If you think you've come a long way, baby, since mankind skulked in caves, you should speak to Marvin Harris, the sad-eyed anthropologist. His conversation is not cheering. Comforting beliefs in inevitable progress, the triumph of technology and the ascent of man - those encouraging old fictions - crumble as he talks.

"Things don't just get better - that's not the way it works," he says. He is a man who takes the long view. Where others detect ancient technological victories - in the discovery of planting say - Harris sees disasters. "History suggests," and by history he means the past 30,000 years, "that things don't just improve. They frequently get worse."

Harris, the Columbia University professor who lectured here last night at the Smithsonian Institution, is the author of a book published last year called "Cannibals and Kings: The Origins of Cultures." In it he replaces "the old onwards-and-upwards Victorian view of progress" with another story that is far less optimistic.

"Our culture is not the first technology that has failed. Nor is it the first to reach its limits of growth. The technologies of earlier cultures failed again and again, only to be replaced by new technologies. And limits of growth have been reached and transcended only to be reached and transcended again."

The cycle he describes does not sound much like progress. New technologies, says Harris, develop not to make life nicer but to repair the damage done by others in the past.

Take farming, for example. Conventional wisdom holds that early stoneage hunter-collectors lived lives of savage misery until, one happy day, a genius dropped seed in a hole and taught us how to plant. Soon men settled down.

"People no longer had to move constantly in search of game, and the new leisure gave them time to think. This led to further and more rapid advances in technology and thus more food - a 'surplus above subsistence' - which eventually made it possible for some people to turn away from farming and become artisans, priests and rulers." - That is an often-heard account of the agricultural revolution.

No, says Marvin Harris, it was not like that at all. Man adopted agriculture because he had no choice.

He had squandered his resources. His hunting had become so skillful that 13,000 years ago many of the larger animals he lived on - "the wooly mammoth, wooly rhinoceros, steppe bison, giant elk, European wild ass, and a whole genus of goats" - suddenly became extinct. With the "megafauua" gone, his choice was plant or starve.

His life had no improved. On the contrary, writes Harris, "Stone-age populations lived healthier lives than did most of the people who came immediately after them: During Roman times there was more sickness in the world than ever before, and even in early 19th-century England the life expectancy for children was probably not very different from what it was 20,000 years earlier . . . "Two thirds of the people alive today are involuntary vegetarians. In the stone age everyone maintained a high-protein, low-starch diet. And the meat wasn't forzen . . . The notion that paleolithic populations worked round the clock in order to feed themselves now also appears ludicrous."

The hunters of the stone age were, in general, a healthy lot. Skeletal remains show that 30,000 years ago the average man was taller than in 1960. "Tooth loss shows a similar trend. In 30,000 B.C. adults died with an average of 2.2 teeth missing; in 6500 B.C. with 3.5 missing; during Roman times with 6.6 missing."

"For the past five or six millennia," writes Harris, nine-tenths of all the people who ever lived did so as peasants or as members of some other servile caste or class."

Hunter-collectors were not dumb, they could have planted had they wished to. "The Shoshoni and Palute of Nevada and California returned year after year to the same stands of wild grasses and tubers, carefully refrained from stripping them bare, and sometimes even weeded and watered them." Though they "discovered" agriculture, they preferred to hunt.

"Last night on television I saw an ad about the Alaska pipeline," Harris remarked. "It's the same story. The technology is staggering, but it is useful only because the oil 30 feet below the surface of Pennsylvania has been used up. We keep doing it. We intensify our efforts, deplete essential resources, and then develop new technologies in order to repair the damage that we've done."

"As for amenities such as good food, entertainments, and esthetic pleasures, early hunters and plant collectors enjoyed luxuries that only the richest of today's Americans can afford," writes Harris. "I'm knocking myself out," he says, "writing these damn books, trying to get myself to the coast of Maine."

Harris is a city boy, he grew up in Brooklyn, but in Maine he's found a refuge. His studies have convinced him that most of us are swept along by the technological intensification and environmental depletion cycles that few of us discern. Food is at the root of it, food and population. It was when man had to deal with too little of the former, and too much of the latter, that his cultures changed.

"Are you married?" Harris asked. "I'm not surprised," he said. 'People these days think it's somehow 'mod' not to get married, to have informal relationships, to delay the kids. They might not know it, but they're responding to the same cost-benefit factors that, in most societies, influence behavior."

"We know," he adds, "that stone-age men had the full repertoire of human thoughts, sadnesses and joys - the cave paintings show us that - and yet that good life changed. That's an important message to get across. There is, in the U.S., a general belief that technology will somehow lead us to a promised land. We're going to have to learn."

Yet Harris ends his book on a note of cautions optimisim. "While the course of cultural evolution is never free of systemic influence, some moments are probably more open' than others. The most open moments, it appears to me, are those at which a mode of production reaches its limits of growth and a new mode of production must be adopted. We are rapdily moving toward such an opening. In life, as in any game whose outcome depends on both luck and skill, the rational response to bad odds is to try harder."