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Evolutionary Musings

“My ague has left me in such a state of torpidity that I wish I had gone thro’ the process of natural selection.” wrote Erasmus Darwin in his letter to his grandson, Charles Darwin.1 In the same letter, he stated that On the Origin of the Species was “the most interesting book” he had ever read. The book, however, also received harsh criticisms, and that bipolar opinion on the book that continues to this day. Indeed, I don’t think there has been any book in human history that generated such polarized opinions. I suspect the reason for this polarization stems from the word evolution, a word, ironically, Darwin didn’t use in the first edition of On the Origin of the Species. In this essay, I want to go over some arguments for and against evolution.2 As to why I am writing about evolution, I will come back to it at the end of this essay.

Fossils of Crinoid columnals found in Utah (courtesy: Wikipedia)
The Fossil Record: If evolution occurred, then one would expect to see intermediate forms among different groups of organisms, a piece of puzzle Darwin could not provide. However, the field of paleontology—a branch of science that studies fossil animals and plants—was rudimentary in his time, and researchers since then discovered many intermediate forms between different groups of animals. For example, intermediate forms between amphibians and reptiles, and between reptiles and mammals have been found.  

Like photos of our lives, these persevered remains of organisms give up a snapshot of evolution. If species B evolved from species A, then we will find fossils of species A in older rocks and species B in younger rocks. For example, the fossil record shows that simple microbial life forms date back to 3.5 billion years while organisms more complex than bacteria date back to 2 billion years. Multicellular organisms, on the other hand, can be only found in younger rocks. In other words, the fossil record shows us that the organisms have evolved through several billion years from simpler life forms.   

Comparison of bat and mouse forelimb (Courtesy: Wikipedia)

Common Structures: Common ancestry can explain why the limbs of humans, mice, and bats are so strikingly similar.  While a wing of a bat, a forelimb of a mouse, and an arm of a human carry out very different tasks, they all have the same basic components that came from a common four-limbed vertebrate ancestor. 

Interestingly, structures without any apparent function have also been detected in animals. These unused structures, called vestigial structures, are residual parts from a common structure. Hind leg bones in whales and wings on flightless birds are examples of vestigial structures.

Distribution of Species: When Darwin visited the Galapagos, he was fascinated by the remarkable variation finches inhabiting those islands showed. Ultimately, that led him to conceive the theory of natural selection that explained how the diversity in finches could have arisen through environmental adaptation and geographical separation. Indeed, the geographic distribution of organisms can be best explained by the combination of evolution and the movement of tectonic plates over geological time. For example, the mammalian populations of North and South America evolved in isolation until a link (the isthmus of Panama) connected those two continents approximately 3 million years ago. Some mammals of South American origin like the armadillo, porcupine, and opossum migrated to the south, while the mountain lion and other North American species migrated to the south through that connection.

Common traits in embryos: All vertebrate embryos, including humans, exhibit gill slits and tails at some point in embryonic development., and this can be best explained by common ancestry. These structures disappear in adult terrestrial animals, but remain functions in aquatic animals like fishes and some amphibians. 

Similarly, similar genes are active in a wide variety of organisms—from fruit flies to worms to mice to humans—early in development. The genes, derived from a common ancestor, influence body segmentation or orientation in these diverse groups of animals. 

Evidence from Molecular Biology: The near universality of the genetic code (read this for more information), which is used to translate nucleotide sequences into amino acid sequences, suggests that all organisms on earth had a common ancestor. Moreover, proteins in all organisms are composed of the same set of 20 amino acids. 

The genetic similarities between organisms also suggest common ancestry. Furthermore, the degree of differences in genomes can indicate the divergence of the genetic lines. For example, we share roughly 96% of our DNA with the chimpanzees. Moreover, that genetic difference is approximately 60 times less than the difference between us and mice. So, compared to mice, we and the chimpanzees had a more recent common ancestor. 

Let’s now get into some arguments against evolution, and I am going to stick to arguments that are connected with vision.

No one has seen evolution occur: We have. Antibiotic-resistant bacteria, for example, are products of evolutionary forces. Due to genetic variations, some bacterial cells would be resistant to antibiotic treatment and will survive. These cells would then produce thousands of daughter cells that would be mostly antibiotic-resistant as well. Since bacteria have a short generation time, within a short period, that entire colony of bacteria would become antibiotic-resistant. Here, the antibiotic is selecting which bacterial cells will survive to drive evolution. The emergence of insecticide-resistant mosquitoes is another example of organisms evolving to adapt to environmental challenges. 

Evolution can’t explain eyes: This argument assumes that complex organs or biological processes can only function if all components are present and functioning today. That, however, is not true as complex biochemical systems/structures can be created from simple systems/structures through natural selection. Jawed fishes, for example, have more complex hemoglobin than jawless fishes, but have simpler hemoglobin compared to that in mammals. 

The development of eyes, similarly, can be explained by evolutionary forces. Specifically, eyes may have evolved independently through the history of life. The steps most likely started with simple light-sensitive retinula cells found on eye stop (in flatworms, for example). Individual photosensitive units and light focusing lenses then appeared in insects, which eventually led to the formation of an eye with a single lens that focused images onto a retina. Thus, through gradual steps, different types of eyes have evolved.

Now that I have gone over these arguments, let me now give one reason why evolutionary biologist Theodosius Dobzhansky wrote an essay titled Nothing in Biology Makes Sense Except in the Light of Evolution.3 During drug development, a potential drug is usually first tested on mice to determine its efficacy and safety. If we didn’t think we and mice are connected by evolution, would there be any point in using mice to test drugs or in biology experiments? 

1. https://charles-darwin.classic-literature.co.uk/the-life-and-letters-of-charles-darwin-volume-ii/ebook-page-14.asp

2.This essay was mainly based on the following article: https://www.ncbi.nlm.nih.gov/books/NBK230201/

3. https://en.wikipedia.org/wiki/Nothing_in_Biology_Makes_Sense_Except_in_the_Light_of_Evolution


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