Unlocking the mysteries of life on planet Earth; Life Itself: Its Origin and Nature, by Francis Crick. New York: Simon & Schuster. 192 pp. $12.95. Life on Earth: A Natural History, by David Attenborough. Boston: Little, Brown & Co. 319 pp. $22.50.

From start to finish, Sir Francis Crick's imaginative book raises an insistent question, ''Should we take it seriously?'' After all, the notion that earthly life started from microbes sent here by alien beings - so-called directed panspermia - while not particularly new, is not exactly mainline biological thinking.

But Sir Francis has a lot going for him. His scientific credentials are impeccable. He won his Noble Prize for his share in unraveling the structure of the DNA molecule, which carries the genetic code. More important, he handles his speculations with a strict scientific discipline while leaving out none of the fun.

He says his main aim is to sketch out the background knowledge one needs to speculate sensibly about organic life's origins - knowledge of how the universe, Earth, and its life forms have evolved as well as how biology's genetic mechanisms work. This he does with an elegant simplicity and wit that let any interested layman into the discussion.

Then he outlines the outrageous notion that he and his colleague Leslie Orgel have been promoting: At a time long ago on a world far away, intelligent beings sent off a rocketload of microbes. This eventually seeded the primitive Earth with its first living organisms.

As Sir Francis notes, the known facts could accommodate this theory. But they in no way support it, although some of the facts could be interpreted as mildly suggesting it. It stands simply as a scientifically respectable alternative to the theory that organic life evolved out of so-called pre-biotic chemistry. The known facts accommodate this theory. But, again, they do not directly support it. The key facts that could be decisive - direct geological evidence of what happened during Earth's first billion or so years have yet to be discovered.

To answer the question raised in the opening paragraph, yes, one should take the book seriously as an example of how a first-class scientist deals with a momentous but slippery problem. As to the notion of directed panspermia itself, well, readers would best be advised to enjoy it as an intriguing speculation.

David Attenborough, on the other hand, is on somewhat more solid ground. Approaching his subject as a journalist, he marshals what is known about the development and wondrous variety of Earth's animal life to focus on humanity's place in this vast drama. His book is as sweeping and as beautifully illustrated as is the BBC-TV series, now airing on PBS, from which it springs.

It also presented a formidable challenge to condense ''three thousand million years of history into three hundred pages,'' Attenborough notes. He adds that he could only succeed in this by picking out the main threads of the story and sticking with those. This means, inevitably, that any specialist could complain of what has been neglected or of where emphasis has been placed. But as an overview for the general reader, it works.

You can enjoy this book on several levels. The most obvious is pictorial. The illustrations alone capture your attention when browsing through. Then, in reading, you can gain an impressionistic view of the development and range of animal life and some appreciation of how zoologists go about understanding it. Finally, there is what I suspect to be the author's underlying purpose - a vision to be gained of the interrelatedness of earthly life and of humanity's special place within it.

This is not the old hubris in which people have seen themselves as beings apart from the rest of nature. It is simply to recognize, as Attenborough puts it, ''that no species has ever had such wholesale control over everything on earth, living or dead, as we now have.'' He adds: ''That lays upon us, whether we like it or not, an awesome responsibility. In our hands now lies not only our own future, but that of all other living creatures with whom we share the earth.''

This is a book to be enjoyed and pondered. In a quite literal sense it will give you a new perspective on life.