Millennia ago, humans realized that greatly different animals (deer, birds and fish, for example) possess body organs and systems very similar to their own, but could not explain why. Over the centuries various explanations were proposed, some theological, some scientific; two centuries ago most people accepted the theological interpretation—that life in its different forms was Created. Papers read to the Linnean Society in 1858 written by Charles Darwin and Alfred Russel Wallace (who had independently reached similar conclusions) did little to change this situation—attendees simply did not understand the importance of what they were hearing. However, when Darwin’s book On the Origin of Species by Means of Natural Selection, was published a year later, evolution became a topic of discussion for every learned person and things changed forever.
It is easy to understand what immediately happened. Darwin and his ideas were ridiculed by almost everyone; scientists said he could not prove what he was saying, and the religious said that mankind was created—as proven by texts in the Bible. Humans simply could not be “descended from an ape.”
Darwin’s work, and its attendant publicity, resulted in widespread use of the words “evolution” and “natural selection.” These terms are sometimes treated as though they hold the same significance—they do not. One is a fact, the other is a theory, and we should take a moment to discuss the differences between the two.
There is plenty of evidence to show that evolution occurred—is still occurring—and that all life on this planet is interrelated, with a common ancestry. Palaeontologists study fossils of once-living organisms, and their work demonstrates that the bones and structures of ancient life forms gradually changed over time. Comparative studies of the physical and systemic structures of living plants and animals uncover the same kind of gradual change. Genetic mapping adds to the information obtained, and shows beyond any possible doubt that links between living species, and between living and extinct species, exist. Weiner, after discussing work done by Seymour Benzer and his wife in the 1980s, noted that flies, worms, seeds, yeasts and bacteria possess thousands of very similar genes or gene sequences. This could only have occurred if they all had a common ancestor. (In fact, a pre-Cambrian common ancestor must have existed, well over 540 m.y.a., for such widely separated species to possess so many similar genes.) Moreover, as Weiner pointed out, the genomes of mice and men (and women, if it needs to be stated) are about the same size and contain corresponding genes. (de Duve actually states that the evidence showing all Earthly organisms to be descendants of one common ancestor is “overwhelming.”)
That evolution occurs is something animal and plant breeders have known and profited from for centuries, and it is a fact that few educated people today dispute.
However, we can never be certain that we know all of the factors that cause evolution to occur, so any explanation of why evolution occurs may someday be modified or even overturned. While we have what we think is a very good idea, there could be additional or different reasons why evolution happens, so scientists continue to call this very good idea a theory. Natural selection is Darwin’s very good idea, and it has withstood all manner of challenges to its ability to explain and to predict. But it can be thrown into doubt, and even discredited, any time a fact of evolution is found that it cannot explain. (All scientific explanations are like this; any or all of them may one day be shown to be inadequate or inaccurate, and we remind ourselves of this limitation by calling many of them theories. All will remain theories forever.We continue to call Einstein’s masterpiece “The General Theory of Relativity,” but few state that it is just a “theory,” or that atomic bombs cannot exist.
“Laws,” too, including the Conservation Laws of physics, can be overthrown if negating proof is discovered.
(Never knowing if any “fact” or theory is entirely correct is a consequence of living within a [presumably] closed system. See the Developing A Universal Religion/Gödel’s Theorem, General Systems Theory, and The Conservation Laws for elaboration.)
Thus, that evolution occurs is a fact; however, the explanation why evolution occurs will always be called a theory. This, presumably, is why controversy continues. A few people, wilfully or mistakenly, capitalize upon the word “theory” to imply that the concept is untrue, and that evolution does not occur. What they might better state is that the natural selection explanation of why evolution occurs is a theory, good only until some better explanation is found.
Returning to the theme of this section, it is estimated that some two billion species have evolved on Earth during the past six hundred million years (the period for which we have some of the best fossil records, and during which all of our land life developed). Today, about 99.9% of these are extinct.
The two million species that exist today exhibit a multiplicity of forms. Variations range from the large, most-obviously complex, multi-system animals, down to the minute, single-celled, relatively simple bacteria. As might be expected, it is the tiniest of these which demonstrate the greatest diversity and resilience. The habitats of bacteria range from the plus 91°C boiling hot springs of Yellowstone Park, to the minus 50°C super-cooled brines found in the Antarctic. Bacteria also flourish under tremendous pressures on the ocean floor, spread prolifically throughout the soil we walk upon, waft through the air we breathe, and luxuriate in every kitchen.
When the papers written by Darwin and Wallace were first read, no one had seen evolution occurring. Today, experiments demonstrating its thesis can be conducted using fruit flies in high school science laboratories. Evolution was thought to be too slow to be witnessed in nature, but the real challenge to demonstrating its ubiquitous occurrence comes from the need to detect and measure small changes over a number of generations while also recording every possible factor that might relate to (or be causing) such changes.
Some of the earliest decisive documentation of evolution occurring in the wild was obtained by Peter and Rosemary Grant through studies of “Darwin’s” finches on the relatively isolated Galápagos island, Daphne Major. For more than two decades, the Grants, with the help of many colleagues, captured, numbered, precisely measured, banded, catalogued and released, almost every finch that lived on the island (sometimes only a few hundred, sometimes several thousand in a year). This period included years of drought, as well as wet and more normal years. In this manner, they recorded the features of close to 100,000 finches, together with many details about their varying habitats.
These records were run through computer programs that sought correlations between the number and variety of finch, and changes in their environment such as rainfall, seed plant variety and abundance, and so on. Drought years drastically reduced the number of softer seeds, leaving the number of hard-shelled cactus seeds about the same. Finch species with large, strong beaks that were able to crack hard-shelled seeds, survived in stable numbers during those years, as might be expected. However, measurements of surviving members of the other finch species showed that only those whose beak was larger than average for their species were surviving and reproducing. The net result was that the beak size of each finch species drifted toward a larger and stronger shape during drought years. This drift continued in successive generations for as long as the drought continued. Wet years produced the opposite effect, and resulted in a drift within each species toward a finer beak structure (because the cactus plants began to die, and thinner beaks could better retrieve the smaller seeds of other plants that fell into the many tiny cracks in the island’s volcanic rocks).
Others have conducted parallel work. John Endler, for instance, working with guppies in various South American countries, observed natural generational variations in colour which were brought about by changes in the environment. Dark water favoured brilliance (better to attract females); light water favoured camouflage (better for hiding from predators).
Any change in an environment may affect species living within that environment. An accumulation of adaptations within one species eventually produces what becomes described as a new species. Collections of plant, animal and insect fossils in museums and universities around the world show that time, environmental change, and geographical separation are all that are needed for species to evolve from old into new.
Changes over time cause descendants to either diverge (i.e. increase in difference, one from another), or converge (i.e., increase in similarity). The factors that promote species divergence are predominately food (which favours the development of tools—for instance, the beak—that better exploit the particular kind of environment which supplies that food) and sex (which favours the development of partner-attracting displays or like-attracting-like matings). The forces that promote species convergence include the presence of enemies (which favours the development of camouflage and herd behaviours, the latter because there is safety in numbers) and the physical features of the environment being exploited. Hybridization, whereby closely related species merge genes, can produce fairly rapid convergence or divergence. The prevailing environment determines which outcome predominantly survives.
Evolution, we now realize, is often not a slow and gradual process. It can occur in small or large bursts, and these may be followed by long ages of slow consolidation. A common sequence is as follows: an environmental calamity occurs, followed by a rapid collapse in food supplies. The calamity can be localized and relatively insignificant (a fallen rock, for instance) or something very pervasive (the eruption of an immense volcano, the impact of a large comet, or the rapid development of an ice age, for example). Each environmental change causes the death of some or perhaps almost all of the existing, previously well-adapted species. The decline in numbers of some species (or the environmental change itself) opens niches that were previously occupied, blocked, or non-existent, and this provides opportunities for suitably different members of surviving species to thrive.
The mutations that make life’s evolution and continuance possible need not be large, as the work with Darwin’s finches demonstrates. However, even drastically mutated offspring may survive and flourish under some kinds of environmental change. Evolutionary change following extinctions is rapid, because many of the previously well-adapted (and presumably competing species) completely die out. This may open niches accommodating to some of the more extreme variants (that might not otherwise have survived); without competition, they may now proliferate. Evolutionary change slows down again just as soon as successfully adapted species fill all available energy niches.
Massive extinctions have not been uncommon in our planet’s history. Two of the more infamous were probably caused by asteroids or comets hitting the Earth. The first of these occurred around 208 m.y.a., creating the environment that early dinosaurs exploited to become the Earth’s dominant animals (this impact produced the changes that mark the junction between the Triassic and Jurassic periods). The second collision happened around 65 m.y.a., and ended the dinosaurs’ supremacy. Other extinctions occurred around 438 m.y.a., 367 m.y.a., and 245 m.y.a. Each of these cataclysms resulted in the demise of more than fifty percent of the prevailing marine species, and an equal or greater percentage of the existing land species.
Records of growth from tree rings as well as ice core samples show that large calamities, due to one cause or another, have also occurred relatively recently. The most prominent events happened around 3200, 2300, 1628, and 1159 BCE; the most recent took place in 535 CE.
Catastrophes rapidly and radically transform the planet’s various environments, and life forms that do not adapt (i.e., evolve) do not survive. (And, as a corollary, it must be emphasized that if life does not continue to evolve it will not continue to survive, for it is certain that changes will occur to life’s environment in the future just as often as they have in the past. Moreover, it is worth noting that the environment currently changing the most rapidly is not the Earth’s biosphere, it is our human mental environment—a change brought about by the mix of facts, ideas, opinions, fantasies, beliefs, etc., that worms its way into our thoughts every day. To survive, we must discover how to adapt to the changes occurring there.)
Fossil records show gross evolutionary changes in Homo’s body structures that took thousands of years to develop. Detecting subtle changes necessitates making fine measurements, and we currently lack the detailed records covering several generations that would unequivocally demonstrate human evolution in action. Doubtless, as computer record-keeping increases in scope and depth (particularly if DNA profiles are to be stored), we will soon have plenty of evidence to show that, like all else that lives, humans evolve, and evolve continuously.
Humans have always acted to minimize the effects of events that may influence their evolution. We store food and survive most food shortages; if we did not do this, the average body mass and size of H. sapiens would drift downwards. We capitalize on niches and specialize in occupations; if we did not do this, our numbers would decrease because there would be too many competing within each energy niche (read money, thus food, for energy) and fewer would survive. We stress universal literacy and education; if we did not do this, the total number of energy niches would decline over time. Together, forces such as these select for particular skills: musical, mathematical, artistic, and so on. In effect, ever since we have had the ability, we have acted in a manner that influences the way we evolve. Genetic engineering is about to vastly extend this ability.
- Every copy of Darwin’s book was sold the first day it came out. It has been called “the book that shook the world.”
For a readily accessible series of more recent discussions about Darwin and life’s evolution, visit www.pbs.org and link to “evolution” at www.pbs.org/wgbh/evolution.
- Eukaryotic organisms (i.e., plants and animals) possess intracellular structures called mitochondria which process chemical molecules obtained from food to release their energy. Mitochondria possess their own DNA (called mitochondrial DNA, or mDNA) which is passed directly from mother to child and does not vary between generations unless some random mutation occurs. The mutations that do occur can be used to trace a species’ history, as well as relationships between different species. By this means, the progression from one original organism to subsequent divergent organisms can be uncovered.
- Scientists, for instance, have mapped the entire genome of the Archaean microbe known as Methanococcus jannaschii. This information, together with the genomes of representatives of the Prokarya and Eukarya kingdoms, may eventually allow us to find the genes common to all living things—that is, some of the genes possessed by the universal ancestor of life on this planet. It may only be a matter of time before a map can be drawn that will show definitive interconnections between all Earthly life forms. This will concurrently trace the major features of the full evolutionary route to Homo sapiens. (Genetic tracing becomes difficult in bacteria, however, because they are able to transfer genes laterally, i.e., directly from one to another. This suggests that we may have to be content with tracing life’s ancestry back no further than bacteria.)
Scientists have already used mitochondrial DNA taken from five major ethnic groups which make up the current global human population to trace the ancestry of Homo sapiens back to a time between 140,000 and 290,000 years ago. We have all evolved from one or another of about one hundred or so woman, the “original Eves,” who lived in Africa. (And we should really repaint any of our pictures involving Eve that do not show her having very dark skin.)
- Weiner, Time, Love, Memory, 184. Surprisingly, the human genome contains around 30,000 genes, only about twice the number of genes possessed by a worm or a fruit fly. Several hundred of our genes turn out to be identical to those found in simple bacteria.
- Weiner, ibid, 206.
- de Duve, Vital Dust, 112.
- Human change today is likely to be occurring most rapidly in the mental, rather than the physical, arena. The most “mentally alert” individuals are the most likely to provide the broadest environment for their children to experience. These children will, as a consequence, likely learn more, in depth and variety, than their peers, thus becoming potentially better equipped for success in later life. Whether or not the descendants of the “most mentally alert” will create a sub-division that eventually becomes genetically built into H. sapiens’ future will depend upon the environment—it must continue to provide a niche where this behaviour is rewarded by reproductive success. For instance, if humans eventually move out into space, it is likely to be the most mentally able that are chosen to go. If these space colonizers do succeed and multiply, then this kind of speciation may become a wide-spread, potentially dominant, reality.
(Because social programs support the survival and reproduction of all, the genes of the “most mentally alert” individuals are unlikely to dominate on this planet in the foreseeable future [because individuals possessing such genes currently tend to have fewer children than others]. The fact that rational behaviour acts to eliminate the genes that result in this behaviour suggests that there is something irrational [possibly its sustainability] about the environment our current social programs create.)
- Natural selection states, essentially, that offspring are never identical, and that those possessing advantageous variations are more likely than their less-advantaged siblings or peers to survive and procreate. These advantages are thereby passed in greater numbers to the next generation, and this causes all species to change over time. There is not much to dispute about any of these postulates.
- A certain amount of knowledge is needed to understand and appreciate what the theory of natural selection tells us about evolution. However, even those without such schooling still require some kind of explanation to account for life’s beginning and the presence of humankind. Creationism was developed to offer an explanation of sorts. It is an ancient idea that attempts to explain the unknown in a simple way. (All religions, if they are to be taken seriously, must explain how and why things are as we find them to be.) Unfortunately, Creationism ignores or attempts to refute too many evolutionary facts to be credible to anyone with an educated and impartial mind. Moreover, a belief in Creationism (like all beliefs) installs the opinion that one knows just as much as (and, often, even more than) is known by those who can call upon mountains of solid evidence that supports a different view.
That a few hold creationist views wouldn’t particularly matter, if it were not for the fact that their belief forces them to influence what is taught to children. Currently, schools in the American states of Alabama, Kansas, Nebraska, New Hampshire, New Mexico, Ohio, Tennessee, Texas, and Washington must teach that evolution is deemed to be no more significant than the belief that Creationists hold to be true (in spite of the mountains of credible evidence that support the former, and none that supports the latter). Still other schools deliberately leave evolution entirely off the curriculum to avoid controversy; they resort to teaching facts alone, and say nothing about the simplifying and edifying explanation that makes the existence of all we see in nature so logical and understandable.
In our world so dependent upon scientific knowledge, Creationism is a capricious belief to support, and it is very likely to limit the future success of its believers. This may not matter to adults, but it hampers children, who have many years to live in a techno-medical society. Of course, in the long run, the fallacy is self-correcting—after all, we live in a universe where survival of the fittest gives preference to those whose actions fit the facts. Unfortunately, as noted in footnote 7, it can only confer preference to those who act rationally when the immediately controlling environment is a rational one. This appears not to be the situation in a number of U.S. school boards.
It may be necessary to articulate that science neither opposes nor supports religion—it simply tries to uncover and understand the facts as they are found to be. To refute the millions upon millions of pieces of evidence that reveal that life evolved (and that humans are just one consequence of this evolution) is foolhardy. Rational individuals might better ask themselves which is most likely to be the truth—that which was originally written by a few wishing to promote a particular belief, or that for which evidence can be found in tangible form, everywhere, by anyone who cares to look.
Read Robert T. Pennock, Tower of Babel: The Evidence against the New Creationism (Boston: MIT Press, 1999) for a scholarly refutation of creationist ideas.
- A single DNA change in a one-celled life form will have a more profound effect, more often, than a single change in a many-celled life form. Thus, although many mutations may be inconsequential and some may be fatal, the few that are neither can result in the rapid diversification and adaptation of simple life forms. This is a common phenomenon in hospitals, where environments hostile to pathogens are routinely maintained—so this is where strains of bacteria able to resist the latest antibiotics keep cropping up.
- Darwin was ill at home and Wallace was collecting abroad at the time.
- See Jonathan Weiner, The Beak of the Finch: A Story of Evolution in Our Time (New York: Alfred A. Knopf, 1994). A lovely book for anyone to read.
- This chain of events is known as “punctuated equilibrium,” and has been popularized by Stephen Jay Gould, an influential evolutionary biologist and widely read author of many books.
- The fallen rock perhaps forces a nourishing stream to forge a different channel. The fine dust thrown into the atmosphere from a volcanic eruption or a comet’s impact might block sunlight for several years. Extensive ice sheets can prevent plant growth for centuries.
- Research suggests that biological recovery following any wide-spread ecological extinction takes an average of ten million years for complex animals, a relatively short period of time on the geological scale used to date fossils. (Recovery can be a matter of days, or even hours, for rapidly reproducing organisms such as bacteria.)
- Sediments formed around 245 m.y.a. have recently been found to hold carbon “buckyballs” that contain trapped helium and argon gases which are present in a ratio similar to that found in carbon-based meteorites. This adds support to the theory that the effects of a sizable comet or asteroid impact caused the massive extinction that wiped out over 90% of all extant species (and marks the Permian-Triassic Boundary). This extinction eliminated much competition and provided the niches that some lizards exploited during the following twenty million years as they slowly evolved into the earliest forms of dinosaurs.
Other environmental calamities may have occurred several times between 750 and 570 m.y.a. An analysis of carbon-12 to carbon-13 ratios in sedimentary layers formed in ancient oceans shows that life came to a standstill four times during that period (see the August 28, 1998 edition of Science). It is postulated that the Earth was entirely covered with ice during these times, resembling a planetary snowball. What subsequently happened may have been as follows. Life survived (as multicellular algae and seaweeds) in the small pockets that formed where volcanoes and hot springs maintained some warmth. Meanwhile, the same volcanoes continuously pumped carbon dioxide into the atmosphere, and a greenhouse environment slowly developed. After some tens of millions of years (during which time life in the warm pockets diversified as it variously adapted to each pocket’s particular environment), the greenhouse gases triggered periods of planetary warming. About 565 m.y.a. most of the ice covering the Earth melted, and the pockets opened up. The life forms released from different zones would then have been able to cross-fertilize, and in the warm, nutritionally rich environment, with minimal competition, evolution would have run rampant. This could have spawned the broad diversity of ancestral multicellular plants and animals that we find in fossil form from this period, and begun the Cambrian Age.