by Kevin Padian
Department of Integrative Biology & Museum of Paleontology
University of California, Berkeley CA
What is Paleontology? The Fossil Record and Evolution
A bit of history: the progression of life through time, as shown in the fossil record, was well known and generally accepted in the early 1800s, long before Charles Darwin ever sailed on the Beagle. People in England and on the Continent knew that life had changed through time, and that the deeper one went in the rock column, the more different from living forms the fossils became. Since that time the fossils have always documented evolution; the question in Darwin's time was the mechanism or mechanisms that could cause the change.
Are fossils rare or abundant?
The fossil record as we know it today is strongest in the last 550 million years of life, but we do have fossils reaching back as far as 3 billion years and beyond. Most of these fossils are of what we would call invertebrate animals, that is, those without backbones, like clams, starfish, sponges, and trilobites. Individual fossil types are commonly preserved with those of other species in groups called assemblages. These assemblages reflect species that commonly live together or died together, or at least were preserved together. The assemblages are characteristic of very specific intervals in geologic history, and of very specific environments within those time intervals; for example, the snails and clams on a tidal mudflat from the Miocene Epoch, or the corals and sponges from a Jurassic reef. These assemblages change from time, preserved in the rock record, in a regular and non-random way; they are not found out of place or sequence, any more than a tri-corner hat from the Revolutionary War is found in an Egyptian tomb.
Fossils and assemblages of land plants and animals are frequently abundant, too, but not as much as the marine invertebrates; they tell their own story about the evolution of life on land.
Is the fossil record complete?
Paleontologists are often asked whether we have complete sequences of fossils, and how many of our forms are truly transitional. Of course our fossil sequences are not complete, any more than any human historical sequence is complete. The genealogy of the Bible, the teachings of Jesus, and the works of Aristotle are incompletely known, even though they occurred near thousands of years ago, whereas the fossil record encompasses billions of years. But as with a book missing many pages, or a film with many of its frames cut out, the plot and the sense can largely be reconstructed. Besides, for numerous shorter intervals the fossil record is amazingly complete in some respects, and this gives us a very good picture, we think, of how evolution generally operates at this scale.
Evolution of Whales
A good example of recent years is the early evolution of whales. Over 50 million years ago, the ancestors of whales were land animals, four-legged mammals with sizable heads and carnivorous teeth. From their skull and ankle bones we can tell that they were related to the group of mammals that today include camels, antelope, hippos, and giraffe. But they didn't look like them, and they didn't look like whales either. Our indication of their relationship to whales comes from features of their skull bones, particularly in the ear region. Their later relatives show features more like those of living whales, but the features did not change in lockstep. The snout became longer and the nostrils receded backwards; the backbones became more mobile in an up-and-down direction; the hands became larger and paddle-like, while the hindlimbs grew smaller and became disconnected from the backbone. Still, the ankle bones remained completely characteristic of the land-living mammals from which whales descended. Eventually, the skeleton became too modified for whales to walk on land anymore. They were fully aquatic creatures. And through all this transition, oxygen isotopes in their bones reflect an increasing contribution of seawater to their diets, as their ecology changed from terrestrial to aquatic to near-shore to deep-water environments.
Between a half-dozen and a dozen species of early fossil whales are known from Egypt, India, and Pakistan that bear the characteristics that bridge the transition from land to sea. Yet we do not regard any of these species as the actual direct ancestor of later whales. Each has its own particular features that indicate that it is somewhat off the line that led to today's whales. But we learn as much about the characteristics of early whale evolution from these fossils as we would learn about the life and times of your ancestors from going back to historical records in the places they came from, even if your own great-great-great-grandparents were not mentioned specifically. We don't look for "missing links" or direct ancestors in the fossil record; we don't need to, and the likelihood of finding them in any case is vanishingly small.
Does the fossil record support the theory of evolution?
Do other lines of evidence support our ideas about evolution gleaned from the fossil record? Very much so. The geographic distributions of plants and animals today and through time show how migrations and the spread of continents have paralleled climate change and evolutionary diversification. Radiometric dates -- which are based on principles of physics and chemistry, not on geology and paleontology -- consistently give us independent confirmation of the ages of fossils and their deposits that have been reckoned on the basis of geology and paleontology. The oxygen isotopes in the bones of whale ancestors, just mentioned, provide another independent assessment of their change of habitat through time. And genetic evidence continues to both confirm and enlighten our ideas about the relationships of living and fossil organisms, just as paleontology provides important information about the dates of appearance and divergence of different groups in the fossil record.
For an example of this last point, we return to the whales. For many years paleontologists and mammalogists tried to figure out the closest relatives of whales, living and fossil. A number of ideas were tossed around, but no answers seemed definitive. Then two interesting things happened, both in the last 10 years. First, molecular geneticists analyzed the genomes of whales and many other kinds of mammals, and determined that the closest relatives of whales were hippos. This answer was not readily accepted by paleontologists, because the first whales were well known from over 50 million years ago, whereas hippos did not appear in the fossil record until about 20 million years ago (and they would be hard to miss). The geneticists replied that their answer would explain several features that hippos and whales share in common, such as large size, aquatic habits, and loss of hair. However, unfortunately, the first hippos were not large and were not aquatic (we don't know about their hair).
This impasse persisted for a while, and then the second interesting thing happened. A new analysis showed that an ancient group of mammals called anthracotheres, which lived after the first whales and before modern hippos, are the closest fossil relatives of hippos, who appear to have descended from them. And these animals, in turn, are closely related to whales. The confusion was resolved: hippos are the closest living relatives of whales, but they have separate ancestries going back 50 million years. So it turns out that even apparent impasses can be resolved by further research and analysis, and genetics and paleontology, even when they at first seem to be pointing to very different answers, usually converge on the same conclusion after all.
The fossil record gives us many examples of the transitions of features that are involved in major adaptive changes, particularly when the origin of new major groups is involved. We know, for example, that the first feathers are found in small dinosaurs that could not fly. They were hair-like and fairly short, and were used for insulation. Later, they grew longer and began to branch, and probably had additional functions in display or camouflage, because they were differently colored. Eventually these early feathers became organized into central stalks with vanes on either side, much as we see in birds today; but they were still too small to propel the animals in flight. Preserved skeletons show us that these feathered dinosaurs use their feathers, in part, to shelter eggs as they were brooding them on their nests. Only with the origin of birds were feathers used in flight. This concept of change in function, which we call exaptation, is one of the most important features of evolution.
Is evolutionary change fast or slow? Does it occur in leaps or through gradual steps?
Paleontologists are also often asked whether the fossil record supports rapid or slow change. In other words, is the pace of change "gradual" or "punctuated"? The answer is yes -- yes to both. To begin with, in Darwin's time the word "gradual" meant "step-like," much as rows of seats are set in a theater, and the word was used that way as often as it was to suggest "slow and steady" change. We see both kinds of change in the fossil record -- both extremes and many gradations in between. It is pointless to argue about words, but valuable to quantify and compare the rates of change when we can. However, neither rate of change is a challenge to Darwin's ideas, because he used the word gradual in both senses.
What is consilience?
Paleontological theory, like theory throughout evolutionary biology, is continually modified through new discovery. It both supports and is supported by lines of evidence from other sciences. We call this principle consilience, and it is a fundamental part of scientific analysis, because it enables us to test individual lines of evidence against others, to see if they point to the same conclusion. The patterns seen in the fossil record have been tested for two centuries now, both by new fossil discoveries and by new discoveries from population biology, genetics, and molecular biology, as well as physics, chemistry, and geology. The fact that all these sciences work together and have converged on the same conclusions is an indication of how strong our understanding of the fossil record and its place in evolutionary theory really is.
Related Links
See Kevin Padian's testimony in the Kitzmiller trial for more about the science of paleontology.