Will the Dinosaurs Return?
When the American Museum of Natural History's paleontologist William Diller Matthew published his book Dinosaurs in 1915, no one understood how the famous Mesozoic creatures originated or went extinct. Both the beginning and end of the "Age of Dinosaurs" were mysterious. Yet, tucked away in a footnote, Matthew made a suggestion that dinosaur-like animals might return someday.
Matthew based his speculation on the idea that the ancestors of dinosaurs resembled modern day lizards. His summary was somewhat contradictory—the long-legged, "more or less bipedal" dinosaur ancestors he described would have looked significantly different than living reptiles—but Matthew affirmed that the immediate precursors of dinosaurs "were probably much like the modern lizards in size, appearance, and habitat." Following from that connection, he wrote:
If some vast catastrophe should today blot out all the mammalian races including man, and the birds, but leave the lizards and other reptiles still surviving, with the lower animals and plants, we might well expect the lizards in the course of geologic periods to evolve into a great and varied land fauna like the Dinosaurs of the Mesozoic Era.
Matthew was not the first to suggest that dinosaurian creatures might reappear. In the first volume, published in 1830, of his influential book Principles of Geology, the Scottish geologist Charles Lyell linked the character of the Earth's fauna to climate over long geological cycles. The dinosaurs and other prehistoric reptiles had thrived during a long-lasting summer, and should the Earth's geology again approach the state it had been in during that time, lush forests inhabited by dinosaurs and pterosaurs might return. Life was closely tied to climate, Lyell thought, and climate was regulated by gradual geological change.
(This was not an evolutionary connection, but an argument that particular kinds of organisms were closely tied to certain climates. The mechanism by which dinosaurs might reappear was not specified by Lyell.)
Lyell's idea of Iguanodon and Megalosaurus once again roaming the earth was lampooned by his colleague Henry De la Beche in a cartoon called "Awful Changes." Set in some unknown future time, a highly-intelligent Ichthyosaurus takes up the mantle of paleontologists and, presenting a human skull to an assembly of marine reptiles, says: "You will at once perceive that the skull before us belonged to some of the lower order of animals; the teeth are very insignificant, the power of the jaws trifling, and altogether it seems wonderful how the creature could have procured food." The thought that ichthyosaurs, dinosaurs, or pterosaurs would suddenly reappear someday was absurd.
Neither did Matthew escape criticism. The naturalist John Burroughs was incredulous at the suggestion that something like "Brontosaurus" might wallow in stagnant, warm swamps in a hypothetical future. In a letter printed in Natural History, the magazine published by the AMNH, Burroughs rhetorically asked: "Does not the evolutionary impulse run its course? Can or will it repeat itself?" Burroughs answered with an emphatic "no." Dinosaurs were specialized to the unique conditions of their time—part of the gradual evolutionary development of the planet—and the Earth could no more revert to a previous state, the naturalist concluded, than a fruit reverse the process of ripening.
Matthew's lengthy response was printed right below the critical letter. The offending passage was simply a bit of speculation, Matthew replied, and it had nearly been cut out of the manuscript. Yet, since Burroughs was inspired to write a letter about it, clearly the suggestion that dinosaur-like organisms might reappear was a subject of interest worth considering.
In fashioning his reply, Matthew followed in Lyell's tradition. Life had not been moving in a straight-line direction from beginning to an unforseen end. Our planet has instead gone through a series of cyclical changes which have influenced the evolution of life on Earth. Evolution has proceeded along some "upward steps," Matthew conceded, but these have been within the context of environments that have appeared over and over again through time. Dinosaurs, for example, were thought to inhabit a world generally similar to that of the present day despite all the intervening changes between their time and ours, and so the objection that the world was too different could be set aside.
But Matthew was not proposing the return of true dinosaurs. He was considering the evolution of dinosaur-like creatures from modern reptiles. If mammals and birds were suddenly wiped out, then the field would be open for reptiles to proliferate and evolve:
Certainly such an expansive evolution of the lizards with their higher competitors removed would not cause the huge Brontosaurus to reappear on earth. But it might—if we accept the modern theory of geologic history—bring about the appearance of gigantic wading or amphibious reptiles equally huge and equally innocuous, although probably not at all like a Brontosaur in appearance.
Life will continue to change over time. That is inevitable. How life will evolve is another matter. There is no pre-determined evolutionary pathway or trajectory. The history of life on Earth is heavily influenced by contingency—what came before provides context for what comes after—and there is no inherent direction that guarantees the reappearance of dinosaurs or dinosaur-like animals.
Strangely, though, we now know that the origin of dinosaurs at least partially owes to a devastating mass extinction that took place approximately 251 million years ago. Our own ancestors and close relatives among the weird and wonderful synapsids were the dominant terrestrial vertebrates just prior to this time, but they were almost entirely wiped out. This set the stage for a proliferation of other creatures, including the ancestors of dinosaurs. Then, 65 million years ago, the non-avian dinosaurs were wiped out in another mass extinction, allowing for the evolutionary radiation of mammals. These rare, global-scale catastrophes wipe out some species and fortuitously provide new opportunities for surviving lineages. If we were to suffer another event right now, of the kind Matthew wondered about, who knows what life would look like 10, 50, or 100 million years from now?