This diagram shows a selection of rock layers, or stratigraphic columns, from the Koobi Fora geologic formation on the eastern shore of Lake Turkana in Kenya. This area is a ridge of sedimentary rock where researchers have found more than 10,000 fossils, both human and other hominins, since 1968. These fossils aid the scientific investigation of human evolution.
Lake Turkana has a geologic history that favored the preservation of fossils. Scientists suggest that the lake as it appears today has only been around for the past 200,000 years. The current environment around Lake Turkana is very dry. Over the course of time, though, the area has seen many changes. The climate of the region was once more humid, which may have been favorable for early humans and hominins to have flourished there.
All lakes, rivers, and streams carry sediment such as soil, sand, and volcanic matter. This sediment eventually settles on the bottom of lake beds or deposits at the mouth of rivers in an alluvial fan. This process of material deposition and erosion as well as the rise and fall in the lake levels due to environmental changes slowly added layers to the geologic record found in the Turkana Basin. Over time the sediment solidified into rock. Bones of ancient humans, our hominin ancestors, and other animal species were buried in the sediment, and eventually became fossilized and preserved in the rocks.
The area has also been dominated by different landscapes over the span of Turkana’s history—flood plains, forests and grasslands, an active volcano, and lakes. In the Koobi Fora formation, bands of sedimentary rock are interspersed with layers of tuff, a sign of times when tectonic and volcanic activity dominated the landscape. Typically, the ash, pumice, and other materials that spew from volcanoes either fall straight back to the earth, or are carried away by air currents or rivers and streams. This volcanic matter eventually settles and over time is compacted to form a special type of sedimentary rock called tuff.
Tectonic activity has had other impacts on research in the Koobi Fora region. During the Pliocene geologic epoch (5.3 million to 2.6 million years ago), tectonic activity left blocks of land at higher elevations than the surrounding land. This allowed for erosional forces to expose rock that was buried long ago. These processes also exposed the fossils buried within those layers of rock.
The layers of volcanic rock are extremely important to reconstructing the history of the Turkana Basin because they allow scientists to calculate the age of hominin fossils found in the region. The volcanic material in tuff is well-suited for radiometric dating, which uses known decay rates for specific unstable isotopes to determine the age of the rock that contains that isotope. Feldspar crystals found in the tuff layers contain an unstable isotope of potassium that can be used for this dating method. The field of archeology often uses carbon isotopes, which are much more common, but the field of paleontology often uses a potassium-argon dating technique because it can be used to date much older rock material. Over time, the unstable potassium isotope (40K) from the rocks decays into a stable isotope of argon (40Ar). The ratio of the stable argon isotope formed from decay to the unstable potassium isotopes tells scientists when the tuff layer cooled and solidified into rock.
Knowing the dates of the tuff, scientists can then estimate a date for the fossils. Fossils above a specific layer are inferred to be younger than that layer, and those below are older, in line with the law of superposition, a key scientific principle of stratigraphy.
Dating of the fossils contributes to a clearer timeline of evolutionary history. Older methods of dating were more subjective, often an educated hypothesis based on the evidence available. However, the fossils in the Turkana region can be dated more accurately because they are found in the sedimentary rock between datable layers of tuff. Although radiometric dating of the tuff is scientifically valid, difficulties still exist. For example, the isotopic “clock” read by scientists can be reset under a variety of conditions, such as from an extreme heating event.
Extending the Learning
The fossils found in the Turkana Basin support the theory of human evolution and the theory that humans originated in Africa before migrating to other places. Consider the age and different species of fossils found in the area. Using your knowledge of evolutionary theory, construct an argument that explains these connections.
fan-shaped deposit of eroded material, usually sediment and sand.
chemical element (gas) with the symbol Ar.
type of dark volcanic rock.
a dip or depression in the surface of the land or ocean floor.
skull of a vertebrate, usually referring to the part that encloses the brain.
to reach a conclusion based on clues or evidence.
process of silt and sediment building up in an area.
area where two or more tectonic plates are moving away from each other. Also called an extensional boundary.
(55-34 million years ago) epoch of the Tertiary Period in the Cenozoic Era.
act in which earth is worn away, often by water, wind, or ice.
to guess based on knowledge of the situation or object.
change in heritable traits of a population over time.
type of mineral used in ceramics, cleaning products, and archaeological dating.
remnant, impression, or trace of an ancient organism.
having to do with the physical formations of the Earth.
Great Rift Valley system
series of faults and other sites of tectonic activity stretching from southwestern Asia to the Horn of Africa.
tribe of the hominid family of primates, distinguished by erect posture, bipedal movement, large cranial capacity, and use of specialized tools. Human beings are the only living hominins.
containing a large amount of water vapor.
statement or suggestion that explains certain questions about certain facts. A hypothesis is tested to determine if it is accurate.
atom with an unbalanced number of neutrons in its nucleus, giving it a different atomic weight than other atoms of the same element.
upper jaw or jawbone.
chemical element with the symbol K.
type of igneous rock with many pores.
having unstable atomic nuclei and emitting subatomic particles and radiation.
method of dating material such as rocks that compares the amount of a naturally occuring isotope of an atom and its decay rates. Also called radioactive dating.
relationship between numbers or numerical values.
long, narrow elevation of earth.
solid material transported and deposited by water, ice, and wind.
rock formed from fragments of other rocks or the remains of plants or animals.
atom with an unbalanced number of neutrons in its nucleus (isotope) that is not radioactive, or decay naturally.
study of rock layers and layering.
movement of tectonic plates resulting in geologic activity such as volcanic eruptions and earthquakes.
massive slab of solid rock made up of Earth's lithosphere (crust and upper mantle). Also called lithospheric plate.
type of rock formed from hardened volcanic ash.
atom with an unbalanced number of neutrons in its nucleus (isotope) that is radioactive, or decays by emitting particles from its nucleus. Also called a radionuclide.