The dramatic search for mankind's roots: two books; Lucy: The Beginnings of Humankind, by Donald C. Johanson and Maitland A. Edey. New York: Simon & Schuster. $16.95.; American Genesis, by Jeffrey Goodman, PhD. New York: Summit Books. $11.95.

Discovering humanity's origins is one of the most fascinating and frustrating of scientific pursuits. Legends abound. Speculation is rife. Evidence is hard come by and open to varying interpretions. But when speculation is founded on the evidence and interpretations are disciplined by incisive, if not always good-natured, scholarly debate, you at least have a rational framework in which to consider this awesome subject.

These two books, each in a quite different way, help one acquire such a framework.

Lucy is the name given by Donald Johanson to the ancient hominid whose 3.5 million-year-old remains he and his colleagues found in Ethiopia. It was part of a rich find that has excited anthropologists and shed much new light on human roots. But that light reveals meaning only as the forms it outlines fit into the pattern of human evolution that has been laboriously sketched over many decades.

"Lucy," the book, is about this larger pattern of human emergence over the past several million years in Africa. Working with the skilled science writer Maitland Edey, Johanson takes readers into the heart of this research -- explaining its technical intricacies, conveying its adventure. The collaborative authorship has produced a text that moves with the speed and drama of a first-rate novel. After all, the thrill of discovery and the rivalries and debates of the discoverers are the stuff of which novels are made.

At the same time, readers get a solid (layman's) grounding in the science of this discovery. Some, if not all, of the interpretation and conclusions presented may fall by the way in years ahead. But the factual understanding gained will always be a useful guide in understanding the new information that is bound to continue to come to light as scientists literally dig for humanity's beginnings in Africa.

"American Genesis," is a more idiosyncratic, but equally exciting, book. Here the scene is largely North America and the time scale fairly "recent" -- the last 250,000 or so years.

Jeffrey Goodman is making a case for a radical thesis. He reverses the traditional view that American Indians came to this continent across the Bering land bridge during times of low water. Instead, he envisions the Indians as migrating the other way -- the first modern men to appear in Asia and Europe. Cro-Magnon man, who displaced the Neanderthals, he says, was really American Indian man.

Goodman draws on evidence gathered mostly within the past 15 or so years. The late Louis S. B. Leakey of African fame had inspired research which now seems to have confirmed his intuition that the Indians have been in North America far longer that the 10,000 to 12,000 years that had been supposed. Indeed, they may go back hundreds of thousands of years.

Although still controversial, the evidence for such antiquity is being accepted increasingly by anthropologists. Less acceptable is Goodman's thesis of America as the birthplace of modern man. Goodman makes no secret of this. While he argues his case forcefully, he is careful to point out that others do not agree with him. He presents the evidence to his readers with care, showing the difficulties of interpretation and distinguishing between what seems legitimate inference and what is pure speculation.

It is this scholarly care that makes his book worth reading. You can join him in his speculation and profit from sound anthropological knowledge at the same time.

Both "Lucy" and "American Genesis" are provocative books. Humanity has deep roots. And, as Goodman particularly points out in recounting the skills and knowledge gained by the Indians, we have a long record of high achievement which is only beginning to be appreciated. All of which puts out much vau nted "space age" technology in a salutary perspective.