Fossilized Evolution And Speciation

To show gradual evolutionary change within a single lineage, you need a good succession of sediments, preferably laid down quickly (so that each time period represents a thick slice of rock, making change easier to see), and without missing layers (a missing layer in the middle makes a smooth evolutionary transition look like a sudden "jump").

Very small marine organisms, such as plankton, are ideal for this. There are billions of them, many of them have hard parts, and they conveniently fall directly to the sea floor after death, piling up in a continuous sequence of layers. Sampling the layers in order is easy: you can thrust a long tube into the sea floor, pull up a columnar core sample, and read it (and date it) from bottom to top.

Tracing a single fossil species through the core, you can often see it evolve. Figure 4 shows an example of evolution in a tiny, single-celled marine protozoan that builds a spiral shell, creating more chambers as it grows. These samples come from sections of a 200-meter-long core taken from the ocean floor near New Zealand, representing about eight million years of evolution. The figure shows change over time in one

Malmgren And Kennett 1981

figure 4. A record of fossils (preserved in a sea-floor core) showing evolutionary change in the marine foraminiferan Globorotalia conoidea over an 8 million-year period. The scale gives the number of chambers in the final whorl of the shell, averaged among all individuals from each section of the core. (After Malmgren and Kennett 1981.)

figure 4. A record of fossils (preserved in a sea-floor core) showing evolutionary change in the marine foraminiferan Globorotalia conoidea over an 8 million-year period. The scale gives the number of chambers in the final whorl of the shell, averaged among all individuals from each section of the core. (After Malmgren and Kennett 1981.)

85 90 100 120 140

Mean Thoracic Width (microns)

85 90 100 120 140

Mean Thoracic Width (microns)

figure 5. Evolutionary change of thorax size in the radiolarian Pseudocubus vema over a period of two million years. Values are population averages from each section of the core. (After Kellogg and Hays 1975.)

trait: the number of chambers in the final whorl of the shell. Here we see fairly smooth and gradual change over time: individuals have about 4.8 chambers per whorl at the beginning of the sequence and 3.3 at the end, a decrease of about 30 percent.

Evolution, though gradual, need not always proceed smoothly, or at an even pace. Figure 5 shows a more irregular pattern in another marine microorganism, the radiolarian Pseudocubus vema. In this case geologists took regularly spaced samples from an 18-meter-long core extracted near Antarctica, representing about two million years of sediments. The trait measured was the width of the animal's cylindrical base (its "thorax"). Although size increases by nearly 50 percent over time, the trend is not smooth. There are periods in which size doesn't change much, interspersed with periods of more rapid change. This pattern is quite common in fossils, and is completely understandable if the changes we see were driven by environmental factors such as fluctuations in climate or salinity. Environments themselves change sporadically and unevenly, so the strength of natural selection will wax and wane.

Let's look at evolution in more complex species: trilobites. Trilo-bites were arthropods, in the same group as insects and spiders. Since they were protected by a hard shell, they are extremely common in ancient rocks (you can probably buy one in your nearest museum shop). Peter Sheldon, then at Trinity College Dublin, collected trilo-bite fossils from a layer of Welsh shale spanning about three million years. Within this rock, he found eight distinct lineages of trilo-bites, and over time each showed evolutionary change in the number of "pygideal ribs"—the segments on the last section of the body. Figure 6 shows the changes in several of these lineages. Although over the entire period of sampling every species showed a net increase in segment number, the changes among different species were not only uncorrelated, but sometimes went in opposite directions during the same period.

Unfortunately, we have no idea what selective pressures drove the evolutionary changes in these plankton and trilobites. It is always easier to document evolution in the fossil record than to understand what caused it, for although fossils are preserved, their environments are not. What we can say is that there was evolution, it was gradual, and it varied in both pace and direction.

Marine plankton give evidence for the splitting of lineages as well as evolution within a lineage. Figure 7 shows an ancestral plankton species dividing into two descendants, distinguishable by both size and shape. Interestingly, the new species, Eucyrtidium matuyamai, first evolved in an area to the north of the area from where these cores were taken, and only later invaded the area where its ancestor occurred. As we'll see in chapter 7, the formation of a new species usually begins when populations are geographically isolated from one another.

There are hundreds of other examples of evolutionary change in fossils—both gradual and punctuated—from species as diverse as

figure 6. Evolutionary change in the number of pygidial ribs (segments on the tail section) of five groups of Ordovician trilobites. The number gives the population average at each section of the three-million-year sample of shale. All five species— and three others not shown—displayed a net increase in rib number over the period, suggesting that natural selection was involved over the long term, but that the species did not change in perfect parallel. (After Sheldon 1987.)

figure 6. Evolutionary change in the number of pygidial ribs (segments on the tail section) of five groups of Ordovician trilobites. The number gives the population average at each section of the three-million-year sample of shale. All five species— and three others not shown—displayed a net increase in rib number over the period, suggesting that natural selection was involved over the long term, but that the species did not change in perfect parallel. (After Sheldon 1987.)

mollusks, rodents, and primates. And there are also examples of species that barely change over time. (Remember that evolutionary theory does not state that all species must evolve!) But listing these cases wouldn't change my point: the fossil record gives no evidence for the creationist prediction that all species appear suddenly and then remain unchanged. Instead, forms of life appear in the record in evolutionary sequence, and themselves evolve and split.

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figure 7. Evolution and speciation in two species of the planktonic radiolarian Eucyrtidium, taken from a sediment core spanning more than 3.5 million years. The points represent the width of the fourth segment, shown as the average of each species at each section of the core. In areas to the north of where this core was taken, an ancestral population of E. calvertense became larger, gradually acquiring the name E. matuyamai as it became larger. E. matuyamai then reinvaded the range of its relative, as shown on the graph, and both species, now living in the same place, began to diverge in body size. This divergence may have been the result of natural selection acting to reduce competition for food between the two species. (After Kellogg and Hayes 1975.)

"MISSING LINKS"

Changes in marine species may give evidence for evolution, but that's not the only lesson that the fossil record has to teach. What really excites people—biologists and paleontologists among them—are transitional forms: those fossils that span the gap between two very different kinds of living organisms. Did birds really come from reptiles, land animals from fish, and whales from land animals? If so, where is the fossil evidence? Even some creationists will admit that minor changes in size and shape might occur over time—a process called microevolution—but they reject the idea that one very different kind of animal or plant can come from another (macroevolution). Advocates of intelligent design argue that this kind of difference requires the direct intervention of a creator.7 Although in The Origin Darwin could point to no transitional forms, he would have been delighted by how his theory has been confirmed by the fruits of modern paleontology. These include many species whose existence was predicted many years ago, but that have been unearthed in only the last few decades.

But what counts as fossil evidence for a major evolutionary transition? According to evolutionary theory, for every two species, however different, there was once a single species that was the ancestor of both. We could call this one species the "missing link." As we've seen, the chance of finding that single ancestral species in the fossil record is almost zero. The fossil record is simply too spotty to expect that.

But we needn't give up, for we can find some other species in the fossil record, close cousins to the actual "missing link," that document common ancestry equally well. Let's take one example. In Darwin's day, biologists conjectured from anatomical evidence, such as similarities in the structure of hearts and skulls, that birds were closely related to reptiles. They speculated that there must have been a common ancestor that, through a speciation event, produced two lineages, one eventually yielding all modern birds and the other all modern reptiles.

What would this common ancestor have looked like? Our intuition is to say that it would have resembled something halfway between a modern reptile and a modern bird, showing a mixture of features from both types of animal. But this need not be the case, as Darwin clearly saw in The Origin:

I have found it difficult, when looking at any two species, to avoid picturing to myself, forms directly intermediate between them. But this is a wholly false view; we should always look for forms intermediate between each species and a common but unknown progenitor; and the progenitor will generally have differed in some respects from all of its modified descendants.

Because reptiles appear in the fossil record before birds, we can guess that the common ancestor of birds and reptiles was an ancient reptile, and would have looked like one. We now know that this common ancestor was a dinosaur. Its overall appearance would give few clues that it was indeed a "missing link"—that one lineage of descendants would later give rise to all modern birds, and the other to more dinosaurs. Truly birdlike traits, such as wings and a large breastbone for anchoring the flight muscles, would have evolved only later on the branch leading to birds. And as that lineage itself progressed from reptiles to birds, it sprouted off many species having mixtures of reptile-like and bird-like traits. Some of those species went extinct, while others continued evolving into what are now modern birds. It is to these groups of ancient species, the relatives of species near the branch point, that we must look for evidence of common ancestry.

Showing common ancestry of two groups, then, does not require that we produce fossils of the precise single species that was their common ancestor, or even species on the direct line of descent from an ancestor to descendant. Rather, we need only produce fossils having the types of traits that link two groups together, and, importantly, we must also give the dating evidence showing that those fossils occur at the right time in the geological record. A "transitional species" is not equivalent to "an ancestral species"; it is simply a species showing a mixture of traits from organisms that lived both before and after it. Given the patchiness of the fossil record, finding these forms at the proper times in the record is a sound and realistic goal. In the reptile-to-bird transition, for instance, the transitional forms should look like early reptiles, but with some birdlike traits. And we should find these transitional fossils after reptiles had already evolved, but before modern birds appeared. Further, transitional forms don't have to be on the direct line of descent from an ancestor to a living descendant—they could be evolutionary cousins that went extinct. As we'll see, the dinosaurs that gave rise to birds sported feathers, but some feathered dinosaurs continued to persist well after more birdlike creatures had evolved. Those later feathered dinosaurs still provide evidence for evolution, because they tell us something about where birds came from.

The dating and—to some extent—the physical appearance of transitional creatures, then, can be predicted from evolutionary theory. Some of the more recent and dramatic predictions that have been fulfilled involve our own group, the vertebrates.

ONTO THE LAND: FROM FISH TO AMPHIBIANS

One of the greatest fulfilled predictions of evolutionary biology is the discovery, in 2004, of a transitional form between fish and amphibians. This is the fossil species Tiktaalik roseae, which tells us a lot about how vertebrates came to live on the land. Its discovery is a stunning vindication of the theory of evolution.

Until about 390 million years ago, the only vertebrates were fish. But, thirty million years later, we find creatures that are clearly tetrapods: four-footed vertebrates that walked on land. These early tetrapods were like modern amphibians in several ways: they had flat heads and bodies, a distinct neck, and well-developed legs and limb girdles. Yet they also show strong links with earlier fishes, particularly the group called "lobe-finned fishes," so called because of their large bony fins that enabled them to prop themselves up on the bottom of shallow lakes or streams.

figure 8. Invasion of the land. A land-dwelling tetrapod (Acanthostega gunnari) from Greenland, about 365 million years ago. An early lobe-finned fish (Eusthenopteron foordi) from about 385 million years ago, and the transitional form, Tiktaalik roseae, from Ellesmere Island, about 375 millon years ago. The intermediacy of Tiktaalik's body form is mirrored by the intermediacy of its limbs, which have a bone structure in between that of the sturdy fins of the lobe-finned fish and the even sturdier walking limbs of the tetrapod. Shaded bones are those that will evolve into the arm bones of modern mammals: the bone with darkest shading will become our humerus, and the medium- and light-shaded bones will become the radius and ulna, respectively.

The fish-like structures of early tetrapods include scales, limb bones, and head bones (figure 8).

How did early fish evolve to survive on land? This was the question that interested—or rather obsessed—my Chicago colleague Neil Shubin. Neil had spent years studying the evolution of limbs from fins, and was driven to understand the earliest stages of that evolution.

This is where the prediction comes in. If there were lobe-finned fishes but no terrestrial vertebrates 390 million years ago, and clearly terrestrial vertebrates 360 million years ago, where would you expect to find the transitional forms? Somewhere in between. Following this logic, Shubin predicted that if transitional forms existed, their fossils would be found in strata around 375 million years old. Moreover, the rocks would have to be from freshwater rather than marine sediments, because late lobe-finned fish and early amphibians both lived in fresh water.

Searching his college geology textbook for a map of exposed freshwater sediments of the right age, Shubin and his colleagues zeroed in on a paleontologically unexplored region of the Canadian Arctic: Ellesmere Island, which sits in the Arctic Ocean north of Canada. And after five long years of fruitless and expensive searching, they finally hit pay dirt: a group of fossil skeletons stacked atop one another in sedimentary rock from an ancient stream. When Shubin first saw the fossil face poking out of the rock, he knew that he had at last found his transitional form. In honor of the local Inuit people and the donor who helped fund the expeditions, the fossil was named Tiktaalik roseae ("Tiktaalik" means "large freshwater fish" in Inuit, and "roseae" is a cryptic reference to the anonymous donor).

Tiktaalik has features that make it a direct link between the earlier lobe-finned fish and the later amphibians (figure 8). With gills, scales, and fins, it was clearly a fish that lived its life in water. But it also has amphibian-like features. For one thing, its head is flattened like that of a salamander, with the eyes and nostrils on top rather than on the sides of the skull. This suggests that it lived in shallow water and could peer, and probably breathe, above the surface. The fins had become more robust, allowing the animal to flex itself upward to help survey its surroundings. And, like the early amphibians, Tiktaalik has a neck. Fish don't have necks—their skull joins directly to their shoulders.

Most importantly, Tiktaalik has two novel traits that were to prove useful in helping its descendants invade the land. The first is a set of sturdy ribs that helped the animal pump air into its lungs and move oxygen from its gills (Tiktaalik could breathe both ways). And instead of the many tiny bones in the fins of lobe-finned fish, Tiktaalik had fewer and sturdier bones in the limbs—bones similar in number and position to those of every land creature that came later, including ourselves. In fact, its limbs are best described as part fin, part leg.

Clearly Tiktaalik was well adapted to live and crawl about in shallow waters, peek above the surface, and breathe air. Given its structure, we can envision the next, critical evolutionary step, which probably involved a novel behavior. A few of Tiktaalik's descendants were bold enough to venture out of the water on their sturdy fin-limbs, perhaps to make their way to another stream (as the bizarre mud-skipper fish of the tropics does today), to avoid predators, or perhaps to find food among the many giant insects that had already evolved. If there were advantages to venturing onto land, natural selection could mold those explorers from fish into amphibians. That first small step ashore proved a great leap for vertebrate-kind, ultimately leading to the evolution of every land-dwelling creature with a backbone.

Tiktaalik itself was not ready for life ashore. For one thing, it had not yet evolved a limb that would allow it to walk. And it still had internal gills for breathing underwater. So we can make another prediction. Somewhere, in freshwater sediments about 370 million years old, we'll find a very early land-dweller with reduced gills and limbs a bit sturdier than those of Tiktaalik.

Tiktaalik shows that our ancestors were flat-headed predatory fish who lurked in the shallow waters of streams. It is a fossil that marvelously connects fish with amphibians. And equally marvelous is that its discovery was not only anticipated, but predicted to occur in rocks of a certain age and in a certain place.

The best way to experience the drama of evolution is to see the fossils for yourself, or better yet, to handle them. My students had this chance when Neil brought a cast of Tiktaalik to class, passed it around, and showed how it filled the bill of a true transitional form. This was, to them, the most tangible evidence that evolution was true. How often do you get to put your hands on a piece of evolutionary history, much less one that might have been your distant ancestor?

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