By Robert Hazen, George Mason University
Many independent lines of evidence provide convincing proof that life has changed over more than 3.5 billion years of Earth history, but no form of evidence is more compelling than fossil record, which is the evidence of past life preserved in rocks.
Is Evolution Real?
Comparative anatomy indicates evolution. We can compare the structure and function of bones. We also have vestigial organs. These are sure signs that we have evolved, that life has changed over time.
We have the shared cellular structures, the shared molecular structures that are common to all life. And, of course, we have the common, shared genetic code of all life forms. Indeed, if we look in detail at various proteins, at various genes in our body, we can see the accumulated effects of change over long time periods as we compare different organisms, where the same gene functions in similar ways in all those different organisms. There is an entire body of evidence that documents evolution.
This is a transcript from the video series The Joy of Science. Watch it now, on Wondrium.
Fossils
The primary source of evidence for evolution is fossils; it is the fossil record, the unambiguous testimony of the rocks. Fossils are evidence of past life. They’re usually preserved in layers of sedimentary rock. Most fossils are preserved as hard parts—the hard parts of animals, or of plants, such as shells, bones, wood, and the like.
Fossils also occasionally occur in many other forms. We have tracks and trails left in mud; dinosaur footprints, or trails of small invertebrate animals, as they crawled across mud. There are casts, or impressions, that are left by mold. The organism itself has disappeared, but the impression of the organism when it died is left.
We have insects preserved in amber, mineralized dung, and leaves preserved in shale—in some cases places where we can actually unfold the layers of shale and pick up entire leaves, even as they were preserved in ancient rock. The fossil record refers to all the varied fossils that have been unearthed all over the world. The record is vast and is constantly growing. It’s remarkable in the clarity of the portrait it gives of how life has changed over time.
Evidence of Evolution of Individual Organisms
The geological record, the fossil record, also reveals evolution on finer and finer scales. We can observe individual types of organisms as they change, layer by layer, through rock sequences, and this is another fascinating aspect of paleontology.
In one set of these rocks, a set of trilobites were found in Ordovician rocks near St. Petersburg, Russia. They beautifully illustrate this point of gradual change through layers of rock.
Trilobites
What you find in a series of trilobites from this era, about a 3-million-year sequence of layered, very beautiful fine-grained limestones, is that there’s a gradual change in the appearance of the trilobites.
At the very base, at the bottom of these rocks, you find a very common species of trilobite, one that’s found around the world, called asaphus. A trilobite had small, nubby eyes; it typically burrowed in mud, and may have scavenged in the mud for remains of dead animals. This is a very common kind of trilobite around the world.
The Evolution of Trilobites
On these particular rocks, as you go up this rock section, as you get younger and younger, you see a gradual change. About a third of the way up through that section, you find similar trilobites, asaphus species; but they suddenly have eyestalks, maybe 1/4 inch long. These trilobites burrowed a little bit deeper into the mud, and therefore, perhaps were able to escape predators more easily as they looked for food.
As you go even farther up in the section, you find trilobites with longer eyestalks, 1/2 inch or more; the same basic species, or same basic genera, but longer eyestalks. When you go to the very top of the sequence, you find astonishing trilobites, with eyestalks up to 1 inch long; apparently, an evolutionary adaptation to escape predators.
This is a complete sequence: at the bottom of this section you don’t find long eyestalks; 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.
Expanding Fossil Record
It must be emphasized that the fossil record is constantly growing and expanding. For example, in 1998, three entirely new genera of dinosaurs 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.
Can Fossils Have Limitations?
Undoubtedly, and year after year, more discoveries will be made, and the fossil record will become ever more complete. That’s the nature of science—that it keeps building on its past understanding.
The vast majority of individual organisms, especially those without hard parts, left no trace at all in the fossil record. Their atoms, their molecules, get decayed and reintroduced in the environment; they’re just recycled over and over again. In fact, our bodies are rich in atoms and molecules that were once part of other organisms, perhaps organisms that lived billions of years ago.
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
Fossils are evidence of past life. They’re usually preserved in layers of sedimentary rock. Most fossils are preserved as hard parts—the hard parts of animals, or of plants, such as shells, bones, wood, and the like.
The vast majority of individual organisms, especially those without hard parts, left no trace at all in the fossil record. 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.
The fossil records that hold testimony to trilobites evolution were found in Ordovician rocks near St. Petersburg, Russia.
