Evidence for Evolution

In this section, you'll learn about different kinds of evidence that have been used to support the idea that evolution occurs. Early evolutionary thinkers made some early observations—from geology, the fossil record, biogeography, and comparative anatomy. Evidence from these sources showed that the earth was much older than previously thought, and that the kinds and distributions of organisms had changed over time. Other kinds of evidence, such as developmental and genetic data, have come to light more recently, and provide even more overwhelming evidence that evolution is real. All the sources of evidence discussed here point to one inevitable conclusion: that all living things are related to each other. All are descended from a common ancestor, and over time, evolution shaped the diversity of organisms that populate the planet today. But don't take our word for it—check out the evidence and decide for yourself.

History of Evolutionary Theory

Before we tackle evidence of evolution head-on, let's briefly review the history of evolutionary thought. Recall that early ideas about the natural world were heavily influenced by Plato and Aristotle, who believed that species were fixed and unchanging. Early Christians built upon this idea, and concluded from the biblical account of creation that God had created each of these species individually. The belief in divinely created, unchanging species dominated for centuries, but by the 18th century, the tide had begun to turn in favor of evolution. At this point, people still weren't quite sure how evolution happened—Charles Darwin and Alfred Russel Wallace hadn't thought of natural selection yet, and Jean-Baptiste Lamarck's inheritance of acquired characteristics was the most commonly accepted mechanism for evolutionary change. Early believers in evolution might have been wrong about how evolution happened, but props to them for at least understanding that it did happen. What evidence led them to abandon old ideas about species being fixed entities, and to conclude that evolution had occurred?

Geology

The first important piece of evidence came from geology. We know now that the earth is old—4.6 billion years, to be exact—but this was not always common knowledge. Prior to the Enlightenment, most people accepted biblical accounts of creation, which placed the origin of the earth around 6,000 years ago. 

Now, 6,000 years might sound like a lot, but here was the problem: thinkers in the mid-1700's sat in their favorite armchairs, reading accounts from ancient scholars describing species of animals and plants in Europe 2,000 years earlier. Then they stood up, stretched, and went outside for a walk, and observed those same species virtually unchanged! They came back inside, and over a nice snack, thought to themselves, "If species haven't changed in 2,000 years and the earth is only 6,000 years old, there's no way enough time has passed for evolution to have happened." Enter James Hutton, the father of modern geology.

James Hutton (1726-1797), a Scottish physician by training, had a keen interest in the natural world. In the process of preparing his land for farming, he became interested in geology. Geology is the study of the materials and processes that shape the earth, as well as the earth's history. Hutton realized that wind and water wear down rock, and that over immense periods of time, deposits of those sediments form new layers of rock, which in turn eventually erode again. He also realized the importance of volcanic processes in shaping the earth's features. 

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List four sources of evidence for evolution: ____________________________________________________________

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Hutton's observations of these geological processes led him to the concept of uniformitarianism—the idea that the same gradual processes that are at work today, like erosion and sedimentation, were at work in the past. Over long periods of time, these processes can produce huge changes in the earth's features—think of the Grand Canyon. Uniformitarianism replaced catastrophism—the idea that sudden, short-term, catastrophic events were responsible for shaping the earth's surface.

The principle of uniformitarianism was popularized by another prominent Scottish geologist, Charles Lyell (1797-1875). Lyell's most famous work, Principles of Geology, built upon the conceptual framework that Hutton had established, providing additional evidence that the earth's features had been shaped by gradual processes still observable today. Lyell was a geology nerd—upon getting married, he took his new wife on a geological expedition for their honeymoon, and no, she did not immediately divorce him once they got home. Smooth move, Lyell. Also on the topic of Lyell's social life, he and Darwin were friends, and Darwin's musings on natural selection were profoundly influenced by the idea of uniformitarianism.

Hutton and Lyell laid important groundwork for evolutionary thought; geology and the principle of uniformitarianism were critical in showing that the earth had, indeed been around long enough for evolution to occur. Once the earth's antiquity was established, it was no longer reasonable to reject evolution on the grounds that the earth was simply too young.

Fossils

For early evolutionists, fossils were a second important source of evidence for evolution. Fossils show evidence of organisms that were once alive. Most people immediately think of bones, teeth, and shells when they think of fossils, but there are many other kinds of fossils as well. Some fossils, like the ones mentioned above, are the mineralized remains of organisms, but imprints of those objects count as fossils too. Fossils can also be things like burrows, footprints, tracks, or coprolites—fossilized feces.

Fossil discoveries led to a number of important observations in the mid to late 1700's. First, an English geologist named William Smith (1769-1839) realized that certain fossils always showed up in the same kinds of rock, even if they were found in different locations around England. Moreover, both the fossils and the surrounding rock occurred in layers, and these layers usually stacked up in the same order no matter where they were found. 

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Stratigraphy—the study of layers of rocks and sediments—had been around for a while before Smith, but he was the first person to apply the principles of stratigraphy to the problem of dating layers. Smith realized that, if two different sites had the same rock layer containing the same fossils, they must come from the same time period! He called the predictable layering of fossils the Principle of Faunal Succession, because the animals (fauna) in the layers always occurred in the same order (succession). This principle was very useful, because it allowed people to correlate layers from different locations, and allowed them to figure out roughly how old a layer was even if surrounding layers were missing. Today, we can also date sediments directly using radiometric dating.

A second important observation was that fossils in layers closest to the surface (and therefore, the youngest layers) most closely resembled living organisms, whereas fossils in lower, older layers of rock looked substantially different from living forms. Some of these organisms in older layers no longer existed on the planet—they had gone extinct! Extinction of a species occurs when all members of that species die. All these observations led to the undeniable conclusion that life had changed over time, and that some species had kicked the bucket, while others had sprung into existence. These ideas were inconsistent with the creation story, and provided strong support that evolution had occurred.

Fossils are also important because they show transitional forms. That is, they show intermediate stages as an organism evolves from one thing into another, or as one kind of organism diversifies into many kinds of organisms. Transitional fossils provide direct evidence that life forms change over long periods of time.

Biogeography

Biogeography is the study of geographical distributions of organisms – that is, where stuff lives. Before evolutionary thought really took off, people assumed a creator had made all the organisms on earth and had plunked them down wherever they currently live. From a biogeographical perspective, there's a problem with that reasoning…let's use a camel as an example. 

Camels love the desert, right? There are deserts all over the world – Asia, Australia, Africa, North America, and South America. Yet camels are only native to Asia. Sure, you can find camels in other places now, but it's just because people moved them there. If a creator made a camel for living in the desert, why restrict them just to Asia?

Darwin was a big fan of biogeography, and spent quite a bit of time thinking and writing about it as he traveled the world. Just as we saw with the camel, Darwin noted that the physical environment was clearly not the only factor determining where a given species lived; the same exact habitat on two different continents might have completely different kinds of organisms. He considered this observation evidence for evolution. 

What was his reasoning? As we said before, if a creator made all the living things on earth, you would expect to see them distributed anywhere they could survive. Yet there are no camels in Arizona, no giraffes in Hawaii, and no chimpanzees in Florida—or at least, they're not native to those places. However, if we accept that species evolve, then each species must have had a region of origin, or a place where they evolved from an ancestor. From there, that species and/or any closely related species could spread out. Thus, we would only expect to find them in the region they originated or in areas they could feasibly reach. 

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Darwin also noticed that different kinds of organisms on one continent were usually more similar to each other than to organisms on other continents. For example, South America has some crazy kinds of rodents—take the capybara, which, at over 100 pounds, could probably squish a lot of dogs. Anyway, the capybara hangs out in the water a lot, so we might expect it to be pretty similar to something like a beaver. Even though the beaver and the capybara live in similar habitats, each one is more similar to other rodents that live in its own geographic area. Again, evolution can explain this pattern. If a group of related species all descended from a common ancestor, you'd expect them to share many similarities and to cluster together geographically, even if they evolved different lifestyles. 

Comparative Anatomy

Comparative anatomy is another important and compelling source of evidence for evolution, and there's no better place to think about it than at the zoo. Let's take a little mental field trip. 

Upon arriving at the zoo, we immediately hit the concession stand for some popcorn, or maybe one of those giant roasted turkey drumsticks, because like any great form of entertainment, anatomy is best enjoyed with a snack. We proceed immediately to the primate enclosure, where some New World monkeys jealously eye what's left of our food. We notice that they share a lot in common with us—their faces, their little hands, the shape of their skulls—compared to a dog or a flamingo or a snake, we have a LOT in common with monkeys, and this is immediately evident when we see them.

Next, we move on to the reptile house, where we check out a giant tortoise. Unlike the monkey, the tortoise has a lot of features that we just can't relate to. Its skull is pretty small compared to the rest of its body; it has thick, scaly skin, and what's up with that shell? 

Still holding the remains of our turkey drumstick (mostly just the bone now), an idea suddenly occurs to us: despite the differences between humans, monkeys, tortoises, and the unlucky turkey, there are some features we all have in common. For example, we all have two arms and two legs, skulls, vertebrae, and ribs. These features are called homologous structures. 

We say that structures are homologous in two or more organisms when those structures came from a common ancestor. They might look really different because they have changed over time to be useful in different environments, but originally, they came from an ancestor we all shared. The bones that make up our arms make up a turkey's wings. The bones that we call ribs in humans, monkeys, and turkeys actually make up the tortoise's shell. That's right—from our last common ancestor, the same bones went to make up the rib cage in the human species and changed over time to provide shelter and protection to tortoises and their relatives. 

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1) Salamander, 2) Frog, 3) Turtle, 4) Aetosaurus, 5) Pleisiosaurus, 6) Ichthyosaurus, 7) Mesosaurus, 8) Duck.

If these structures all look so different due to millions of years of evolution in wildly different environments, how do we know they're homologous to each other? How can we be sure they came from a common ancestor? Let's use one of the lower leg bones (tibia) as an example. We can tell tibiae are homologous in humans, monkeys, tortoises, and turkeys because they always connect with the upper leg bone (femur) and with the bones of foot, and similar muscles connect them to other bones. 

There are certain bumps and grooves on all the tibiae, and although those features might be a little different from tibia to tibia, they're there, and in roughly the same places. Finally, scientists who study development know that the tibia originates in the same way in all these different animals. All these lines of evidence support the conclusion that tibiae are homologous in humans, monkeys, tortoises, and turkeys—that is, a long time ago, a common ancestor had a tibia, and over time, evolution has modified the tibia for different animals in different environments. 

Vestigial structures are another class of anatomical features that provide evidence for common ancestry and evolution. Vestigial structures serve no apparent purpose in species that possess them at present, but may have been important in their ancestors. For example, whales have pelvis (hip) bones, but they don't need them, since they don't walk; in other words, these hip bones are completely useless to modern whales. However, the fact that whales have hip bones even though they don't need them suggests that their ancestors might have walked on land. Fossil evidence shows this to be correct. 

In sum, comparative anatomy shows how seemingly disparate kinds of organisms actually share many fundamental similarities, and strongly supports the notion that those similarities are derived from a common ancestor. The differences we see in modern organisms are the result of changes over time, as organisms adapt to their environment; in other words, evolution.

Using the image above, describe three similarities in the bones between these species:

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Development

We already touched on development a little bit when we talked about homology, because homologous structures are especially easy to identify at the early stages of development. This is because, among vertebrates, embryos all look kind of similar at their really early stages. If you have two features that look really different in adults (like the wings of a bird and the arm of a human), studying their development can help you to see that the bones making up wings and arms are homologous, which means they shared a common ancestor.

Sometimes, embryos also reveal rudiments, which are the remains of an ancestral feature. Rudiments are kind of like vestigial structures, except that vestigial structures show up in adults, and rudiments are only briefly visible during the embryonic period. For example, baleen whales don't have any teeth—instead, they use their baleen like a giant sieve to capture plankton to eat. Interestingly, before they are born, they show evidence of tooth formation—those little teeth are rudiments. They go away by the time the baby whale is born, but the fact that they're present in the womb tells us that the ancestors of baleen whales had teeth, and that, over evolutionary time, they lost their teeth and acquired baleen.

Many modern scientists focus on developmental pathways, or on genes that regulate development, and these, too, provide evidence for evolution. Really different organisms—jellyfish and aardvarks, for example—have strikingly similar genetic pathways for development. For example, Hox genes are a group of genes that affect the formation of body segments—and all animals, regardless of shape, have them! Studying how mutations affect Hox genes also sheds light on how so many different kinds of animal shapes exist on the planet. For example, mutations in Hox genes can cause a certain kind of body part—like a leg or a wing—to grow in the completely different place. Over evolutionary time, different animals have evolved different numbers and kinds of Hox genes, leading to a diversity of body shapes and types. Studying these genes and understanding how they differ in jellyfish, aardvarks, and other animals can help us understand how certain kinds of physical traits came to be, and makes it clearer than ever that all these different organisms are related to each other.

Molecular Biology

Although the tools of modern molecular biology weren't available to early evolutionary thinkers, we now have at our disposal even more evidence that supports evolution. For starters, we know that similar chemistry, structures and processes underlie all cells. Consider, for example, that all cells, regardless of what animal they come from, have mitochondria. This important cellular organelle is the site for cellular respiration in almost all eukaryotic organisms. Ribosomes are another great example—cells from all three domains of life (Eukarya, Bacteria, Archaea) possess these structures, which help to assemble proteins based on the organism's genetic code. Ribosomes from cells of these three domains actually do show differences, but nonetheless, the process of protein synthesis basically works the same way among all living things.

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Molecular biology also contributes to our understanding of evolution by revealing degrees of relatedness between different organisms. We just noted that some structures and processes are conserved across all organisms, providing evidence for shared ancestry a long time ago. But modern genetics can also help us parcel out more recent evolutionary events. By figuring out the sequences of certain genes and comparing them in different organisms, scientists can figure out who is most closely related to whom. This is because mutations occur every time our genetic material is replicated; it doesn't happen very often, but every once in a while a base pair might be added, deleted, or changed. It is estimated that 300 new mutations are introduced into the human genome every generation—that is, you have 300 new mutations compared to your mom and dad! Assuming these mutations occur at a somewhat constant, predictable rate, we can reason that the more time that has passed since two organisms diverged from each other, the more different their DNA will be. Just think, taking into account 300 new mutations every generation, your child would have 600 new mutations compared to your parents and your grandchild would have 900 new mutations. Over time, these accumulate in the genome, adding to the differences observed between organisms. In other words, more similar DNA sequences can mean that two organisms are closely related, and very different sequences indicate that they split from each other a longer time ago. 

For aficionados of gigantic Ice Age mammals, mammoths provide a cool example of how gene sequencing reveals information about the evolution of elephants and their relatives. In 2006 an international group of scientists sequenced genes from extinct wooly mammoths—itself a remarkable feat. Mammoths are often found in permafrost, extremely cold soil, which provides ideal conditions for preserving DNA. The research team compared mammoth gene sequences to two different kinds of elephants living today: modern Asian and African elephants. They discovered that the closest living relatives of mammoths are Asian elephants. In other words, mammoths and Asian elephants share more of their DNA and a more recent common ancestor than modern Asian and African elephants. 

Evolution in Action

In case you're not convinced yet that evolution really happens, we can actually witness evolution occurring right before our eyes, or, at least over a relatively short period of time. 

Dogs are a great example of evolution in action; think about how many shapes, colors, and sizes of dog are out there. Snoopy. Lassie. Rin Tin Tin. Pluto. Goofy. Odie. Bolt. We could go on and on. All those dogs (if you can figure out Odie's breed) are members of the exact same species: Canis lupus familiaris. That is, all dogs, whether Chihuahua or Great Dane, are capable of interbreeding and producing fertile offspring—there may be some logistical complications involved here. This great diversity of dog breeds descended from gray wolves, and estimates from genetic data indicate that domestication of wolves may have started up to 130,000 years ago, even though archaeological evidence of dog domestication only goes back about 17,000 years. 

In the time since dogs have been domesticated, a process called artificial selection has occurred. Artificial selection works kind of like natural selection, in that only individuals with certain traits get to have offspring. The main difference is that in artificial selection, humans are consciously selecting certain traits, and with natural selection, individuals with traits allowing survival and reproductive success do the best. 

How did artificial selection produce the huge variety of dog breeds we see today? At some point in the not too distant past, some people were nuts about white, fluffy fur. They always picked the dogs with the lightest colored coats and the softest fur, and allowed those animals to breed with each other. After generations and generations of selection for those traits, we find ourselves with poodles. The same can be said about bulldogs, hounds, golden retrievers, and every other breed of dog. 

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Many of the traits that make dogs great pets were selected by the same processes. For example, dogs are affectionate, playful, loyal, and attentive to humans. Recent research suggests that dogs are better at reading human body language than wolves and chimpanzees. All these attributes and skills are probably due to selection for those traits by humans. In sum, by selecting desirable traits at each generation and allowing only dogs with those traits to reproduce, humans have effectively created all the breeds of dogs we're familiar with today, and have made dogs excellent companions. Hooray for man's (and woman's) best friend!

Artificial selection in dogs proceeded over thousands of years. Even though that might seem like a long time, it's a relatively small interval in the grand scheme of evolutionary time. We can see other, shorter-term examples of evolution, too. Consider the evolution of pesticide resistance in crops—when farmers douse their plants in chemicals to kill insect pests, pests quickly become resistant to those chemicals—sometimes in just a few years. The same can be said about bacteria and antibiotic resistance.

Vocabulary Review

Find nine words in the article you circled as difficult or you didn’t understand. List and then define these words.

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