Emil Arsov

Where We're Going      

1. Why do some species survive while others go extinct?     

Extinction is often caused by a change in environmental conditions. When conditions change, some species possess adaptations that allow them to survive and reproduce, while others do not. If the environment changes slowly enough, species will sometimes evolve the necessary adaptations, over many generations. If conditions change more quickly than a species can evolve, however, and if members of that species lack the traits they need to survive in the new environment, the likely result will be extinction.

2. Does evolution proceed toward increasing complexity? 

In the approximately 3.8 billion years since life originated on Earth, evolution has resulted in many complex organisms and structures. The human brain and stereoscopic eyes are just two examples. At the same time, simpler organisms like algae, bacteria, yeast, and fungi, which arose several billion years ago, not only persist but thrive. The presence of single-celled organisms alongside complex organisms like humans testifies to the fact that evolution within a given lineage does not necessarily advance toward increasing complexity. When more complex organs are advantageous, complex organs have arisen. Single-celled organisms, however, fill many roles, or niches, much better than any multicellular organism could, and so they remain in a relatively stable state of adaptation.

3. If fish became amphibians through the process of evolution, then why do fish still exist?        

Fossil evidence clearly shows that amphibians descended from one group of ancient fishes whose thick, bony fins gradually evolved into limb-like appendages. Other species gave rise to the kinds of fish that inhabit oceans, lakes, and streams around the world today. Fish, like all living creatures, continue to evolve. This evolution is not toward a life on land, but instead toward successful use of the underwater environment. There are countless ecological opportunities under water, which is why fish still exist. When the earliest ancestors of modern amphibians left the water, they found many new opportunities on land. As amphibians and other land creatures diversified, however, fewer and fewer opportunities existed for newcomers.

 4. Could apes ever evolve into some other humanlike creature?   

It is possible that in many millions of years present day apes could evolve into some other humanlike species. It is, however, very improbable. First of all, humans did not evolve from any of the species we know as apes today. At some point 5 to 8 million years ago, the common ancestor of humans and modern apes diverged to form the two separate lineages we know today. The species at the end of these lineages are a result of a very specific combination of selection pressures and genetic mutations over millions of years. This same combination is highly unlikely to occur ever again.

5. Are humans influencing the process of evolution? 

Most scientists would agree unequivocally that humans have greatly affected the process of evolution, from the rise of antibiotic and pesticide resistance to the largely human-caused increase in the extinction rate. Our effect on the process of evolution even extends to our own species' evolution. Technology and culture have protected us to a great extent from the selective pressures that drive evolution, allowing many people -- especially those in developed nations who, without medical intervention, would not live to reproductive age -- to pass their genes on to the next generation. Other scientists note that technology and culture have changed but not eliminated the role of natural selection on our species. We now adapt to crowding, pollution, and new disease rather than the necessity to escape from large predators. Humans will change in the future, but are unlikely to evolve into a new, separate species because no human group is truly isolated anymore, given our transportation systems. Without genetic isolation, there is no further opportunity for speciation among humans.

6. If humans evolved from apes then why are there still apes?     

Humans did not evolve from present-day apes. Rather, humans and apes share a common ancestor that gave rise to both. This common ancestor, although not identical to modern apes, was almost certainly more apelike than humanlike in appearance and behavior. At some point -- scientists estimate that between 5 and 8 million years ago -- this species diverged into two distinct lineages, one of which were the hominids, or humanlike species, and the other ultimately evolved into the African great ape species living today.

How We Know   

1. Is there evidence for evolution?    

In the 150 years since Darwin proposed the theory of evolution by natural selection, a mountain of evidence has accumulated to support the theory. A greatly expanded fossil record since Darwin's time, the discovery of DNA and the process of genetic replication, an understanding of radioactive decay, observations of natural selection in the wild and in laboratories, and evidence in the genomes of many different organisms, including humans, have all bolstered the validity of the theory of evolution.

2. How can you know what happened millions of years ago if no one was there to see it?

Evidence and observation are the building blocks of all scientific inquiry; evolutionary science is no different. Evidence in the form of the fossil record, geological formations, and genetics attest to change having taken place and give clues to how evolution works. The theory of evolution puts these clues together into a cohesive explanation of the diversity of living things. Like all theories, the theory of evolution relies on tangible evidence as well as inference for those things that can't be observed directly. It is important to remember that Earth itself contains evidence of life in the past, and that this evidence provides critical support for the theory of evolution.

3. Does the fossil record tell us the whole story?

Opponents of evolution point to gaps in the fossil record as proof that the theory is invalid. They say the fossil record fails to show what are called "transitional forms," generally the in-between stages as one type of creature evolved into another. The fossil record certainly has gaps, mostly because the conditions required to create fossils have been rare ever since life began on Earth. A very small percentage of animals that have lived and died ever became fossils. Thus, many pieces of the puzzle are missing; some will never be found. Nonetheless, we have many, many fossils that illustrate evolutionary transitions between fish and amphibians, between reptiles and mammals, between dinosaurs and birds, and in many lineages such as whales and horses. And new fossils continue to reveal transitional forms that some said don't exist.

What It Means to Evolve      

1. How can one species "turn into" another?         

One species does not "turn into" another or several other species -- not in an instant, anyway. The evolutionary process of speciation is how one population of a species changes over time to the point where that population is distinct and can no longer interbreed with the "parent" population. In order for one population to diverge enough from another to become a new species, there needs to be something to keep the populations from mixing. Often a physical boundary divides the species into two (or more) populations and keeps them from interbreeding. If separated for long enough and presented with sufficiently varied environmental conditions, each population takes its own distinct evolutionary path. Sometimes the division between the populations is never breached, and reproductive isolation remains intact purely for geographical reasons. It is possible, though, if the populations have been separate for long enough, that even if brought back together and given the opportunity to interbreed they won't, or they won't be successful if they try.

2. How can evolution produce complex organs like the eye?        

In the process of natural selection, individuals in a population who are well-adapted to a particular set of environmental conditions have an advantage over those who are not so well adapted. These individuals pass their genes and advantageous traits to their offspring, giving the offspring the same advantages. Generation after generation, natural selection acts upon each structure within an organ like the eye, producing incremental improvements in the process. Each tiny change in a structure is dependent upon changes in all the other structures. In this way, individual parts of a system evolve in unison to be both structurally and functionally compatible. Eventually, over thousands and sometimes millions of years, the small improvements add up -- the simple, systematic process has produced an almost unfathomably complex organ. Recently, scientists have found clues to the evolutionary pasts of some of the most complex organs, helping to clarify how this process works.

3. Does evolution stop once a species has become a species?

Evolution does not stop once a species becomes a species. Every population of living organisms is undergoing some sort of evolution, though the degree and speed of the process varies greatly from one group to another. Populations that experience a major change in environmental conditions, whether that change comes in the form of a new predator or a new island to disperse to, evolve much more quickly than do populations in a more stable set of conditions. This is because evolution is driven by natural selection, and because when the environment changes, selective pressures change, favoring one portion of the population more heavily than it was favored before the change.

4. Is evolution happening now?     

Evolution is always happening, though often at rates far too slow to be observed in a matter of days, weeks, or even years. The effects of evolution can be felt in almost every aspect of our daily lives, though, from medical and agricultural dilemmas to the process of choosing a good mate. In medicine, there's the question of how long the antibiotics we take now will remain effective, given the relatively fast rate at which bacteria can evolve resistance to drugs. In agriculture, the need to protect this year's crops is pitted against the concern that doing so will set the stage for insects to evolve pesticide resistance. For all of us, there is the issue of decreasing biodiversity, as most scientists believe that life on Earth is currently undergoing a mass extinction in which 50 percent or more of species will die out. These are just a few examples of ways in which evolutionary processes affect our daily lives.         

A Matter of Time          

1. How long does evolution take?   

Even though evolution is taking place all around us, for many species the process operates so slowly that it is not observable except over thousands or hundreds of thousands of years -- much too long to witness in a human lifetime. There are cases in quickly reproducing life forms like bacteria and fruit flies, however, where evolution can be seen happening in a matter of weeks for the bacteria and many months for the flies. In these cases the relatively large number of generations in a given period of time is key, since evolutionary change occurs incrementally from one generation to the next. All else being equal, the more generations you have, the more quickly evolution happens.

2. Can you observe evolution happening?

Because for many species, humans included, evolution happens over the course of many thousands of years, it is rare to observe the process in a human lifetime. Usually only laboratory scientists studying quickly reproducing life forms, like single-celled creatures and some invertebrates, have the opportunity to see evolutionary change happen before their eyes. All of us can and do experience the indirect effects of evolution nearly every day, however. One of the more important evolutionary concerns facing humans today is the emergence of antibiotic-resistant microbes. A battle against bacteria that we have been winning with medicine for the last 50 years or so is now an even race, according to some scientists -- because of the rapid rate of bacterial evolution. Similarly, the use of pesticides in agriculture has driven the evolution of resistant insects that require more or harsher chemicals to be killed. Scientists studying Galapagos finches have seen evolutionary changes in beak size and shape in just a few years. Major evolutionary transformations take much, much longer.

3. How old is Earth?    

Earth is approximately 4.55 billion years old. Radiometric dating, a method that measures the level of radioactive decay in rocks to determine how old they are has consistently aged moon rocks and meteorites at 4.4 to 4.6 billion years old. Ages of these types of rocks provide the most accurate estimates of the age of Earth and the rest of the solar system because they have not been subjected to the same forces that recycle Earth's crust. As new land forms along cracks between the planet's continental plates, old rocks are destroyed. Thus, the oldest rocks on Earth may not exist anymore.

The oldest dated minerals, at 4.0 to 4.2 billion years old, are tiny zircon crystals found in sedimentary rocks in western Australia.

 4. How long has life existed on Earth?    

The oldest known fossils are approximately 3.5 billion years old, but some scientists have discovered chemical evidence suggesting that life may have begun even earlier, nearly 4 billion years ago.

5. How long ago did dinosaurs exist?      

Dinosaurs existed between 230 million years ago and 65 million years ago, but none of the known dinosaur species existed for this entire time period. Throughout the group's existence, individual dinosaur species were evolving and going extinct. Some species diverged and gave rise to other species, while others disappeared. A mass extinction event at the end of the Cretaceous period,

65 million years ago, ended the reign of dinosaurs on Earth. Recently, many scientists have come to the conclusion that, while dinosaurs may have disappeared, one dinosaur lineage had evolved into birds long before the extinction event that wiped out the other dinosaurs -- and so, in a sense, dinosaurs are still around today.

6. How long ago did humans become human?   

The oldest known hominid, or humanlike species, has been dated at 4.4 million years old. Another species, which is yet to be confirmed as a hominid, has been dated at 6 million years old. Scientists estimate that the hominid lineage diverged from the ape lineage 5 to 8 million years ago. Homo sapiens, the species to which we belong, has existed for about 100,000 years.

Darwin     

1. Who was Charles Darwin? 

Charles Darwin was born on February 12, 1809, in Shrewsbury, England. His father, Robert Darwin, was a physician, the son of Erasmus Darwin, a poet, philosopher, and naturalist. Charles's mother, Susannah Wedgwood Darwin, died when he was eight years old.

At age 16, Darwin left Shrewsbury to study medicine at Edinburgh University. He later enrolled in Cambridge University to prepare for a career as a clergyman in the Church of England. After receiving his degrees in 1831, Darwin accepted an invitation to serve as an unpaid naturalist on a five-year scientific expedition to South America aboard the HMS Beagle. This voyage and Darwin's later research formed the basis for his theory of evolution by means of natural selection, detailed in his book On the Origin of Species, published in 1859.

Darwin continued throughout most of the rest of his life to publish his research and writings on biology. In his later years, Darwin was plagued by fatigue and intestinal sickness, thought by some historians to have been caused by Chagas' disease, contracted during his travels in South America. He died on April 19, 1882, and lies buried in Westminster Abbey.

2. Why was Darwin's idea considered dangerous?     

Darwin's theory of evolution by natural selection was considered dangerous in 19th-century England because it threatened the prevailing views of the Anglican Church and society at large. If extrapolated beyond its role as a scientific theory, it seemed to run counter to the notion of humankind's supremacy and God's hand in the world order. The same fear exists today. Evolution can seem to be in direct conflict with deeply held belief systems of how life came to exist on Earth.

Why It's Important      

1. How can the evolution of one species affect the evolution of another?             

No species exists in a vacuum; every form of life on Earth interacts over time with other organisms, as well as with its physical environment. For that reason, the evolution of one species influences the evolution of species with which it coexists by changing the natural selection pressures those species face. The classic examples of this sort of evolution, called coevolution, are predator-prey and host-parasite relationships.

One such predator-prey relationship exists between garter snakes and a species of salamander-like amphibian called a rough-skinned newt. In parts of the midwestern United States, garter snakes prey on newts, and probably have for thousands of years. In that time, over countless generations, the newts have evolved a powerful defense: a toxic chemical that they secrete through their skin. Where garter snakes are concerned, however, this defense mechanism has only been marginally successful. Generation after generation, as the newts became more poisonous, the snakes also evolved, developing greater tolerance to the newt's toxin. The result of this coevolutionary process, played out over countless snake and newt generations, is a chemical more toxic than almost any other natural substance on Earth, and a population of snakes that are seemingly immune to the toxin's effects.

2. What does it mean when some scientists say humans have stopped evolving?    

Technology and culture have protected us to a great extent from the selective pressures that drive evolution, allowing many people, especially those in developed nations who would otherwise not live to reproductive age, to pass their genes on to the next generation. In addition, human groups are no longer isolated; they travel the globe. Without genetic isolation, there is no further opportunity for speciation among humans. On the other hand, natural selection is a function of environmental change, and our physical, biological, and cultural environments have changed tremendously. Humans face, for example, new diseases like HIV/AIDS that can greatly impact survival and reproduction. Human populations may also be undergoing evolutionary changes of which we're not yet aware.

3. If extinction is a natural part of life on Earth, why should we care about protecting endangered species?     

If the mass extinction most scientists agree we're experiencing now is allowed to continue, it will be the first time in the history of life on Earth that a single species would be responsible for such a catastrophe. And although extinction is a natural process that has occurred many times in the distant past, it's a process that would be in our best interest to avoid. The extinction of just one species can dramatically impact many others, and like all creatures, humans rely heavily on other species. It is therefore impossible to predict how we might ultimately be affected by a mass extinction.

4. How does evolution affect me in my daily life?     

We can and do experience the indirect effects of evolution nearly every day. One of the more important evolutionary concerns facing humans today is the continual evolution of antibiotic-resistance in bacteria. The successful medical battle we have waged against bacteria for the last 50 years is now an even race, according to some scientists. Similarly, the use of pesticides in agriculture has driven the evolution of resistant insects, requiring the use of harsher chemicals in greater quantity to kill them.

Evolution on Trial       

1. What is "intelligent design," and is it science?        

"Intelligent design theory" is built on the belief that evolution does not sufficiently explain the complexity that exists in life on Earth and that science should recognize the existence of an "intelligent designer." Proponents assert that their criticism of evolution is scientific, not religious. But the various aspects of intelligent design theory have not yet been subjected to the normal process of scientific experimentation and debate, nor have they been accepted by the scientific community. No research supporting the claims of intelligent design has ever been published in any recognized, professional, peer-reviewed scientific journal. Finally, the question of whether there is an intelligent designer is untestable using the methods of science, and therefore is not a scientific claim.

2. What is "creation science"? Is it a real science?      

Proponents of "creation science" hold that special creationism -- the conviction that God created the universe, including humans and other living things, at one time in the relatively recent past -- can be supported using the methods and theory of science. Scientists from many fields have examined these ideas, however, and have found them to be scientifically insupportable. For example, evidence for a very young Earth is incompatible with many different methods of establishing the age of fossils and geological formations. Furthermore, because the basic proposals of creation science are not subject to test and falsification, these ideas do not meet the criteria for science.

3. Wouldn't it be fair to teach evolution and "creation science" and/or "intelligent design" in public schools?    

The Federal courts have ruled that creation science is not science at all, but a religious concept. Therefore it is not appropriate content for a science classroom. More to the point, evolution studies, like other sciences, are founded on a growing body of observable, reproducible evidence in the natural world, whereas "creation science" is based on accounts written in the Bible and "intelligent design" is not yet supported by scientific evidence. Teaching evolution alongside these other approaches would imply that creation science and intelligent design theory are as rigorously tested as evolution, and they are not.    

The Basics 

1. What is evolution?   

Biological evolution refers to the cumulative changes that occur in a population over time. These changes are produced at the genetic level as organisms' genes mutate and/or recombine in different ways during reproduction and are passed on to future generations. Sometimes, individuals inherit new characteristics that give them a survival and reproductive advantage in their local environments; these characteristics tend to increase in frequency in the population, while those that are disadvantageous decrease in frequency. This process of differential survival and reproduction is known as natural selection. Non-genetic changes that occur during an organism's life span, such as increases in muscle mass due to exercise and diet, cannot be passed on to the next generation and are not examples of evolution.

2. Isn't evolution just a theory that remains unproven?     

In science, a theory is a rigorously tested statement of general principles that explains observable and recorded aspects of the world. A scientific theory therefore describes a higher level of understanding that ties "facts" together. A scientific theory stands until proven wrong -- it is never proven correct. The Darwinian theory of evolution has withstood the test of time and thousands of scientific experiments; nothing has disproved it since Darwin first proposed it more than 150 years ago. Indeed, many scientific advances, in a range of scientific disciplines including physics, geology, chemistry, and molecular biology, have supported, refined, and expanded evolutionary theory far beyond anything Darwin could have imagined.

3. Are all species related?      

Yes. Just as the tree of life illustrates, all organisms, both living and extinct, are related. Every branch of the tree represents a species, and every fork separating one species from another represents the common ancestor shared by these species. While the tree's countless forks and far-reaching branches clearly show that relatedness among species varies greatly, it is also easy to see that every pair of species share a common ancestor from some point in evolutionary history. For example, scientists estimate that the common ancestor shared by humans and chimpanzees lived some 5 to 8 million years ago. Humans and bacteria obviously share a much more distant common ancestor, but our relationship to these single-celled organisms is no less real. Indeed, DNA analyses show that although humans share far more genetic material with our fellow primates than we do with single-celled organisms, we still have more than 200 genes in common with bacteria.

It is important to realize that describing organisms as relatives does not mean that one of those organisms is an ancestor of the other, or, for that matter, that any living species is the ancestor of any other living species. A person may be related to blood relatives, such as cousins, aunts, and uncles, because she shares with them one or more common ancestors, such as a grandparent, or great-grandparent. But those cousins, aunts, and uncles are not her ancestors. In the same way, humans and other living primates are related, but none of these living relatives is a human ancestor.

4. What is a species?    

Members of one species do not normally interbreed with members of other species in nature. Sometimes, members of different species, such as lions and tigers, can interbreed if kept together in captivity. But in nature, geographic isolation and differences in behavior, such as choice of habitat, keep these sorts of closely related animal species apart. Similarly, closely related species of plants can sometimes be hybridized by horticulturists, but these hybrids are rarely found in nature. A species, then, is defined by science as a group of interbreeding or potentially interbreeding populations that is reproductively isolated from other such groups.

5. What do genes have to do with evolution?    

Genes are the portions of an organism's DNA that carry the code responsible for building that organism in a very specific way. Genes -- and, thus, the traits they code for -- are passed from parent to offspring. From generation to generation, well-understood molecular mechanisms reshuffle, duplicate, and alter genes in a way that produces genetic variation. This variation is the raw material for evolution.

6. What role does sex play in evolution? 

Sexual reproduction allows an organism to combine half of its genes with half of another individual's genes, which means new combinations of genes are produced every generation. In addition, when eggs and sperm are produced, genetic material is shuffled and recombined in ways that produce new combinations of genes. Sexual reproduction thus increases genetic variation, which increases the raw material on which natural selection operates. Genetic variation within a species -- also known as genetic diversity -- increases a species' opportunity for change over successive generations.

7. Is evolution a random process?  

Evolution is not a random process. The genetic variation on which natural selection acts may occur randomly, but natural selection itself is not random at all. The survival and reproductive success of an individual is directly related to the ways its inherited traits function in the context of its local environment. Whether or not an individual survives and reproduces depends on whether it has genes that produce traits that are well adapted to its environment.

8. Are evolution and "survival of the fittest" the same thing?     

Evolution and "survival of the fittest" are not the same thing. Evolution refers to the cumulative changes in a population or species through time. "Survival of the fittest" is a popular term that refers to the process of natural selection, a mechanism that drives evolutionary change. Natural selection works by giving individuals who are better adapted to a given set of environmental conditions an advantage over those that are not as well adapted. Survival of the fittest usually makes one think of the biggest, strongest, or smartest individuals being the winners, but in a biological sense, evolutionary fitness refers to the ability to survive and reproduce in a particular environment. Popular interpretations of "survival of the fittest" typically ignore the importance of both reproduction and cooperation. To survive but not pass on one's genes to the next generation is to be biologically unfit. And many organisms are the "fittest" because they cooperate with other organisms, rather than competing with them.

9. How does natural selection work?       

In the process of natural selection, individuals in a population who are well-adapted to a particular set of environmental conditions have an advantage over those who are not so well adapted. The advantage comes in the form of survival and reproductive success. For example, those individuals who are better able to find and use a food resource will, on average, live longer and produce more offspring than those who are less successful at finding food. Inherited traits that increase individuals' fitness are then passed to their offspring, thus giving the offspring the same advantages.

10. How do organisms evolve?       

Individual organisms don't evolve. Populations evolve. Because individuals in a population vary, some in the population are better able to survive and reproduce given a particular set of environmental conditions. These individuals generally survive and produce more offspring, thus passing their advantageous traits on to the next generation. Over time, the population changes.

 11. Does evolution prove there is no God?

No. Many people, from evolutionary biologists to important religious figures like Pope John Paul II, contend that the time-tested theory of evolution does not refute the presence of God. They acknowledge that evolution is the description of a process that governs the development of life on Earth. Like other scientific theories, including Copernican theory, atomic theory, and the germ theory of disease, evolution deals only with objects, events, and processes in the material world. Science has nothing to say one way or the other about the existence of God or about people's spiritual beliefs.

Where We Came From 

1. Did we evolve from monkeys?       

Humans did not evolve from monkeys. Humans are more closely related to modern apes than to monkeys, but we didn't evolve from apes, either. Humans share a common ancestor with modern African apes, like gorillas and chimpanzees. Scientists believe this common ancestor existed

5 to 8 million years ago. Shortly thereafter, the species diverged into two separate lineages. One of these lineages ultimately evolved into gorillas and chimps, and the other evolved into early human ancestors called hominids.

2. How did humans evolve?  

Since the earliest hominid species diverged from the ancestor we share with modern African apes, 5 to 8 million years ago, there have been at least a dozen different species of these humanlike creatures. Many of these hominid species are close relatives, but not human ancestors. Most went extinct without giving rise to other species. Some of the extinct hominids known today, however, are almost certainly direct ancestors of Homo sapiens. While the total number of species that existed and the relationships among them is still unknown, the picture becomes clearer as new fossils are found. Humans evolved through the same biological processes that govern the evolution of all life on Earth. See "What is evolution?", "How does natural selection work?", and "How do organisms evolve?"

3. Is culture the result of evolution?         

A society's culture consists of its accumulated learned behavior. Human culture is based at least partly on social living and language, although the ability of a species to invent and use language and engage in complex social behaviors has a biological basis. Some scientists hypothesize that language developed as a means of establishing lasting social relationships. Even a form of communication as casual as gossip provides an ingenious social tool: Suddenly, we become aware of crucial information that we never would have known otherwise. We know who needs a favor; who's available; who's already taken; and who's looking for someone -- information that, from an evolutionary perspective, can mean the difference between failure and success. So, it is certainly possible that evolutionary forces have influenced the development of human capacities for social interaction and the development of culture. While scientists tend to agree about the general role of evolution in culture, there is still great disagreement about its specific contributions.

4. How are modern humans and Neanderthals related?     

There is great debate about how we are related to Neanderthals, close hominid relatives who coexisted with our species from more than 100,000 years ago to about 28,000 years ago. Some data suggest that when anatomically modern humans dispersed into areas beyond Africa, they did so in small bands, across many different regions. As they did so, according to this hypothesis, humans merged with and interbred with Neanderthals, meaning that there is a little Neanderthal in all modern Europeans.

Scientific opinion based on other sets of data, however, suggests that the movement of anatomically modern humans out of Africa happened on a larger scale. These movements by the much more culturally and technologically advanced modern humans, the hypothesis states, would have been difficult for the Neanderthals to accommodate; the modern humans would have out-competed the Neanderthals for resources and driven them to extinction.

5. What do humans have in common with single-celled organisms?      

Evolution describes the change over time of all living things from a single common ancestor. The "tree of life" illustrates this concept. Every branch represents a species, each connected to other such branches and the rest of tree as a whole. The forks separating one species from another represent the common ancestors shared by these species. In the case of the relatedness of humans and single-celled organisms, a journey along two different paths -- one starting at the tip of the human branch, the other starting at the tip of a single-celled organism's branch -- would ultimately lead to a fork near the base of the tree: the common ancestor shared by these two very different types of organisms. This journey would cross countless other forks and branches along the way and span perhaps more than a billion years of evolution, but it demonstrates that even the most disparate creatures are related to one another -- that all life is interconnected.

6. What happened in the Cambrian explosion?

Life began more than 3 billion years before the Cambrian, and gradually diversified into a wide variety of single-celled organisms. Toward the end of the Precambrian, about 570 million years ago, a number of multicelled forms began to appear in the fossil record, including invertebrates resembling sponges and jellyfish, and some as-yet-unknown burrowing forms of life. As the Cambrian began, most of the basic body plans of invertebrates emerged from these Precambrian forms. They emerged relatively rapidly, in the geological sense -- over 10 million to 25 million years. These Cambrian forms were not identical to modern invertebrates, but were their early ancestors. Major groups of living organisms, such as fish, amphibians, reptiles, birds, and mammals, did not appear until millions of years after the end of the Cambrian Period.