A remarkable search for roots; I Sought My Brother: An Afro-American Reunion, by S. Allen Counter and David L. Evans.Cambridge, Mass.: The MIT Press. 276 pp.$19.95.

There can be no full appreciation of ''I Sought My Brother'' without a sense of the courage, vision, and faith that brought it into being. Subtitled ''An Afro-American Reunion,'' it is the story of a series of extraordinary visits made by the black American authors S. Allen Counter and David L. Evans to the rain forests of Suriname, the newly independent black republic on the north-central coast of South America.

Driven by a profound sense of ancestral calling, they went there to seek out the communities descended from the enslaved Africans who rebelled against their Dutch taskmasters and fled to the jungle in the latter half of the 18th century. The result of that series of pilgrimages is presented in a sometimes stunning ''pictorial essay'' - an essay that would have been greatly enhanced by the use of maps.

Allen Counter, a professor of biological sciences at Harvard University, was the initiator of the project. Powerfully affected by the resurgence of interest in Afro-American history in the 1960s and determined to find a usable past, he decided to seek out the locations of peoples of African descent in the Western Hemisphere. From the outset, it was a venture of the heart.

Especially interested in those groups who were relatively untouched by modernization, he learned about the ''Bush Negroes'' of Suriname and determined that he would go into ''the deepest part of the jungle to live among the bush people who wanted nothing to do with the outside world and have been essentially isolated.'' Counter said, ''I wanted to find out how much of their original African culture they had retained because that is important to all Afro-Americans.''

After many rebuffs from funding agencies - primarily because he was not technically ''qualified'' as an ethnographer or anthropologist for such an undertaking - Counter raised the money from his own resources and those of his family and friends. He found a companion in Donald Evans, a senior admissions officer at Harvard and an electrical engineer. In the summer of 1972, totally inexperienced in the ways of such journeys, they set out on the first of what would amount to seven voyages into the rain forests of Suriname. With the cooperation and assistance of the government of the country, they eventually made their way 200 miles south of the capital city of Paramaribo into the heart of the forest.

''I Sought My Brother'' is the condensed account of those experiences from 1972 to '78. Although there are some disadvantages in their decision to present the seven expeditions as one visit in the book, the authors are nevertheless successful in conveying to us a sense of the powerful emotional and spiritual experiences that were central to their courageous adventure. For instance, when at the end of several days of perilous boat travel on that first venture, they finally met the people they had come so far to find, they were overwhelmed by the thought that they had ''found our living ancestors, our pre-slavery bloodline, still alive and well and proud.''

The life, vitality, and wholeness of the most isolated Afro-American river communities in Suriname are well-documented by Counter and Evans. Looking at the many pages of photographs, both in black and white and in color, it is easy to understand why they said, ''our eyes kept telling us that we were in Africa, but in our minds we knew that we were in South America.''

Interestingly, one of the traditions of the villagers was that they had never been taken from Africa, but had been carried by the European slavers to some remote part of their native continent, far from their ancestral villages. Thus, just as Counter and Evans came to them as part of a pilgrimage, so one of the leaders said of them, ''They are our African brothers and they will show us the road home.''

In a sense, the heart of the book is a sometimes moving account of that shared seeking of the way. The two North American black men report on their careful observations of family life, community organization, medicine and religion (which are one), nutrition, language, music, and artisanry, as well as the basic beauty and humanity of the people in their everyday lives.

They in turn share their own gifts, ranging from the music of Otis Redding and Aretha Franklin (which led to marvelous community dances), to the distribution of hearing aids for those who needed them.

So the sharing went on. However, one of the ironies of the experience was built into the very act of visiting and sharing. For during the time of their sojourns the authors noted with sadness that ''the expansion of Western civilization,'' through such forces as the mining industries, was breaking down the protective isolation of their living ancestors. ''They are losing control of their lives and their livelihood,'' Counter and Evans report. Indeed, in some villages closest to such Western influences, they have already lost many of their young men to the jobs and other attractions of the encroaching forces of industrialization.

Paradoxically, Counter and Evans - even while they sincerely bemoan the fact that ''the Bush Afro-Americans are losing the fight against modern influences'' - are themselves part of those modern influences. As a result, both the authors and their beloved kinspersons may soon have to face the great possibility that ''the road home'' now stretches out before us all. With their great sensitivity, they may already realize that the homeward road insistently presses us toward a future that we must all create, imbued with the same courage, faith, commitment, and vision that Allen Counter and Donald Evans gave to their reunion with the past.

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