How is it possible that something as complex and wondrous as a human being could develop from the primordial slime that existed billions of years ago? We are so different from bacteria and viruses that evolution seems impossible. Actually, when we examine the chemistry and structure of human cells, we see that we aren’t that different from bacteria and viruses. There are a great many clues to our bacterial origins; for example, the basic homology or structure of our DNA is the same as that of bacteria, and, in fact, this is what makes it possible to identify criminals through DNA testing. We use bacteria to multiply tiny quantities of human DNA to make the characteristic patterns of elongated black dots on paper that help investigators solve crimes. The DNA of humans is folded and wrapped around itself in a pattern that resembles the pattern found in bacterial DNA, and there are large sections of base pairs in our DNA that are the same as the patterns found in the DNA of bacteria and other more complex organisms.
One may think that the basic structure of our bodies, the layout of our arms and legs, and the organization of our organs are different from, say, a worm’s, but when you compare the spinal column of humans and the segmented body parts of worms, here, too, you will see many similarities. It is apparent that nature likes to reuse successful inventions over and over again.
Ontogeny recapitulates phylogeny
As a human embryo develops in the womb, it goes through stages that reveal the debt we owe to earlier life forms for the evolution of certain bodily structures. (Scientists call this process, “Ontogeny recapitulating phylogeny.”) Early in the development of the human fetus, it passes through a stage where it develops what scientists call a notochord, or a piece of cartilage that simple marine animals have in place of a regular backbone. As the human fetus develops further, this notochord becomes a fully-formed backbone. The fetus also briefly exhibits gill plates at one stage in its development; these gill plates eventually become our chin and jaw bones.
The organization of similar groups of bodily structures is controlled by supergenes,* which are an important group of genes that are responsible for the development of the main subsections of the bodies of organisms. Small changes in these supergenes can cause very large changes in the physical structure of an organism, and this is how whole new species can develop rather quickly on an evolutionary time scale.
All of these genetic changes are overseen by a process called natural selection, which can best be described as anything that improves the survival advantages of an individual organism or of an entire species. If a change in the internal or external structure or in the behavior of an organism makes it better able to evade predators, find food more efficiently, or find a mate more readily, then this new feature will lead to more offspring with the same trait being produced by that individual. Some of these mutations have the opposite effect and make the organism less likely to survive. These changes are gradually weeded out, and the species as a whole benefits. As the environment changes, those individuals that are most flexible and adaptable to the changes survive and prosper.
There can sometimes be an explosion of new species when there is a dramatic change in the environment. This is exactly what happened about 600 million years ago during the Cambrian Era on Earth when the atmosphere began to contain much more oxygen with the rise of photosynthetic organisms. Nature invented aerobic metabolism (metabolism using oxygen) to take advantage of this new development; aerobic metabolism produces much more energy per molecule of food than the older anaerobic method of metabolism (i.e., fermentation). Also, around this time, nature invented shells and skeletons made from calcium carbonate. These two new developments sparked a huge increase in the number of new species in what scientists call the “Cambrian Explosion.”
Sceptics who feel that evolution could not possibly be responsible for the incredible diversity of life forms on this planet should consider how there are now 400 or so breeds of dogs. All of these breeds have been created through selective breeding by humans from a single wolf-like ancestor in only a few tens of thousands of years. Imagine then if you had billions of years to develop new species, which is exactly what nature had.
More evidence supporting evolution is found in the fossil record. For example, as you travel down from the rim of the Grand Canyon to the Colorado River a mile below, you pass through millions of years of sedimentation until you reach rock layers that are more than two billion years old. The fossils in the surrounding sedimentary layers gradually become simpler and simpler until the soil around you at the bottom of the Canyon contains only the fossilized remains of single-celled organisms.
Perhaps the most astounding fact about evolution is that there are now an estimated ten million to one hundred million species of organisms on the Earth. There may be as many as a million different species of bacteria alone. The book, Bergey’s Manual of Systematic Bacteriology, lists 4,000 species of bacteria. In a pinch of soil, there are probably about a billion bacterial cells and probably over 10,000 colonies or 8,000 different types of bacteria. In the mouths of human beings, there are about 500 different species of bacteria.
A truly remarkable fact about life on the Earth is this: the biochemistry involved in the life functions of bacteria and those of all other organisms is basically the same; we humans and all life forms on this planet are truly one.
HOW DOES EVOLUTION WORK?
To illustrate how evolution works with one small example, take the case of the Golden Trout, the state fish of California. These beautiful fish are found high in the Sierra Nevada mountains, west of the tiny town of Lone Pine, California. The Golden Trout are descended from the native Rainbow Trout, one of the most popular game fishes in the world.
During a previous Ice Age, the Sierras were covered by large glaciers. As the climate warmed, these glaciers melted and flooded California’s Central Valley where the present-day towns of Fresno and Bakersfield are. Ancestral Rainbows, closely related to the Redband Trout, which still exist today in small pockets around the West, came up from the Valley and eventually colonized the streams high in the Sierras via the Kern River. As the climate continued to warm, the lower Kern River dried up and isolated these Rainbows from other Rainbows farther downstream. Eventually, because of different environmental conditions and through inbreeding, these fish took on the spectacular coloring that they now possess.†
The Golden Trout has existed as a separate species for only a few tens of thousands of years and can still interbreed with Rainbow Trout. This is an indication that the Golden Trout and the Rainbow Trout share a common ancestor and that they are genetically similar. Given more time, closely-related species like this become more genetically different and soon become unable to interbreed, or if they can interbreed, their offspring are sterile (just like donkeys and horses are bred together to produce mules, which are sterile).
When the same process that created the Golden Trout occurs on a
continental scale, we get a whole country full of remarkable animals, such as those found in Australia. The continent
of Australia was at one time joined to other continents, but over millions of years, gradually drifted away from
them and became isolated. The plants and animals on the Australian continent were able to evolve in "splendid isolation"
(as paleontologist George Gaylord Simpson said) from other life forms around the world, which is how such unusual animals
as the duck-billed platypus, the kangaroo, and the koala bear developed.
* Supergenes = a set of closely linked genes that, because they are near each other on the chromosome, are rarely broken up by recombination and are usually passed on to the next generation as a unit (from The Wisdom of the Genes by Christopher Wills, 1989, Basic Books, a division of Harper Collins Publishers).
† Scientists think that the Golden Trout's spectacular coloring provides better UV protection at the higher elevations where they live.
1) The basic shape and chemistry of bacterial DNA and our DNA is the same. There is a unity to all life on this planet.
2) Nature doesn’t reinvent the wheel. It uses successful inventions over and over again.
3) Individual members of species are all slightly different. Mutations in DNA create changes, some of which are good, some are neutral, and others are bad. Mutations that help the animal survive better result in more offspring with that characteristic being produced.
4) A dramatic change in the environment or a relocation of a species can cause a rapid increase in new species, especially in small, isolated populations. A good example of this on a small scale is the Golden Trout of California, which is derived from the Rainbow Trout. A much larger example is the unusual flora and fauna of Australia.
© Copyright 2019. Phil Matsumoto. All rights reserved.
Last updated on 10/29/2019.