1 History of Evolution and the Geologic Timescale

The Geologic Timescale and the Inevitability of Scientific Ideas

The scientific process demonstrates a strength found in the progression and inevitability of ideas, particularly regarding the history of evolution and life on Earth. The geologic timescale serves as the fundamental framework for this history, categorizing Earth's past into nested hierarchies. The broadest divisions include the Hadean, Archaean, Proterozoic, and Phanerozoic Eons. Within the Archaean, we find the Eoarchaean ($4031\,Ma$), Palaeoarchaean, Mesoarchaean, and Neoarchaean ($2500\,Ma$). The Proterozoic is divided into the Palaeoproterozoic, Mesoproterozoic, and Neoproterozoic. Significant biological milestones are marked across this scale, such as the emergence of prokaryotes approximately $4000\,Ma$, multicellular life at $1600\,Ma$, Animalia between $650\,Ma$ and $600\,Ma$, and land plants near $500\,Ma$. The Phanerozoic Eon, beginning $538.8\,Ma$, encompasses the Palaeozoic, Mesozoic, and Cenozoic Eras. Major life forms within this eon include non-avian dinosaurs and early Mammalia (spanning approximately $243\,Ma$ to $66\,Ma$) and the emergence of Hominins roughly $2\,Ma$.

Historical Perspectives on the Age of the Earth

Prior to the late 17th century, geologic time was largely perceived through the lens of historical human time. In 1654, Archbishop James Ussher of Armagh, Ireland, calculated the Earth's age by totaling generations recorded in the Old Testament. He concluded that the Earth was formed on October 23, $4004\,BCE$, which would make the planet $6030\,years$ old by the year 2026. Scientific estimates evolved significantly with Lord Kelvin (William Thomson, 1st Baron Kelvin), who posited that the Earth began as a "red-hot globe." Kelvin estimated the Earth's age to be between $20$ and $400\,million\,years$, eventually settling on a range of $20\,million$ to $40\,million\,years$. His model assumed the Earth was cooling from a molten state and that the Sun was no older than $20\,million\,years$. However, his calculations were limited because the discovery of radioactive decay and its internal heat generation was not published until the year of his death, 1907. The definitive modern age of the Earth was determined in 1956 by Clair Patterson, a University of Iowa alumnus associated with Cal Tech. Using lead isotope dating, he calculated the age to be $4.550\,Billion\,years$ (with a margin of $\pm 70\,Million\,years$). Patterson later became a prominent activist for the removal of lead from gasoline.

Principles of Absolute and Relative Dating

Geologic time is measured using two distinct methods: absolute age and relative age. Absolute age represents the numerical age in years before the present (e.g., $4.54\,Ga$). Relative age is a qualitative method developed centuries ago that determines the sequence of events without providing a specific numerical date, utilizing names for time intervals to establish older versus younger relationships. A historical comparison illustrates this: while relative dating might list the World War II as occurring after World War I, absolute dating assigns the specific years $1939-1945$ and $1914-1918$ respectively. This logic applies to geological strata, where relative ages assign order to events and absolute ages assign exact dates.

Steno’s Principles and the Foundations of Stratigraphy

In 17th-century Florence, Italy, Nicolas Steno established the fundamental principles of stratigraphy, which is the study of the order and relative position of strata (rock bodies) and their relationship to the geologic timescale. The Principle of Original Horizontality states that all strata or bedded rocks are deposited in a nearly horizontal orientation. If strata are observed in a dipping or tilted position, it implies that tectonic tilting must have occurred at some point after the initial deposition. While standard, there are rare exceptions to this rule, such as windstream lines in sand dunes where erosion and deposition create angled layers, or reef core structures involving back reef facies and talus slopes. The Principle of Superposition dictates that in an undisturbed sequence of strata, the oldest beds are located at the bottom, with higher beds being progressively younger. This principle extends to fossils contained within the rock; those in higher layers are younger than those found in lower layers.

Uniformitarianism and the Continuity of Natural Processes

James Hutton ($1726-1797$) introduced the concept of Uniformitarianism, arguing that the solid parts of current landmasses are composed of materials produced by the sea and other natural operations. He concluded that the land is not a "simple and original" creation but a composition formed by "second causes." Hutton observed that current processes—such as tides, currents, and seafloor operations—existed in previous worlds of sea and land. He postulated two requirements for land permanence: the consolidation of loose materials into masses and the elevation of these masses from the sea floor to their current positions. Famous for his statement, "We find no vestige of a beginning – no prospect of an end," Hutton emphasized that geologic change occurs through natural, observable operations. Charles Lyell ($1797-1875$) later popularized this with the phrase, "The present is the key to the past," asserting that the physical, chemical, and biological processes operating today are identical to those that operated in the geologic past.

Biostratigraphy and the Beer Can Archaeology Analogy

To understand how fossils date rock strata, one can look at the "evolution" of beer containers as described by D.B.S. Maxwell in "Beer Cans: A Guide for the Archaeologist." Just as fossils change through time, design features of artifacts provide precise age estimates. Pre-1900 beer was stored in bottles with cork stoppers. In 1900, the crimped metal cap was introduced. A major "evolutionary leap" occurred in 1935 when Kruger Cream Ale from Virginia became the first canned beer. Pre-WWII saw the popularity of short cone cans. In 1962, the "snap-top" (pull tab) was introduced, followed by the Coors push-button opening in 1972. The 1980s saw the narrow-mouth "stay-on-tab," and 1997 introduced the currently ubiquitous "wide-mouth" can. By identifying these features in a site's "strata," archaeologists can date 20th-century events to within five to ten years. In geology, fossils function exactly like these design changes, allowing scientists to correlate the age of rocks based on the specific biological "artifacts" they contain.

Regional Geology: Iowa’s Bedrock and Fossil Record

Iowa's bedrock geology serves as a practical application of Steno's principles and biostratigraphy. The state's bedrock reveals different geologic systems based on age: Cambrian ($505-530\,Ma$), Ordovician ($439-505\,Ma$), Silurian ($415-435\,Ma$), Devonian ($355-385\,Ma$), Mississippian ($325-353\,Ma$), Pennsylvanian ($298-320\,Ma$), Jurassic ($160\,Ma$), and Cretaceous ($74-102\,Ma$). In Johnson County, the Phacopid "lens-face" trilobite from the Devonian Age is found in higher elevations around Lake McBride, while the Encrinurid trilobite is found in deeper river valley incisions. Moving to the Mississippi Valley in Northeast Iowa, Asaphid trilobites are found on the valley sides, whereas the lowest (and therefore oldest, per Steno) parts of the valley contain Dikelocephalid trilobites from the Cambrian Period. This sequence—Cambrian, Ordovician, Silurian, and Devonian—demonstrates how fossils and relative positioning build the regional geologic record.

Taxonomic Units of Geologic Time

Geologic time is categorized into two sets of units. Chronostratigraphic (time-rock) units refer to the actual physical strata deposited during a specific interval, categorized as Erathem, System, Series, and Stage. Geochronologic (time) units refer to the interval of time itself, categorized as Era, Period, Epoch, and Age. Boundaries between these units (such as systems or stages) are defined globally by "driving a spike" at a single type-locality. The primary divisions of the timescale are as follows: the "Precambrian" spans from $4.54\,Ga$ to $539\,Ma$, representing approximately $80\%$ of Earth's history despite having very few fossils. The Phanerozoic Eon ("visible life") spans from $539\,Ma$ to today and is divided into three eras: the Paleozoic ("ancient life," $539-252\,Ma$), the Mesozoic ("middle life," $252-66\,Ma$), and the Cenozoic ("new life," $66\,Ma$ to Present). A common mnemonic to remember the geologic periods is: "Come Over Some Day, Maybe Play Poker, Try Jeff’s Crazy Pizza, Nacho Queso," representing the Cambrian, Ordovician, Silurian, Devonian, Carboniferous (Mississippian/Pennsylvanian), Permian, Triassic, Jurassic, Cretaceous, Paleogene, Neogene, and Quaternary periods.