Crinoid columnals (Isocrinus nicoleti) are shown from the Middle Jurassic Carmel Formation at Mount Carmel Junction, Utah. Wikimedia Commons
By the early 1800s, many scientists already recognized life on Earth was ancient, had changed over time, and fossils represented prehistoric extinct creatures. Evolution wasn’t a new idea, but understanding how it worked awaited Charles Darwin’s 1859 publication of “On the Origin of Species.”
Today, much of the ongoing research and evidence for evolution involves genetic, molecular and protein studies. But when Darwin and Alfred Russel Wallace were working out natural selection as the major mechanism of evolution, genetic inheritance and molecular biology were unknown. They had little to go on other than observations of living species and the fossil record.
A major shortcoming of the fossil record was its incomplete nature, a problem well recognized by Darwin. Although much more extensive today, with important transitional forms added as predicted, gaps in the evolutionary record remain common and widespread.
Ideally, the fossil record should offer an unbroken series of gradually changing fossilized organisms, linking every phylum and species ever known. Like a movie, each fossil would represent an individual frame of the film which, when played, displays the complete history of evolution on Earth.
In reality, the record with its gaps is more like an old damaged movie, with many random frames missing. You can understand the plot with difficulty and pick out the main characters, but it’s hard to follow all the twists and turns or make out minor characters.
These gaps aren’t surprising when considering the odds against fossils forming and enduring for millions of years.
Circumstances must be just right for fossilization. Hard-bodied animals with bones, teeth or exoskeletons have the advantage, with soft-bodied animals only ever found in a few fossil sites around the world. Muscle, fat and soft tissues quickly decay, scavengers eat or scatter remains, and bacteria and fungi typically destroy any remains.
Since weather and exposure destroy tissues, where an animal (or plant) lives and dies plays a key role. Tropical jungles rarely form fossils, desert areas slightly more often, and ocean bottoms or flood zones where sediments deposit most frequently. Rapid burial by mud or volcanic ash that protects the organism from destructive forces is ideal, as long as temperature, pH and oxygen levels are right.
If scavengers, decay, weather and environment all cooperate, the body must now remain buried and undisturbed while ground water percolates past. Water carries dissolved minerals from the soil which gradually replace the original atoms in the tissues.
As millennia pass the remains fossilize, and the surrounding sediment gradually converts to rock by pressure and time. Having already beaten incredible odds, the fossil and surrounding area must now remain intact for up to hundreds of millions of years despite earthquakes, volcanic activity, erosion, tectonic plate movement and other disasters.
Over eons a small fraction of fossil-bearing rocks eventually become exposed. Erosion and the elements immediately begin to destroy and scatter the fossils, making identification increasingly difficult. Only in the last couple hundred years have scientific naturalists been around to find and study those fossils that surface.
Discovered fossils are likely to be those of the most common and longest existing animals. Transitional forms like Tiktaalik, an intermediate between fish and the earliest land animals, will be fewer in number and arise only in isolated areas. This makes their fossilization and discovery even more improbable.
It’s easy to see why gaps are common in the fossil record, but every year they become slightly smaller and fewer in number. Returning to the movie analogy, more frames are continuously being added and the film increasingly easy to follow. Even better, modern genetics and molecular studies supply the accompanying script to help predict and interpret the story line.
What would Darwin think if he could view the fossil record today, along with advances like carbon dating, genetics and modern biologic testing? He figured out the role of natural selection in evolution using only the limited evidence of his time and groundbreaking ideas. I doubt he’d see a problem with the fossil record.
Lifelong Oregonian Fred Schubert, a The Dalles biologist, has a lifelong interest in general science and science writing. Feel free to submit comments on this article or suggestions for new topics to firstname.lastname@example.org.