Geologic Background to Paleoanthropological Research in the Ethiopian Rift System

WIC Dec 22, 2000

INTRODUCTION

Based on current scientific evidence, East Africa is the cradle of mankind.  The geological history of Northeast Africa is briefly highlighted, in order to understand why the rift valley formed there and why it created the most conducive environment for the proliferation of life, including the origin and evolution of our ancestors in the region. 

The African continent drifted northward while rotating slightly counter clockwise from 74 to 46 million years ago.  At about 50 million years ago, most of the land surface in Northeast of Africa was above sea level and the ocean was slowly moving away to the east and north.  Ancestral Indian Ocean covered the eastern half of Saudi Arabia, Yemen, eastern Ethiopia, Djibouti, and northern Somalia.  During this time, Saudi Arabia and Yemen were still part of Africa.  At about 46 million years ago, the equator passed through central Ethiopia just south of Addis Ababa, northern Somalia, and southern Sudan.  Today, Ethiopia is located north of the Equator between 3 and 15 degrees latitudes. 

Major volcanic eruptions started in southern and southwestern Ethiopia at about 50 million years ago, thus causing the Ethiopian land surface to rise, and the sea to move eastward.  The volcanic eruption that started in the vicinity of Arba Minch and the Akobo River basin in southern and southwestern Ethiopia, respectively, continued to ravage the rest of Ethiopia.  By about 29 million years ago, most of Ethiopia and Yemen were covered by about 1 million cubic kilometer of volcanic material, thereby forming the Ethiopian and Yemen Plateau.  Following widespread volcanism and uplift in Ethiopia, the region presently occupied by Kenya and Tanzania was subjected to similar geologic process of voluminous eruptions and mountain building.  The continuous migration and flow of voluminous molten rocks from the interior weakened the upper part of the earth in East Africa, heralding the birth of the Red Sea, the Gulf of Aden, and the Great East African Rift Valleys.  At about 24 million years ago, the ancestral Red Sea rift valley was formed above sea level, whereas the Gulf of Aden Rift Valley was mostly under water.  Moreover, the northern and east-central parts of the Afar Depression were also fully developed by about this time.  By about 15 million years ago, both the Red Sea and the Gulf of Aden Rift Valleys were under water, and the East African Rift Valley, which forms the third arm was fully developed.  Thus, the NE-SW-trending broad Ethiopian Rift Valley separated the Ethiopian Plateau, which formed as a result of voluminous volcanic eruption, spanning millions of years into the Northwestern and Southeastern Ethiopian Plateau.  After the formation of the rift valley, most of the volcanic activity was confined to the valley floor and the adjacent bounding highlands.  Today, the Ethiopian Rift valley consists of the Afar Depression, which contains the active Erta Ale volcano, the Main Ethiopian Rift, which runs from the vicinity of Awash-Arba to Arba Minch-Konso, the Chow Bahir or Weyto Basin, and the broad Omo River Basin.  The simplified and brief narrative described above highlights the complex processes that led to the present topographic profiles of Ethiopia and the surrounding regions. 

The widespread occurrences of paleoanthropological records related to human evolution and culture in Ethiopia, Kenya, Tanzania, and Malawi owe their existence to the formation of the Great East African Rift Valley, a major and unique continental fracture system on the surface of the earth.  Within Africa, the rift valley stretches from the Red Sea coast to Mozambique for more than 3000 km, and averages about 50 to 60 km in width.  Almost all of the known paleoanthropological sites that have given us information about our ancestors dating back to about 5 million years ago are located within the floor of the East African Rift System.  This is not a coincidence; rather the rift basins created an ideal environment for the proliferation of animal and plant life.  Abundant freshwater rivers and lakes, open grasslands and woodland forest habitats, and moderate climates regulated and protected by mountain ranges on both sides of the rift floor sustained dynamic ecosystems for millions of years.  Major rivers drained from the adjacent highlands into the rift valley floor, forming inland lakes and swamps.  Sediments, dead animals, and plants were carried by these floods and buried in water-saturated sediments within the rift valley.  Sediments eroded from the plateau and ashes and lavas from volcanoes within the rift floor were responsible for the quick burial and preservation of dead animals and plants. 

Volcanic products interbedded with the fossil-bearing sediments provided age constraints for the geologic processes and fossil remains, including hominid origins and evolution, their tools, and diet.  Such remains have no chance of being preserved on the plateau because of constant erosion.  However, the landscape in the rift floor is unstable and constantly modified because of earth movements. For example, low-lying areas, which received fossil-rich sediments for millions of years are suddenly pushed up above the lowland environment and become source of sediments themselves.  Another cycle is initiated whereby erosion begins and exposes fossil remains contained in the uplifted land surface.  As a result, the East African Rift System provides unique setting for paleontological and archaeological investigations of human origins and evolution.  Skeletal and cultural remains of hominids have been recovered from many locations within the basins of the East African Rift Systems in Ethiopia, Kenya, Tanzania, Uganda, and Malawi.  Today, the paloanthropological and archaeological sites in Ethiopia have provided the oldest and most diverse hominid remains and artifacts compared with any other place on earth. 

Important paleoanthropological and archaeological sites ranging in age from about 5 million years to the present occur within the Ethiopian Rift System in the Omo Basin (Shungura, Usno, and Fejej), the Main Ethiopian Rift (Konso-Gardula, Bilate River valley, Gademotta, Gadeb, Melka Kunturè, Kesem-Kebena, and Chorora), and the Afar Depression (Mulu Basin, Middle Awash, Gona, and Hadar).  Most of these localities occur within the rift floor.  Gadeb and Melka Kunturè are located along the eastern and western rift margins of the central part of the Main Ethiopian Rift, respectively.  The Paleoanthroplogy Inventory Project of Ethiopia discovered some of Ethiopia's most important paleontological and archaeological localities such as Fejej, Burji, Konso-Gardula, Bilate, and Kesem-Kebena in the late 1980s.  This project was initiated to survey vast, previously unknown areas of the rift basins and inventory their paleoanthropological resources.  The survey indicated the tremendous fossil potential of the rocks exposed in the Ethiopian Rift System.  The remarkable preservation of animal and plant remains was possible because of quick burial by sediments and volcanic products.  These sediments washed from the volcanic highlands on both sides of the rift floor provided the necessary chemical components for the preservation of fossils during burial.  There is a strong link between rapid fossil-rich sediment accumulation and chemical process during burial, and fossil preservation. 

Location map of paleoanthropological sites in the Ethiopian Rift System

The interplay between depositional and erosional processes driven by volcanic eruption, uplift, and subsidence of the rift floor created numerous gaps in the fossil record, particularly in the important time period between 10 and 5 million years, which is pertinent to the understanding of the origin of our species.  Ethiopia holds the distinction by having the most complete and uninterrupted rock accumulation within the rift floor compared with other countries traversed by the East African Rift System.  As a result, the paloanthropological and archaeological sites in Ethiopia have provided the oldest and most diverse hominid remains and artifacts, spanning millions of years. 

The geological background of the most important paleoanthropological and archaeological localities in the Ethiopian Rift System is briefly highlighted, beginning with the broad Omo River Basin of southwestern Ethiopia.  The brief narrative is intended to demonstrate the role of rift basins in the accumulation of fossil-bearing sediments, burial and preservation of fossils, uplift and subsidence of basin floor, and exposure of fossils by erosion.  Systematic exploration within the valley floor leads to discovery and collection of diagnostic fossils, which are taken to the National Museum in Addis Ababa for cleaning, description, and curation.   

The Omo Basin

 The north-south-trending Omo-Turkana Basin in southwestern Ethiopia and northern Kenya has provided an unparalleled record of hominid and archaeological remains, spanning the last 4 million years.  The Omo-Turkana basin began to subside at about 15 million years ago and was fully developed by about 7 million years ago.  The Omo Basin in southwestern Ethiopia is filled with river and lake sediments carried from the Ethiopian highlands by major rivers ancestral to the Omo River.  The fossil-bearing sediments occur on both sides of the border in Ethiopia (Mursi, Shungura, Kibish, and Usno areas) and Kenya (Koobi Fora and Nachukui areas).  The Shungura Formation of the Omo Group in Ethiopia is about 760 m thick and occurs north of the modern Omo River delta.  This sequence contains abundant volcanic ash layers, which are used to determine the age of the geological processes and the fossils contained in the rock sequence.  Based on the fossil record, the Omo Basin contained animals that evolved from closed woodland to more open grassland environment over the last 4 million years.

Panoramic view of the Fejej plain along the eastern part of the Omo River delta.  The view is looking west from within the rift valley toward the western highlands.  The mountain ranges are oriented north south and mark the western boundary of the Omo Rift valley (Photo, Giday WoldeGabriel @1989).

About 25-30 km north and upstream from the Omo delta, fossil-bearing sediments of the Usno Formation are about 172 m thick, and are similar to the Shungura Formation rocks.  Early hominids (4 million-years old or younger) and archaeological (less than 2 million years old) remains have been collected from the fossil-bearing sediments of the Shungura, Usno, and Fejej localities in the lower Omo Basin. The ages of these paleoanthropological remains were established by dating and by chemical correlations of volcanic ash and lavas interbedded with the fossil-rich rocks.  The discoveries of hominid remains within the sedimentary sequence of the Omo-Turkana Basin, spanning the last 4 million years suggest that volcanic eruptions, uplift and subsidence of the rift floor, and flooding had minimal impact on the long-term habitation of hominids and other animals in the region.  The animals and plants were able to adapt to the changing environment with time.  

Southern part of the Main Ethiopian Rift

 In the vicinity of Arba Minch, the southern part of the Main Ethiopian Rift is divided into the Ganjuli and the Galana valleys by the Amaro Mountain range.  Rock successions along the floors of both branches are poorly exposed because of recent sediment accumulations, lakes, and vegetation cover.  Fieldwork in 1989 at the Burji locality, at the southern end of the Amaro Mountains discovered fossil-bearing deposits beneath volcanic rocks.  A primitive species of mastodon (elephant-like) that was also identified in places as far north as Turkey and plant remains were discovered within the rock sequence.  The biological evidences suggested the age of these fossils and for the formation of basins in the southern part of the Main Ethiopian Rift to be 17 to15 million years .

 A new locality with important hominid and archaeological remains was also discovered in the KonsoGardula region, southwest of Arba Minch.  Unlike the Omo Basin, this fossil locality is small and occurs in the vicinity of Konso town, at the southern end of the Main Ethiopian Rift.  The fossil and artifact bearing rocks are about 50 m thick and range in age between 1 and 2 million years.  Geologic and archaeological evidence suggest hominid habitation along a lake margin.  However, rising and tilting of the land surface and subsidence along the Ganjuli basin floor during the last 2 million years modified the landscape.  As a result, the fossil and artifact bearing rocks of the Konso-Gardula region were exposed by erosion.  A joint Ethiopian and Japanese research team firmly established the first African record of the Acheulean stone tool tradition attributed to Homo erectus, ranging in age between 1.4 and 1.7 million years ago. 

North-south-trending mountains separate the Konso site from the Shungura and Usno Formations of the lower Omo basin and the Chow Bahir rift valley.  However, the southern part of the Main Ethiopian and the Chow Bahir Rift valleys are connected by the Segen River valley, a major tributary of the Weyto River that drains into Chow Bahir, east of the Omo River delta.  No detailed or comprehensive paleoanthropological survey has ever been conducted to determine evidence of hominid occupation and/or migration from the lower Omo Basin across the Chow Bahir Rift to the Konso area at the southern end of the Main Ethiopian Rift.

Panoramic view of the Chow Bahir (Weyto) Rift valley in southwestern Ethiopia, looking west toward the Hamar Mountains.  The valley is more than 20-km wide.  Widespread fossil-rich sediments occur in the rift floor, but access is limited because of dense vegetation (Photo, Giday WoldeGabriel @1997).

Northward along the rift floor and east of Sodo, the Bilate River and its tributaries expose young (less than 1 million-year old) sediments interbedded with volcanic rocks.  These volcanic rocks erupted from the Duguna and Corbetti volcanic centers east of Sodo and north of Lake Awasa, respectively.  A preliminary survey conducted in the area by the Paleanthropological Inventory of Ethiopia in 1989 indicated widespread fossils and stone-tool production localities. 

 Stone tools manufacturing quarry in the Bilate River valley a couple of kilometers east of the small town of Bedessa along the road from Boditti to Yirgalem in Sidamo region, south-central Ethiopia.  The obsidian (black rock fragments) are interbedded within a thick soil horizon.  Downstream from this outcrop, the red soil with obsidian fragments is covered by basaltic lava flows.  Widespread obsidian rocks are present at the southern foothills of the Duguna Mountain, located five to ten kilometers to the right of this picture (Photo, Giday WoldeGabriel @1989).

Central part of the Main Ethiopian Rift

 Unlike the Omo, Konso-Gardula, and Bilate Basins, geologic sections in the basin floor of the central part of the Main Ethiopian Rift between Awasa and Nazreth are poorly exposed.  Although the rift valley was formed between 10 to 15 million years ago, the rift floor is covered by lakes, younger sediments washed from the adjacent mountains bounding the rift valley, and volcanic rocks from the Aluto, Shalla, Gadamota, and other smaller volcanic centers that span in age from about 20,000 years to1 million years ago.  Few, mostly relatively young paleoanthropological sites have been identified and studied within this part of the rift valley.  Vertebrate animal remains are rare in these localities; this may simply be related to poor exposure of fossil bearing deposits.  Alternatively, an environment that was dominated by deep lakes that formed in actively subsiding basins probably did not support abundant life because of inaccessibility.

Panoramic view of the Butajira area east of Lake Ziway in central Ethiopia about 160 km southwest of Addis Ababa. The view is from the top of the western rift margin north of the town of Butajira.  The rift floor is covered by lava flows from linear volcanic centers shown on the left side of the picture (Photo, Giday WoldeGabriel @1984).

A Middle Stone Age (35,000 to >180,000 years) archaeological site was discovered in the vicinity of the Gademotta volcanic center, west of Lake Ziway.  The Middle Stone Age sites occur within old soil horizons.  Water-level fluctuations of ancestral Lake Ziway appear to have greatly influenced human habitation in the area during the last 1 million years.  However, the continuous occurrences of artifacts in old soil horizons suggest that some of the sites remained above water, and habitation was not disrupted by volcanic ashes and lava flows from nearby volcanoes and rising lake levels. 

 Other contemporaneous paleoanthropological localities were discovered in the Gadeb and Melka Kunturè areas along the eastern and western boundaries of the central part of the Main Ethiopian Rift valley, respectively.  The availability of water appears to have been a major factor in the occupation of these sites more than a million years ago.  Both of these sites are outside the rift floor, unlike most of the important archaeological sites in Ethiopia.  It is suggested that widespread volcanic eruptions and high lake-level stands in the adjacent rift floor during the last 1.5 million years appears to have made the rift floor less hospitable, and the occupation of sites outside the rift floor possible.   

 The rift floor between the Gademotta Middle Stone Age locality near the town of Ziway and the Kesem-Kebena Acheulian sites, north of the Awash National Park in the northern part of the Main Ethiopian Rift also contains less than 1 million-year old volcanic centers.  Except for the Melka Kunturè Paleolithic site at the headwaters of the Awash River along the western rift margin southwest of Addis Ababa, no other paleoanthropological site has been reported from this part of the rift floor.  The 1.5 million-year old Melka Kunturè Paleolithic site is in an erosional valley cut by the modern Awash River.  It contains evidence for hominid occupation and vertebrate remains in river sediments.

Northern part of the Main Ethiopian Rift

 The Main Ethiopian Rift broadens northward into the Afar Depression.  Several new localities with vertebrate fossils and Acheulian and Late Stone Age artifacts were discovered in the Kesem-Kebena area along the foothills of the western rift margin by the Paleoanthropological Inventory of Ethiopia in 1989.  The fossil-rich deposits and interbedded volcanic rocks of the Kesem-Kebena region range in age between 1 million to more than 3.7 million years.  These deposits also contain Acheulian stone tools that are about 1 million years old.

Fossil-rich sediments and volcanic ash and lava flows in the Kebena River valley along the western foothills of the western rift margin.  The black layers in the upper part of the photograph are volcanic flows that were displaced by earth movements (fault).  The rock sequence ranges in age between 1 and 4 million years (Photo, Giday WoldeGabriel @ 1989)

Across from the Kesem-Kebena region, widespread fossil-rich sediments occur in the Chorora area southwest of the town of Asbe Teferi along the foothills of the eastern rift margin.  Diverse animal and plant fossils older than 10 million years were identified there.  Moreover, much younger archaeological remains were collected from other fossil-bearing sediments in the area.

The fossil- and stone tool-rich sediments of the Chorora area along the foothills of the eastern rift margin west of the main Harar highway between the towns of Mieso and Asebe Teferi in eastern Ethiopia.  The light-colored patches are altered volcanic rocks (Photo, Tim White @1990).

Southern Afar Depression

 Paleoanthropological studies in the southern Afar Depression (Mulu Basin, Middle Awash, Gona, Hadar, etc.) yielded an unparalleled record of early hominid biology and culture (stone tool technology) compared with any other place on earth, including known sites along the full length of the East African Rift System.  The Mulu Basin is the newest paleoanthropological area discovered in the middle 1990s. A new hominid species that is different from those collected in the Middle Awash and the Hadar regions was discovered and described by an Ethiopian paloanthropologist Yohannes Haile Selassie in the last three years.  Preliminary analysis of volcanic samples interbedded within the fossil-bearing sediments range in age between 3 and 4 million years.  The age of the sequence and access to widespread exposures within the Mulu Basin make the area as one of the potentially productive sites for paleoanthropolgy research in the future.

Fossil-rich sediments in the Galili area of the Mulu Basin, located east of the town of Gedamaitu along the main highway to Gewane in eastern Ethiopia.  The sediments are capped by a 3.4 million years old lava flow.  Ethiopian Paleoanthropologist Yohannes Haile Selassie discovered a new 4 million-year old human ancestor here.

The Middle Awash region, located north of the Mulu Basin contains the most complete geological record of volcanic eruption, basin formation, and fossil and archaeological remains in the southern Afar Rift and beyond. The research is directed by a joint Ethiopian and American scientists. So far, more than 200-m thick fossil-rich sediments and interbedded volcanic rocks that range in age from about 6 million years to the present have been mapped.  The Middle Awash region underwent several cycles of environmental changes during this time.  A major inland lake covered most of the Middle Awash from about 6 to 4.4 million years ago except for brief and widespread volcanic eruption at 5.2 million years ago.  As a result, the land surface was pushed up, and a large part of  the landscape was above water, beginning at about 4.4 million years ago.  Hominids and other land animals roamed the area until about 4 million years ago, when it was submerged again and covered by another inland sea. During this time, violent underwater volcanic eruptions dominated the region for several thousand years. Geologic processes related to volcanic eruptions, subsidence, and uplift of the rift floor dictated these cyclic environmental changes.

Panoramic view of the fossil-rich sediments of the East and West Side of the Middle Awash a few kilometers on both sides of the Awash River (dark band in upper half of photo) north of the town of Gewane in eastern Ethiopia.  The view is looking east from the top of a ridge.  The different rocks range in age from about 5 million years to the present.  The light-colored bands are altered volcanic ashes (Photo, Giday WoldeGabriel @1992).

The fossil-rich rocks of the Middle Awash east of the Awash River yielded A. Afarensis  remains, ranging in age between 3.4 and 3.9 million years.  Other hominid fossils that are about half a million years old were also discovered in the area.  Fossil-rich sediments in Aramis and Bouri areas on the west side of the Awash River yielded new hominid species of the 4.4 million-year old A. ramidus and the 2.5 million-year old A. Garhi, respectively.  Correspondingly, typical Acheulean stone tools were discovered on the East Side, whereas the West Side provided early Acheulean and Middle Stone Age artifacts.  Moreover, the oldest evidence (2.5 million years) of stone-tool usage for skinning, cutting, and crushing of bones and meat was discovered within a couple of hundred meters from the A. Gahri site at Bouri.  More hominid species have been collected from the Middle Awash study area than any other place in the Great East African Rift System, and such discoveries make it the most prolific paleoanthropological and archaeological site in the world.

In the Gona area north of the Middle Awash, younger geologic deposits, containing the oldest stone tools were discovered and described by Ethiopian archaeologist Dr. Sileshi Semaw.  The stone tools dated to about 2.5 million years are made from volcanic rocks, and occur interbedded within fossil-bearing deposits.  So far, no hominid remains have been discovered there in close association with the stone tools.

Archaeological excavation site at Gona west of Hadar.  The oldest stone tools dated between 2.5 and 2.6 million years old were discovered at this area (Photo, Sileshi Semaw @ 1998).

Eastward from Gona, the Hadar paleoanthropological site contains more than 250 m of fossil-rich sediments and interbedded volcanic ash, ranging in age from about 2 to 3.4 million years old.  Major rivers flowing from the highlands to the ancestral Hadar lowlands deposited the fossil-rich rocks.  It has been explored and studied by a team of American researchers for more than 25 years.  The Hadar region is one of the richest fossil sites in the southern Afar Depression.  The remains of dozens of individuals of Australopithecus afarensis including the partial skeleton of the 3.2 million-year old "Lucy" or "Dinknesh" were collected from this area, beginning in the early 1970s.  Over the years, more hominids, and other prolific mammalian fossils and stone tools were collected from the Hadar area.

River and lake sediments in the Hadar area.The famous "Lucy" or "Dinknesh " fossil was discovered this area. Volcanic ash layers are visible as white bands in the rock sequence (Photo Tim White)       

Abundant animal and plant fossil remains, including hominids collected along the full length of the Ethiopian Rift system, suggest that the rift valley environment created productive ecosystems during the last 18 million years.  The volcanic rocks interbedded within the fossil-rich sediments provide temporal framework for calibrating and sequencing of the hominid origins and evolution. Moreover, detailed chemical analysis on volcanic rocks from different fossil localities compliments the dating results.  The laboratory results facilitate correlations among different geologic sections across the landscape, forming the basis for delineating the complex geologic processes responsible for the formation of the Ethiopian Rift System and the environmental conditions that were associated with it. 

The Ethiopian Rift System started more than 25 million years ago.  Most of the rift valleys are filled and covered with sediments brought in by rivers, whereas some of them contain fine-grained sediments that accumulated in lakes.  The fossil record clearly reflects the environment that the animals inhabited millions of years ago.  In sediments that formed in a lake environment, animals like hippos, crocodile, fish, etc., are dominant, whereas those found in river sediments consist of land animals like cattle, horses, monkeys, elephants, etc., including early human ancestors.  Sediments derived from the highlands and uplifted landscape within the rift floor aided in the burial and preservation of dead animals and plants by providing secondary minerals released from altered rock fragments. 

The most important paleoanthropological sites listed above represent only a fraction of the over all land surface of the rift floor of the Ethiopian Rift system.  There is great potential for major discoveries of paleontological and archaeological records at new sites provided detailed and systematic explorations are conducted by experienced and qualified teams of Ethiopian and foreign researchers.  Ethiopia holds the key to the understanding of hominid origins and evolution.

Despite widespread volcanic eruptions within the Ethiopian rift floor for the last 6 million years, evidence on the impact of volcanic eruptions on animals and plants, including the early human ancesstors and the environment is not readily apparent.  However, historical and/or modern analogs illustrate the potential for regional and sometimes global destructive effects of major volcanic eruptions.  Certain areas along the Ethiopian rift system were subjected to such cataclysmic volcanic eruptions in the last one million years.  Because of such extensive geological evidence throughout the Ethiopian Rift System, mitigation based on detailed geological studies of some of the youngest volcanic centers close to major urban centers is adviseable.