Fossils and Evolution: Fossilization, Darwinian Theory, and Punctuated Equilibrium

A fossil is any remains, trace, or imprint of any plant or animal that once lived on Earth. Such remains of past life include body fossils, which preserve hard or soft body parts, and trace fossils, which record biological activity such as footprints, tracks, and burrows. The oldest known body fossils are 3.4-billion-year-old remnants of early bacteria, whereas chemical traces of life may extend back to 3.8 billion years.

The conditions that lead to fossilization occur so rarely that only an estimated ten percent of all species that have ever existed are preserved in the fossil record. Consequently, the record of life and evolution is highly incomplete. To be preserved, life-forms become mineralized after death, with organic tissues typically replaced by calcite, quartz, or other minerals during burial and diagenesis. Fossils are relatively common in shallow marine carbonate rocks, where organisms that produced calcium carbonate shells are preserved within a carbonate matrix.

The fossil record has been used to test, modify, and support evolution—a concept traditionally viewed as a slow, gradual process describing how life has changed on Earth from simple single-celled organisms to the complex biosphere of today. A more precise definition of biological evolution is a sustained change in the genetic makeup of populations over generations, leading to the emergence of new species. The field of evolution was pioneered by Charles Darwin in his works On the Origin of Species (1859) and The Descent of Man (1871), and it is now a multidisciplinary science incorporating geology, paleontology, biology, and, with neo-Darwinism, genetics.

Darwin sailed on the HMS Beagle (1831–1836), making numerous observations of life and fossils worldwide, which led to his theory of natural selection. In this theory, species with favorable traits have a better chance of survival. The main tenets are that species reproduce more than necessary, but populations remain stable due to a constant struggle for food and space, so only the fittest survive. Darwin proposed that traits contributing to an individual’s survival are passed to descendants, thereby propagating favorable traits. However, Darwin lacked a good explanation for why some individuals possess favorable traits while others do not; this evidence came later with genetics and the recognition that mutations cause changes in character traits. Sequential inheritance of mutation-induced changes over generations can lead to species transformation and, eventually, the evolution of new species. Darwin’s natural selection works by the gradual elimination of less successful forms, favoring those with beneficial mutations.

Modern evolutionary theory recognizes two major styles of change. Macroevolution describes changes above the species level and the origin of major groups, whereas microevolution concerns changes below the species level and the development of new species. Another major development over the past century relates to the rate of evolutionary changes as preserved in the fossil record. Darwin thought evolution progressed slowly, with one species gradually transforming into another, but the fossil record supports only a few examples of such gradualism (notable cases include changes in Ordovician trilobites and Cenozoic horses).

Fossil evidence demonstrates that nearly all species persist with little change for long periods of geologic time, followed by sudden disappearance and replacement by entirely new species. In other cases, new species appear abruptly without the disappearance of existing ones. Biologists initially viewed such apparent rapid change with skepticism, attributing it to an incomplete fossil record, but many complete records confirm that these rapid changes are real. A new evolutionary paradigm named punctuated equilibrium, advanced in the 1970s by Steven Jay Gould and Niles Eldredge, explains these sudden evolutionary shifts.

Physical or geographic isolation of some members of a species—such as during supercontinent breakup—can separate populations, alter their environment, and effectively isolate some members under conditions that select for change. A small, isolated group may possess a mutation that favors their new environment, allowing them to survive. When supercontinents collide, many previously separated species encounter one another and must compete for food and space; only those best suited to that particular environment survive to reproduce, leading to extinction of others.

In other cases, major environmental catastrophes such as meteorite impacts and flood basalt eruptions cause extreme planetary changes, triggering mass extinctions. Relatively minor or threatened species that survive may suddenly find themselves with traits that favor their explosion into new ecological niches and their subsequent dominance in the fossil record. This pattern underscores the importance of environmental perturbations in shaping the course of evolution.

FURTHER READING: McKinney, Michael L. Evolution of Life: Processes, Patterns, and Prospects. Englewood Cliffs, N.J.: Prentice Hall, 1993.
Stanley, Steven M. Earth and Life Through Time. New York: W. H. Freeman, 1986.