Tracing the story of life on earth

''Mega-books'' - those popular spinoffs from such TV series as ''Civilisation ,'' ''The Ascent of Man,'' and ''Cosmos,'' to name a few - represent an unfortunate publishing phenomenon, according to David Attenborough.

Ironically Mr. Attenborough has formed that opinion even though he is author of this season's new ''mega-book'' - ''Life On Earth'' (Boston: Little, Brown & Co., 319 pp. $22.95), which ties in with a BBC television series of the same name premiering in America in January. I spoke with Mr. Attenborough when he was in New York recently publicizing the book.

''Society is losing something in the process of converting television series into books,'' explains the author who is also a zoologist and TV producer, and whom whom PBS viewers may remember as the host/narrator of ''The Tribal Eye.''

''Fundamentally I think it is very important that a large number of books be published, lots of different voices heard. If we get into a situation where big publishers want only 'mega-books' . . . tied in with a television show, then that is really diminishing publishing, . . . a bad thing.''

Asked which he would recommend, if a person were forced to choose between reading his new book or watching the television show, Mr. Attenborough squirms like a schoolboy, and invokes a slight British stammer:

''I think . . . they are very different things, and they serve different purposes. The word is not interchangeable with the image. If I wanted someone to get an idea of the glory of the mating dance of the bird of paradise, I would suggest that he look at the television. If I wanted someone to understand why it should be that there are so many birds of paradise in New Guinea, I would suggest that he read the book.But the book and the series, in a sense, had to be almost independent of each other. If you made a documentary and had a commentary which made perfect sense without the pictures, it probably would be a rotten documentary, because you should use only the words which are needed to amplify the images on the film. So, a printed commentary would be incomprehensible.''

Although Mr. Attenborough has a university degree in zoology from Cambridge, he doesn't consider himself an authority on the subject. ''What that book and series are really about,'' he explains, ''is not blindingly new thoughts on the history of the animal kingdom. It wasn't newness I was after. Of course, I have to be zoologically competent. . . . But in this instance what was really required was sombody who actually knew something about presenting animals on television. And for 10 years before I came to BBC in 1952, the major thing I did was make natural history films.''

What does he hope the project will accomplish?

''If you show a frog to people, they say, 'It's a funny thing, and it hops, and it croaks.'

''But if you say, 'The frog is an amphibian, and 150 million years ago amphibians were the most highly advanced vertebrate life on earth, and their ancestors were fish, and their fins have become more muscular so they crawled and hopped, and this creature's voice may have been the first voice ever heard on earth above the hum of mosquitoes, and this creature was the one who actually brought back-boned animals onto land, so this was ultimately your ancestor' - they suddenly see frogs in a very different light.''

Asked why he considers it important for people to know such things, he remarks: ''At the risk of sounding portentious, the whole history of mankind - the thing that makes human beings human - is a divine inquisitiveness. Mankind's curiosity about the world around him is a human characteristic. You see it in babies; you see it in primitive man; you see it in us. We all want to make sense of our surroundings. And this is what this book, this series, is all about.

''In a sort of pragmatic sense, the idea of evolution in biology has probably little practical use. But, in fact, it has revolutionized man's thought, given us a completely new attitude toward the world. The concept of evolution has invaded not only our views about the animal world, but our views of politics, society, the way in which man, himself, has developed.''

Mr. Attenborough feels in the course of his research for this book he experienced some exhilarating moments of revelation. There was the time, for example, ''when we were filming dinosaur bones in a quarry in Bavaria. . . . We filmed a slab of limestone, which, of course, was once the mud at the bottom of a lagoon. In it was the crumpled image of a pterodactyl - very beautiful. It had fallen into the lagoon. And there were the tracks of something else, where the mud had been stirred up - something had crawled along. We tracked it just as if it were alive today. And there at the end of the track was the dead horseshoe crab.

''To all intents and purposes it was identical to the horseshoe crabs alive today; later we went to the Chesapeake Bay and saw so many that you could have walked along the seven-mile beach on their backs without touching sand - they were coming up to breed. That sort of continuity of life and the continuity of impulses is an extraordinarily moving thing. I hope I managed to transmit that feeling to the printed page.''