History of the Theory of Evolution

By Eilene Lyon

This is the follow-up article to Science and Belief.

Darwin’s Observations on the Natural World

Charles Darwin formulated his hypothesis of evolution by natural selection based on observations he made during his trip around the world on the Beagle in the 1830s. Some of those observations:

• Island groups had wildlife differing from mainland populations
• The wildlife in the Cape de Verd island group was different than that found in the Galapagos. That in Cape de Verd was similar to the African continent. That in Galapagos was similar to the American continent.
• Within an island group, animals were similar, but distinguishable from island to island. The Vice-Governor of the Galapagos remarked to Darwin that if someone brought him a giant tortoise, he could tell exactly which island it had come from.
• Fossil remains in South America bore resemblances to living animals. The Toxodon fossil, in particular, had features that seemed to combine those of many different extant species, possibly being a distant ancestor to them all.

Darwin did not coin the term “evolution” or even use it in his book The Origin of Species. An earlier evolutionary theory proposed by the French naturalist, Jean-Baptist Lamarck, was essentially debunked by Darwin’s work.

Because of its potentially explosive nature in a religious, Victorian-era society, Darwin did not immediately publish his ideas. In fact, he sat on them for 20 years. It was due to the development of a nearly identical hypothesis by the younger Alfred Russel Wallace that Darwin was essentially forced to publish or be scooped.

What is the Theory of Evolution?

The essential elements in evolutionary theory are:

• Organisms are capable of producing many more offspring than will survive
• Though the offspring inherit traits from the parents, they are all slightly different
• Natural selection leads to survivors who will pass on their traits
• Reproductive isolation eventually leads to greater differentiation

I’d like to point out a couple caveats. “Species” is an artificial concept that will likely never be clearly defined, because evolution is an ongoing process. We know a duck will not be able to produce offspring with a rabbit, but a wolf can certainly reproduce with a domestic dog. Perhaps in the distant future, dogs and wolves will not be able to interbreed, due to reproductive isolation.

Another point is that heritable traits are not “good” or “bad” and do not necessarily lead to more complexity. Traits are passed on from organisms that are well-adapted to their environment, and thus more likely to survive.

I’m reminded of the joke about two guys encountering a grizzly bear. One stops to tighten the laces on his boots and his friend says, “What are you doing? You can’t outrun a grizzly!” The one tying his laces replies, “I don’t need to outrun the grizzly. I just need to outrun you!”

The Role of Genetics

Since Darwin and Wallace presented their hypotheses in 1859, advances in science have never disproved them. That is why we now have a Theory, some even call it a Law, of Evolution. Our understanding of DNA helps explain the mechanism of heritable traits.

Offspring receive 50% of their DNA from each parent (except in the cases of clones – we’re talking sexual reproduction here). But because each offspring receives a different 50% from each parent, the siblings exhibit variation.

Also, mutations are regularly occurring in DNA from generation to generation. As a population becomes reproductively isolated (for many possible reasons), the exchange of DNA with other populations ends and each population produces its own mutations which are passed down, separately.

Humans, Apes, and Monkeys

So, now to answer the question, “If humans descended from monkeys, why are there still monkeys?”

The short answer is, of course, that humans did not descend from monkeys. Humans are most closely related to apes, in particular bonobos and chimpanzees. But we didn’t descend from them, either. All modern primates descended from an ancient common ancestor, much like the descendants of Darwin’s “Toxodon.”

Why don’t we know exactly what that ancestor was? As Bill Bryson put it, “It isn’t easy to become a fossil.” We have found many hominid ancestors in the fossil record and eventually one may be found that we could call “the missing link,” though that is a misnomer. But most species simply never made it into the fossil record.

One thing that science has been able to do is come up with estimates as to how long ago that common ancestor existed. This is because the rate of mutations in DNA can be calculated, and that rate can be used to devise a timeline.

Chimps and humans share 96% of their DNA. This is one-tenth of the difference between the DNA of mice and rats. That’s pretty darn similar.

The timeline for when various primate lines became reproductively isolated, using something called cladistics, is shown in this graphic:

I hope I have been able to make this explanation easy to follow, even for non-scientists. I appreciate any feedback to make this article better.

Feature image: Photo by Luca Ambrosi on Unsplash

Sources:

Darwin, Charles. 2000. The Voyage of the Beagle with an Introduction by H. James Birx. Great Minds Series, Prometheus Books, Amherst, New York.

Ricklefs and Miller. Ecology 4^th Ed. 2000. W. H. Freeman and Company, New York. pp. 10 – 13.

https://en.wikipedia.org/wiki/Darwinism

https://news.nationalgeographic.com/news/2005/08/0831_050831_chimp_genes.html