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A Kansan's Guide to Science, page 3 of 4
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The History of the Earth and the History of Life

How old is the Earth, and what is the evidence for the Earth's age?

Scientists have determined that the Earth is about 4.5 billion years old. This is older than the oldest rocks native to Kansas, which are buried deep beneath the surface and can be recovered only by drilling. Even older rocks, however, have been found in the state. These are meteorites, rocks that have literally fallen from the sky. Because meteorites differ so much from the rocks geologists typically find in Kansas, they are relatively easy to identify, and they provide one clue to the age of the Earth. Meteorites are found all over the world. Because all parts of the solar system are thought to have formed at the same time, meteorites must be the same age as the Earth. Meteorites have been dated to about 4.5 billion years old. The oldest rocks that formed on the Earth are 4 billion years old.

The way scientists can date ancient rocks like Kansas meteorites is through the use of radioactive isotopes. These are atoms that decay at predictable and constant rates into other stable atoms. The relative concentrations of some of these isotopes within a rock can be used with considerable accuracy to determine the age of that rock. Using this technique, scientists have shown that very old rocks--3.5 million years or older--occur on all the continents and that meteorites and moon rocks are about 4.5 to 4.6 billion years old.

How do geologists talk about geological time?

Geologists have divided the 4.5 billion years of geological time into smaller parts using the geological time scale (table 1). As we have discussed, geologists have determined the age of the Earth by studying the concentrations of radioactive isotopes in certain rocks. Only certain types of rocks, however, can be dated using radioactive isotopes. In many cases, the best way to determine a rock's age is to look at the fossils contained within that rock. Many of the rocks in Kansas, like those found along many of our state highways and in stream banks, have abundant fossil remains. Geological time is divided into a number of eras and periods, each characterized by a specific set of fossils, the remains of organisms that lived in those remote geological times.

The oldest rocks, which were formed before large animals had evolved, are termed Precambrian rocks. Precambrian rocks do not contain many fossils, and most of the fossils that occur in them are rather small--in fact, most are microscopic in size. On top of the Precambrian rocks, which are older than about 540 million years, lie the Cambrian rocks. The term Cambrian comes from the Roman word (Cambria) for Cymru, which is the Welsh word for their native land. Thus Cambrian rocks were first described from Wales in the United Kingdom, but they have since been found on all the continents. The rocks outside Wales are recognized as Cambrian rocks because they contain the same kinds of fossils as the Cambrian rocks in Wales.

The process of determining the ages of rocks and relating them to each other is called correlation. Correlation involves looking for similar types of fossil organisms in rocks from different parts of the world. Sometimes a single key fossil species may reveal a rock's age. This is like dating a historical event by the presence of a key person in a particular place. For example, if we know that a house in Kansas was once occupied by John Brown, we know that house must have been in existence in the 1850's. Other times, a rock's age will be revealed by the presence of several key species. In Kansas the Reagan Sandstone, which lies deeply buried below the surface, was deposited during the Cambrian Period. On the basis of the fossils it contains, which were recovered from drilling, it can be correlated to other sequences of Cambrian rocks, both in Wales and elsewhere. Because some of the rocks containing Cambrian fossils are closely associated with rocks that can be dated using radioactive isotopes, we can date the fossil-bearing rocks to around 500 million years. In Kansas, it turns out that Cambrian rocks supply an important part of the oil and gas produced in the state of Kansas, not a bad heritage. Furthermore, the Reagan Sandstone lies directly on top of the even older Precambrian rocks that are present beneath the surface everywhere in Kansas.

The geologic history of the Earth is divided into major segments but not all of that history is preserved here in Kansas (see table 1). There were prolonged periods during which rocks were deposited, but there were also long times when no rocks were deposited in Kansas, although rocks of those ages are present in the other parts of the country. The oldest rocks preserved at the surface in Kansas are from the Mississippian Period, which lasted from about 360 to 320 million years ago. These rocks are found in the southeastern part of the state. In eastern Kansas, the alternating sandstone, shale, and limestone beds were deposited during the Pennsylvanian and Permian Periods, which lasted, respectively, from about 320 to 285 million and from about 285 to 245 million years ago. To the west are rocks that were deposited during the Cretaceous Period, from about 145 to 65 million years ago, most notably the Niobrara Chalk beds. Still farther west on the High Plains of Kansas is the Ogallala Formation, which was deposited during the Tertiary Period and is less than about 5 million years old (fig. 2).

Table 1--Kansas geologic timetable.

geologic time chart

Fig. 2--Generalized geologic map of Kansas.

generalized geologic map, with cross section that follows (roughly) I-70

What evidence do we have for the history of life?

Fossils are our best evidence of past life. Fossils are the remains of long-dead organisms preserved in rocks. For an animal or plant to become a fossil, it has to be buried soon after it dies. After burial, the enclosing material may slowly harden, sometimes over many millions of years, and be transformed into rock. The remains of the organism also undergo chemical changes. The final product is a chemically altered form that preserves the original animal or plant to a greater or lesser degree. The fossil record provides an excellent chronicle of life's history, so that we now understand it quite well. Of course, not all animals are perfectly preserved, so the fossil record is far from perfect.

Incidentally, although many ancient organisms have been preserved as fossils, other ancient organisms are the source of the oil that powers our cars and the coal that drives our power plants. The state of Kansas is particularly well endowed with rich reserves of oil and coal derived from ancient life, much of which lived more than 100 million years ago. Conservative estimates suggest that Kansas has oil reserves measured in billions of barrels, a quantity that provides both a reasonable idea of the number of organisms that lived in the past and also tangible evidence of the contributions of these ancient organisms to the state's economy (fig. 3). Coal reserves in Kansas also are significant, although less important economically at the present time. Coal is derived from the remains of ancient swamps and forests that once covered Kansas. These forests contained a wide variety of now-extinct plant groups. Millions of tons of coal are known to occur in Kansas, especially in the southeastern part of the state. These are a further testament to the wealth of past life.

Fig. 3--Oil is a byproduct of fossil remains. This oil well, in Saline County, is one of many found throughout the state.

oil pumpjack

When did life first appear?

Early in its history, the Earth was a molten, volcanically active sphere that was inhospitable to any kind of life. Moreover, at that time, the Earth was bombarded by many more meteorites and asteroids than in more recent times. Just over 4 billion years ago, the Earth began to cool somewhat, and some of the earliest rocks from this time period bear chemical traces of life. Thus, shortly after the Earth cooled, life had already begun to evolve. The oldest fossils known are simple, microscopic organisms. These single-celled organisms, resembling modern pond scum, date from roughly 3.5 billion years ago. Although these earliest organisms were exceedingly simple, we owe them at least one great debt. They helped generate the oxygen in the atmosphere that our early animal ancestors needed. For nearly all of the next 3 billion years (the period called the Precambrian), single cells were the only life that existed. Eight hundred million years ago the first multicellular organisms appeared. These were a type of algae resembling modern-day seaweeds. Near the end of the Precambrian, by 550 million years ago, the world's oceans were populated by such multicellular animals as sponges, jellyfish, snail-like organisms, and a variety of wormlike organisms together with unusual flattened organisms called the Ediacara. The rocks containing these early multicellular organisms are exposed in many parts of the world, but unfortunately cannot be found in Kansas.

What was the Cambrian explosion?

Roughly 540 million years ago a key event occurred, sometimes referred to as the Cambrian explosion. Early in the Cambrian Period, abundant, complex animals began to evolve. Over a period of about 30 million years, the ancestors of nearly all the major groups of animals living today appeared in the fossil record. Many kinds of early animals are now extinct, and we know them only through the study of fossils. Some of the extinct forms were truly bizarre. Others of the early animals are intermediate between the different modern animal groups. That is, their overall form is close to what we would predict for the ancestors of each of these great groups of animals. Others, while primitive, clearly belong to familiar modern groups: clams, snails, arthropods (the group that contains today's spiders, lobsters, and insects), and our own group, the vertebrates. Although these ancestral species are now extinct, their descendants are still with us today and comprise the groups of animals with which we are familiar in the modern world.

We do not know what caused the Cambrian explosion, but it is being actively researched by a number of geologists and paleontologists who are developing a good understanding of those events that occurred so long ago..

The Cambrian explosion is important for several reasons. The organisms did not appear all at once, but over 30 million years, an extremely long period of time from our point of view. Thus, it was not an instantaneous event. Compared to the long, Precambrian interval when only single-celled organisms dominated the Earth, however, the pace of this evolution seemed to be quite explosive indeed. The Cambrian explosion is an excellent example of how the rate of evolution has varied in the long history of life. Sometimes evolutionary change occurs quickly; at other times it moves slowly. This is analogous to human history and the history of nations. Witness the relatively sudden toppling of the Communist regime in the former Soviet Union. It happened in a few years, relatively quickly com-pared to the previous 70 years of that regime's stability. Thus, the Cambrian explosion does not contradict the fact that evolution has happened. Instead, it shows that the rate at which evolution occurs is not constant. Although, as mentioned above, no Cambrian rocks are found at the surface in Kansas, rocks dating from shortly after the Cambrian explosion are found buried deep beneath the surface.

What happened after the Cambrian explosion?

The next key event in the history of life was the evolution of plants and their invasion of land, roughly 420 million years ago during the Silurian Period. Within a few tens of millions of years, forests covered large parts of the Earth's surface that was above sea level. Shortly thereafter, our own group, the vertebrates, invaded the land. Fish had lived in the oceans for nearly 100 million years and were extremely diverse. New species eventually evolved that had the ability to move onto land, although they returned to the water to mate and lay their eggs, just as modern-day amphibians do--the frogs, toads and salamanders. Eventually, tens of millions of years later, some vertebrates evolved to become fully terrestrial, laying their eggs on land. The animals that evolved this ability are the ones we now refer to as reptiles. Kansas has one of the best fossil records of these early reptiles, which lived in the Pennsylvanian and Permian Periods. They have been recovered from Garnett, Kansas, in rocks that preserve what was once an ancient lagoon. Several kinds of early reptiles lived in this lagoon, including Petrolacosaurus (figs. 4, 5). This Kansas reptile is close to the ancestral form of the reptile group that spawned the dinosaurs, crocodiles, and birds. Also living in that 300-million-year-old Kansas lagoon in close association with Petrolacosaurus was another early reptile called Edaphosaurus (fig. 6). It was part of a separate lineage that gave rise to modern mammals.

Fig. 4--Skeleton of Petrolacosaurus, an early reptile from the Pennsylvanian of Kansas (University of Kansas, Museum of Natural History, Special Publications, Reisz, 1981).

skeleton of early reptile

Fig. 5--Reconstruction of Pennsylvanian-aged lagoon in central Kansas (University of Kansas, Paleontological Contributions, Peabody, 1952).

sketch of Pennsylvanian lagoon showing small reptiles and scorpians

Fig. 6--Edaphosaurus (left) from the Pennsylvanian of Kansas is a transitional link, along with its close relative Dimetrodon (right), between early reptiles and other groups more closely related to modern mammals (The Age of Reptiles, a mural by Rudolph F. Zallinger, Peabody Museum of Natural History, 1989, used with permission).

Two large reptiles, with striking fins on their backs; dimetrodon is bulkier and has larger head and teeth.

Not all the periods are represented in the Kansas geological record. This is partly because sea level has varied dramatically throughout earth history. Sometimes the continents have been flooded by shallow seaways. At other times the continents have been largely exposed as they are today. During times of inundation, sediments were deposited on the sea floor. Over millions of year, these accumulated sediments formed massive layers of limestones, shales, and sandstones. In general, when the land is above sea level, rocks are not deposited. Thus, in the future, Kansas will have relatively few rocks that preserve a record of the present day.

Shortly after the plants evolved, sea level rose and fell periodically across the North American continent. In Kansas, as sea level rose and fell, the surface environment alternated between shallow swamps and shallow oceans. It was at this time, during the Pennsylvanian Period, that much of the coal in Kansas was formed, in large swamps teeming with plants. After the plants died, they were covered by water and mud and sand. As layers of sediment accumulated, the decaying plant material was compacted, eventually forming coal. At other times, when the water level rose, shallow seas covered Kansas and much of the rest of the country as well. During this time, limestone was deposited. This limestone is now exposed as the yellow-brown rocks that crop out along the side of many highways and country roads in eastern Kansas. Later, during the Permian Period in Kansas, the seas began to dry up. As the seawater evaporated, it left behind thick layers of salt. This salt now is mined commercially, especially near Hutchinson, Kansas.

Still later, to the west, after a long interval during which the land was above sea level, the water level once again rose, covering much of the continent in the Cretaceous Period. Chalks and sandstones from this time period occur throughout central and western Kansas. (The sandstones are a rich source of water for Kansas, and wells are often drilled into these rocks.) The Cretaceous chalks formed because the Kansas sea also contained tiny microorganisms. As these small organisms died, their skeletons rained down onto the seafloor and over millions of years formed the extensive chalks that crop out throughout much of western Kansas. The white cliffs of Dover in England are a similar type of chalky rock that was deposited in the same type of environment and at the same time. The Cretaceous rocks contain very different organisms from those found in Pennsylvanian rocks. The Cretaceous chalk contains large clams that evolved to look completely different from those that lived during the Pennsylvanian Period. The chalk also contains ammonoids, which are now extinct but were relatives of modern-day squids and octopuses (fig. 7). The ammonoids also lived during the Pennsylvanian Period, but the ones that lived at those times were much more primitive and had much simpler shells than the Cretaceous forms. The most noteworthy fossils in the Cretaceous rocks of Kansas, however, are the fossils of animals with backbones. Among these were the giant mosasaurs, great meat-eating marine reptiles that in Kansas grew up to 20 feet long (fig. 8). Giant predatory fish and sharks as much as 15 feet long also roamed the Kansas seas (fig. 9).

Fig. 7--The fossil ammonoid Aegoceras, a marine predator with a spiral shell went extinct along with the dinosaurs 65 million years ago (University of Kansas, Paleontological Contributions, Donovan and Forsey, 1973).

Black and white photo of ammonoid shell.

Fig. 8--The mosasaur Platecarpus was a giant marine reptile that grew up to 20 feet long. Species like Platecarpus were abundant in Kansas seas 70 million years ago. They are close relatives of modern-day snakes (Kansas Geological Survey, Educational Series 6, Evans, 1988).

Black and white drawing of mosasaur.

Fig. 9--Fifteen-foot-long fossil fish Xiphactinus from the Cretaceous of western Kansas, with a smaller fossil fish Gillicus preserved inside its abdominal cavity (University of Kansas, Paleontological Contributions, Bardack, 1965).

Black and white photo of fossil fish.

The skies above the Kansas Cretaceous seas also were rich in life. Pterosaurs, some of them giants with wing spans reaching almost 20 feet, glided through Kansas skies, scooping fish from the water (fig. 10). These reptiles, close relatives of the dinosaurs, evolved flight independently from the modern feathered birds. Along with these pterosaurs, however, birds flew through Kansas skies and fed on ancient Kansas fish; these were true, feathered birds. Several species of these early birds are known from Graham County, Kansas. Some of these were flightless species, such as Hesperornis and its close relative Parahesperornis (fig. 11), stood nearly 5 feet tall and had large feet, like modern diving birds, such as the loon, which migrates through Kansas in the spring and autumn. Unlike the loon and other modern birds, it had teeth set in its mouth, and its bones were not hollow. These were characters inherited from reptilian ancestors that had not yet been lost in the early evolution of birds.

Fig. 10--Reconstruction of Pteranodon, a species of pterosaur, a winged reptile from the Cretaceous of Kansas (University of Kansas, Paleontological Contributions, Brower and Veinus, 1981).

Drawing of Pteranodon skeleton.

Fig. 11--Parahesperornis, an early fossil bird from the Cretaceous of western Kansas (L. Martin, Transactions of the Kansas Academy of Science, 1984, v. 87).

Drawing of Parahesperornis skeleton.

Why do scientists describe the history of life as a branching tree?

Playing out over these immense time scales are two distinct phenomena. First, life was continually evolving, casting out new branches representing new lineages of organisms like a great tree. At the same time, however, many lineages went extinct as if nature were pruning the tree of life. The kinds of animals and plants that populate the Earth have been changing continually. Truly cataclysmic events often led to mass extinctions, some of them eliminating more than three quarters of all life on Earth. Recovery was slow, but it always occurred. Often the survivors of these extinction events were kinds of organisms that had not been especially important before the mass extinction. Part of the reason that Kansas fossils from the Pennsylvanian Period differ from those of the Permian Period is because a time of extinction separates these two intervals. In addition, these fossils differ because evolutionary change had occurred.

What happened in Kansas during the Cretaceous-Tertiary mass extinction?

The Cretaceous Period ended 65 million years ago, and the Tertiary Period began. The boundary between the two periods is marked by the Cretaceous-Tertiary mass extinction. Before the extinction occurred, dinosaurs were the dominant terrestrial organisms of their day. Three distinct species of plant-eating dinosaurs, two with thick bony armor on their backs and one with a broad duck bill, are known to have roamed the state of Kansas (fig. 12). The largest Kansas dinosaurs grew up to 20 feet in length. During the Cretaceous the mammals, the group that includes our own species, were small and rodentlike, but even more primitive than today's rodents. Fossil mammals are found rarely in Cretaceous rocks, and they probably were of marginal importance in the dinosaurs' world. Seemingly overnight, this all changed. A large meteorite roughly 5 miles across struck the Earth near what is now the Yucatan peninsula of Mexico. The catastrophic impact and ensuing darkness eliminated the large terrestrial dinosaurs and many other kinds of animals as well. The small mammals, arguably less well adapted than the dinosaurs, somehow survived. Within a few million years after the mass extinction event, they had evolved into a diverse array of animals, including bats, whales, and horses. Our own lineage, the primates, also evolved shortly after the extinction event. From the start of their evolution, the primates were distinguished from the other mammals by their relatively large brains and good eyesight.

Fig. 12--Silvisaurus, an armored dinosaur from the Cretaceous of Kansas (University of Kansas, Paleontological Contributions, Eaton, 1960).

Drawing of Silvisaurus dinosaur.

We also can see evidence of the Cretaceous-Tertiary mass extinction transition on a more narrow scale in Kansas. Before the mass extinction, several types of giant marine reptiles, including the mosasaurs (fig. 8), swam in the shallow seas that covered parts of Kansas. These mosasaurs went extinct along with the dinosaurs, and the seas they lived in have since retreated, but their close cousins, the snakes, populate what is now the prairie of Kansas. The skulls and backbones of snakes are very similar to those of mosasaurs: they share many anatomical features because they share a close common ancestor. Mosasaurs can best be thought of as the close evolutionary kin of the snakes that were very well adapted to the Kansas environments of 70 million years ago. Similarly, snakes, including the rattlesnake, are well adapted to the modern-day prairie environment.

An evolutionary continuity stretches across the 70-million-year transition from sea to prairie in Kansas. The history of life and its evolution as shown in the fossil record now are well understood and full of variety. The tree of life is a diversely branching tree that has been populated by millions of species, almost all of which are now extinct. Some of the major branches of this tree have vanished through the incessant processes of extinction; other major branches are still with us today.

How did our relatives, the primates, evolve from swinging in trees to walking the plains?

From our perspective, another key evolutionary transition occurred roughly 4 million years ago in Africa. Primates originally were tree dwellers and had a stooped posture. Some primates from this time show profound similarities to chimpanzees in the structure of their face and hands and in the size of their brains, but their hips show clearly that they walked with an upright, erect posture--much like that of modern humans. These are the first known fossil hominids, the narrowly defined group of organisms that includes modern humans and several other extinct species. The evolutionary transition happened during a time when the climate in Africa and in much of the rest of the world was becoming cooler and drier. Patches of trees were becoming more widely spaced, requiring longer travel times to move between them, a situation that favored an upright gait.

We can identify some fossils as being links in the evolutionary chain between a living animal and its older ancestors in the fossil record. We expect such links to have a combination of features intermediate between an ancestor and its descendants. The earliest hominid fossils fulfill this expectation. In fact, a series of transitional links forms a chain from the earliest primates to the most primitive hominids and from these earliest hominids to our modern species of humans (fig. 13). The general trends are the evolution of larger brains and such specializations of the hands and limbs as our opposable thumb, which enables us to manipulate tools. Within the hominid family tree, some branches do not lead to humans at all. Some of the so-called australopithecine hominids developed large teeth and jaws, probably for crushing plants. The australopithocines went extinct more than a million years ago. Hominids are known from rocks in Africa, Asia, and Europe. We have no record of these evolutionary transitions here in Kansas because the group was evolving elsewhere.

Fig. 13--Hominid skulls showing evolutionary transitional forms. The earliest Australopithecus have a combination of apelike, humanlike, and intermediate features. The earliest species of Homo have intermediate features between Australopithecus and modern humans. All skulls show an overall trend towards larger brain size; used with permission of artist Darwen Hennings, National Academy of Science, 1999.

5 hominid skulls showing trend toward larger brain size, other changes.

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Kansas Geological Survey, Educational Resources
Placed online Oct. 2004. Originally printed 2000.
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