By Robert Hazen, George Mason University
Fossils are the evidence of past life preserved in rock. They provide an amazing view of almost 4 billion years of biological change. We can view that change by looking at successive layers of rock, epoch by epoch, in successive layers. As new fossil species are discovered, month by month, that fossil record becomes more detailed and more complete, and the gradual evolution of life becomes much more firmly established.
Record of Evolution of Trilobites
The fossil record refers to all the varied fossils that have been unearthed all over the world. The record is vast, and that record is constantly growing. It’s remarkable in the clarity of the portrait it gives of how life changed over time.
The fossil record reveals evolution on a finer scale. We can observe individual types of organisms as they change, layer by layer, through rock sequences, and this is another fascinating aspect of paleontology. One such set of rocks is a set of trilobites that were found in Ordovician rocks near St. Petersburg, Russia. They beautifully illustrate this point of gradual change through layers of rock.
It depicts a complete sequence: at the bottom of this section you don’t find long eyestalks in the trilobites; at the top of the sequence you don’t find anything but long eyestalks. Clear evidence that life has changed in detail, gradually over time, as you go from the bottom to the top of this 3-million-year sequence.
This is a transcript from the video series The Joy of Science. Watch it now, on Wondrium.
Did the Whales Evolve, Too?
The evolution of whales provides a more recent example of this gradual-change process. Though they are mammals, whales have no legs. They seem so different in form, and in lifestyle, from other living mammals that it really seems difficult to imagine how they could evolve; anti-evolutionists often use whales as proof that evolution has not occurred, and could not occur.
We should read a quote, from this creationist literature, that goes against the idea that whales evolved: “Darwinists rarely mention the whale, because it presents them with one of their most insoluble problems. They believe that somehow a whale must have evolved from an ordinary land-dwelling animal, which took to the sea and lost its legs. A land mammal that was in the process of becoming a whale would fall between two stools. It would not be fitted for life on land or sea, and would have no hope of survival.”
Gaps in Fossil Record
Another quote, of a similar vein, says: “There are simply no transitional forms in the fossil record between marine mammals and their supposed land-mammal ancestors. It is quite entertaining, starting with cows, pigs, or buffaloes, to attempt to visualize what the intermediates may have looked like. Starting with a cow, one could even imagine one line of descent which prematurely became extinct due to what might be called an udder failure.”
These arguments do make a point about the fossil record. There are gaps, there are things missing, and the commentators are correct; one would expect to have intermediate forms. So what do paleontologists do? They go out and they find those intermediate forms. Indeed, in the last decade, paleontologists have found several of these missing links between land animals and whales.
Intermediate Forms as the Missing Links
Moving back in time, one such intermediate form is the 35-million-year-old Basilosaurus. It’s a sleek, powerful, toothed whale, and a recent discovery of a complete fossil in Egypt included tiny, vestigial hind legs; bones of a complete hind leg that belonged to this whale. This is a feature without any obvious function for the whale; it’s a vestigial limb that was gradually disappearing, but was still present 35 million years ago.
An even more primitive whale, Rhodocetus, has more exaggerated hind legs, not unlike a seal. Then, finally, came the discovery, in 1994, of the new genus Ambulocetus, the walking whale. This beautiful creature is a true intermediate between land mammals and sea mammals; it has features of both.
Evolution, being a robust theory, makes innumerable predictions about intermediate forms that should be present in the fossil record. The fossil record now is not complete; each year we discover more and more, and we fill in the gaps. Each new discovery generally fills in a slot between previous missing links, and so we get an ever more complete picture of how life has changed over immense spans of time.
Growing Database of Fossil Records
The fossil record is constantly growing and expanding. For example, in 1998, three entirely new genera of dinosaurs discovered were discovered, dinosaurs that had never been seen before. One of these was especially remarkable: it was a bizarre, large species with a crocodile-like narrow snout with big, sharp teeth, especially designed for eating fish. This is the first known dinosaur that specialized as a fish eater.
In future, more discoveries will be made, and the fossil record will become ever more complete. That’s the nature of science. It keeps building on its past understanding. Only very rarely did an individual die and become buried quickly enough, into the right chemical environment, to be preserved in stone. So the fossil record is admittedly a very incomplete record of past life.
Common Questions about Fossils
Though they are mammals, whales have no legs. They seem so different in form, and in lifestyle, from other living mammals that it really seems difficult to imagine how they could evolve; anti-evolutionists often use whales as proof that evolution has not occurred, and could not occur.
Moving back in time, one intermediate form of whale is the 35-million-year-old Basilosaurus. It’s a sleek, powerful, toothed whale, and a recent discovery of a complete fossil in Egypt included tiny, vestigial hind legs; bones of a complete hind leg that belonged to this whale.
The fossil record is constantly growing and expanding, and year after year, more discoveries will be made, and the fossil record will become ever more complete.
