2.3The Origins and Diversity of Life

In addition to taking the first steps toward the scientific method and hypothesizing about events from the past, the Greek philosophers also developed a keen appreciation for the study of natural history. Aristotle, for example, distinguished among 500 species of birds, mammals, and fish, and he wrote extensively on the anatomy and movement of animals. He also proposed a taxonomy of nature—a classification system of life—that led from polyps at the lowest level to humans at the pinnacle. This would later be called “the great chain of being,” or scala naturae. According to this linear classification system, each species occupied a link in a chain of ever-increasing complexity. This concept influenced Western thinkers for more than 2,000 years and Islamic scholars, like Ibn Khaldun, wrote about it 800 years ago (Malik 2019). On the scala naturae, every organism represented a unique link in the chain, and each link represented a different level of complexity. In this framework, different organisms could not share comparable degrees of complexity. Likewise, in this view, each specific link on the chain of being would remain forever fixed—precluding the possibility that organisms might change. Both of these misconceptions would have to be overcome before evolutionary biology could emerge as a science.

The late eighteenth and early nineteenth centuries brought new theories to explain the origins of life and the diversity of species. Erasmus Darwin (1731–1802), an English physician, philosopher, and the grandfather of Charles Darwin, was one of the first to propose the idea of evolutionary change in his book Zoonomia (Darwin 1796; King-Hele 1998).

Erasmus Darwin argued that all life evolved—although he did not use that word—from what he called a “single living filament” (Darwin 1796). For Erasmus Darwin, this living filament had been modified in endless ways, over millions of years, to produce the life that he saw around him. He also hypothesized that humans had initially walked on four limbs and, even more remarkably, that we had descended from another primate species. This was a radical idea at the time.

After Erasmus Darwin, Robert Chambers (1802–1871), a Scottish geologist, writer, and publisher, presented a more formally developed set of ideas on how new species originate in his 1845 book, Vestiges of the Natural History of Creation (Chambers 1845). In the section of his book on what today we would call evolution, Chambers highlighted two critical points: (1) the composition of species has changed over time, and (2) this change was slow, gradual, and unlinked to catastrophes (Mayr 1982). From these ideas, Chambers outlined his principle of progressive development, in which he hypothesized that new species arise from old species. One aspect of Vestiges that often goes unnoticed is that Chambers thought not in terms of individuals so much as populations—groups of individuals of the same species that are found within a defined area and, if they are a sexual species, interbreed with one another. Chambers was perhaps the first to recognize that, in the parlance of modern evolutionary biology, populations evolve; individuals do not.

Robert Chambers and his Vestiges influenced a broad range of readers. The book was widely read by scientists and laypeople alike, including a young Abraham Lincoln (Herndon and Weik 1893). Vestiges would eventually sell an astonishing 100,000 copies (Secord 2000). For all its success, the greatest deficit in Chambers’ book was that it didn’t explain why new species come into being.