Mary Leakey, the paleontologist whose discovery of human-looking footprints in pre-Stone Age Africa has astonished the world discussed her findings yesterday at the National Geographic Society.

As early as 1976 some rather wide-looking prints were discovered in the Laetoli region of northern Tanzania and after much analysis and searching of the interior brain, Dr. Leakey concluded they were hominid footprints.

But then in 1977 further prints were found, more convincing than the first one. And last year still more were found, clearer still.

"Has any competent authority questioned the basic assumption that these are human-type footprints?" I asked.

"Not that I've heard of," she said, looking very composed in her brown wool suit and gray silk stockings.

"If you haven't heard any arguments, I imagine no such arguments have been made," I said, and Dr. Leaskey beamed like a woman who has already cashed a million bucks worth of gold and is therefore not much rattled by somebody questioning whether the gold was genuine.

"It's really beyond argument," she said.

Through laboratory dating of material found both above and below the prints (which were preserved in briefly damp volcanic dust) the footprints are known to be between 3.6 and 3.8 million years old, and the presumed significance of the prints is that they are the oldest known indications that man, or their ancestors, or their somewhat related hominid types, actually walked erect.

"And one bipedalism is established," she went on, it frees the hands for manipulation. But there was to be the active inquiring brain. Without that, two free hands don't get you far."

"Is it possible," I asked, "that the brain began developing, resulting in erect posture, rather than following it.?"

"I don't think so," she said. "I think the brain work followed."

"Idle hands are the devil's workshop," said another reporter, introducing the dark thought that the brain developed in order to further human mischief.

"Tell me," said a third fellow, "what you think the importance of these discoveries are, to modern men?"

"Very little," said Leakey, "to give you an honest answer. It's just that I myself am very curious about the past."

Of course, as I observed afterward, if you're going to entertain questions like that, what good is anything? We are born, we propagate, we die, and all learning, all speculation is somewhat beside the point in the long run. But we live, after all, in the short run, and surely it's more agreeable to speculate on our probable origins millions of years ago than to just grunt and chase after one more possum for supper.

"I still have my four dalmatians," said Leakey, admiring two of them shown in a diorama in Explorers Hall. I suggested dogs are a great comfort to people who live in utter wildernesses, even if they're not actually very helpful in day-to-day life.

"But they are quite helfpful," she said, "because when you have your nose to the ground (meaning the human scientists) you don't know what you'll run into."

The dogs keep an eye out, in other words, for any saber-toothed tigers or beasts that might approach and endanger the explorers. There is no telling, she said, what you will run into in the wilds of Africa.

A somewhat inelegant footnote to the discovery of the 1977 prints reveals the plain fact that some of the important scientists were chasing about in the field throwing dried elephant dung at each other. Exactly as people do on fancy house parties in the Virginia and Maryland countyside, when a fellow suggests a hike through the pasture to the woodlands, and the whole party is diverted tossing renmants of cows' earlier digestions.

Mankind is the species that throws cow or elephant turds in sport, sometimes leading to important discoveries:

One of the scientists, dodging a particularly well-aimed missle, found his eyes near the ground -- and behold; the ancient footprint was thus seen, showing the importance of elephants.

It saddens her that no tools have been found, and now she is reasonably sure these creatures had not yet formed the mental concepts and technical level to make them.

She said the prints suggest three individuals, perhaps a man, woman and child and the child may well have been carrying something heavy at its side. She said no fossil teddy bears have been found and (somewhat severely) said she hardly expected any would be.

Among the things you find are scorpions and cobras, not that they seem to have daunted Mary Leakey.

She was born in London in 1913, daughter of a landscape painter, and great-great-granddaughter of an early student of paleontology, John Frere, who was the first to conclude that certain crude stones were in fact tools of early man.

At the age of 11, Mary visited the ancient caves of Cabrerets in France, where the Abbe Lemozi guided her and gave her a respect for serious work in archaeology. Lather at the University of London she studied geology, prehistory, and drawing (which was extremely useful in recording Stone Age tools she subsequently discovered).

She visited Africa in the mid-'30s with a family member interested in missionary work. She met (and in 1936 she married) Louis S. B. Leakey, Kenyan-born anthropologist who was the son of missionaries.

In those days she smoked cigars and dressed casually so that she was commonly mistaken for Dr. Leakey's brother and was often introduced as "Mr. Leakey."

Her husband died in 1972, but as early as 1935 the two of them explored Laetolil deposits, though without much success. Two later expeditions were not productive, either, but Mary Leakey never could get this region, a two-day trip from the Olduvai Gorge, out of her head.

Life was not always idyllic. She suffered from pneumonia, malaria, appendicitis and a parasitical ailment that left her incapacitated for months.

At the time the Leakeys, and various others also working in Africa, began their researches, man was believed to date from no further back than half a million years.

Her husband's work at Olduvai Gorge attracted wide interest and controversy for years, as he pushed the date of man or man-like creatures back further and further in time. It was Mary Leakey herself who in 1942 discovered an important cache of stone axes, and Mary herself who in 1959 discovered the skull of a near-man, 1.75 million years old (which she spent 18 months restoring from small fragments).

It was only in 1975, however, following the discovery of a humanoid tooth in 1974, that serious surveys of the Laeotil deposits were made by her team, which has been supported since 1959 by the National Georgraphic Society.

These deposits, she says, are unique, consisting of thin layers of volcanic dust that, for the space of only three or four weeks, were sufficiently wet from rain that they took footprints, not only of man-like creatures, but of many other animals that passed over them.

Then they were covered thickly with additional dust and, in the course of time, plants, which eventually eroded to uncover the ancient traces.

She will spend some time lecturing, first at Mary Washington College at Fredericksburg, Va., and then in other cities.

I asked if she needed any money for her work, and if the talks might lead to grants or contributions.

"Of course we can use money. Are you kidding?"

But money alone, needless to say, is hardly a substitute for a well-aimed elephant party, or for the fun and games that lead (and never mind the solemn brows of killjoys) to all human advance.