East Side Story: The Origin of Humankind

The Rift Valley in Africa holds the secret to the divergence of hominids from the great apes and to the emergence of human beings

by Yves Coppens

umans are creatures whose roots hard de Chardin. As an invited speaker, one radiating from both the Asian and lie in the animals. Accordingly, I gave a talk on the French paleontolo­ the African tropiCS. Hwe find ourselves at the tip of gist and philosopher's scientific work. The gentleman seemed interested, one of the branches of an immense Although this aspect of Teilhard's writ­ thanked me and left. Several months tree of life, a tree that has been devel­ ing is often forgotten by biographers, later I received a letter of invitation to a oping and growing ever more diverse who are essentially interested in his conference in Rome that he proposed over a period of four billion years. From philosophical texts, he produced more to hold in May 1982. My questioner an evolutionary standpoint, it is impor­ than 250 scientific reports over the had been none other than Carlos Cha­ tant to locate the place and the time coUrse of 40 years. His opus includes gas, president of the Papal Academy that our branch separated from the rest articles on the structural geology of of Sciences! In search of subjects that of the tree. It is these questions that Jersey, Somalia, Ethiopia and China; on would have both current interest and the present article attempts to answer. the Paleocene and Eocene mammals of important philosophical implications, When, where and why did the branch Europe; on the Tertiary and Quaternary he had considered what I had said and that led to us, the genus Homo, diverge mammals of the Far East; on the fossil had organized, under the aegis of his from the branch that led to ouT dosest men of China and Java; on the south­ institution, a confrontation between pa­ cousin, the genus Pan, or the chimpan­ ern African australopithecines (a kind leontologists and biochemists. zee? Because this parting of the ways of prehuman, one that was already That meeting did take place and, al­ seems to unfold several million years hominid, but not yet Homo); as well as though discreet, its influence on scien­ before Homo, properly speaking, was on the Paleolithic and Neolithic tools of tific thought was considerable. Two sig­ born, the issue of our precise origin also all those countries. nificant facts, one paleontological and needs to be addressed. When, where and A member of the audience, whom I one biochemical, were presented to the why did Homo appear in the bosom of did not know at the time, came up to participants. The first was the announce­ a family, Hominidae, that was well plant­ me after my talk and congratulated me ment by David Pilbeam, professor of ed in its ecosystem and well adapted to very courteously, admitting that he had paleontology at Harvard University, that its environment? not known about this technical aspect his research group had discovered, in I first realized in 1981 that it might of Father Teilhard's work. He asked me the Upper Miocene levels of the Potwar be possible to find answers to these several questions about this science of Plateau in Pakistan, the first known face questions. The occasion was an inter­ evolution that I practiced and about its of a ramapithecid. This face resembles national conference in Paris organized state of development. My visitor ended an orangutan's much more closely than by UNESCO to celebrate the 100-year this short interview with a precise ques­ it does a chimpanzee's face. Pilbeam's anniversary of the birth of Pierre Teil- tion: Is there at present an important data were particularly important because issue that is still being debated in your the ramapithecids had for many years field? been considered by some paleoanthro­ Yes, I responded, there is a problem pologists to be the first members of YVES COPPENS specializes in the study of chronology, as is often the case in the human family. of human evolution and prehistory. He The second fact presented was a received his degrees from the Sorbonne, historical sciences. Biochemists, struck where he studied vertebrate and human by the great molecular proximity be­ statement by Jerold M. Lowenstein of paleontology. A member of many orga­ tween humans and chimpanzees, place the University of California at San Fran­ nizations, including the French Acade­ the beginning of the divergence of these cisco that active proteins had been dis­ my of Sciences and the National Acade­ two groups some Jhree million years ;Co,,-er.e(LinLh~deDJaLriiJ:ii~rial of a ra­ my of Medicine, Coppens is currently ago. This discipline also assignsastrict­ .mapithecid. He had determined that chair of paleoanthropology and prehis­ ly African origin to humanity. In con­ activity by injecting extract from, the tory at the College of France in Paris. He trast, the field of paleontology describes ramapithecid teeth into a rabbit, where is also known for having done 20 years of extensive fieldwork in Africa, particu­ a divergence that dates as far back as it brought on the fOITnation m antibod­ larly in Chad and Ethiopia. 15 million years ago. Paleontologists ies. Lowenstein then told us of the in­ also postulate a broad origin, that is, disputable reaction ofthese antibodies

88 SCIENTIFIC AMERICAN May 1994 RIFf V AU.EY cuts across eastern Africa from north to south, of ours into two groups. The western party thrived in forests created by tectonic forces eight million years ago. The cbang­ and became our closest cousins, the chimpanzees. The east­ ing landscape and mountain boundaries divided an ancestor ern population evolved on the savanna and became human. to the antigens of orangutans. This attributes, biochemists examine molec­ Indeed, it made complete sense, as so strong reaction made it dear that some ular details. They look at DNA, at the often happens when one has found the of the ramapithedd proteins were still proteins and cl!romosomal maps of cur­ solution to a problem. The origin of hu­ preserved and that the creature seemed rent species- elements that are not usu­ manity, as the molecular biologists had related to orangutans. ally conserved in fossils. Their work suspected, appeared to be Africa, and Before the discovery of the ramapith­ helps paleontologists, who can then ar­ Africa alone. The question of our fami­ edd face, scientists had procured only range species in order of complexity ly's place of birth seemed settled. some of this genus's teeth and jaw frag­ and compare their protein maps. The But the question of the date of this ments. Although these features were progression from simple to complex birth remained to be addressed. Sever­ certainly interesting, it is necessary to and the sequence that emerges repro­ al paleontologists present at this con­ know that all the bones of a skeleton duces, in some fashion, the evolution gress continued to defend the great an­ do not carry information of equal value. of creatures in the fossil record. In the tiquity of the hOminids, whereas the These pieces were less significant than case of the ramapithecid, however, bio­ molecular biologists extolled the extra­ the orbit area and the nose and upper chemistry had made, as never before, a ordinary brevity of the independent jaw region found in the new Pakistani foray back in time by examining fossil part of our branch. The most generous piece. Paleontologists use such fadal proteins. of the paleontologists had arrived in fossils to draw anatomical comparisons Circumstances had come together in Rome convinced of the IS-miIlion-year with similar or contemporary fossils. A such a way that we could finally put history of our family. The most extreme simple comparison of the face of this the ranJapithedd in his place. This hom­ of the molecular biologists were sure ramapithedd, an orangutan and a chim­ inoid had been known to be Eurasiatic, that three million years, at most, would panzee clearly revealed the similarities and he remained so. Now that his rela­ measure the length of existence of the between the ramapithedd and the tionship to the great ape of ASia, the human family. Both sides canle to the orangutan. orangutan, had been brought to light, conclusion-made, of course, with only Rather than comparing anatomical the geographic picture became clear. the most serious considerations possi-

SCIENTIFIC AMERICAN May 1994 89 Common ancestor of Pan and Homo Deinotherium Gomphotherium Hipparion The Omo River Sequence LATE MIOCENE AROUND EIGHT MILLION YEARS AG I) ble-that seven and a half million years such a way before because it takes time ecus-near Lake Garusi, an area also was a good span. I dubbed this conclu· to study and identify fossils. Its impli· called Laetoli, in Tanzania. In 1955 an· sion "the prehistoric compromise." cations were vast, particularly when other Olduvai expedition led by Leakey The two paleontological and bio­ coupled with the information from the revealed a single australopithedne chemical announcements of the Rome ramapithecid and the newfound con­ tooth. These modest discoveries, how­ meeting were not the only crudal items sensus on dates. ever, did not command much interest. that came to light in the early 1980s. The entry of paleoanthropologists into Another set of results further clarified eastern Africa was actually an ancient t was not until the 1960s that the our understanding of human origins. affair. In 1935 Lows Leakey's expedi­ world eagerly turned its attention Twenty years of excavations in eastern tion to Olduvai Gorge in Tanzania dis­ I to eastern Africa. In 1959 Mary lea­ Africa (between 1960 and 1980) had covered remains attributed to Homo key found at Olduvai an australopithe­ finally yielded a mass of information in erecrus. In 1939 the German team of cine skull eqwpped with all its upper which could be sought evolutionary se­ Ludwig Kohl-Larsen found fossils that teeth. This skull could be absolutely quences and patterns. This extensive were named Praeanthropus africanus­ dated to about two million years ago material had not been looked at in later considered to be Australopith· by the volcanic tuff below which it had been enveloped. The new hominid was named Zinjanthropus; it was a small· brained bipedal hominid spedes that went extinct about one million years ago. After that significant finding, expe· ditions started to arrive in abundance: a new team came each year for the first 12 years, and each one excavated for 10 or 20 seasons. Never before had such an effort been deployed by paleontolo­ gists or paleoantbropologists. The results reflected the investment. Hundreds of thousands of fossils were discovered, of which about 2,000 were hominid remains. Yet, despite the con­ stant work of preparation, analysis and identification of these fossils as they were unearthed, it is understandable that it was not until the 1980s that the first complete inventory of these thou­ sands of finds was published. It is pre­ cisely this new information that, when added to the data received at the Rome conference, became essential to solving the mystery. What emerged so clearly was that there was absolutely no sign of Pan, or one of its direct ancestors, in eastern Africa during the time of the australo­ VEGETATION AND CUMATE vary dramatically on either side of the Rift Valley: pithednes. Molecular biology, biochem­ wet western woods (dark green) give way to eastern grasslands (yellow). Reflect­ istry and cytogenetics continued to ing these ecological differences, which arose millions of years ago, chimpanzees demonstrate that humans and chimpan­ are distributed only to the west (stippling), whereas hominid fossils are found only zees were molecularly extremely close, to the east (cross-hatching). which meant, in evolutionary terms,

90 SCIENTIFIC AMERICAN May 1994 Gazella Giraffa Hippopotamidae Australopithecus Deinotherium Ceratotherium Hyaenidae Nyanzachoerus Hipparion Machairodonlinae

APOUND SIX MILLION YEARS AGO LOWER LOTHAGAMIAN (LOWER PUOCENE) FIVE MILLION YEARS AGO

that they had shared a common ances­ devoted to chimpanzees and gorillas biogeographical province in which the tor not very far back in time. geologi­ showed a significant group of territo­ common ancestors of the future Ho­ cally speaking. And field-workers had ries. including all the large forested re­ minidae and Panidae lived. Then. about just revealed that Hominidae. as of sev­ gions of tropical Africa. but stopped. eight million years ago, a tectonic crisis en or eight million years ago, were pres­ almost without overflow. at the great arose that entalled two distinct move­ ent in Ethiopia. Kenya and Tanzania. furrow that cuts perpendicuiarly across ments: sinking produced the Rift Val­ But during the same period. this region the equator from north to south: the ley. and rising gave birth to the line of had not seen the least sign of the fami­ Rift Valley. All the hominid sites that peaks forming the western rim of the ly Panidae. no precursor of the chim­ dated to more than three million years valley. panzee and no precursor of the gorilla. ago were fmUld, without exception, on The breach and the barrier obviously Even though one cannot base a hypoth­ the eastern side of this furrow. Only disturbed the circulation of air. The air esis on a lack of evidence. the striking one solution could explain how. at one masses of the west maintained. thanks absence of these Panidae where Hom­ and the same time. Hominidae and Pan­ to the Atlantic. a generous amount of inidae were abundant represented a idae were close in molecular terms but precipitation. Those of the east, com­ sufficient contrast to cause concem­ never side by side in the fossil record. ing into collision with the barrier of the all the more so because the 200.000 to Hominidae and Panidae had never been western rim of the Tibetan plateau, 250.000 vertebrate fossils that had together. which also was rising. became orga­ been collected constituted a statistical I therefore suggested the follOWing nized into a seasonal system, today base with a certain authority. model. Before Hominidae and Panidae called the Jllonsoon. Thus. the original I had been thinking about this puz­ had separated. the Rift Valley did not extensive region was divided into two, zle during the conference in Rome. A constitute an irregularity sufficient to each possessed of a different climate quite simple explanation came to mind divide equatorial Africa. From the At­ and vegetation. The west remained hu­ when I opened an atlas marking the lantic to the Indian Ocean. the African mid; the east became ever less so. The distribution of vertebrates. The map continent constituted one homogeneous west kept its forests and its woodlands;

COMPARISON OF 1HREE HOMINOID SKUllS illustrates the the African apes. the chimpanzee (right). Indeed. this very proximity between two of the creatures. The ramapithecid comparison led paleontologists to reject the Eurasiatic rama­ (center) found in Pakistan resembles the great ape of Asia, pithecids as close ancestors of humans and to focus on an the orangutan (left), much more closely than it does one of African origin.

SCIENTIFIC AMERICAN U 1994 91 . Crocodilus Australopilhecus Ceratotherium Giraffa Hyaena Hippotraginae Enhydriodon Machairodontinae Lepus Nyanzachoerus Gomphotheriidae

FIVE MILLION YEARS AGO UPPER LOTHAGAMIAN (LOWER PLIOCENE) 3.5 MILLION YEARS AGO

the east evolved into open savanna. date and in one way or another was then continues from about three mil­ By force of circumstance, the popula­ familiar with the event or its conse­ lion years ago to today, with the emer­ tion of the common ancestor of the quences, but no interdisdplinary effort gence of another subfamily, the homi­ Hominidae and the Panidae families had brought them all into a synthesis. nines. The hominines moved extensive­ also found itself divided. A large west­ Adrian Kortlandt, a famous ethologist ly, from eastern Africa across the entire ern population existed, as did a smaller from the University of Amsterdam, had planet. The last of the australopithe­ eastern one. It is extremely tempting to thought about such a possible scenario, cines coexisted for about two million imagine that we have here, quite sim­ but without any paleontological -sup­ years with the first of these hominines, ply, the reason for the divergence. The port, some years before. which have only one genus, Homo. western descendants of these common The hypothesis lacked only a name. The emergence of this hominine sub­ ancestors pursued their adaptation to Three years later I was invited by the family can be seen in a remarkable se­ life in a humid, arboreal milieu: these American Museum of Natural History ries of geologic beds and fossils found are the Panidae. The eastern descen­ in New York City to present the 55th along the banks of the Dmo River in dants of these same common ances­ James Arthur Lecture on the Evolution Ethiopia. And, not surprisingly, because tors, in contrast, invented a completely of the Human Brain. I also assumed a this is the second part of the East Side new repertoire in order to adapt to visiting professorship at the Mount Sinai Story, the role of climate proves to be their new life in an open environment: School of Medicine of the City Uni­ as powerful a force for change three these are the Hominidae. versity of New York. The idea of giving million years ago as it did eight million This uncomplicated model has the this model a title that would be easy to years ago. advantage of explaining why Homini­ remember and that would honor my The Dmo River tale began at the turn dae and Panidae are so close in a ge­ hosts came to me then. I called it the of this century, when a French geograph­ netic sense and yet never together geo­ East Side Story. ic expedi tion proposed to cross Africa graphically. It also has the advantage of It is possible that the East Side Story diagonally, from the Red Sea to the At­ offering, by means of a situation that is has answered the first volley of ques­ lantic. The Viscount du Bourg de Bozas at first tectonic and then ecological, a tions: the when, where and why of our directed the expedition. Having depart­ variant of the situation found on is­ divergence from Panidae. Dur phyletic ed from Djibouti in 1901, the explo­ lands. Compared to complex solutions branch, the one that now bears us, was ration was to end dramatically in the about the movements of Hominidae marked off from the rest of the genea­ death of its leader from malaria on the from the forest to the savanna or about logical tree of living creatures eight mil­ banks of the Congo. The team nonethe­ the movements of Panidae from the sa­ lion years ago in eastern Africa by rea­ less brought back from the journey, vanna to the forest, the Rift Valley the­ son of geograQhic isolation. The need which followed the original itinerary, a ory is quite straightforward. for adaptation tothiileWiUibitat of the fine harvest of fossils. Among the col­ It was only later, when I was reading savanna, one that was drier and more lection was a group of vertebrate re­ the work of geophysidsts, that I learned bare than the preceding one, promoted mains gathered in what was then Abys­ that the activity of the Rift Valley some further.genetic divergence. sinia, on the eastern bank of the lower eight million years ago was well known. valley of the Dmo River. The Dmo lies Reading the studies of paleoclimatolo­ he second series of questions is on the eastern side of the Rift Valley. • gists fortified me with the knowledge more intricate: the when, where Intrigued by this yield, which was ,de­ that the progressive desiccation of east­ Tand why of the appearance of scribed in two or three articles and if ern Africa was also a well-known event, the genus Homo in the family Homini­ Emile Haug's geologic treatise in 1911, whose starting point had been placed dae. The past eight million years dur­ Camille Arambourg decided at the be­ at about eight million years ago. Final­ ing which our branch of the tree has ginning of the 1930s to conduct a new ly, reading the declarations of paleon­ grown have revealed themselves to be expedition. Arambourg, future profes­ tologists further reassured me, because more complex than one might have sor of paleontology at the National they placed the emergence of eastern imagined. The story begins with the di­ Museum of Natural History in Paris, African animal life- a fauna labeled versification of a subfamily, the aus­ reached the Dmo and stayed eight Ethiopian, to which the australopithe­ tralopithecines. These creatures made months in 1932. He returned to Paris cines belong- at about eight or 10 mil­ very modest movements from eastern with four tons of vertebrate fossils. lion years ago. Each disdpline knew this Africa to southern Africa. The story The next major operation-the Dmo

92 SCIENTIFIC AMERICAN May 1994 Ga/ago Loxodonta Austra/opithecus Elephas Giraffa Came/us Suidae Hyaena sregodon Panthera Kobus Diceros

3.'. MILLION YEARS AGO LOWER SHUNGURIAN (UPPER PLIOCENE) 2.5 MILLION YEARS AGe

Research Expedition-was undertaken paleontological community of this clear 3.3 to 2.4 million years ago, that the between 1967 and 1977. It was cata· correlation between the evolution of whole earth cooled and that eastern lyzed, in part, by the bone rush of the the climate and the evolution of the Africa became dry. (Laetoli and Hadar 1960s and 1970s, described earlier, hominines. I did so in a note to the Pro­ were too old, Olduvai was too young which had followed the 1959 find by ceedings of the Academy of Sciences in and East Turkana presented a strati­ Mary Leakey at Olduvai. A series of re­ Paris and in a communication to a graphic gap at that pOint, so they could searchers conducted the to-year Omo congress in London at the Royal Geo­ not offer the same demonstration.) We expedition in stages. In 1967 Aram­ logical Society. The reaction was very know this fact through several other bourg and I worked on the site with skeptical. tests conducted in various regions of Louis and Richard Leakey and Frands Of all the great eastern African pale­ the world. Clark Howell. Between 1968 and 1969 ontological Sites, the strata of Orno were This climatic crisis appears clearly in Richard Leakey left the expedition, and the only ones that could have permit­ the fauna and flora records of the Orno Arambourg, Howell and I continued the ted such observations. This site alone sequence. By indexing, both qualitative­ work. Finally, from 1970 until 1976, offered a continuous sedimentary col­ ly and quantitatively, the animals and Howell and I dug there alone (Aram­ umn that ran from four million years plants gathered in the various levels, bourg died in 1969). ago to one million years ago. It is pre­ we can interpret the differences that From the very first expedition, the cisely between three and two million emerge from these species, with regard stratigraphy of this site was eminently years ago, or to be very exact between to changes in the environment. visible, a superb column more than 1,000 meters deep. The fauna contained in these beds appeared to change so markedly as it progressed from base to summit that the site was obviously ca­ pable, even at mere glance, of telling a story. When dating by potassium-argon and by paleomagnetism finally became available, so that a chronological grid could be placed on this sequence, the history became clear. Starting four million years ago (the age of the oldest Omo level, the Mursi formation) and ending one million years ago (the age of the most recent level, the top of the Shungura formation), the climate had clearly changed from hu­ mid to distinctly less humid. As a con­ sequence, the vegetation had evolved from plants adapted to humidity to those capable of thriving in a drier cli­ mate. The fauna had also changed from one suited to a brushwood assemblage to one characteristic of a grassy savan­ na. And the Hominidae, subject like the other vertebrates to these climate fluc­ tuations, had changed from so-called gracile australopithecines to robust MARY AND LOUIS LEAKEY examine the Zinjanthropus skull and upper jaw at australopithecines and, ultimately, to Olduvai Gorge in Tanzania in 1959. Their discovery of a hominid fossil at this site humans. led to a bone rush: paleontologists flooded in, and hundreds of thousands of fos­ In 1975 I informed the international sils were excavated in subsequent decades.

SCIENTIFIC AMERICAN May 1994 93 Panthers Deinotherium Damaliscus Diceros Girsffa Dinofelis Australopithec L',<; Loxodonta Homotherium Lepus Phaoochoerus Hipparion Equus Hyae:IB

2.5 MILLION YEARS AGO UPPER SHUNGURIAN (UPPER PUOCENE) 1.8 MILLION YEARS AGO

We know, for example, that the cheek herbivores becomes more digitigrade priate caution, of course, a method called teeth- that is, the premolars and mo­ in open habitats in which they are more actualist; in other words, we believe lars- of herbivore vertebrates have a vulnerable: one runs better on tiptoe that the varieties of animals or plants tendency to develop and become more than in boots. A certain number of ana· we are considering acted then as they complex when the diet becomes more tomical features corresponding to very act today. grassy and less leafy. nus change takes precise functions can also be good in­ Many examples demonstrate this tran­ place because grass wears down the dicators: the tree-dwelling feet of some sition to a drier environment, and they teeth more than leaves do. We know rodents or the feet of others that are are extraordinary in their agreement. also that the locomotion of these same adapted to digging. We use, with appro- As one moves from the older strata on

HUMAN UNEAGE begins about eight million years ago, with the diver­ gence, from a shared ancestor. of the australopithecines and the chim­ panzees. The australopithecines are a complex grouP. and paleoanthro­ pologists continue to debate the classification of its members. To dis­ tinguish those in this family tree, the unorthodox term ·pre· Australopithecus" describes the more archaic hominids, the prebumans that came next are called Ausrralopithecus and the robust forms of these later species are called Paranthropus.

COMMON ANCESTOR

Homo habilis

LATE MIOCENE PL!OCENE PLEISTOCENE 8 7 6 5 4 3 2 o MILLIONS OF YEARS AGO

94 SCIENTIFIC AMERICAN May 1994 Girsffa Came/us Equus Deinotherium Elephas Panthers parous Ga/ago Oama/iscus Phacochoerus Panthera leo Kobus Lepus Homo Ceratotherium

1 3 MILLION YEARS AGO PLEISTOCENE ONE MILLION YEARS A

the bottom to the younger strata on lens from species that grow in humid fossil skulls, done by Ralph L Holloway the top, there is an increase in the hypo conditions are abundant-they include of Columbia University. At the same sodonty-that is, in a tooth's height·to· Celtis, Acalypha, Olea and Typha. In time, the changes caused Hominidae to width ratio-among Elephantidae (ele· the more recent strata, however, these retain an upright stance as the most phants close to the ones living in Asia pollens diminish conSiderably or even advantageous and to diversify the diet today), Rhinocerotidae (specifically the disappear from the record, whereas pol· whUe keeping it essentially vegetarian. white rhinoceros), Hipparion (ancestors lens from Myrica, a plant typical of dry The second adaptation led in two di· of the horse), Hippopotamidae (precur· climates, appear. The number of pollens rections: a strong physique and a nar· sors of the hippopotamus) and some transported by the wind, called alloch· row, specialized vegetarian diet for the pigs and antelopes. In other words, tone pollens, dwindles from 21 percent large australopithecines and a large brain these groups exhibited the increasing at the bottom, where the forest edge is and a broad·ranging, opportunistic diet complexity that we assodate with a shift near the Omo River, to 2 percent at the for humans. from a diet of leaves to a diet of grass. top, where the Omo was low and the Some hundreds of thousands of years The Suidae, or precursors to s\vine, also forest edge far away. later, it was the latter development that show an increase in the number of The story with the hominids is simi· proved to be the more fruitful, and it is cusps on their molars as they evolved. lar. They are clearly represented by Aus· this one that prevailed. With a larger On the lower strata are many an· tralopithecus afarensis on the lower stra· brain came a higher degree of reflection, telopes-including Tragelapbinae and tao But the younger strata on the top reo a new curiOSity. Accompanying the ne· Reduncinae, which live among shrubs. veal A. aethiopicus, A. boisei and Homo cessity of catching meat came greater All these creatures must have lived in habilis. The oldest spedes of australo­ mobility. For the first time in the histo· an environment of wooded savanna pithecines, the graciles, are more en· ry of the hominids, humanity spread close to water. On the top levels the sconced in tree· filled habitats than are out from its origin. And this mobility is true horse, Equus, appears, as do the the more recent species, those called the reason that in less than three mil· high· toothed warthogs, Phacochoerus robust. As for humans, we are unques· lion years, humanity bas conquered and Stylochoerus. We also see the de· tionably a pure product of a certain this planet and begun the exploration velopment of the swift antelopes, Meg· aridity. of other worlds in the solar system. alotragus, Bealragus and Parmularius, I called this climatic crisis "the animals found on open grasslands. (H)Omo event" using the simple play On the bottom, three species of small on words of Omo and Homo, because it FURTIlER READING Galago, or monkey, and the two Chirop· permitted the emergence of humans­ EvOLt.mON DES HOMINIDEs ET DE LEuR tera, Eidolon and Taphozous, indicate a an event that affects us quite specifical· ENvIRONNEMENT AU COURS DU PuO· well·developed forest and a dense sa· ly-and because it was the Omo se· PliISTOCOO DANS LA BASSE vAlliE DE vanna. This conclusion is supported by quence that revealed it for the first L'QMO EN ETHloPlE. Yves Coppens in the large number of Muridae rodents, time. Some years later the same data Comptes Rendus Hebdomadaires des such as Mastomys, as well as the roo were reported from South Africa. Seances de "Academie des Sciences, Vol. dents Grammomys, Paraxerus, Thryon· 281, Series D, pages 1693-1696; De· omys and Golunda. At the top, the roo hus, it appears strikingly clear cember 3, 1975. EARliEST MAN AND ENvIRoNMfNfs IN dents Aethornys, Thal/omys, Coleura that the history of the human TIlE LAKE RUDOLF BASIN: STRATIGRA­ and Gerbil/urus in conjunction with Jac· family, like that of any other fam· T PHY, PALEOECOLOGY AND EvOLUTION. ulus and Heterocephalus, the Chirop· ily of vertebrates, was born from one Edited by Yves Coppens, F. Clark How· tera, and the Lepus, or hare, replace the event, as it happens a tectonic one, and ell, Glynn U. Isaac and Richard E. F. previous inhabitants. All the later roo progressed under the pressure of an· Leakey. University of Chicago Press, dents inhabit dry savanna. other event, this one..clirnatic. 1976. Pollen specimens on the bottom indio These changes can be but quickly REcrm AnvANCES IN TIlE EvOLUTION OF PRiMATES. Edited by Carlos Chagas. cate 24 taxa of trees, whereas the top is summarized here. Essentially, the first Pontifida Academia Scientlarum, 1983. characterized by 1 L At the bottom, the adaptation changed the structure of L'ENvIRONNEMENT DES HOMINIDEs AU ratio of pollens from trees to pollens the brain Iiut did not increase its vol· PuO·PIiISTOCENE. Edited by Fondatlon from grasses equals 0.4. But at the top, ume, as suggested by the interpreta· Singer·Polignac. Masson, Parts, 1985. it is less than 0.01. At the bottom, pol· tion of endocasts, latex rubber casts of

SCIENTIFIC AMERICAN May 1994 9S