Those three, striding upright across a vast East African plain in the shadow of an active volcano some 3.6 million years ago, could they have been a male and a female leading a little one by the hand -- an early precursor to the human family?
"It's purely hypothetical, but that's a possible interpretation," says Mary D. Leakey, world renowned paleoanthropologists, whose team found the trio's footprints at Laetoli in northern Tanzania last summer. "I think that may well be the true explanation. . . .
"All we can say is that we have three bipedal creatures who were walking along together, not hurrying, but walking quite calmly, not rushing away from some catastrophic event, but just walking due north across the duty, ashy plain."
What their incredible 73-foot-long trail of sharply-etched prehistoric footprints, identical to those of modern man, says to Mrs. Leakey is that these creatures were definitely hominids -- and therefore ancestors of hmankind -- walking with the same upright, free-swinging gait modern man does.
"On any grounds there is no real Rubicon you can cross over and say that on one side is man and on the other is not man. The whole evolutionary process is so gradual. But I myself believe that the Laetoli hominids were in the direct line of human evolution. . . . They were probably at a stage of evolution where you can't call them man. They were probably before man became man. But they are hominids."
These footprints and the numerous jawbones and teeth found at Laetoli rank as the oldest known human-like remains. Only the jaw bones and teeth known as Ramapithecusm found in Pakistan and India are older, dated at somewhere between 7 and 9 million years old.
The Laetoli discovery is especially exciting because it provides evidence that ancient man was bipedal far earlier than anthropoligists had speculated.
Mrs. Leakey's son, Richard E. Leakey, also an anthropologists, has been fossil hunting and finding for more than a decade at Lake Turkana in Kenya. He has also made spectacular discoveries.
"Material found by Richard," Mrs. Leakey says, "is about 2.6 million years old. Limb bones there show that the stance was upright and they were bipedal. And material found at Hadar in the Afar region of Ethiopia by Dr. Johanson certainly shows an upright stance and bipedalism at about 3 million years." Dr. Ronald C. Johanson is the curator of physical anthropology and the director of scientific research at the Cleveland Museum of Natural History.
Another extraordinary thing about finding the Laetoli prints is that they reveal not just the bone structure of early mortal man but the exact outline of the flesh -- the bare foot.
When they were first uncovered, Mrs. Leakey says her initial reaction was "a certain degree of disbelief. We all felt slightly incredulous about it because they were so entirely human in appearance. None of us had really expected footprints of that age to be so similar to our own."
For this discovery and many other startling finds unearthed during 37 years of dedicated work with her celebrated husband, the late Louis S. B. Leakey, Mrs. Leakey recently came to Boston to receive the 1980 Bradford WAshburn Award, given annually by the Boston Museum of Science to honor outstanding contributions to public understanding of natural science.
The exacting excavation and recording methods she pioneered are now in general use.
The Leakeys received the National Geographic Society's Hubbard Medal in 1962 for "revolutionizing knowledge of prehistory by unearthing fossils of earliest man and giant animals in East Africa."
The British couple began their work in 1935 at Olduvai Gorge, a deep gash across the Serengeti Plain about 30 miles north of Laetoli, which exposes layer upon layer of habitation ranging from 400,000 years ago at the top down to about 2 million years ago at its lowest level which the Leakey's have designated "Bed 1."
Year after patient year they clambered up and down this 30-mile long gorge under the full force of the equatorial sun during Tanzania's dry season -- July, August, and September. With dental picks, paint brushes and calipers they painstakingly ferreted out, identified, and measured thousands of fossils. They uncovered a circile of stones which Mrs. Leakey calls "the very earliest human structure any where in the world."
The effervescent Louis was the irrepressibly enthusiastic leader of the team. But it was Mary, equally eager but working quietly, who made the major finds.
Her first sensational discovery at Olduvai happened in 1959 when she turned up "Zinjanthropus" (man of East Africa). For 24 years the Leakeys had found outstanding examples of early stone tools but no trace of the hominids who had lived there. Spotting what she recognized as skull fragments lodged in a rock slide, she looked up higher only to see parts of two immense teeth. (Jaw bones are tougher, harder bones and, like teeth, she explains, seem to preserve best.) Hurrying back to camp, she shouted to her husband, "I've found him -- found our man!" For 18 months Mrs. Leakey labored to piece back together again the 400 fragments of this early Humpty Dumpty.
"Zinj," whose designation was later corrected to Australopithecus boiseim (australo means "southern," pithecus means "ape," and "boisei" refers to the Boise Fund at Oxford University) was the first Australopithecine skull found at Olduvai and the first example of this creature to be dated by potassium-argon dating. Originally, the skull was thought to be no older than 600,000 years. But the deposit in which it was found was dated at an astonishing 1.7 million years old. That determination radically changed then prevailing views about the time scale of human evolution.
This discovery revealed the immense potential of the Olduvai site, and inspired the National Geographic Society to begin giving substantial financial support to the Leakeys, support that continues to the present time.
"Zinj," was not, however, Mrs. Leakey stresses, the toolmaker of Bed 1. The wielder of volcanic rocks and water-worn stones with parts knocked off to make a sharp cutting instruments was a smaller creature with a larger brain relative to his body size who was a contemporary of "Zinj."
"We called him Homo habilis,m or the "handy man," because he was the toolmaker of Bed 1," Mrs. Leakey says. Finding him the very next year -- 1960 -- was, she believes, an even more important scientific discovery. Anatomists, she says, deduced that this creature, dating back 2 million years, was bipedal, with the same free striding gait of modern man.
Still more important, Mrs. Leakey believes, is her team's discovery of the footprints at Laetoli, which indicate that man-like creatures were striding upright at least a million and a half years earlier than previous hard evidence indicated.
The prints were discovered sandwiched between layers of petrified volcanic ash. A rare sequence of climatic conditions appears to have been responsible for crystallizing these brief moments in prehistoric times.
Dr. Richard L. Hay, a geologist at the University of California at Berkeley, interprets the event as taking place during a very short period, probably no more than the onset of a single rainy season. Several times the volcano Sadiman , towering the east, erupted, spewing dry ash over the part of the Serengeti plain called Laetoli for the red lilies that grow there in profusion. Had a wind sprung up, it could have blown the ash away. Heavy rains might have obliterated the footprints. Instead, gentle showers fell, moistening the ash, making each layer cohesive soon after it was deposited.
Creatures in the region, apparently unintimidated by the heaving volcano, continued ambling across the plain, leaving hundreds of foot and hoof prints behind the harden in extraordinary detail into volcanic rock.
Dr. Hay believes that the ash contained soluble salts and that when it became damp through rain or dew, the salts dissolved enough to make it cohesive. Abundant hares left behind their hippity-hop prints, and guinea hens their footmarks. Even raindrops, falling on the dusty surface, pockmarked the area. There is evidence, too, that large animals -- elephants and rhinos -- drawn to Laetoli during the rainy season even as they are today, skidded and slipped in the moistened ash.
The first traces of the hominid trail -- extending in a relatively straight line from south to north -- stirred in the Laetoli team intense interest and excitement when dating of the petrified ash beneath the prints registered at 3.8 million years ago and that above them at 3.6 million.
Up to this point, the earliest footprints of primitive man ever found had been those of Neanderthal man, who dates back a mere 80,000 years. To the paleoanthropologist, that's like yesterday.
In 1978 when the northern section of the trail was uncovered, the Leakey party was under the impression that the prints were left by two individuals. They reasoned that the larger prints on the right may have been left by a male, walking ahead, and that the smaller prints on left may have been those of a female, following.
Reporting the discovery in the April, 1979, National Geographic, Mrs. Leakey makes this commentary on the smaller footprints:
"Incidentally, following her path produces, at least for me, a kind of poignant wrench. At one point, and you need not be an expert tracker to discern this, she stops, pauses, turns to the left to glance at some possible threat or irregularity, and then continues to the north. This motion, so intensely human, transcends time. Three million six hundred thousand years ago, a remote ancestor -- just as you or I -- experienced a moment of doubt."
Clearing the trail southward last summer, Mrs. Leakey and her assistants uncovered under four inches of volcanic ash prints in a far better state of preservation. This gave the anthropoligists much more detail than they had had the previous year.
"Now we know that there were three individuals and not two," Mrs. Leakey explains. "It became quite clear that although the trail of 39 prints on the left is of a single individual the trail of 31 prints on the right is of dual prints in which two individuals were walking in tandem, with one in front being followed by a second, who put his or her feet in the footsteps of the leading individual.
"I think now it is likely that the broader foot leading on the right was that of a male, being followed perhaps by a female, and that one or the other was holding onto and leading the juvenile on the left."
Mrs. Leakey says she can't possibly explain why the individuals on the right walked as they did, "but it is a fact that it happened."
When young gorillas and young chimpanzees are playing, she points out, they often walk one behind the other. In this soil, she says, they would have left knuckle marks. "But there is absolutely no trace of any such mark."
By measuring the hominids' foot lengths as a clue to their height, Mrs. Leakey reports that "we can probably say with some confidence that these creatures were between 4 and 5 feet tall."
Their feet were narrower across the toes than the prints of some of her African staff members who have been used to going barefoot -- but not as narrow as those of people today who have worn shoes since growing up.
Mrs. Leakey doubts that today's Laetoli natives are direct descendants of these ancient creatures. "You are dealing in millions of years," she points out , during which there may have been many population changes.
As yet Mrs. Leakey has given non name to the individuals whose remains and footprints she has found at Laetoli because, she says, "I do not believe the material has been studied in enough detail."
She believes that after the ancient Ramapithecusm comes the Laetoli creatures; that they are followed by a large creature uncovered by Dr. Johanson in Ethiopia. These are followed in turn by Homo habilism (the handy man), Home erectusm (erect man, found by Richard Leakey) and finally Homo sapiensm (knowing man or modern man) -- with Australopithecusm (including "Zinj") branching off the direct line of human evolution quite early.
Not all anthropologists fit the Laetoli material into the family tree of human evolution as Mrs. Leakey does.
Dr. Johanson and an associate, Dr. Tim D. White, assistant professor of anthropology at the University of California at Berkeley have published a paper in science magazine in which they group together their Afar material and the Leakey's Laetoli material in one new species which they are calling Australopithecus afarensism (southern ape from Afar). "I think they have done so without sufficient basis in fact," argues Mrs. Leakey. "I think it is a premature conclusion because they have not demonstrated that the two are, in fact, identical. They have merely stated that in their opinion they are."
Not only that, she contends, "but there is a question as to whether all of the material from the Afar region belongs to a single species or whether there are two species represented." The two sites are about 1,000 miles apart.
They have chosen, she reports, as the type for Astralopithecus afarensism a worn lower jaw found by her team at Laetoli. "I think," she protests, "that it is not desirable to use as a type specimen for afarensism a specimen from Laetoli which is over 1,000 miles distant."
The Leakey's originally visited Laetoli in the 1930s, but abandoned the site in favor of Olduvai Gorge, which they felt was a richer field for digging. The gorge has been explored a bit by German anthropologists in 1911, when Tanzania was known as German East Africa, but the workers did not turn up any traces of human existence. It was not until 1974 -- two years after Louis Leakey passed on -- that Laetoli again sprang into prominence at the Leakey camp at Olduvai. A neighbor offered to build Mrs. Leakey a stone house at the Gorge. In gathering sand for the construction, he found a lot of fossil animal teeth in the nearby Garusi River.
"We decided to try to find out where these teeth were coming from," Mrs. Leakey recalls. Her team wandered up the river bed and came upon exposures of the beds at Laetoli which they had never seen before. Nearly 40 years of weathering had exposed them. Teeth they found there looked comparatively recent by their standards -- perhaps 200,000 years old.
"On one visit we found a human or a hominid tooth. That was very exciting," Mrs. Leakey recalls. Taking samples of the lava which overlies the flat gray fossil beds, they discovered to their astonishment that they dated at 2.4 million years -- 400,000 years earlier than the earliest deposit of Olduvai. "That was enough to make me decide to have a first season there in 1975," Mrs. Leakey says. "We then got dates of the fossils themselves and they proved to be 3.6 to 3.8 million years."
Though thousands of tools have been found by the Leakeys at Olduvai, not one has been turned up at Laetoli, indicating that although the Laetoli hominids were upright and freestriding, leaving their hands free, they were not not tool makers or users.
"There seems to have been an enormous gap in time between the adoption of bipedal gait and the first formalized toolmaking," she says. "One wonders why this should have been. It seems to me personally that it could well be that the brain developed rather more slowly. And until it had developed and was in a position to direct the hands, they were rather useless appendages. But that is just my private theory."
The seeds of Mrs. Leakey's richly productive career in anthropology were sown when she was 11 and her father took her to see the prehistoric caves in Francehs Dordogne region. Later she attended lectures on archeology and geology at University College, London. Possessing an unusual talent for drawing, she copied artifacts for her own amusement. Dr. Gertrude Caton-Thompson, a British archeologist, invited her to illustrate a book on stone tools she was writing. It was she who introduced the Leakeys to each other.
All three Leakey children live in Kenya. Jonathan, the eldest, is a herpetologist. He runs a snake farm and tourist camp at Lake Baringo in Kenya. Richard is director of the National Museums of Africa in Kenya. And the youngest, Philip, an assistant minister of environment and natural resources, has just been elected to the Kenya Parliament.
Mrs. Leakey spends most of her time at Olduvai Gorge, rarely visiting her home in Nairobi, Kenya, thriving on the isolation and privacy of the gorge. This year she plans to spend most of her time writing -- finishing the last of two volumes on the Olduvai findings and "putting in hand" a report on Laetoli.
However exciting the potential for discovery may be, Mrs. Leakey takes a matter-of-fact approach to her work. "An excavation is an excavation," she says. "It is something to be carried out. You do a job as you do any other sort of job. You just carry on. When it's finished, if you get good results, you are very happy. If you don't, well, you go and do another one."
The footprints at Laetoli still stretch southward, waiting to be uncovered, waiting for Mary Leakey to carry on the job.