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This diagram shows a selection of rock layers, or stratigraphic columns, from the Koobi Forageologicformation on the easternshoreof Lake Turkana in Kenya. This area is aridgeofsedimentary rockwhere researchers have found more than 10,000 fossils, both human and other hominins, since 1968. These fossils aid the scientific investigation of humanevolution. 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 morehumid, which may have been favorable for early humans and hominins to have flourished there. All lakes, rivers, and streams carrysedimentsuch as soil, sand, and volcanic matter. This sediment eventually settles on the bottom of lake beds or deposits at the mouth of rivers in analluvial fan. This process of materialdepositionanderosionas well as the rise and fall in the lake levels due to environmental changes slowly added layers to the geologic record found in the TurkanaBasin. Over time the sediment solidified into rock. Bones of ancient humans, ourhomininancestors, 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 oftuff, 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 activityhas 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 forradiometric dating, which uses known decay rates for specificunstable isotopes to determine the age of the rock that contains thatisotope.Feldsparcrystals found in the tuff layers contain an unstable isotope ofpotassiumthat 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-argondating technique because it can be used to date much older rock material.Over time, the unstable potassium isotope (40K) from the rocks decays into astable isotope of argon (40Ar). Theratioof 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 thenestimatea 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 ofstratigraphy. Dating of the fossils contributes to a clearer timeline of evolutionary history. Older methods of dating were more subjective, often an educatedhypothesisbased 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.

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Examine the stratigraphic column diagram. What relationship seems to exist between the approximate age of the fossils and rocks layers and their depth in the earth?

Following the law of superposition in geology, older fossils and rocks are found in lower strata than younger fossils and rock layers.

Tectonic activity left some areas of land uplifted, and erosional forces from the lake, nearby rivers, and other forms of weathering exposed rock, even older rock layers, as outcroppings in the landscape. This made the fossils easier for researchers to find. The volcanic material in tuff layers also makes it possible to get a more accurate date for the fossils.

Potassium-argon dating is a form of isotopic dating commonly used in paleontology. Scientists use the known natural decay rates for isotopes of potassium and argon to find the date of the rocks. The radioactive isotope converts to a more stable isotope over time, in this case decaying from potassium to argon. If scientists find the ratio of potassium to argon, it tells them how long the rocks have been around by how long the isotopes have been decaying. By understanding the dates of these rocks, scientists can deduce the age of the nearby fossils.

What difficulties might paleontologists and archaeologists have when trying to find and date fossils?

There are many possible answers. One answer based on a common problem encountered by scientists is that fossils are often encased in rocks or are similarly colored, so they blend in with their surroundings. Sometimes, only a small part of a fossil is showing. They might also be buried. Because of these characteristics, field crews have to carefully examine their surroundings to find possible fossils. Fossils might also be fragile or found in small fragments. Archaeologists have to use their skill and patience to put small pieces back together, like a jigsaw puzzle. Although fossil dating is now more scientifically accurate, it still requires skill and experience as scientists have to make educated guesses based on any evidence and the dating available for the layers surrounding the fossils.

The diagram explains that each fossil is given a specific name, such as KNM-ER 1813. Why is a unique fossil name like this important?

Because each name is a unique identification, this helps scientists keep track of where and in what order fossils are found. Doing this helps paleontologists maintain accurate records and piece together the story of human history. In this example, the prefix "KNM-ER" tells us the relative location of where this fossil was found; this stands for Kenya National Museum—East Rudolf, from the former name of Lake Turkana. The accompanying numbers are chronological, meaning that, in this example, our fossil is the 1,813th fossil found in the area.

In 1669, Danish naturalist Nichlaus Steno put forth the idea that horizontal layers of sedimentary rock represent a time sequence of Earth"s geologic history. He realized that older layers of sedimentary rocks are deeper in the earth, and younger ones build on top of them. In geology, this is called the law of superposition. Stratigraphic analysis of the geology in the Lake Turkana region of Kenya showed that landscape has changed over time, and at times there was no lake present at all. Modern Lake Turkana has only been around for around 200,000 years. In addition to those fossils found at Koobi Fora, hominin fossils have been found in three other parts of the Turkana Basin. These are the Shungura Formation, the Usno Formation, and the Nachukui Formation. In the 19th and 20th centuries, some researchers thought Asia rather than Africa held the origins of man, and they investigated dragon bones (fossilized teeth and bones) found in China. One site, Dragon Bone Hill, led to the excavation of Homo erectus remains many call Peking man, which date back to roughly 750,000 years ago. Rock formations or regions, such as Koobi Fora, are often given names based on descriptions, local names, or geographic features. The term Koobi Fora comes from the language of the Gabra people who live near the site. This term describes the area by listing some of its vegetationKoobi Fora means a place of the commiphora, a source of myrrh.


Term Part of Speech Definition

alluvial fan


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.

divergent boundary


area where two or more tectonic plates are moving away from each other. Also called an extensional boundary.


adjective, noun

(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.

radiometric dating


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.

sedimentary rock


rock formed from fragments of other rocks or the remains of plants or animals.




stable isotope


atom with an unbalanced number of neutrons in its nucleus (isotope) that is not radioactive, or decay naturally.



study of rock layers and layering.

tectonic activity


movement of tectonic plates resulting in geologic activity such as volcanic eruptions and earthquakes.

tectonic plate


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.

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unstable isotope


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.