Making Sense of Evolution

Historical Innovations in Science

One of the hardest things to grasp about science is that it is constantly changing. Conceptions of the way the world works are not necessarily permanently fixed in science, and can be amended through the discovery of additional evidence, experimentation, and reexamination of findings. Before Darwin could make the breakthroughs that redefined modern biology and our understanding of the natural world, discoveries in other fields had to occur. This background knowledge will be useful in framing relevant issues that made Darwin’s understandings possible. As far as my students are concerned, many of these findings will already have been discussed or implemented in units prior to this one, but a review may give additional insight or frame the issues for readers of this curriculum unit.

Geology

While it is my intention to have introduced the concepts of geologic time, index fossils, the law of superposition, and even continental drift to my students before engaging in this unit, it might be worthwhile to review the state of geology leading up to Darwin’s breakthrough and the eventual widespread acceptance of evolution. Before the 19^th century scientists were not interested in overturning traditional religiously based views of the natural world. By the 19^th century, these traditional views were impeding the progress of naturalists more interested in fact than upholding a misguided and limiting world view.

In the early part of the 19^th century, scientific consensus surrounding geology began to shift, and Darwin’s contemporaries began to move away from a literal interpretation of the Bible that pegged the age of the Earth to only thousands of years. Scientists, including James Hutton, began to look for natural explanations for geological formations, which included consideration for geologic forces coming from within the Earth due to a molten core, and the pushing-up of granite from below the Earth’s surface.² Hutton’s theory essentially framed a fundamental law of geology about how gradual forces have been changing the surface of the Earth throughout history. This stood in opposition to the traditional notion of special creation, namely that the Earth was created by God and was immutable.

Evidence for these gradual geologic changes are actually easier to find in fossils than just in rock or other geological formations. The discovery of fossilized elephants in North America, Europe, and Siberia by paleontologist Georges Cuvier demonstrated that environments must have changed over time, supporting different living things at different times.³ Cuvier also discovered that fossils allowed geologists to peg the relative age of layers of rock from one location to another even in areas where those fossils might not have been found but were still clearly part of the same strata.⁴ In fact, most layers of rock strata were discovered and labeled before Darwin’s breakthrough in evolution and were labeled based on the fossil discoveries contained within them – with drastic changes in the fossil record demarcated by mass extinctions as well as the appearance of new species in the fossil record.⁵ The geological events and periods of Earth’s history are organized into the geological time scale, and include eras, periods, and epochs.

With the discovery of fossils and a resulting understanding of basic geological principles, scientists were able to organize the fossil record. Branches of life, speciation, and evolution can be seen through the fossil record. The fossil record does a good job of showing that more complex organisms arose from earlier ones. However, the fossil record is far from complete. Some organisms do not fossilize well because they decayed too rapidly for fossilization to occur, or were too soft to make an imprint in rock strata that formed later as sediment accumulated in the area.⁶ Another issue is that only a small fraction of life forms ever become fossils due to localized conditions, but also because fossils have to survive within rocks that are undisturbed for millions of years of geologic changes.⁷ With all of this in mind, Darwin was armed with the fundamental knowledge that vast amounts of time had passed during which the Earth underwent a myriad of geologic changes as well as drastic changes in the animal life that had populated the planet.

Biology

In not too dissimilar fashion from the breakthroughs in geology, the19th century saw changes in how key topics in biology were perceived well before Darwin published his groundbreaking findings in The Origin of Species by Means of Natural Selection in 1859. Just as theological ideas about the age of the Earth had clashed with and perhaps even limited breakthroughs in geology, the same was true for the biological sciences leading up to the 19^th century. Despite fear of reprisal, some scientists and naturalists did consider that life could change through time to become better adapted to survive.⁸ Darwin’s own grandfather, Erasmus, was one such free thinker. Erasmus Darwin died six years before the birth of his grandson. His thinking did left an impression in the scientific community. Likewise, John Baptiste Lamarck was another early proponent of evolution; though he was incorrect about the mechanisms for evolution. Lamarck incorrectly believed that organisms could adapt during their lifetime and then pass along any adaptations directly to their offspring; Darwin would later demonstrate that adaptive variants in a population are ‘born lucky’ and this allows them to leave more offspring because they happen to better match the environmental challenges. Thus, adaptations take place in populations across many generations and natural selection is the guiding force behind incremental changes over time. Despite Lamarck’s confusion regarding the specific mechanism for evolution, he was spot on when he found that: “the fixity of species was an illusion.”⁹

Simultaneously, comparative anatomy was beginning to take shape during the 19^th century. Comparative anatomy simply means taking structures found in different species and comparing them. Sometimes there are striking similarities. Homologous structures provide compelling evidence for evolution because the striking similarities between species demonstrates relatedness due to common ancestry. But the contemporaries of Darwin weren’t just looking at homologous structures between adult specimens of various species, they were also looking at embryos that could be observed at various stages of development. Examples of homologous structures are rife in nature, and include the forelimbs of vertebrates ranging from the human arm, a bird’s wing, seals, horses, and so on – in each you will find a humerus, radius, ulna, carpals, metacarpals and phalanges. This underlying similarity in anatomy is explained by common ancestry. With the study of development through embryos, scientists in Darwin’s day suspected these similar structures arose due to similar developmental pathways.¹⁰ Darwin was the first to proffer that these shared developmental pathways were caused by common ancestry, where vertebrates would have shared a common ancestor that passed down these traits, which became adapted over time for use by different but related species. This conceptual breakthrough was in direct contrast to the once dominant theory of special creation, which held that every organism was created separately, and logically should show no common ancestry.¹¹

Fig. 1: Homologous Structures. Image Credit: Волков Владислав Петрович. CC 4.0

Darwin

Charles Darwin’s fateful journey toward discovering the mechanism for evolution was undertaken in 1831 aboard the HMS Beagle and lasted for five years. His mission was simply to serve as the ship’s naturalist, and he did not set out to restructure the science of biology by writing about natural selection. On this journey Darwin would spend time ashore, record his observations and collect samples.¹² It wouldn’t be until his return to England that he would begin to put the evidence together for natural selection acting on living things and driving the process of evolution. Following his return, Darwin distributed various fossil samples and carcass specimens to well respected experts, but was surprised at their findings.¹³ For example, James Gould informed Darwin that birds Darwin had classified, as entirely different species were in fact closely related species of finches, differing primarily in their beak structure. As the investigations continued, Darwin found that each island had specifically adapted but related species inhabiting them – leading to the conclusion that the animals had adapted to their environments over time, rather than having been specially created to inhabit their environments. Darwin realized that at some point a single species of finch could have flown there from the mainland. As finches arrived on different islands, they became specially adapted to the unique environment of each island thus maximizing their chances of survival.¹⁴

The massive weight of these findings were not lost on Darwin, who began to realize the vast implications of his evidence. If all living things evolved from a common ancestor, then the natural laws that applied to the animal kingdom also applied to human beings, and he realized that monkeys and apes were probably our closest evolutionary relatives.¹⁵ These lines of thinking ultimately led Darwin to refine his thinking and publish The Origin of Species by Means of Natural Selection in 1859.