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Olorgesailie, Kenya

Fieldwork at Olorgesailie, Kenya, seeks to understand the life and times of early humans in the East African Rift Valley over the past 1 million years.

Olorgesailie: Life and Times of the Handaxe Makers

Map showing the location of Olorgesailie, Kenya. Image courtesy of Karen Carr Studio.

Early humans lived in the Olorgesailie region, in what is now the southern Kenya, between 1.2 million and 490,000 years ago. Excavations at Olorgesailie show the habitats and animals these early humans encountered, the handaxe tools they made, and the climate challenges they met.

Olorgesailie is a sedimentary basin located in southern Kenya in the East African Rift Valley. It has been excavated for many years and contains many artifacts that have accumulated over a long time period. Here we will look at finds unearthed in this region dated between 1.2 million and 490,000 years ago. Faunal remains (the fossil bones of animals) tell us which animal species lived in the Olorgesailie area, while the sediments show the changes that occurred in the environment, such as the expansion or drying up of a lake, the presence of rivers, and the eruption of volcanoes.

A handaxe from Olorgesailie (handaxe is ~14cm tall). Image courtesy of James Di Loreto and Donald H. Hurlbert, Smithsonian Institution.Artifacts have been excavated at Olorgesailie since 1942. Olorgesailie is unusual because of the large number of handaxes found there, along with other bifacial tools (flaked on two sides to create an edge).  Archeologists used to assume that excavated sites provide examples of campsites (home bases) or other living spaces typical of modern human hunter-gatherers. Yet sites are difficult to interpret without looking at the factors that can disturb the pattern of artifacts and bones, moving them, destroying items, or rearranging them to mix layers from different time periods. It is also interesting to consider how very old archeological sites provide evidence of behaviors that aren’t quite like those of contemporary or recent humans who forage for food as a way of life.

The latest fieldwork at Olorgesailie is a collaborative effort of the Smithsonian’s Human Origins Program and the Department of Earth Sciences of the National Museums of Kenya. Led by Dr. Rick Potts, the excavations offer a look at how early hominins used the landscape and how they responded to environmental changes.



The frontal bone, including the brow ridge, of the hominin skull from Olorgesailie (KNM-OG 45500). Image courtesy of Jennifer Clark and Richard Potts, Smithsonian Institution.

Who Lived at Olorgesailie?

For years, we knew about the hominins at Olorgesailie through the artifacts they left. The dates of the sediments made it likely that that hominin species responsible for the tools was Homo erectus. Finally, in 2003, the first hominin fossil of Olorgesailie was found. The fossil consists of a brow ridge and front part of the braincase (the frontal bone), the left temporal bone (ear region), and several other fragments of the braincase. The individual was small, with a braincase size (cranial capacity) of perhaps 700 to 800cc. The fossil is from a layer above (younger than) a volcanic layer dated about 974,000 years old, and is estimated to be about 900,000 years old. The discovery site is about 1.5 km east of a large accumulation of handaxes, which come from the same layer as the fossil.


Dating Olorgesailie

Illustration reconstructing life at Olorgesailie. Image courtesy of Karen Carr Studio.

The age of the sediments at Olorgesailie is figured out by applying the single-crystal argon (Ar) method, which can measure tiny quantities of argon-40 and argon-39 gases (40Ar-30Ar) in volcanic crystals and glass.  The quantities of these gases, trapped inside each volcanic particle, results from the consistent decay of radioactive material over time since a particular volcanic eruption occurred.  Comparing these quantities gives a solid estimate of the amount of time that has passed since the eruption.  Thanks to the efforts of the Berkeley Geochronology Center in California, a long sequence of volcanic eruptions has been dated at Olorgesailie, making this site one of the most precisely dated archeological sites in the world for the past 1 million years.  The Olorgesailie sediments also record times when Earth’s magnetic pole reversed direction, particularly at around 992,000 years ago and 790,000 years ago.  These magnetic signals in the sediments also help date fossils and stone tools found in the Olorgesailie layers. Scientists have come to Olorgesailie to try out or refine a variety of dating techniques, including the uranium-series method, the OSL (optically-stimulated luminescence) method, and TL (thermo-luminescence) method.


Where Hominins Lived...and Died

Though many artifacts and faunal remains have been found in the Olorgesailie basin, hominin remains are rare. Where were the hominins living and why has there been only one hominin specimen found in years of excavations?

The answer might lie in where the hominin fossil was found. It was located in sediments near the top of a volcanic ridge known to the excavation team as Lava Hump. Lava Hump connects the lowland basin (where lakes and streams once formed) with the highlands of Mt. Olorgesailie. Since no hominins have been found in the sediments within the lowlands, Rick Potts and his team think that the hominins were living in the highlands around Mt. Olorgesailie. Hominins may have foraged near the lakesides or by the streams for food, and left many artifacts, but died in other places. Highland areas, such as Mt. Olorgesailie, were sources of stone used for making stone tools.

After decades of finding thousands of animal fossils, the Smithsonian research team began excavating along the main volcanic ridge connecting the lowlands to Mt. Olorgesailie.  Within only a few months, the first Olorgesailie hominin fossil was recovered. The fossil shows the bite marks from carnivore teeth along the brow, and just the bits of the braincase were all that remained. It’s quite possible that the hominin toolmakers almost always reached the refuge of the highland, away from the water’s edge, at night when predators often prey on others near watering holes.


Finding Stone to Make Tools

In the earliest part of the Olorgesailie sequence, between 1.2 million and 900,000 years old, many handaxes have been found within about 5 km of volcanic outcrops in the highlands. These outcrops served as sources of stone. Early hominins used as many as 14 different types of volcanic rock, all of which have been identified in the local area. In fact, the Smithsonian team has excavated an actual quarry site, about 990,000 years old, where early hominins went to strike huge flakes off the volcanic outcrop.  At this site, the hominins tested the stone, and begin to turn the best pieces (ones without mechanical flaws) into handaxes. Many thousands of artifacts were uncovered, mostly leftovers from the toolmaking process, right at the source of the stone.

Two types of rock, quartzite and obsidian, are known to have occurred about 48 km and 18 km away from the basin, respectively. Although only tiny amounts of tools made from these two types of rock have been found at Olorgesailie, they indicate that the hominins moved around over at least these distances. By being mobile, the early humans of Olorgesailie could locate food and water throughout the year, and could survive rarer, more drastic changes in their habitat.


Upper Member 1 paleosol (thick, lower brown layer), which can be traced for several kilometers, provides chemical evidence of a broad grassland 990,000 years ago. Image courtesy of Chip Clark, Smithsonian Institution.

One Environment at a Time

Different habitats occurred at Olorgesailie at different times. A thorough knowledge of environmental change comes from studying one layer at a time. An example comes from that part of the Olorgesailie sequence called the upper Member 1 paleosol – an ancient soil, about 10 to 40 cm thick, from which many stone tools and animal bones have been unearthed in the Smithsonian excavations.  About 990,000 years old, this ancient soil contains marks made where plant roots once grew, footprints that record where hippopotamuses and other animals had walked, and of course the remains of early human activity and the bones of more than 20 different species of large mammals and birds. The paleosol also contains carbonate, which precipitated out from the ground water long ago and provides a record of the chemistry of the plants that originally grew in the soil.

A carbonate layer at Olorgesailie. Image courtesy of Jennifer Clark, Smithsonian Institution.In places nearer the equator, like Olorgesailie, grasses have one pathway of photosynthesis that involves 4 carbon atoms, while woody plants (trees and bushes) have a different pathway that involves 3 carbon atoms.  This sets up the distinction between C4 plants (grasses) and C3 plants (trees and bushes). By testing the carbon isotopes in the upper Member 1 paleosol, we learned that most (75 to 100%) of the vegetation 990,000 years ago consisted of C4 plants. This means a landscape that was covered almost entirely with grass, and with some trees. This information makes sense of the fossil animals whose bones were unearthed from the same soil, almost all of which were from grass-eating species whose teeth had become worn down bythe tough silica particles (phytoliths) found in grass.

Traces of roots in a layer of sediment give clues about the vegetation – in this case the wide, vertical rootmarks of reeds and other wetlands vegetation. Image courtesy of Richard Potts, Smithsonian Institution.

In later times at Olorgesailie, C4 grasslands continued to be prominent in the region. Yet the range of habitats between 800,000 and 500,000 years ago included wooded grasslands, where tree cover was greater, and grassy woodlands in which trees were even more prevalent.  In addition, measurement of oxygen isotope values can indicate changes in the amount of moisture over time. These values indicate a moister climate or cooler temperatures compared with today throughout most of the Olorgesailie sequence of sediments over the past 1 million years.


What Animals Did Hominins Encounter?

Hominins encountered a variety of animals at Olorgesailie. Image courtesy of Karen Carr Studio.

Hominins encountered a variety of animals at Olorgesailie.  However, the fossil fauna of Olorgesailie was different from faunas of many other East African fossil sites.  Most of the fossil mammals at Olorgesailie were equids, or zebras, whereas in other places, mostly bovids, or antelope-like creatures predominate. The layer in which the hominin was found also had animals such as Theropithecus oswaldi, a large primate that was a specialized grazer, as was Equus oldowayensis. Some fossil animals, such as amphibians, lived around the lake while others were tied to the grasses.


Hyena Hill

Around 990,000 years ago, one area where early humans visited a lot is the place the Smithsonian team calls Hyena Hill.  The excavations at Hyena Hill include a dense accumulation of stone tools and bones that were cut and broken open for food by early humans. Yet also among the fossil animals from Hyena Hill were…hyenas. The team found an ancient den where hyenas lived at around the same time when early humans were also active in this area. The burrows were located in a hill that excavated by a bulldozer (through sediment known not to contain any fossils or stone tools) in order to gain access to the hyena skeletons buried in the tunnels beneath the ground.


Elephas recki fossil rib with cut marks. Image courtesy of Chip Clark, Smithsonian Institution.

Elephant Butchery Site

One excavation in the 990,000-year-old layer illustrates the kind of opportunities that existed for hominins in the Olorgesailie basin. This is the elephant butchery site, where more than 2300 stone artifacts were found surrounding the bones of an extinct elephant, Elephas recki. Many of these artifacts were sharp flakes. These flakes could have been used to remove flesh from the elephant, as shown by a cut marks on one of the elephant’s ribs, some of the vertebrae, and even the hyoid bone where the elephant’s tongue muscles attached.


Excavation of the elephant butchery site. Image courtesy of Richard Potts, Smithsonian Institution.

The elephant butchery occurred as a small wetlands zone populated by reeds and other marsh plants began to dry out. Hominins brought stones from at least 17 different sources, and engaged in toolmaking right at the site to produce some of the flakes that were used in the butchery. Once the elephant bones were uncovered, the excavation was extended to the area nearby. At first, fewer tools were found, but eventually other butchery sites of zebra and an antelope were found.  This drying wetlands zone must have been a favored place for early humans to find animals to butcher for meat and the nutritious bone marrow.


Olorgesailie Hominins Encountered Many Environmental Changes

Olorgesailie hominins encountered many environmental changes during the Pleistocene at Olorgesailie. The landscape was changed repeatedly. At some points during the basin’s history, an ancient lake was present. The level of water in the lake varied considerably between deep and shallow. At some points, the lake filled the entire basin. At other times, the water level receded, leaving behind wetlands or a dry grassland. At other times, the basin contained river channels rather than a lake. Other environmental changes included the fall of volcanic ash which blanketed the entire landscape. Large-scale climatic fluctuations between arid and moist, volcanic eruptions, and earthquakes were responsible for the many environmental challenges faced by early humans over the past 1 million years.

At least 16 major environmental shifts occurred between 1.2 million and 400,000 years ago. These events would have changed the landscape of the entire basin, relocating or eliminating resources for hominins. However, after at least 12 of these events, archeological remains (stone tools) are found in the next stratigraphic layer, indicating that the toolmakers either survived the event or rapidly re-colonized the area. Other mammalian species seem to have been less able to adjust. Although the hominin handaxe-makers of Olorgesailie were capable of surviving large environmental changes, the end of handaxe technology in the region coincided generally with the extinction of other mammalian species, between 500,000 and 350,000 years ago.


How Do We Know the Lake Level Changed?

Diatoms were sampled from many different layers at Olorgesailie. Here we see finely laminated diatomite layers, along with a gray layer of volcanic ash (just below hammer) that settled into the lake nearly 1 million years ago. Image courtesy of Richard Potts, Smithsonian Institution.

In fact, how do we know there was a lake there at all? The modern landscape is arid, with nothing wet enough to suggest that a large lake sometimes filled the basin. Researchers discovered the existence of the paleolake by looking at the sediment layers in the basin. The thick layers of white sediment at Olorgesailie are made up of the silica skeletons of diatoms, a type of algae that grow in lakes. The species of diatoms have been identified, and the different combinations of diatom species correspond to different water depths, salinity, and alkalinity (pH value). This allowed our team to reconstruct lake level, salinity and pH for numerous intervals in the Olorgesailie sequence.


Red Beds and Climate Variability

There are many indications of fluctuation in the level of moisture and aridity at Olorgesailie.  One intriguing indicator is a series of reddened beds found in the later part of the sequence, between nearly 800,000 and 500,000 years ago. These brightly colored patches of sediment were produced by burning of buried plant matter. In some instances, the reddened sediment is associated with melted diatomite, which required an enormous amount of heat and a complete absence of water. The reddened beds required, then, the accumulation of an abundance of swamp plants, followed by burial (only an underground fire could have produced sufficiently high temperatures to melt the silica in the diatomite), followed by intense drought. The fires may have started as lightning ignited the buried materials, much like peat fires in places today.

Reddened sediment reflects the burning of dessicated plant remains buried within diatomite sediment, indicating a severe climate shift between wet and very dry. Image courtesy of Richard Potts, Smithsonian Institution.














The Fauna Changed as Environments Changed

The species of animals unearthed at Olorgesailie and other locations in southern Kenya changed over time as environmental conditions shifted time and again. Species of baboons, elephant, zebra, pigs, and hippopotamuses that had been very abundant in the region went extinct. They were replaced by closely related species that still survive in East Africa today.

Theropithecus oswaldi mandible from Olorgesailie. Image courtesy of Chip Clark, Smithsonian Institution.Theropithecus oswaldi (baboon), Equus oldowayensis (zebra), Elephas recki (elephant), Metridiochoerus hopwoodi (pig), and Hippopotamus gorgops were very large and specialized grazers (grass-eaters). Hippopotamus gorgops was a specialized aquatic hippo. Its elevated eye sockets allowed it to see while most of the body rested in the water.

However, between about 900,000 and 360,000 years ago, these species became extinct, and were replaced by close relatives. The replacement species were smaller and capable of changing their diets more effectively. Some replacements, such as the zebra Equus grevyi and the elephant Loxodonta africana, are capable of browsing (eating leaves and other high-growing vegetation) or grazing (eating grass and other low-growing vegetation). The baboon Papio anubis is an omnivore capable of living both on the ground and in the trees. Phacochoerus aethiopicus is a warthog that grazes and consumes a variety of other foods including seeds, fruits, shrubs and tubers. Hippopotamus amphibious, the modern hippopotamus, moves long distances over land to find water. Specialized grazers at Olorgesailie became less dominant as environmental changes between wet and dry became more prominent.

The evolution and survival of species that possess more flexible or versatile ways of life provide some insights into human evolution.  Although the handaxe-making hominins were successful for a long time, the handaxe way of life also came to an end in the Olorgesailie region by around 400,000 years ago. They were replaced by others who made smaller and more diverse toolkits.  It seems likely, then, that the emergence of versatile adaptations in both hominins and other mammals was a critical evolutionary response in such highly changeable surroundings.




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Page last updated: September 14, 2018