THE LIVING WORLD
Unit Four. The Evolution and Diversity of Life
14.6. The Evidence for Evolution
The evidence that Darwin presented in The Origin of Species to support his theory of evolution was strong. We will now examine other lines of evidence supporting Darwin’s theory, including information revealed by examining fossils, anatomical features, and molecules such as DNA and proteins.
The Fossil Record
The most direct evidence of macroevolution is found in the fossil record. Fossils are the preserved remains, tracks, or traces of once-living organisms. Fossils are created when organisms become buried in sediment. The calcium in bone or other hard tissue mineralizes, and the surrounding sediment eventually hardens to form rock. Most fossils are, in effect, skeletons. In the rare cases when fossils form in very fine sediment, feathers may also be preserved. When remains are frozen or become suspended in amber (fossilized plant resin), however, the entire body may be preserved. The fossils contained in layers of sedimentary rock reveal a history of life on earth.
By dating the rock in which a fossil occurs, like the one in figure 14.13, we can get an accurate idea of how old the fossil is. Rocks are dated by measuring the amount of certain radioisotopes in the rock. A radioisotope will break down, or decay, into other isotopes or elements. This occurs at a constant rate and so the amount of a radioisotope present in the rock is an indication of the rock’s age.
Figure 14.13. Dinosaur fossil of Parasaurolophus.
Using Fossils to Test the Theory of Evolution
If the theory of evolution is correct, then the fossils we see preserved in rock should represent a history of evolutionary change. The theory makes the clear prediction that a parade of successive changes should be seen, as first one change occurs and then another. If the theory of evolution is not correct, on the other hand, then such orderly change is not expected.
To test this prediction, we follow a logical procedure:
1. Assemble a collection of fossils of a particular group of organisms. You might, for example, gather together a collection of fossil titanotheres, a hoofed mammal that lived between about 50 million and 35 million years ago.
2. Date each of the fossils. In dating the fossils, it is important to make no reference to what the fossil is like. Imagine it as being concealed in a black box of rock, with only the box being dated.
3. Order the fossils by their age. Without looking in the “black boxes,” place them in a series, beginning with the oldest and proceeding to the youngest.
4. Now examine the fossils. Do the differences between the fossils appear jumbled, or is there evidence of successive change as evolution predicts? You can judge for yourself in figure 14.14. During the 15 million years spanned by this collection of titanothere fossils, the small, bony protuberance located above the nose 50 million years ago evolved in a series of continuous changes into relatively large blunt horns.
Figure 14.14. Testing the theory of evolution with fossil titanotheres.
Here you see illustrated changes in a group of hoofed mammals known as titanotheres, which lived between about 50 million and 35 million years ago. During this time, the small, bony protuberance located above the nose 50 million years ago evolved into relatively large, blunt horns.
It is important not to miss the key point of the result you see illustrated in figure 14.14: Evolution is an observation, not a conclusion. Because the dating of the samples is independent of what the samples are like, successive change through time is a data statement. While the statement that evolution is the result of natural selection is a theory advanced by Darwin, the statement that macroevolution has occurred is a factual observation.
Many other examples illustrate this clear confirmation of the key prediction of Darwin’s theory. The evolution of today’s large, single-hoof horse with complex molar teeth from a much smaller four-toed ancestor with much simpler molar teeth is a familiar and clearly documented instance.
Darwin and Moby Dick
Moby Dick, the white whale hunted by Captain Ahab in Melville's novel, was a sperm whale. One of the ocean's great predators, a large sperm whale is a voracious meat- eater that may span over 60 feet and weigh 50 tons.
A sperm whale is not a fish, though. Unlike the great white shark in Jaws, a whale has hairs (not many), and a female whale has milk-producing mammary glands with which it feeds its young. A sperm whale is a mammal, just as you are! This raises an interesting question. If Darwin is right about the fossil record reflecting life's evolutionary past, then fossils tell us mammals evolved from reptiles on land at about the time of the dinosaurs. How did they end up back in the water?
The evolutionary history of whales has long fascinated biologists, but only in recent years have fossils been discovered that reveal the answer to this intriguing question. A series of discoveries now allows biologists to trace the evolutionary history of the most colossal animals ever to live on earth back to their beginnings at the dawn of the Age of Mammals. Whales, it turns out, are the descendants of four-legged land mammals that reinvaded the sea some 50 million years ago, much as seals and walruses are doing today. It's pretty startling to realize that Moby Dick's evolutionary ancestor lived on the steppes of Asia and looked like a modest-sized pig a few feet long and weighing perhaps 50 pounds, with four toes on each foot.
From what land mammal did whales arise? Researchers had long speculated that it might be a hoofed meat- eater with three toes known as a mesonychid, related to rhinoceroses. Subtle clues suggested this—the arrangement of ridges on the molar teeth, the positioning of the ear bones in the skull. But findings announced in 2001 reveal these subtle clues to have been misleading. Ankle bones from two newly described 50 million-year-old whale species discovered by Philip Gingerich of the University of Michigan are those of an artiodactyl, a four-toed mammal related to hippos, cattle, and pigs. Even more recently, Japanese researchers studying DNA have discovered unique genetic markers shared today only by whales and hippos.
Biologists now conclude that whales, like hippos, are descended from a group of early four-hoofed mammals called anthracotheres, modest-sized grazing animals with a piggish appearance abundant in Europe and Asia 50 million years ago.
In Pakistan in 1994, biologists discovered its descendant, the oldest known whale. The fossil was 49 million years old, had four legs, each with four-toed feet and a little hoof at the tip of each toe. Dubbed Ambulocetus (walking whale), it was sharp-toothed and about the size of a large sea lion. Analysis of the minerals in its teeth reveal it drank fresh water, so like a seal it was not yet completely a marine animal. Its nostrils were on the end of the snout, like a dog's.
Appearing in the fossil record a few million years later is Rodhocetus, also seal-like but with smaller hind limbs and the teeth of an ocean water drinker. Its nostrils are shifted higher on the skull, halfway towards the top of the head.
Almost 10 million years later, about 37 million years ago, we see the first representatives of Basilosaurus, a giant 60-foot-long serpentlike whale with shrunken hind legs still complete down to jointed knees and toes.
The earliest modern whales appear in the fossil record 15 million years ago. The nostrils are now in the top of the head, a "blowhole” that allows it to break the surface, inhale, and resubmerge without having to stop or tilt the head up. The hind legs are gone, with vestigial tiny bones remaining that are unattached to the pelvis. Still, today's whales retain all the genes used to code for legs— occasionally a whale is born having sprouted a leg or two.
So it seems to have taken 35 million years to evolve a whale from the piglike ancestor of a hippopotamus— intermediate steps preserved in the fossil record for us to see. Darwin, who always believed that gaps in the vertebrate fossil record would eventually be filled in, would have been delighted.
The Anatomical Record
Much of the evolutionary history of vertebrates can be seen in the way in which their embryos develop. Figure 14.15 shows three different embryos early in development, and as you can see, all vertebrate embryos have pharyngeal pouches (that develop into gill slits in fish); also every vertebrate embryo has a long bony tail, even if the tail is not present in the fully developed animal. These relict developmental forms strongly suggest that all vertebrates share a basic set of developmental instructions.
Figure 14.15. Embryos show our early evolutionary history.
These embryos, representing various vertebrate animals, show the primitive features that all vertebrates share early in their development, such as pharyngeal pouches and a tail.
As vertebrates have evolved, the same bones are sometimes still there but put to different uses, their presence betraying their evolutionary past. For example, the forelimbs of vertebrates are all homologous structures; that is, although the structure and function of the bones have diverged, they are derived from the same body part present in a common ancestor. You can see in figure 14.16 how the bones of the forelimb have been modified for different functions. The yellow- and purple-colored bones, which correspond in humans to the bones of the forearm and wrist and fingers, respectively, are modified to make up the wings in the bat, the full leg of the horse, and the paddle in the fin of the porpoise.
Figure 14.16. Homology among vertebrate limbs.
Homologies among the forelimbs of four mammals show the ways in which the proportions of the bones have changed in relation to the particular way of life of each organism. Although considerable differences can be seen in form and function, the same basic bones are present in each forelimb.
Not all similar features are homologous. Sometimes features found in different lineages come to resemble each other as a result of parallel evolutionary adaptations to similar environments. This form of evolutionary change is referred to as convergent evolution, and these similar-looking features are called analogous structures. For example, the wings of birds, pterosaurs, and bats are analogous structures, modified through natural selection to serve the same function and therefore look the same (figure 14.17). Similarly, the marsupial mammals of Australia evolved in isolation from placental mammals, but similar selective pressures have generated very similar kinds of animals.
Figure 14.17. Convergent evolution: many paths to one goal.
Over the course of evolution, form often follows function. Members of very different animal groups often adapt in similar fashions when challenged by similar opportunities. These are but a few of many examples of such convergent evolution. The flying vertebrates represent mammals (bat), reptiles (pterosaur), and birds (bluebird). The three pairs of terrestrial vertebrates each contrast a North American placental mammal with an Australian marsupial one.
Sometimes structures are put to no use at all! In living whales, which evolved from hoofed mammals, the bones of the pelvis that formerly anchored the two hind limbs are all that remain of the rear legs, unattached to any other bones and serving no apparent purpose (the reduced pelvic bone can be seen in the figure in the “Today’s Biology” reading on the previous page). Another example of what are called vestigial organs is the human appendix. The great apes, our closest relatives, have an appendix much larger than ours attached to the gut tube, which holds bacteria used in digesting the cellulose cell walls of the plants eaten by these primates. The human appendix is a vestigial version of this structure that now serves no function in digestion (although it may have acquired an alternate function in the lymphatic system).
The Molecular Record
Traces of our evolutionary past are also evident at the molecular level. We possess the same set of color vision genes as our ancestors, only more complex, and we employ pattern formation genes during early development that all animals share. Indeed, if you think about it, the fact that organisms have evolved from a series of simpler ancestors implies that a record of evolutionary change is present in the cells of each of us, in our DNA. According to evolutionary theory, new alleles arise from older ones by mutation and come to predominance through favorable selection. A series of evolutionary changes thus implies a continual accumulation of genetic changes in the DNA. From this you can see that evolutionary theory makes a clear prediction: Organisms that are more distantly related should have accumulated a greater number of evolutionary differences than two species that are more closely related.
This prediction is now subject to direct test. Recent DNA research allows us to directly compare the genomes of different organisms. The result is clear: For a broad array of vertebrates, the more distantly related two organisms are, the greater their genomic difference. This research is described later in this chapter on pages 298 and 299.
This same pattern of divergence can be clearly seen at the protein level. Comparing the hemoglobin amino acid sequence of different species with the human sequence in figure 14.18, you can see that species more closely related to humans have fewer differences in the amino acid structure of their hemoglobin. Macaques, primates closely related to humans, have fewer differences from humans (only 8 different amino acids) than do more distantly related mammals like dogs (which have 32 different amino acids). Nonmammalian terrestrial vertebrates differ even more, and marine vertebrates are the most different of all. Again, the prediction of evolutionary theory is strongly confirmed.
Figure 14.18. Molecules reflect evolutionary divergence.
The greater the evolutionary distance from humans (as revealed by the blue evolutionary tree based on the fossil record), the greater the number of amino acid differences in the vertebrate hemoglobin polypeptide.
Molecular Clocks. This same pattern is seen when the DNA sequence of an individual gene is compared over a much broader array of organisms. One well-studied case is the mammalian cytochrome c gene (cytochrome c is a protein that plays a key role in oxidative metabolism). Figure 14.19 compares the time when two species diverged on the x axis to the number of differences in their cytochrome c gene on the y axis. To practice using this data set, go back about 75 million years ago to find a common ancestor for humans and rodents—in that time there have been about 60 base substitutions in cytochrome c. This graph reveals a very important finding: Evolutionary changes appear to accumulate in cytochrome c at a constant rate, as indicated by the straightness of the blue line connecting the points. This constancy is sometimes referred to as a molecular clock. Most proteins for which data are available appear to accumulate changes over time in this fashion, although different proteins can evolve at very different rates.
Figure 14.19. The molecular clock of cytochrome c.
When the time since each pair of organisms presumably diverged is plotted against the number of nucleotide differences in cytochrome c, the result is a straight line, suggesting that the cytochrome c gene is evolving at a constant rate.
Key Learning Outcome 14.6. The fossil record provides a clear record of successive evolutionary change. Comparative anatomy also offers evidence that evolution has occurred. Finally, the genetic record exhibits successive evolution, the DNA of organisms accumulating increasing numbers of changes over time.