Despite a starring role in the 1978 Clint Eastwood film "Every Which Way but Loose," numerous successful advertising spots and a host of behavioral and other similarities to humans, the orangutan gets shortchanged.

		"THE RED APE: ORANGUTANS AND HUMAN ORIGINS" By Jeffrey H. Schwartz Westview Press. ($27.50)

Confined to the steamy jungles of Borneo and Sumatra, these long-haired, pot-bellied, tree-dwelling great apes are intelligent. Scientists studying them in captivity have reported that orangutans are pensive problem solvers. Whereas chimpanzees take the trial-and-error approach, orangutans sometimes go off to the side and think before solving the puzzle.

Some are tool users, fashioning leaves into makeshift megaphones for long-distance calls in the forest, and, according to the field scientist Carel von Schaick, they've exhibited signs of culture, too.

Sadly, though, the orangutan (the name comes from the Malay words for "man of the forest") is an endangered species.

The dwindling of any species is unsettling, but if Jeffrey Schwartz, a professor of physical anthropology and the history and philosophy of science at the University of Pittsburgh, is correct, the orangutan's decline would be particularly shameful.

Contrary to other scientists in his field, who exalt the chimpanzee, Schwartz believes that the orangutan is, in fact, mankind's closest living relative.

He first proposed the idea in a paper in the scientific journal Nature in 1984, then followed that up with "The Red Ape," a book designed to spread his ideas beyond the scientific community.

Now, 20 years later, he has issued a thoroughly revised and updated version of "The Red Ape." But it's not the passionate defense of his theory that you might expect. In fact, in the second sentence of the preface, he concedes:

"Although, as you'll read, there were a few moments when the possibility of humans being more closely related to orangutans than to chimpanzees might have been taken seriously, this suggestion has long been dismissed."

Not exactly the hard sell you might expect from someone who has been labeled an iconoclast.

Throughout the book, Schwartz espouses his theory humbly, even cautiously. At many times he seems more interested in revealing the flaws in what he portrays as an almost conspiratorial preference for the chimp as man's next of kin than convincing the reader that he's right about the orangutan.

It's not that he has lost confidence in the validity of his ideas. Instead, Schwartz appears to be more concerned about science itself than his pet theory.

What really seems to irk him is that scientists haven't taken more seriously the questions he has raised and, in a larger sense, what this says about his field as a whole.

As the book progresses, it becomes clear that "The Red Ape" is not so much about orangutans. More important, Schwartz gives the reader a deep look into how scientists think and how they draw their conclusions -- sometimes erroneously -- from existing data.

"The Red Ape" isn't a journey into the swamps of Sumatra but, as Schwartz describes it, a tour "through the historical maze of theories and counter-theories of who's related to whom in the search for our closest living relative."

The first half of the book is mostly a review, from Darwin and Huxley forward, of evolutionary theory. This is interesting stuff, and Schwartz's ability to approach the issues as a philosopher of science makes for some fascinating insights, but it's his insider's look at the field that yields the best material.

He disdains the tendency of headline-hunting scientists to proclaim every new fossil find "man's oldest relative." And there are some unsettling anecdotes about how certain overly possessive contemporaries ("No, it's my fossil, and you can't see it") stunt research.

The book really starts to pick up around the halfway point, when he covers his own work and the origin of his orangutan theory. His writing, which is often dry and weighed down by terminology, comes to life when he reviews the various reactions to his ideas, especially the critical response of his contemporary Collin Groves.

Their back-and-forth comes across as a scientific version of "he said, he said." You can also sense Schwartz's frustration regarding the widely accepted preference for determining relatedness based on genetic data without reference to morphology or the shape of fossils. The much condensed version of the debate:

If chimps and humans are so alike, why do they look so different? And why do orangs and humans have so much in common?

Morphology, he argues, no longer gets the respect it deserves. Reading Schwartz is an intellectual journey, at times a difficult one. He often seems to be writing more for his colleagues than the general reader; I wish I'd discovered the very helpful glossary in the back a little earlier.

In the end, you might not find yourself standing in Schwartz's corner, but, as he writes early on, "At the very least, you will also get to know a fascinating relative of ours: the orangutan."