GG101 Evolution Through Time

GEOLOGY/GEOPHYSICS 101 Program 13

EVOLUTION THROUGH TIME

This piece of petrified wood still shows the growth rings of the tree that it came from. The atoms that once formed thewood have been replaced by other atoms as the wood lay buried in the stream bed. Instead of oxidizing and decaying, these atoms were replaced one by one by molecules of silica,maybe over a hundred thousand years.

Since the silica molecules occupy the same locations as the former carbon, hydrogen,oxygen, and nitrogen atoms, they retain the structure of the original wood. Fossils like these tell us everything we know about the plant and animal life of the distant past. By tracing how life forms have changed throughout geologic time,we get a picture of the history of life on Planet Earth. This is, after all, the only life anywhere in the universe as far as we know, and our own species shares a common heritage with all life, hard as that may be for us to acknowledge, and we should learn as much about life on Earth as we can if we want to understand ourselves.

From the fossil record, we also learn that few, if any, species are eternal. Even the mighty dinosaurs, representatives of which were dominant on Earth for a hundred million years have perished. You see, Earth is indifferent to life, which exists only on the planet's terms to begin with. Although life may have a small amount of influence on the atmosphere and the hydrosphere to some degree, there are external factors which keep going, and life must adjust to it in order to survive. The sun keeps on shining, and the plates keep on moving, and the winds keep on blowing while life of amazing variety struggles and adapts to survive in many different environments which change as geological processes move around the Earth's crust.

There are two main themes to today's lesson.

It's important to remember as we study evolution through time, that a particular time or a particular sequence of rocks has a unique collection of organisms as species come and go. This is true now as it was true in the past, and paleontologists can recognize the age of a particular rock or a rock strata by reading its fossil assemblage. It's sort of in the same way that if you're listening to a collection of oldies on the radio, you could pinpoint the date from which songs were played.

We have learned from studying the fossil record that the complexity of organisms increases with time as new life forms have developed to take advantage of the available energy on the Earth's surface. Life started with simple one celled organisms and became increasingly complex until in the recent past mammals evolved, which represent today the most complex forms of life on Earth.

We also want to note here that ecosystems which involve interactions between life and the environment are complicated systems which have evolved along with life and represent four billion years worth of trial and error to try to get things right.

You should have already read the lesson assignment for today. It's important to note here that there's not a specific corresponding chapter in the text for this lesson. This was added sort of as an extra for this series, "Earth Revealed," and it's an important topic, evolution through time, which is ordinarily covered in the second semester of a geology course, but it fits in nicely with the material, and I'm glad they put it in this sequence. Because there's no specific text assignment, the study guide chapter is longer, so you should spend more time with the study guide but also use the text to review important concepts, so you

As usual, I'll review those objectives with you briefly. In general, this lesson will help you

In a course like this, we can't really do justice to the process of Evolution. All we can really do here is to try to understand how it relates to Earth's history and Earth's processes. This is one area where the boundaries between the sciences of biology and geology become kind of hazy. There are many good books on evolution. You can ask the librarian or check the library catalog, and, also, a college course in biology will go into much more detail than we can do here though I'll try today to summarize the main points of evolution by natural selection and hope from this you can get a sense of what causes life to change and how life changes.

First, I want to note that fossils of various types are the main form of evidence that we have that evolution has occurred. A "fossil" is simply the preserved remains of prehistoric life. A common misconception is that dinosaurs represent the most common form of ancient life, dinosaurs as exemplified by these models. Really, dinosaurs are not the most common, they're simply the most spectacular form of life, mostly because of their large size and the fact that they dominated for such a long period of time, a hundred million years or so.

The interesting part of understanding dinosaurs is trying to figure out what caused them to disappear rather abruptly in geologic time. After a hundred million years of dominating life on Earth, they suddenly disappeared. I mean "suddenly" in geologic terms over a very brief period of time, perhaps over a few tens of thousands of years.

In the last lesson, we saw how you could use faunal succession combined with uniformitarianism and superposition to show how life changes through time. The evidence that we have is mostly from marine sedimentary rocks, and this is simply because there are many more organisms in the sea than on the land, and so many are more preserved, and, also, the conditions for preserving organisms or fossils is better on the sea floor than on land.

In fact, we have only very limited evidence of fossils of land animals, especially in recent eras of mammals and humans, but we still have enough even in recent times of land mammals to get a general sense of the changes that have taken place.

Another thing to note here is that throughout geologic time, there must have been many more organisms than we'll ever find fossil evidence of because only a small fraction of organisms are preserved, and not only that, but mostly it's only the hard parts of organisms, such as bones, and teeth, and shells that are preserved, and many organisms, especially those present in early Earth's history didn't have hard parts at all. They were soft parts, things like jellyfish.

Not only that, but there are rather limited conditions that can preserve a fossil, and these don't occur in most places. The video shows you good examples of the types of fossils and fossilizations, so I won't go into that in detail, but you should pay attention to this section in the video.

Okay, even with the limited evidence that we have, it leads us irrevocably to the conclusion that life has changed over time. The fossils show us how life has changed, not necessarily the mechanism of change, but how the forms have changed from simple to complex.

Okay, the Theory of Evolution by Natural Selection was put together by Charles Darwin. Now, Darwin set sail on around the Earth voyage in 1831. Darwin at that time was 22 years old. He was looking for a career. He had dropped out of medical school because he didn't like it, so he signed on as the naturalist on the H.M.S. Beagle, whose job it was to explore the Southern Hemisphere and to map the coast of South America and generally just observation of conditions in the New World.

The things Darwin saw on this voyage began to make sense to him, largely because he was armed with Lyell's conception of the immense length of geologic time. It served as a model and to which Darwin could fit the observations that he made. When he returned to England in 1836, he quietly searched for evidence of his theory of evolution. As he thought about his own experiences and read of the ideas of others regarding competition for resources, survival of the fittest, natural selection, and evolution, the details of the workings of evolution by natural selection became evident to him.

Hearing that another naturalist, his name was Alfred Wallace, had come upon a similar theory of evolution by different means. Darwin asked Charles Lyell to present his and Wallace's ideas jointly to the World Society of London, which Lyell did, and it, at the time, provoked much controversy, and Darwin had to be defended very adamantly by both Lyell and Thomas Huxley,who was another friend of Darwin's at the time.

In 1859 they convinced Darwin to publish a book, The Origin of Species, in which he presented the theory and the massive evidence that he collected over the 20 years since he had returned from the Beagle's voyage.

See, on the voyage, Darwin was exposed to the immense diversity of life that we find in the tropics, and science at that time was mostly a temperate climate endeavor. Darwin lived in England. He also, then, saw the sparse life that was present in the very harsh environment of the Magellan Straits at the southern tip of South America. Probably the most stimulating of all the things Darwin saw were in the Galapagos Islands.

The Galapagos Islands are islands very similar to Hawaii; they're about the same age, a million years or so, which lie along the edges of three plates off the coast of South America. Here Darwin found finches, a type of bird that's very common the world around, but the finches on the Galapagos Islands showed very unusual adaptations.

They resembled those on the continent of South America in many ways, but it's as if their features were distorted. Some of the had very long beaks, for example. Some of them had developed hard beaks for cracking nuts. Along the Galapagos, these finches had filled niches and habitats that were filled by other organisms on the South American mainland. There were seed eaters and fruit eater and bug eaters, and some of the finches lived in the ground; some lived in trees; some lived in cliffs. Darwin concluded from this that the variety of finches had evolved from mainland finches that were probably blown there by accident some time in the distant past and isolated by 650 miles of ocean, so that the unique new breeding population developed on the island, which later evolved into these new finches. In the absence of competition from other species, and also the absence of predators that would eat the finches, the new species evolved then to take advantage of these unfilled niches on the new islands.

The young age of the islands is what really provoked Darwin and provided dilemma for him, and I want to note here that part of the development of the theory of evolution came about from Darwin's understanding of the young age of Galapagos, so he had to know to begin with that the islands were young.

You see, the common belief at the time was the biblical interpretation of creation, which indicated that the creation event was a one time thing that happened many years ago; yet here on the Galapagos Islands were many organisms that were found on these young islands and nowhere else on earth, and it seemed to Darwin that this implied that there must have been a separate creation event just for the Galapagos Islands; in fact, when he traveled to other pacific islands, he also saw on these islands organisms which were present only there.

So you see the dilemma: Either God had created new organisms every time he created a new island, which was contrary to what the Bible had said had happened as far as creation goes, or else these new species must have evolved from existing species by some sort of process.

In modified theory, Darwin's Theory still presides, and it fits very nicely into the plate tectonics model. Plate tectonics, in fact, explains some of the previously unexplained features of the theory.

Okay, I'd like to take a minute or two to detail what Darwin said in his Theory of Evolution by Natural Selection and see if we can get a sense of how it works, so that we can understand how life evolved from these simpler organisms.

We observe when we look at organisms of a given species that no two individuals are exactly alike, and here the word "species" really means the ability to interbreed and produce fertile offspring, so any two creatures which are capable of interbreeding and producing offspring that can also breed, are said to be of the same species.

There are different definitions of species, but I'll use the word species in that sense, so no two individuals are identical although they may be similar. Now the differences between individuals may be obvious differences, or they may be subtle differences. Obvious differences might be differences in color or differences in striping or size, but the subtle differences might be things like subtle differences in morphology, the length of a toe, or even more subtle differences in the biochemistry of the origanism; in other words, how its internal chemistry functions.

So in addition to the fact that no two organisms are alike, Darwin also noted that life is a struggle for survival, and only the fit survive. You see, many more offspring are produced in the natural world that ever have a hope of surviving, so we use the word "fit" here simply to mean successful in their environment, and really we mean "successful" is the ability to reproduce; in other words, to live long enough to reproduce, to get enough energy from the environment that you can successfully reproduce.

So those organisms which are the most fit produce the most offspring, and, again, only a fraction of those offspring survive to reproduce again because they must compete for limited resources within a particular niche. The process of reproduction passes on characteristics and traits from parent to offspring, including those traits which made the parent successful in their particular environment, and at the same time, a conservative genetics limits the variation within a species, so that the differences between individuals in the species are not too great.

Darwin's theory, then, provides a mechanism for how this works. Genetics provides the basis for the passage of traits, and even though genetics wasn't really understood and worked on until early in the twentieth century, Darwin was aware that traits can be inherited and passed on from one generation to the next.

Though natural selection, that is, limiting the number of offspring that can survive creates adaptive pressure, and, at the same time, geologic processes provide motivation for evolutionary change.

Here's how that works. I used the word "adaptive" pressure. You see, when an organism is living in a particular environment, it's tuned with that environment; that is, it's capable of surviving in that particular environment, and If the environment changes, the organism either has to adapt to the new changes, or else it won't be successful, and it will eventually die out, so each species, then, is adapted to a particular environment, and we use the words "habitat" or "niche" to define this environment, and, briefly,

There are several types of changes that can cause changes in the traits of a given population, things like geographical isolation, for example, if two branches of a particular group of organisms are separated from each other, they no longer interact, so they start to develop separately.

Another of these already mentioned is the changing environment. When a changing environment happens, organisms that are not able to adapt simply die out, and those which can adapt pass on those adaptive traits, then, to their offspring.

Another kind of change is mutation. "Mutation" basically is a mistake in the genetic information, and I'll come back and look at these types of changes again in a couple of minutes.

You see, the reason why we're interested in this is that geographic isolation and changes in environment are geological phenomena; whereas, mutations are biological, but like other geologic processes, gradual changes magnified over long periods of time can produce radical changes, which leads to what Darwin calls "speciation" or the creation of a new species, so evolution, then, depends upon interactions between the environment and a gene pool. The "gene pool" is simply all the genes that are contained in a particular group of organisms.

Let's see if we can get a little bit of a sense of how these processes interact. Okay, a population of organisms belonging to a given species carries a gene pool. The gene pool is maintained by inbreeding but is purified by outbreeding; that is, a particular group of organisms that if they only bred within themselves would tend to magnify certain recessive traits, and mostly these recessive traits are harmful; some are neutral; and very few are useful.

We might say that a successful population has stabilized its gene pool and established a niche; in other words, it's a form of ecological equilibrium. The niche that this particular population has carved out for itself provides enough food for that population to maintain itself or grow in size, and at the same time, inbreeding and outbreeding are balanced to maintain a relative uniformity within the species.

Now, let's look at the two kinds of changes that can occur because, you see, environments on the Earth are never stable. They're never stable because the Earth is tectonically active and constantly changing. They're driven on one hand by the internal heat engine; driven on the other hand by the external heat engine, so here are examples of things that can happen: Uplift and subsidence may affect temperature, rainfall, and drainage patterns, and a particular region may go from a mountaintop to seafloor or from seafloor to mountaintop and in fairly short periods of geologic time, short periods again meaning millions or tens of millions of years.

The other thing to note here is that environmental changes occur, there's a difference between individuals adapting and populations adapting. You see, an individual has a limitation in the way it can adapt, and most animals don't really have the ability to adapt very much to the changes in environment.

If you're a fish, for example, living on the continental shelf, and the continental shelf becomes uplifted to dry land, the fish, individually, doesn't have the ability to adapt, but populations do have the ability to adapt because of the large gene pool, so we might say here that possibilities reside in the gene pool, and these possibilities manifest themselves, when necessary, to survive in a new environment, so individuals who are well adapted to one environment may not be well adapted to a new environment, but the offspring they produce might carry traits which help them to adapt better to that new environment, so that over time the population as a whole becomes more adapted to the new environment.

I'll come back and talk about some more of this after the video, but I think this is a good time to take a break and watch the video.

Major funding for Earth Revealed was provided by the Annenberg CPB Project.

Our planet is teeming with a tremendous diversity of life forms, and each year many new species are discovered. There is much to learn about how this living world evolved; yet fossil evidence of earliest life suggests a world once dominated by microscopic organisms. How the present variety of life evolved through time remains one of the most intriguing issues in science.

This is algae, one of the simplest organisms on Earth, and it's been around an extremely long time. From fossil evidence we know that algae has been on Earth at least 1.8 billion years. Even older and more primitive organisms have been found in rocks from all over the world. The probable fossil remains of blue green algae in bacteria have been found in rocks nearly three billion years old in Australia, in Africa, and even here in North America.

The study of fossils has enabled scientists to identify thousands of species in plants and animals that once inhabited the Earth, but the picture of ancient life is far from complete, especially our understanding of life's origins.

An enormous challenge for scientists is to fill in the gaps in our knowledge of life's history, and one of the best ways to start is to go to the rocks themselves and literally dig for clues. These clues exist in the fossilized remains of past life.

Fossils can be formed in several ways. After some organisms die, they are quickly buried by sediment.As the sediment turns to stone, the remains decay or dissolve, and a hollow cavity called a "mold" is left behind Molds are among the most common fossils, but other types also occur in abundance. Buried bone and shell material may survive intact within sedimentary rock for millions of years; however, much of this material crystallizes or is replaced by minerals deposited from ground water.

Casts preserving the original forms of the organism result. Like molds, casts are numerous. Evidence showing the activity of past organisms is also found in the form of fossil tracks, burrows, and nests. That fossils exist at all is startling considering the vigorous processes of erosion and plate tectonics acting on Earth's surface.

The preservation of fossils is probably best in the ocean where you have a superabundance of organisms, so that when you have many millions of little critters that can fall on the sea floor, many tens or even hundreds or thousands may be preserved, so probably our best fossil record comes from marine sedimentary rocks when the organisms are small and the chances for preservation are best.

Terrestrial life, however, is not as dense as it is in the ocean, and because of the chemical weathering and predation, scavenging, and so on by other animals, the record, especially of vertebrates, is not really as complete as would be desirable. Therefore, the record of life that does survives from the geologic past is skewed toward marine organisms.

Of these, organisms with bones or shells are most likely to be fossilized. Only rarely do the softer body parts leave fossil impressions, and so almost all ancient organisms lacking hard parts have faded into obscurity. There are, however, a few spectacular exceptions.

One of these is stromatolites. These lumpy algal mats were the dominant life form throughout much of Earth's history but are now restricted to a few small areas of warm, shallow water. Stromatolitic algae added immense amounts of oxygen to Earth's early atmosphere with important consequences for the evolution of life. Fossil stromatolites appear as pillar-shaped layers of sedimentary debris.

The debris was trapped in the hairlike filaments of the algae when it was still alive. Algal tissue itself is not preserved. Paleobiologist Stanley Aramic specializes in the study of these ancient life forms whose fossils date back as much as three and a half billion years.

This area of the Alexander Hills in Eastern California contains an interesting sequence of stromatolites. Here in the crystal spring formation we have a sequence of stromatolites with other kinds of sediments that are recording an interesting history on the interplay of microorganisms in sediment. These columnar stromatolites represent the time when the microorganisms were living in shallow water, trapping and binding the sediment producing the columnar morphologies that are so characteristic of stromatolites.

Then the environmental conditions change somewhat where coarser sediment sands came in and stalked the growth of stomatolite. Probably the sediments were being deposited too rapidly, and the cyanobacteria just couldn't keep up with this rapid rate of sedimentation. Then, conditions changed and went back to quieter water again with the precipitation and accumulation of calcium carbonate, and then the conditions continued and above here the columnar stromatolites again begin to develop and then higher up these coarser sediments come in an stalk stromatolite growth.

So it's by studying the interplay of the microorganisms with the sediment, and the sequences in which these things occur that a geologist can get an understanding of the environment and hear the history that went on with the deposition of these rocks in the crystal spring formation.

The cell structure of stromatolites and all early life forms was very simple, a structure biologists call "Procaryotic." "Procaryotic cells duplicate by splitting into exact replicas of themselves. Later in the fossil record more complex eucaryotic cell structure appears.

Eucaryotic reproduce sexually, resulting in variations in newly created cells. Most modern life forms, including animals, are eucaryotic.

Prior to sexual reproduction organisms reproduced by division of cells, and if you have simple division, asexual reproduction, you're essentially having the production of clones of the original organism carrying the original organism's DNA as long as there's no mutation involved or anything like that, and essentially there's very little chance with the exception of mutation for evolution because you simply have the same organism being cloned generation, after generation, after generation.

When you finally have sexual reproduction, you have, for the first time, the chance for DNA to combine, to have a brand new organisms that contains some of its parents, of course, but is itself different from either parent, and so you have a whole new ability for these organisms to change.

About 600 million years ago, many complex eucaryotic life forms developed with geological suddenness. Though sexual reproduction no doubt accelerated this appearance of new life, no one knows exactly why it happened. The first appearance of numerous fossils in rocks of this age marks the end of the long Precambrian Era and the beginning of the Paleozoic Era. This event is known as the "Cambrian Explosion." The preservation of these fossils was made possible by the fact that life, for the first time, included abundant hard parts. The significance of the Cambrian Explosion is that this marks the first appearance, the widespread appearance, of hard shelled organisms.

Undoubtedly, some of the earlier Precambrian organisms mutated and evolved hard parts, but for some reason those hard parts were an adverse thing, probably having to do with availability of oxygen. Also, those earlier creatures just absorbed oxygen through the tissues of their skin. If you can imagine a jellyfish or an earthworm, it has a large surface area, through which it can absorb oxygen. A hard part would inhibit that, so in the early part of the Earth's history, we have much less oxygen in the atmosphere than we do now.

By the Cambrian, it may have been then necessary for animals to have hard parts in order to survive because of other animals eating them, so if you have a hard shell, you were less attractive and harder to eat, so you might survive a little better than something like an earthworm or a jellyfish.

The Cambrian Period marks the beginning of what is known as the Paleozoic Era of geologic time. Conditions on Earth during the Paleozoic were ideal for the evolution of life. Within the first 50 million years, all of the major groups of organisms which still survive today had evolved, including the first vertebrates, our most ancient ancestors. During the Paleozoic, continents were drifting toward the Equator transforming cold glaciated terrain into warm, shallow seas. Organisms thrived and diversified in this environment. The bottoms of these shallow seas were literally carpeted with invertebrate animals, including brachopods, trilobites and primitive corals. Above these swam the first fishes.

By middle Paleozoic time, plants and animals had made their way out of the oceans and began to populate the land. The first appearance of life out of the sea was possible in large part because the planet's atmosphere had become oxygen rich due to photosynthesis of stromatolitic algae.

As oxygen built up, an ozone layer formed high in the atmosphere shielding the Earth's surface from deadly solar radiation. Although this process was well actually underway in the Precambrian, it was not until the mid Paleozoic that life was sufficiently developed to take advantage of the dry land habitats the ozone layer protected.

At the same time oxygen increased in the atmosphere, another atmospheric gas, carbon dioxide, decreased because carbon dioxide had become an important building block of life. And a lot of the carbon dioxide is trapped in organisms in seawater and actually eventually becomes limes oozes on the sea floor, which may eventually become limestones, so organisms are very much a part of the control of our environment.

The plants take in the carbon dioxide and give off oxygen, so the plants, the limestones, the ocean, the atmosphere, human beings, horses and camels, the sycamore tree outside are all interrelated in a big cycle called the "carbon cycle."

The late Paleozoic was a time of great global scale changes. It was during this time that almost all of the continents were tectonically assembled into a single supercontinent called "Pangea." This immense land mass nearly spanned the Earth from pole to pole forming a tremendous north to south barrier that locked ocean circulation and severely disrupted the climate.

The continental collisions that created Pangea also transformed continental margins from shallow seas rich with life into mountain ranges similar to the Alps and the Himalayas. These changes in climate and terrain in late Paleozoic time had a grave effect on the global ecosystem. It triggered the largest mass extinction in all of Earth history.

In this tragic finale to a geologic era, over 90 percent of all known species of life disappeared. The small percentage of species that survived the mass extinction at the end of the Paleozoic inhabited a world of opportunities moving into environments now free of competitors.

The fossil record suggests that individual species gave rise to a multitude of new life forms each well suited to its particular environment.Mass extinctions have occurred throughout the history of life. The impact of these mass extinctions in some ways could be viewed almost as taking out a lot of organisms and allowing for newer organisms to evolve and repopulate the Earth with different species.

The arrival of new species in the aftermath of the Paleozoic mass extinction heralds the next geologic era, the Mesozoic. Evolution of the Mesozoic was critical for life on Earth for it was at this time that life was transformed from ancient to more modern forms. Reef building corals appeared on continental margins, and large reptiles joined the fishes as swimming predators. Later in the Mesozoic the flowering plants and assiduous trees first evolved on land as well as flying reptiles and the first bird. Mammals were also present, but they were small and inconspicuous, but despite this proliferation of life, the Mesozoic Era is best known as the Age of the Dinosaurs, for it was these so-called terrible lizards that dominated the land for some 140 million years.

Dinosaurs captured the public's imagination from the time their fossils were first excavated in the early Nineteenth Century. Soon the rush began to collect the remnants of these ancient creatures. Expeditions spanned the globe uncovering everything from dinosaur skulls to fossilized dinosaur eggs. Piece by piece, a spectacular view of Earth's past inhabitants gradually took shape. Dinosaurs were seen as solitary cold blooded creatures, who moved through their tropical world at a sluggish pace,but recent data, together with a better understanding of biology, have prompted scientists to rethink this view.

Fossil tracks indicate many of these creatures must have been swift footed. Dinosaur fossils have also been discovered in places that were dry and had cool climates during the Mesozoic. This and other evidence suggests these animals were warm blooded. Predatory dinosaurs apparently required large amounts of food for survival, suggesting the high rate of metabolism typical of warm blooded animals.

Finally, far from living solitary lives, some dinosaurs lived in nest building herds caring for their young. The dinosaurs might still dominate our world today even more diverse and spectacular than ever before, but for another drastic environmental change 65 million years ago, which triggered yet another round of worldwide mass extinctions. Geologic clues suggest several natural phenomenon may have been responsible for the change. Dust from extremely intense volcanic eruptions possible cooled the atmosphere enough to change Earth's climate and seriously disrupt the food chain. Or a large asteroid may have struck the Earth cooling it as smoke and dust blocked out sunlight. Whatever the cause, many species were no longer well suited for their environments. The mass extinction at the end of the Mesozoic not only exterminated the dinosaurs, it ushered in the present era of geologic time, the Cenozoic.

Cenozoic time is marked by intense tectonic changes, which helped to direct events in evolution. For example, when Australia became tectonically detached from other continents, mammals riding aboard this drifting land mass proceeded on their own evolutionary path, resulting in the unusual forms we see in Australia today, such as the duck billed platypus, the spiny anteater, and the kangaroo. The ecological crisis of the Mesozoic extinction and the tectonic activity that followed helped set the stage for events in evolution, which ultimately allowed mammals to dominate the land.

An extraordinary record of life in the Cenozoic has been found in a rather unexpected place, buried beneath the streets of one of the world's largest cities. At Rancho La Brea in Los Angeles, natural tar pools have trapped thousands of animals. The tar has preserved an especially complete record of prehistoric life from the late Cenozoic Era.

The last few million years of Earth history; in fact, the last 2 million years, has generally been considered the Ice Ages. It is that setting that governs what we see here at Rancho La Brea. We are in the last half of the last glacial period. Just before modern climates come about, the oldest remains here at Rancho La Brea of vertebrates, terrestrial vertebrates, are about 36, 38,000 years old, and the fossil record here extends from that period up to about 10,000 years ago.

The La Braya tar pits have yielded fossils ranging from microscopic plants and pollen scores to giant mammoths and bisons. For me the most fun is not so much finding the fossil, it's identifying that suddenly this isn't just a lump of bone; suddenly this is a saber toothed cat skull. This is a bison femur, something like that, and there's a thrill that goes through you when suddenly you realize this isn't just a fossil. This is a particular kind of animal that died here, and that's very exciting for me. As the site is excavated, the positions of every fossil are carefully measured and cataloged. At the end of the excavation we want to be able to reconstruct exactly how these bones came to be deposited, what happened to this animal between the time the animal died in the asphalt and the time that we dig it up here, so we hope to be able to do when we take measurements on each bone is to reconstruct how this bone has moved through time in the deposit and put it back with the rest of the animal, so that we can determine what happens to these individuals as they decompose and as the pit is deposited through time.

After excavation, the bones are taken to a nearby lab for analysis. Examining the material around the bones is as important as identifying the bones themselves. All of the soil surrounding that large bone when it's cleaned off will be gone through underneath the magnifying glass looking for the small bones, and plants, and shells, and insect material. Microfossils are that tool that you need to really reconstruct an environment. They give you much more information about rainfall and climate changes than large amounts do.

Large animals didn't necessarily live here. They just died here; whereas, a mouse or a gopher probably spent its entire life right within the area of the park, so it is very important that we do collect all of these small things. Through careful identification of fossils collected at the tar pits, scientists have been able to reconstruct the former habitat at Rancho La Brea.

Forty thousand years ago the Los Angeles Basin was a sagebrush plain dotted with groves of cypress and pine and stream side woodlands. A great abundance and diversity of animals lived here, including deer, dire wolves, saber toothed cats, ground sloths, and mastodons. Most of the larger mammals in this ecosystem have since become extinct. The fossils at Rancho La Brea represent a very brief span of late Cenozoic time, just 25 to 30,000 years. Though few ancient life localities are as rich as Rancho La Brea, other Cenozoic fossils provide spectacular evidence of evolution spanning millions of years. The case of the horse is especially stunning.

Appearing near the beginning of the Cenozoic the first horses were no larger than dogs and had four toes on each foot. They lived in woodlands. Then, about 40 million years ago a new ecosystem appeared on earth, the grasslands. Moving in to forage this environment, the horse grew larger, and threw time developed hooves to replace toes. This permitted horses to run swiftly with better chance of survival in the open.

The idea that species gradually changed to better suit their natural habitats was first described by Charles Darwin some 150 years ago. The main thing that Darwin contributed to our idea of life in the past and what happened was the method of evolution. Prior to Darwin's idea of evolution, we knew that plants and animals changed. We knew that they varied over time, but we didn't really have an idea as to why they changed, and Darwin gave us the method.

Darwin observed that within every population of animals there is a range of characteristics, be it different heights, agility, or hair color. Those animals whose features most exposed them to danger tend to die out leaving few offspring. Those animals with more favorable features survive and through reproduction pass on these characteristics to future generations, but features favorable at one time won't be favorable during another, and so animals evolve. Yet Darwin's model has run into some difficulty.

Darwin was a gradualist, and that is, he thought that evolution proceeded very slowly, and because of that his theory of evolution required a great deal of geological time; however, when paleontologists carefully examined the fossil record looking for those intermediates that would occur between species, they couldn't find these intermediates.

Most paleontologists blame lack of intermediates on an incomplete fossil record, but after more careful study there was a realization that these transitional forms simply don't exist. This brought about the idea of punctuated equilibrium hat once a species appeared it lasted for a long time unchanged. Then suddenly in terms of geological time, a new species appeared, and there is really very little, if any, evidence of the intermediates.

Today punctuated equilibrium has been added to Darwin's Theory to account for change seen in the fossil record. Not only can life evolve gradually, but in a world where rapid environmental change sometimes occurs, rapid evolution can also take place.

The history of life over great lengths of time is governed by the need to adapt or die out. During the Pleistocene ice age mammals dominated the continents. These included mastodons, mammoths, ground sloths, and saber toothed cats. They suddenly disappeared about 8,000 years ago. The cause of this mass extinction isn't known, but a likely possibility is the increasing efficiency of human hunters. Our species is, of course, very successful, and we have enjoyed a rapid rise in population since the end of the last ice age, but the evolutionary development of humans is not well understood because human fossils are so rare.

The most famous examples of an ancestral species come from Africa where the first humans most likely evolved. It's sobering, and at the same time exhilarating, to stare into the abyss of geologic time and realized that it all began with tiny and extremely simple organisms.

Organisms like these gave rise to the vast variety of life on Earth. The resilience of life is truly amazing when we consider the many mass extinctions that have occurred throughout Earth history. The latest mass extinction may be taking place at this very moment. The impact of humans on Earth is causing species of both plants and animals to disappear at an astounding rate. Many scientists estimate that a larger percentage of the Earth's species will disappear within our lifetime that were lost during the mass extinction of the dinosaurs.

Some people figure humans as the final product in a long process of organic evolution, but our species is only one branch on the evolutionary tree of life. Organic evolution has operated since life first appeared on Earth, and because geologic processes continually change the conditions of the Earth's surface, we can only imagine what life will be like in the distant future.

Major funding for "Earth Revealed" was provided by the Annenberg C.P.B. Project.

Well, if you thought that the material on evolution I presented before the video was a little bit abstract, I hope the video cleared it up a little bit, but you're not alone in this; in fact, Darwin, himself, thought his Theory of Evolution was abstract, and he hesitated to publish it in the first place because he suspected it would not be understood by most of the people who read it, and he suspected that the people who did read it wouldn't understand it or wouldn't agree with it, and he was really right on both accounts.

Many people did read it; in fact, the book sold out on the first day it was published, and if you had, of course, the first edition of that book today, you'd be very rich. Many people disagree with the theory mostly on principle because for one reason it demystified people. It removed people from a special place in the creation of the universe and put us on the same level, you might say, with other organisms. As a result of this, Darwin remained very private to avoid confrontations. Darwin was aware of the controversy this would cause, and, in fact, that's the reason he asked Lyell to present the paper in the first place because he didn't want to have to deal with the people who would disagree with him.

It's also worthy of note here that Darwin became the subject of much ridicule as the result of this theory. I want to note here also that we have some major evidence that's available today that wasn't available to Darwin.

Just briefly, as you remember, Wegener had postulated that the continents began to drift about 200 million years ago. It's interesting to compare the life forms on the new world and North and South America with those on the old world, most notably are the new world versus old world monkeys, the chimpanzees versus the new world monkeys, which show very different forms.

Another one is right here in the Hawaiian Islands. Various types of snails that are found on adjacent ridges in the Hawaiian Islands show very different traits; n fact, there are different species of snails found on two adjacent ridges, and, you see, for snails, two ridges are separated by only a mile, it might as well be the Atlantic Ocean because the snails are not capable of migrating back and forth, so this separation of the populations allows different species to appear even over fairly short time periods and less than a million years in the case of Hawaii.

One of the things that troubled Darwin himself about the theory of evolution was the ultimate chicken and egg question; that is, if you project backwards that life has become increasingly more, and more, and more complex, where did it begin, and how did it begin, and the question of the origin of life is one of the major problems that scientists are dealing with today.

There are basically only three possibilities. One is that life came from extraterrestrial sources; in other words, that it exists in other places in the universe and somehow was transported to Earth. For various reasons, this theory is not a particularly good one, but people are working on these theories, and some people believe that this might have been the case.

The other, of course, is divine intervention, that God basically zapped the Earth and caused life as portrayed in the Genesis Account.Most geologists today, most scientists today, tend to prefer looking at the possibility that life may have evolved from chemicals that were already here on the early Earth, probably in the sea.

The video showed the result of these Miller-Urey experiments, which showed that if you take the constituents that were present in the early atmosphereand zapped them with electricity or ultraviolet light, that they produce the chemicals of life. Now, there's a big difference between producing the chemicals of life and producing life itself, and many people might say that some divine intervention is necessary in there at some point to make the transition from the chemicals of life to life itself. There are many gaps that are not yet explained, and this question is by no means resolved. What we do know is that the Earth's atmosphere has changed.

Oxygen accumulation in the atmosphere was probably the first major ecological crisis, and it points to the fact that life on Earth is capable of modifying the atmosphere, and, in fact, modifying the atmosphere to the point where it becomes poisonous for those organisms that create it. The first photosynthetic organisms that produced oxygen did not have the ability to not be oxidized by the oxygen, so the oxygen that was produced by photosynthesis was poisonous to those organisms, and a major adaptation was necessary, which today we recognize are enzymes which keep organic materials, such as ourselves from oxidizing in the open air.

Another important effect of the accumulation of oxygen, of course, in the atmosphere, was that it allowed the development of oxygen breathing organisms, so that respiration could evolve,and it also allowed the ozone layer to form, which blocks the surface of the Earth from ultraviolet radiation, which allowed organisms then to exist, not only under water, which also blocks the ultraviolet, but allows them to exist on land surface as well.

We have good evidence that this oxygen revolution occurred because rocks before about 2 billion years old are not oxidized. Beginning about 2 billion years or so, the iron and sediments becomes oxidized and is red in color, and we use this as evidence that oxygen appeared in the atmosphere at a rather late time.

One more thing to note here. There are other connections between evolution and plate tectonics. It's been observed that evolution appears to proceed in a series of small jumps over relatively short time intervals; in other words, everything seems to go along pretty fine with no major changes until suddenly over a very short time period, a whole lot of new species appear,including the mass extinctions that the video talked about.

These rapid changes are followed by long periods of stability. Now, how does this sound familiar? Short changes followed by long periods of stability. We saw this in geologic processes such as mountain building processes, for example. Well, in fact, geologic events tend to occur exactly this way. In evolutionary terms this is called the "punctuated equilibrium," but it applies to both geologic processes and to evolution because plate motions remain consistent for long periods of time, and large areas do remain tectonically stable between cycles of orogeny, for example.

It's these changes resulting from this interaction of geological and biological processes hat we see in the fossil record. The video and study guide detail the forms of life which characterize the major eras, and you should spend some time to learn what those major types of life are. Okay. You know, whether or not the details of evolution are correct is not the issue here. The evidence for the changes in life through its history are undeniable, and evolution at this point is a model which helps us understand how new forms of life developed over time.

Over geologic time small changes can have large results, both in geology and in biology. Some people might say that evolution is more of a geological process than a biological process, but I don't think you can really separate them because life and Earth are so intimately linked it's really hard to tell where one begins and the other ends, and, by the way, if you're having trouble grasping geologic time, you're not alone. Most people can't; in fact, I'm not sure that there's anyone who really can grasp this immense amount of geologic time. That's why I said in the last program that it was the crowning achievement of the human intellect to have figured out this large amount of time, and that's probably why it took humans as a species more than 10,000 years to figure it out. It's not an easy concept, but the more we think about it and being aware of it, I think it helps us to put these changes that occur on the Earth into a proper perspective, including the development of life and the changes that occur.

Well, next time we'll begin to look at Lesson 12, Chapter 9. We'll focus our attention to the materials that make up the Earth; that is, minerals and rocks.

To do this, we'll study atoms and how they combine into minerals, and how those minerals combine into rocks, kind of changing our focus, so for next time you should

Well, until next time, study hard, and I'll see you then.