It has been asserted by some writers who believe in the immutability of species that geology yields no linking forms. This assertion is certainly erroneous…. What geological research has not revealed is the former existence of infinitely numerous gradations connecting nearly all existing and extinct species.
—Charles Darwin, On the Origin of Species
Most people think that the idea of evolution came from studying the fossil record. Although it is true that the change in fossils through time was well established by 1805, none of the early naturalists who studied fossils were driven to the notion of evolutionary change. The leading paleontologist of his time, Baron Georges Cuvier of France, did not accept the wild evolutionary speculations of his peers, such as Lamarck and Geoffroy, and used the fossil record to criticize these evolutionists. Early nineteenth-century ideas about evolution came strictly from living organisms, and paleontology and fossils played little or no role in the debate.
When Darwin published On the Origin of Species in 1859, his arguments were based almost exclusively on evidence from living organisms. Darwin spent two entire chapters appearing to apologize for the incompleteness of the fossil record and for the seeming lack of support it offered for his radical new idea of evolution. Actually, if you read those chapters closely, Darwin very cleverly convinces the reader that the fossil record is exactly as one would expect, given the processes of geology and the vast expanses of time that were already accepted for the age of the earth. In the second of the two chapters, he argues convincingly that the fossil record, as imperfectly known as it was back then, is still strongly supportive of his ideas.
But if the fossil record was not much help to Darwin in 1859, it soon became his chief line of evidence. Only a year after his book was published, the first specimens of the transitional fossil Archaeopteryx were found in Germany, and soon the British Museum had spent a fortune to acquire the first decent skeleton of this classic fossilized transition between birds and reptiles. In the 1870s, American paleontologist Othniel C. Marsh laid out a remarkable series of horse fossils that demonstrated how the entire lineage grew from a small dog-sized form with three or four toes to our modern racehorse. Soon other examples of evolutionary transitions in the fossil record were being described and published, and by 1900 some of the first fossils that belong to our family, but not our species (“Java Man,” now known as Homo erectus), were discovered as well. The early twentieth century brought an incredible explosion of paleontological discoveries as the great museums mounted expeditions to the western United States and Canada, Asia, and Africa to secure great dinosaur skeletons for their exhibit halls, again producing further evidence of evolution in the fossil record.
But the past 30 years have produced some of the greatest discoveries of all, including incredible fossils that show how whales, manatees, and seals evolved from land mammals; where elephants, horses, and rhinos came from; and how the first backboned animals evolved. We now have an amazing diversity of fossil humans, including specimens that show that we walked upright on two feet almost 7 million years ago, long before we acquired large brains. In addition to all this fossil evidence, we have new evidence from molecules that enables us to decipher the details of the family tree of life as never before.
Although scholars in 1859 may have considered Darwin’s evidence from fossils weak, this is no longer true today. The fossil record is an amazing testimony to the power of evolution, with documentation of evolutionary transitions that Darwin could have only dreamed about. In addition, detailed studies of the fossils have even changed our notions about how evolution works and have fueled a lively debate in evolutionary biology about the mechanisms that drive evolution. The fossil record is now one of the strongest lines of evidence for evolution, completely reversing its subordinate status of only 150 years ago. Instead of the embarrassingly poor record that Darwin faced in 1859, we now have an embarrassment of riches.