rule.
The dilemma of applying phylogenetic analyses to study the “Island Rule” raises another issue. Analyses that explore whether the island phenomenon is universal are often performed by pairing one mainland-living species or subspecies with another from an island and comparing the two. This process can be extrapolated in order to understand the general trend within an entire order or even an entire class of organisms. However, relatedness between the mainland and insular-living organisms is often unclear. Because evolution on islands occurs within species or closely related species, it is crucial to have reliable information regarding the relatedness of the groups compared. Additionally, the process of averaging data to represent a single species or subspecies on the mainland or on an island is inappropriate because of the wide range of variation within groups. Therefore, it is important that we, as scientists, think of improved ways to study and compare morphological characteristics in animals from islands and the mainland.
Follow Dr. Yao and Ms. Young as we tackle these issues and more. We hope to spark a new wave of research within anthropology that appropriately uses and applies an understanding of island evolution to humans.
Best wishes,
Colleen and Lu
Data analyst, scientist, and storyteller with a passion for creative and sustainable design solutions. 