Dec. 22, 1997 Genetic detective work at Columbia University has unraveled the complicated lineages of African and Asian macaques, a genus of cocker-spaniel-sized monkeys that is more widespread than any other primate except humans.
New information about how, when and where the macaques spread across three continents over the last 5 million years is expected to tell anthropologists more about how other mammals dispersed and adapted to the same conditions.
Among the many findings is that the sole African member of the genus and the widespread Asian long-tailed macaques have been inaccurately placed among the other macaques. The study, which appears in the January 1998 issue of the Journal of Human Evolution, clearly separates the Barbary ape of Morocco and Algeria from the Asian macaques and finds no particular genetic affinity of the Asian long-tailed macaques with other members of a species group that bears its name. The researchers advocate abolishing that species group as a classification.
The study of all 19 living macaque species confirms that the genus is one of the oldest among Asian monkeys, dating to at least 7 million years ago, and one of the most successful, radiating from its home base in Africa to Europe and across southern Asia, from eastern Afghanistan through Pakistan, India, southern China, Burma, Thailand, Japan, Taiwan and the Indonesian archipelago.
The research was reported by Juan Carlos Morales, associate research scientist at the Center for Environmental Research and Conservation at Columbia, who conducted the genetic analysis, and Don J. Melnick, professor of anthropology and biological sciences and director of the Center, who gathered most of the field specimens for genetic analysis over a period of 20 years and worked with Dr. Morales to interpret the results.
Because it shows how and where macaques spread most rapidly -- throughout Asia over the past 5 million years -- the new work will serve as a road map to Asian primate evolution, and to the evolution of many other mammalian species in Asia during this time, according to its co-authors. Data on macaque distribution at different periods in history will give researchers clues to what factors influenced that distribution, and will be of most immediate use in helping decipher the movements of two other widespread primate groups, the leaf monkeys and the gibbons, the researchers said.
"With each successive genus that is analyzed in this manner, we will get a better and better idea of exactly what was going on in this part of the world and which geological and climatological events were more important than others in shaping the evolutionary history and dispersal paths of a diverse group of organisms, such as primates," Professor Melnick said.
Macaques invaded Asia in what appears to be a series of waves, each time spreading and diversifying across the continent over a relatively short period of time, according to the study's authors. Their rapid differentiation into different species indicates that the environment was conducive to both colonization and diversification. The many isolated species of macaques further indicates that periodic lowering of sea levels allowed the spread and subsequent isolation of macaques on what are now offshore islands. Glaciers and river systems probably also contributed to the isolation of populations and creation of new species.
Macaques are members of the papionins, a group of old-world primates that includes baboons, mangabeys and mandrills. The macaques have adapted to a wide range of conditions, finding ecological niches at sea level and at 12,000 feet, from semi-desert to cold mountain forest to low-lying tropical habitats.
"The macaques have spread to so many environments precisely because they have conserved many of the generalized features of the old-world monkeys," Professor Melnick said. "As good generalists, they have filled many different habitats with only small anatomical alterations of the skull and body. In many ways, macaques are the Darwin's finches of the primate world."
In external appearance, the macaque family has no common defining feature, leading to considerable confusion when appearance has been used to classify them. There has even been some debate on whether macaques are a single genus or two genera, and whether some species now classified as macaques, such as the Barbary ape, rightfully belong in the genus. Until Professor Melnick's and Dr. Morales' work, the accepted classification was the one devised in 1976 by Jack Fooden of the Field Museum of Natural History in Chicago, based on genital appearance. Body size, fur length and coloration, face shape and tail length have also been used by others to identify macaque species.
The Fooden classification, while generally accurate, has placed Macaca fascicularis, the long-tailed macaque, as the immediate ancestor of a small group of Asian macaques that include the commonly-known rhesus monkey, from which the Rh blood factor was isolated, the Japanese snow monkey and the Taiwan monkey. In fact, the more recent Columbia studies find that the various species thought to be closely related to the long-tailed macaque simply share with it ancestral characteristics, and have no special genetic proximity to the long-tail. The real ancestor of the rhesus, Taiwan and Japanese macaque lineage is the rhesus monkey itself, the researchers believe. They advocate abolishing fascicularis as a classification, and placing the long-tailed macaques as a more distant ancestor to most of the mainland and offshore island Asian macaques.
Researchers examined the DNA in cell bodies called mitochondria, because such DNA evolves relatively rapidly and is only transmitted through the maternal line, making it a good indicator of recent genetic changes. They determined the nucleotide composition of a segment of mitochondrial DNA, then compared the results for genetic similarity and shared genetic changes to draw their conclusions. The work was supported by the Center for Environmental Research and Conservation at Columbia University and the National Science Foundation.
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