Fossil Ancestors of Burma
To document the origin of the higher primates, paleoanthropologists have retraced the steps of a 1923 expedition.
 
by Russell L. Ciochon



        "On the road to Mandalay,/ Where the flyin' fishes play,/ An' the dawn comes up like thunder outer China 'crost the Bay!" So wrote Kipling at the turn of the century of the wonders and enchantment of Burma and its city of Mandalay. Today, as enchanting as ever, the road to Mandalay beckons to paleontologists because Burma is the only place in the world that has yielded fossil evidence of an important link in the primate order. There are two groups of primates on the earth today. The higher primates - monkeys, apes, and humans - are the most familiar. They are sometimes called the Anthropoidea, or humanlike primates. The lower primates - the lemurs, lorises and tarsiers - make up the second group. These primitive primates, often called Prosimii, or pre- apes, were the first to evolve and were the forerunners of all later forms. However, fossils showing the beginning of the evolutionary branch leading from prosimians to anthropoids have been hard to come by.
        An early discoverer of the "Burmese link" that demonstrates this transition was the legendary fossil collector Barnum Brown, a paleontologist at the American Museum of Natural History. In the spring of 1923, Brown and his wife, Lilian, arrived in Rangoon, the capital of Burma. They journeyed by river steamer up the broad , muddy Irrawaddy to the port of Pakokku. From there they mounted a mini expedition to the little-explored Ponnyadaung (or Pondaung, as Brown spelled it) Hills, located deep in the teak-bamboo forest of Upper Burma. Outfitted with four bullocks, two small, mat-covered carts, and a pair of sway-backed saddle horses, they rode in search of the varicolored sandstones of the Pondaung Formation, rock deposits that had been discovered earlier in the century by British economic geologists. In the words of Lilian Brown, they were "rainbow-chasing" - following the sedimentary rocks shaded yellow, gray, red, purple, and green, seeking a prehistoric pot of gold.
        When Brown arrived in Burma, virtually nothing was known about the early prehistoric life of southern Asia. The prevailing scientific opinion was that Asia was the mother of all continents, the center of origin for much of the earth's life. Brown had been sent by the American Museum to collect late Eocene (40- to 45-million-year-old) fossils in support of this "Garden of Eden" theory. He specifically sought to collect large skulls and skeletons of extinct animals that could be exhibited at the American Museum.
 

B. BrownIn 1923, Barnum Brown (mounted) collected fossils in Burma's Ponnyadaung Hills. This photograph (here tinted) was printed in Natural History in 1925, with a caption noting that the servant Mari (in the cart) died of malaria contracted on the journey.

        For two months, Brown's small bullock-cart caravan creaked along the dirt tracks, the only roads in this remote part of Burma. Because the resident commissioner of Burma had provided a letter of introduction to all village headmen along the route, Brown was able to camp in villages that were nearest to exposures of the fossil-bearing Pondaung sandstones. One such stop brought him to the outskirts of Mogaung village, where he set up camp with Lilian and their two servants, Mari and Dos. Early each morning Brown would ride off in search of new fossil sites. One day, a short distance northwest of the village, he came upon a locality where he saw a number of small bones and teeth eroding out of the rock. Here he picked up part of a jaw with three teeth - a piece about the size of a fifty cent coin - that belonged to a medium size primate.
       

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Map of Southeast Asia, with detail of the field site.

Barnum Brown probably did not realize that what he had discovered was an early higher primate. But Edwin Colbert, Brown's colleague at the American Museum, did, and in 1937 he named the jaw Amphipithecus mogaungensis (near-ape of Mogaung). Amphipithecus joined the ranks of another Burmese fossil primate, discovered in 1913 by paleontologist G.D.P. Cotter. Cotter had explored the southern exposures of Burma's Ponnyadaung Hills while working for Britain's Geological Survey of India. A description of this specimen (a piece of upper jaw and two pieces of lower jaw) was published in 1927 by Guy Pilgrim, who named it Pondaungia cotteri. Pilgrim thought it could be an Eocene higher primate, but the fragments were too scrappy for precise identification. With Colbert's more confident description of Amphipithecus in 1937, the late Eocene beds of the Ponnyadaung Hills became known as the source of earliest record of the Anthropoidea.
        Anthropologists debated the evolutionary affinities of Amphipithecus and Pondaungia for the next four decades. Were they really the world's earliest higher primates or were they prosimians with a few independently evolved anthropoid like features? Was Asia even the place to look for the origin of the higher primates? More fossil evidence was needed, but no one was able to work in the Ponnyadaung Hills during the years embracing the Japanese invasion, World War II, Burma's achievement of independence, and the emergence of nationalistic policies that followed.
        I first visited Burma in 1975, to discuss the possibility of a joint U.S.-Burmese paleoanthropological research project. With U.S. involvement in the Vietnam War drawing to a close, my proposals fell on receptive ears. After discussing plans with geologists at the Mandalay Arts and Sciences University, I submitted a letter to the Ministry of Foreign Affairs. The reply stated that the Ministry had "no objection to a planned paleontological visit." On my return to the United States I teamed up with Donald E. Savage, a seasoned paleontologist, and in March 1977, having obtained the necessary funding and entry visas, we set off on a two-month research tour of Burma.
        Upon arrival in Rangoon, however, we were only given permission to prospect for fossils along the Irrawaddy River drainage, where the sediments date from the Pleistocene epoch (the last 2 million years of earth history). Ministry officials declared that the area around the more ancient Ponnyadaung Hills was so dangerous that we would need a police escort, which could not be provided at the time. Swallowing our disappointment, we flew north to Mandalay, where we were met by U Ba Maw and U Thaw Tint, our colleagues at Mandalay University, along with a geology student who was to be our field coordinator, Tin Maung Oo (who likes to be called "Tin").
        For the next six weeks we explored a 300-mile section of the Irrawaddy River, collecting fossils along its banks and terraces. This fieldwork was rewarding, but we yearned to visit the Ponnyadaung Hills, a mere 200 miles to the west. To impress upon our Burmese colleagues how eager we were for information about the 40-million-year-old fossil beds, we left our copies of Barnum Brown's field maps and publications with them. They promised to attempt a reconnaissance of the region in the late fall dry season.
        Savage and I returned home and awaited further word on our research proposal to the Burmese government and on results from our Burmese collaborators' visit to the Ponnyadaung Hills. We heard nothing for almost a year when suddenly a small package arrived from Mandalay. It contained plaster casts of jaws and teeth of several Eocene mammals, one of which was a nicely preserved jaw of Pondaungia. U Ba Maw and U Thaw Tint had not only found Brown's localities; they had also succeeded in recovering a new fossil primate. An accompanying letter stated that they had found many more fossil mammal specimens, several of which they thought could also be primates. They encouraged us to return to Burma as soon as possible to visit the localities and to work with them on publication of the finds. In December 1978, arrangements were made, and we departed once again for Burma.
        On our approach into Mingaladon Airport in Rangoon we were asked to set our watches back in accordance with Burma Standard Time. The British businessman sitting next to me intoned, "Set your watch back thirty minutes and turn time back thirty years." Indeed, little has changed in Burma since its independence in 1948. But our minds were set on a much longer time frame, for we were hoping to turn time back 40 million years!
 

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Our knowledge of Amphipithecus comes from fragments of two separate jaws. One portion was found in 1923, another (shown overlapping) in 1977.This drawing of Amphipithecus is based on fossil evidence and an interpretation of the order of evolution of anatomical features found in living higher primates.

        After several days of preliminaries, we made the short hop by air to Mandalay and prepared for our journey to the Ponnyadaung Hills. U Ba Maw and U Thaw Tint could not accompany us on this trip since they were in the middle of university exams, but they sent some of their students with us. Before dawn on December 20, our small caravan - two World War II-vintage jeeps loaded with camping and excavation equipment, four geology students, a cook, a mechanic, two drivers, Tin, Savage, and myself - left for the field.
        To reach the Ponnyadaung Hills we had to cross the two great rivers of Burma. Crossing the Irrawaddy was a simple matter of driving over a steel-girder bridge, but at the Chindwin River our two jeeps had to be loaded onto a flatbed barge, along with chickens, goats, bicycles, and a large number of other passengers. A small tug then towed us across the mile-wide river. After several more hours of driving we reached the Pale Township People's Council, where we checked in and picked up an escort of several armed policemen. Our Burmese colleagues had assured us that the only danger in the Ponnyadaung Hills was an unlikely encounter with a Bengal tiger, but for the sake of appearances, we decided to accept the offer of an escort.
        On our five-hour climb from Pale into the Ponnyadaung Hills, the road deteriorated rapidly from a graded gravel track into a series of potholes and dust bowls interspersed by a washboard. The open scrub-brush and farmland near the Chindwin River quickly gave way to gently rolling forested hills. As we climbed still farther, the road became very steep, and the teak and bamboo forest surrounding us came alive with azure butterflies, screeching parakeets, and scampering red- orange jungle fowl. We encountered young Burmese men driving oxcarts loaded down with teak logs, but not a single motorized vehicle. At the fifty-five- mile post the road became impassable for even a four-wheel-drive jeep. We transferred our supplies onto two oxcarts, each pulled by two grunting oxen, and began the eight-mile hike down an old cart road into Mogaung village. The oxcart wheels creaked and squeaked - but not as a result of neglect, I was told; rather, the local villagers never grease the wheels because they feel the noise keeps away evil spirits.
        Late in the evening of our daylong trek we reached our destination - Mogaung, a village of some nine hundred people. Surrounding the village, which consists of small teak huts raised off the ground on poles, is a bamboo fence. Inside, banana, coconut, and toddy palm trees grow in abundance, and pigs and chickens roam freely. We were lead to the hut of U Gyo, the village headman, where we were fed a traditional snack of bananas and green leaf tea. With Tin acting as translator, U Gyo greeted us graciously and suggested we use the new village schoolhouse as a base camp. It was rice-planting time, and school was not in session. We were exhausted, and after a quick meal of chicken curry, everyone bedded down.
        Early the next morning, with Barnum Brown's field map in hand, we proceeded on foot northwest from Mogaung, through flooded paddy fields, across small streams, and along well-worn paths in the forest. After we had walked about one mile through the lush forest cover, the rainbow- banded sediments of the Pondaung Formation suddenly appeared. The local villagers called these areas kyit chaung, "placed without vegetation." Owing to the chemical properties of the sediments, vegetation, especially fragile young rice plants, is not able to grow in them. In this case the villager's loss is the paleontologist's gain.
 

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A family tree of the primates lists living groups at the top. The major fossil species, some of which left no descendants, are represented vertically by the branching tree. The Anthropoidea, or higher primates, are shown in orange; the Prosimii, or lower primates, which comprise all other primates, form a less unified group (yellow). Some early prosimians, perhaps a group related to lemurs and lorises, gave rise to the Anthropoidea. Amphipithecus and Pondaungia are transitional forms that possess a number of anthropoid features but also retain a few prosimian characteristics.

        Since Barnum Brown had left very precise field notes, we had no trouble finding his localities, and we soon began to accumulate a treasure trove of fossils. The Burmese geology students also showed us the spot where the new primate jaw had been found. Over the next couple of days, Savage and I collected the remains of many extinct animals, including hippo- and piglike anthracotheres, rhinolike brontotheres, small deerlike artiodactyls, rodents, lizards, turtles, fishes, and crocodiles. From studies of this fauna and some associated plant remains, together with an understanding of the type of rocks in which they occur, we can reconstruct the paleoenvironment of this part of Burma in the late Eocene. The sediments were deposited by a medium-size river that drained seaward toward the Burmese Gulf, which in the past was located much farther north. Along the banks of this river, which was partially covered by a forest canopy, anthrocotheres, brontotheres, and small artiodactyls came to drink. Turtles, crocodiles, and fish swam in the river. In the trees above, the ancestors of the higher primates romped.
        On our second night in Mogaung village, the headman, U Gyo, honored us with a visit to "get better acquainted." As spiritual leader and chief administrator of Mogaung, U Gyo had considerable power, but being sixty-five years of age, he had learned to use his position wisely. He spoke of how Mogaung village had changed over the past half century ("actually, very little"). I then asked him when Mogaung had last been visited by Westerners, "people like ourselves". U Gyo thought a while and then began to recount the following story:

 

When I was a young boy of ten, I remember a white man and woman coming on horseback with several oxcarts of supplies. With the help of Mogaung villagers, they set up camp only a few hundred yards from where we are sitting. The man would ride off each morning and return late in the evening with his horse packed with odd-shaped rocks. The woman, who wore pants but was strikingly beautiful, would stay in camp and play with a small dog, whose hair she was constantly combing.

 

We immediately realized that U Gyo was describing Barnum and Lilian Brown. I later showed him a photograph of Brown taken in Burma in 1923, and he confirmed the identification.
 

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Outcrops of the 40-million-year-old Pondaung Formation are bare of vegetation.

        After several more days of fossil hunting around the Mogaung village, we decided to hike to a locality Brown had discovered some six miles to the northeast, near the village of Gyat. We found some excellent exposures near a large lotus pond, which we could recognize from a photograph by Brown published in Natural History in 1925. Unfortunately, we were not able to find any fossils. In the late afternoon we trekked back to Mogaung, where our cook was preparing a special holiday meal of roast chicken and potatoes. It was Christmas Eve. Savage and I rested and sipped a clear, sweet alcoholic drink distilled from the fruit of the toddy palm tree. As the sun sank behind the mountains, the air became chilled, bringing out the smell of the teakwood smoke and frying oil.
        As darkness fell we heard a commotion in the distance. One of our police guards, who had not returned with the field party, burst into camp brandishing his weapon proudly. He led a procession of villagers, two of whom shouldered a rough-hewn pole carrying a small deer. The deer was immediately butchered and some parts roasted that night. The following day we proceeded to our next camp, at nearby Legan village. There, the deer meat provided a magnificent Christmas Day feast for our entire field party and all the village elders.
        After another week of fossil hunting, we journeyed back to Mandalay, where more excitement was in store for us. At the Mandalay Arts and Sciences University we met with U Ba Maw and U Thaw Tint, who produced a box of small fossil jaws they had collected in the Ponnyadaung Hills. They weren't sure what the six pieces were, but they had an idea they might be primates. Savage and I were almost breathless as each jaw was removed from the box. The first was a primate, the original of the cast they had previously sent us. Three of the others also proved to be early anthropoids. U Ba Maw and U Thaw Tint had tripled the early anthropoid sample of Burma. Of the four primate jaws they had discovered, two were Pondaungia and one was Amphipithecus. The fourth may represent a type of primate previously unknown to science. Our Burmese colleagues asked us to help them publish these finds in Western journals.
        After returning to the United States, Savage and I began to study the casts and photographs of the new Ponnyadaung fossils, making comparisons with Brown's Amphipithecus jaw and Cotter's Pondaungia specimens. Since several of the new specimens are more complete than the older fossils, the characteristics of the two species are becoming clearer. Both fossil forms exhibit a combination of lower and higher primate features, with the latter considerably more predominant, indicating that they were at or across the evolutionary transition from prosimian to anthropoid.
        Some of the lines of evidence that point to this conclusion can be illustrated by a look at Amphipithecus, the better known of the two species. For one thing, this gibbon-size animal, probably weighing about twenty pounds, was relatively large in comparison to most lower primates alive in the Eocene or even today. The lower jaw is deep (top to bottom), both absolutely and in relation to the height of the teeth, and this depth extends the full length of the jaw. In the lower primates, the jaw is not as deep and lessens in height toward the front. The Amphipithecus jaw is also very robust (thick). These jaw characteristics relate to the fact that the right and left halves of the lower jaw were fused, unlike those of nearly all extinct and living prosimians, whose jaws move independently as they chew. The fused jaws of anthropoids, which evolved for chewing tougher foods, are strengthened and reinforced to withstand the extra stresses that are placed on them during mastication. As in all anthropoids, the jaws of Amphipithecus are buttressed where they join by two horizontal, shelf-like thickenings of bone, called tori. (In contrast, the minority of Eocene lower primates that have fused jaws exhibit only one torus, suggesting they are unrelated to Amphipithecus.)
        The cusps, or elevations, on the chewing surfaces of the teeth are relatively flat, a trend found in fruit eaters. Most prosimian teeth, instead, have a very crested cutting surface, useful for a diet of insects or leaves. In this respect, Amphipithecus resembles 30- to 35-million-year-old anthropoids found in Egypt's Fayum province (see "Dawn Ape of the Fayum," by Elwyn L. Simons, Natural History, May 1984.) Another important consideration is the number of teeth in the jaw. Among primates in general, there is a long evolutionary trend toward reduction in the number of teeth. Amphipithecus has three premolars, a relatively primitive feature it shares with some prosimians and New World monkeys. Old World monkeys (as well as apes and humans) have two premolars. In this respect, Amphipithecus (or some closely allied species) is a suitable candidate as a forerunner of both the New World and Old World anthropoids - in other words, of all higher primates.
        The original specimen of the Amphipithecus lower jaw contained the root of the canine tooth, the root of the first premolar, the second and third premolars, and the first molar. Luckily, the new specimen has helped complete the picture, since it contains the first and second molars and part of the third. The square shape of the second molar (viewed from above) is significant, because this is characteristic of anthropoids. In contrast, among the lower primates there is a narrowing toward the front of this tooth. The newly discovered first molar has resolved a rather arcane controversy over the possible position of a rearward cusp known as the hypoconulid. A nick in the original specimen, where some believed this cusp has broken off, was not in the position characteristic of higher primates. As it turns out, the new specimen shows there is no hypoconulid cusp at all on the first and second molars. In this feature, Amphipithecus differs from all Old World anthropoids but resembles many New World monkeys.
 

Migration of the Early Anthropoids: The relative positions of continents, oceans, and shallow seas 40 million years ago are reconstructed in a map of the earth's surface. Before higher primates (anthropoids) evolved, lower primates were present in Europe, Asia, North America, and probably Africa. The Burma fossil finds suggest that the higher primates arose from lower primates in Asia. Early forms probably then spread to Africa and, by way of volcanic islands, to South America (red arrows). Those in Africa subsequently gave rise to all the Old World monkeys, apes, and humans, while the New World monkeys evolved on their own in South America.
Another possibility (indicated by the blue band) is that a population of early higher primates came to occupy parts of both Asia and North America, which were connected by a land bridge. When this connection was severed, the New World and Old World groups began to evolve independently, the New World group eventually migrating to South America. No one has discovered fossils of higher primates in North America to support this second hypothesis.

        The fossil finds from Burma suggest that the first higher primates evolved in Asia some 40 to 45 million years ago and spread from there to the other parts of the world. This geographic spread could have occurred by way of a number of routes, but I believe that the most likely sequence was the following one. At the end of the Eocene, early anthropoids, the Ponnyadaung primates or their close relatives, migrated across Asia into Africa by crossing the narrow, swamplike Tethys Sea, which then separated the two continents. Once in Africa, these early higher primates continued to evolve, with some populations becoming ancestors of the 30- to 35-million-year-old Fayum primates of Egypt (and ultimately of all Old World monkeys, apes, and humans). Other populations crossed the then-narrow equatorial Atlantic Ocean by island hopping along a series of volcanic islands. In this way they reached South America and became the ancestors of the New World monkeys.
        The increasing clarity with which Western paleontologists are now able to view these events in primate evolution is owed in large part to the discoveries made by our Burmese colleagues, who continue to search for more fossil evidence. Although I have returned to Burma several times since my memorable visit to Mogaung village, I have not been granted further opportunity to visit the Ponnyadaung Hills. While a field trip to southern China was arranged in 1983, and one to Vietnam is planned for next year (both countries have related geological deposits), the road to Mandalay still beckons.