A sort of "This Is Your Life" for the whole country, the Smithsonian Institution's Kin and Communities Symposium took off with a roar yesterday, featuring everybody from Rosalynn Carter to the McLain Family Band of Berea, Ky.

There was Sen. Hubert H. Humphrey (D-Minn.), the keynoters, who was given the Smithsonian's Joseph Henry Medal. There was Margaret Mead, the great anthropologist. There was Alex Haley of "Roots." And their families: sisters, nephews, grandchildren, all lined up at the back of the Eisenhower Theater state.

They came on casually, which was right, for the family can be a pretty loose group. In fact, Smithsonian Secretary S. Dillon Ripley led the First Lady on while the McLains were playing "The Stars and Stripes Forever" - it sounded like a polka with all those banjos - and Humphrey slipped in later while Dr. Mead spoke and the other people onstage kept beckoning him out of the wings.

The speech was vintage Humphrey. It ranged from Santayana's warning that "those who cannot remember the past are condemned to repeat it" to the senator's own boyhood memories of life over a drugstore and a father who bought radio time to read poetry aloud just because he wanted to.

"Why are we seeing what appears to be a growing disintegration of the family?" he asked, calling this question far more serious even than the energy crisis, a question that looms behind the symptoms of crime and alcoholism and slums and the rest of it.

"Public life leaves little time for family life," he said. "I had lunch with my family only when I had influenza. But we had breakfasts and Sundays."

It takes time, he said, to be a family. Time to listen ("we didn't use the word dialogue; we just talked to each other"), time to love, time to be. He told of learning that his granddaughter Victoria had been born mentally retarded. Gripped the lectern. Shouted it out: "I hate that word institution"

There are homey community shelters today in Minnesota to get people like Victoria "out of the warehouses."

"I believe in the healing power of love," he said, "and I'm no preacher but a sinner."

Drawing on the terrible story of the Ik, that tribe in Uganda that lost its very humanity after being uprooted and artificially disintegrated, Humphrey brought all these elements together somehow: love and morality ("the way you treat God is the way you treat people"), love and the family's survival, the family as the basic element in society, society's urgent need "to remember where we have been as well as where we are to know where we're going."

"Today we have the Now Generation. The problem with Now is that it disappears."

He picked a final quote from a Reynolds Price novel: "We are very plain people. We are the history of the world."

It made a fine watchword for the symposium, which climaxes the Smithsonian's two-year celebration of the Bicentennial.

As President Carter put it in a statement read by his wife, "The continuity and wellbeing of the family is vital to the country . . . and I am confident your findings will be of help to us all."

Interestingly, she wasn't the only wife of a public official who came to stand-in and read a message. Bennetta Washington spoke for the mayor - "I'm leaving out the whereases and things" - who hoped the sessions would improve the quality of life for Washingtonians.

Haley was specific: He urged every family to write down as much as its oldest member can recall, keep a family archive and hold reunions.

Dr. Mead was general: "Think of the center, not the edges," she said. Here we are, all on one planet, sharing the same atmosphere, and we must stop the armaments race, to protect our own children and other people's children . . . "and all this starts with the family . . ."