Caves and Evolution
|The following correction was subsequently made to this article in issue 23 (volume 8.2):|
Two errors appeared in Robert W. Loftin's article, "Caves and Evolution," in Creation/Evolution XXIII, and the author wishes to make the following corrections:
1. On page twenty-two, paragraph four, an incorrect formula was given for calcium bicarbonate. The correct formula is: Ca(HCO3) 2
2. While gypsum is a salt, it was incorrect to say that it is a salt of calcium sulfate, since gypsum is calcium sulfate. This error appeared on page twenty-three, paragraph four.
The Evidence from Geology
Caves are relatively young, as far as the geological features of Earth are concerned. There are several different types of caves, some of them obviously older than others. Ice caves are probably the youngest kind (Balch, 1970). Sea caves are formed by wave action in soft rocks along the coast; they are seldom very deep and usually do not last long (Warwick, 1976). There are caves in volcanic lava flows which are formed when hot lava cools on the outside first while continuing to flow in the middle, thus finally flowing out of the center and leaving a hollow tube. These form at the time of the volcanic activity and may be very young indeed (Wood, 1976).
There is another class of caves, however, which provides strong evidence that Earth is much older than some creationists would have us believe. Strictly speaking, the creationist view is not tied to any particular age for Earth. It is logically possible for a creationist to argue that Earth is very old, even billions of years old, and that the creation took place a very long time ago. However, such arguments are rare among creationists because, despite avowals to the contrary, creationism is linked to certain other beliefs, especially beliefs about the historicity of certain religious texts. For this reason, creationists almost always argue for a very young Earth.
On the other hand, the plausibility of the evolutionist position does depend upon the age of Earth. If Earth is very young, as most creationists insist, there has simply not been enough time for evolution to have taken place. So creationist attacks on evolution often concentrate on the questions regarding the age of Earth. If creationists can show that Earth is roughly ten thousand years old, they prove their case.
One common explanation of the existence of sedimentary rock among creationists is that these sediments were laid down at the time of Noah's Flood (Morris, 1974; Whitcomb and Morris, 1961). Just to cite one example, Henry M. Morris, one of the best qualified and articulate creationists, expresses the point this way:
The creation model, on the other hand, must interpret the [geological] column in terms of essentially continuous deposition all accomplished in a relatively short time—not instantaneously, of course, but over a period of months or years, rather than millions of years.
[Morris, 1974, pp. 111-112]
Even if we suppose, for the sake of argument, that it is true that the great sedimentary deposits in the geological column were laid down by the Flood, a position which strains credibility to the uttermost, the existence of caves in these sediments shows beyond any reasonable doubt that these formations are much older than the supposed date of the deluge. If Carlsbad Caverns, for example, is some sixty million years old, as the most competent geologists assure us that it is (Hartzog, 1987), then the strata in which the cave was formed must be older still.
Carlsbad Caverns, Mammoth Cave in Kentucky, and many other well-known caves here and abroad are limestone caverns. Limestone solution caves are the most numerous kind in the world, far outnumbering all other kinds put together. They are also the largest. The processes by which limestone caves are formed are still very much in evidence today and can be studied in considerable detail. Limestone is all but insoluble in water, so we know that limestone caverns do not simply dissolve out of the rock like sugar dissolves in water (Picknett, Bray, and Stenner, 1976, pp. 213-266). Limestone is actually less soluble in water than granite, and one never finds water-formed caves in granite (Moore and Nicholas, 1964, p. 7).
Despite the fact that limestone caves are called solution cavities, they are literally eaten or etched out of the rock by acid rather than dissolved out like sugar. And compared with dissolution, etching by weak acids is a slow process. The major acid involved is carbonic acid (H,CO3), which is produced when carbon dioxide combines with water. Almost all carbon dioxide comes from the activity of plants and animals in the soil rather than from the air (Moore and Nicholas, 1964, p. 8). Carbonic acid acts on the calcium carbonate (CaCO) in the limestone to produce calcium bicarbonate (CaCO), which is highly soluble in water. The technical name for solution caves is phreatic.
It is important to realize that most limestone caves are not formed by abrasion and erosion in the way that surface features, such as canyons and valleys, are. (Such caves, called vadose caves, do exist but are easily identified because they have a very different shape from solution caves. See Anderson, 1974.) Streams often flow through caves and contribute very slightly to the process, but this is almost always a later, secondary development. This is why the general pattern of cave passages so seldom resembles the pattern of watercourses on the surface. Most of them are nearly level, and right-angle bends are common. There are through caves in the world, in which water runs in one end and out the other, but this is certainly not the most common type (Halliday, 1974, p. 68). Most caves seem to have been formed underwater by the very slow movement of slightly acid water, rather than by erosion, which is a slow enough process in itself.
Carbonic acid is also responsible for the many stalactites, stalagmites, and other deposits which contribute so much to the beauty and interest of limestone caves (White, 1976, pp. 267-327). These features are formed when calcium carbonate in solution in the water is deposited out, but this process is not one of simple evaporation. The air in most caves, even in the most arid regions, is highly moist; therefore, when water soaking down from above reaches the air of the open cave, it does not lose water to the air and leave minerals behind. This is clearly shown by the composition of the deposits, which consists of almost pure calcium carbonate. When the slightly acid water with its dissolved minerals meets the moist air of the cave, a minute amount of the carbon dioxide leaves the water and goes into the air. This process is almost exactly the reverse of the major process of cave formation, for, when carbon dioxide goes into the air, the solution becomes supersaturated and a small amount of calcium carbonate is precipitated out (Moore and Nicholas, 1964).
The growth of cave deposits of this type is slow at best and far from uniform. It depends upon how much water percolates down through the soil, the temperature, and the season of the year. It also depends upon the amount of vegetation growing in the soil over the cave. This is because the carbon dioxide dissolved in the water comes chiefly from the metabolic action of bacteria in the soil. During the growing season, there is more metabolic activity and, thus, more acid (Moore and Nicholas, 1964).
For this reason, caves in warmer, wetter climates are usually larger and have many more cave deposits than colder, drier caves. This is one factor that indicates that the climate around Carlsbad Caverns in New Mexico was once very different from what it is today. It is difficult to see how such a large cave with massive calcite features could have formed in the arid climate with the sparse vegetation which prevails in the area today (White, 1976, p. 281).
When the attention of creationists is directed to what we know about the chemistry and hydrology of cave formation and cave deposits, they sometimes point to some very rapid processes which superficially resemble the calcium carbonate formations I have discussed (Austin, 1980). For example, on the mortared brickwork of old forts and places of that sort, formations which look to the naked eye like stalactites and stalagmites sometimes form in less than one hundred years. However, those formations are composed of gypsum, which is a salt of calcium sulphate. Unlike calcium carbonate, gypsum is moderately soluble in water, which means that transport and recrystallization can take place much more rapidly (White, 1976, p. 304). There is a whole class of cave deposits called evaporate minerals which consist of those minerals which dissolve readily in water. As might be expected, these formations are ephemeral when compared to the carbonates which form all the really large and impressive cave formations. The chemistry of all this is not particularly complex and is very well understood.
To summarize the main points of limestone solution cave formation, it seems that (1) limestone caves must be younger than the rocks they are in, (2) limestone cave formation processes are slow, since they consist primarily of etching by weak acids, and (3) carbonate cave deposits, such as stalactites and stalagmites, in limestone caves start to form after the cave dries out, since deposits result from a loss of carbon dioxide into the atmosphere inside the cave. Thus, cave formation and deposit formation are sequential. The time it takes to form the cave must be added to the time it takes to form the deposits in the cave; only then can one arrive at a rough estimate of the age of the cave.
This does not mean that cave formation cannot be proceeding in one part of the cave while deposits are forming in another part. Mammoth Cave in Kentucky, for example, has five definite levels. It is thought that these correspond to five different water levels of the nearby Green River and, thus, that the five levels formed at five different times (Tilden, 1968, p. 245). Deposits were forming in one level of the cave while other levels were still being etched out of the limestone. The best way to arrive at a rough estimate of the age of this cave would be to add the time required to form each of the five levels together, since they probably formed sequentially, then add the time for deposits to form in the lowest (youngest) level.
From this kind of calculation, we arrive at an age for Carlsbad Caverns of about sixty million years (Hartzog, 1987). The phase of cave deposits began some five million years ago. It takes a long time for a weak acid to hollow out a cave 1,013 feet below the earth's surface and with a big room large enough to contain fourteen football fields and tall enough for a twenty-two-story building. The largest cave deposit is the Giant Dome which is sixty-two feet high and sixteen feet thick. And the surrounding limestone began to be formed about 250 million years ago as a great barrier reef on the edge of an inland sea (Moore and Nicholas, 1964). The fossils of the organic life that formed the reef are still there.
The Evidence from Biology
It is worth noting that caves sometimes contain life forms which are especially adapted for living in the perpetual darkness of the underground environment (Jefferson, 1976). The most relevant types for our discussion are the troglobites, which are species which must spend the whole or a part of their lives in caves and are incapable of survival in other environments. In this, they differ from cave animals which sometimes live in caves or take shelter in caves temporarily but can survive in other habitats. Troglobite is a general term which includes such animals as blind cave salamanders, crayfish, fish, insects, isopods, and so forth (Jefferson, 1976).
Animals in caves provide very interesting subjects for the study of evolution. For one thing, evolution seems to have "gone backward" here in a sense, for highly evolved organs such as eyes are often lacking in cave animals. It has often been pointed out that most mutations are harmful, and, in some environments, poor eyesight is one of the most detrimental of all mutations. In most habitats, animals with poor eyeseight would be eliminated. But in caves where eyesight is no advantage, many animals have lost it. This shows how rapidly an important organ can atrophy and be eliminated by natural selection if it is no longer advantageous (Barr, 1968).
Blind cave animals are descended from sighted animals which formerly lived outside or in the twilight zone of caves. Many of them have vestigial eyes, often only tiny spots or, in some species, fairly complete eyes tightly covered with skin. In some of the salamanders, for which there are two distinct stages of growth, the young stages have eyes which are lost as the animal matures (Brandon, 1971). This is very difficult to explain with the creation model. If the creator created blind cave animals especially for subterranean environments, there does not seem to be any convincing reason why he would have given them eyes at all, of any kind or at any stage of their life cycle.
The geographical distribution of cave animals, particularly the larger ones such as fish, crayfish, and salamanders, also fits the evolution model very well. From the evolution model, one would expect a given cave animal to have a very limited geographical distribution, in most cases confined to a single cave system or, perhaps, a small region with many caves which are connected, perhaps in unobvious ways, or which were formerly connected. In general, troglobionts seem to fit this pattern (Jefferson, 1976).
For example, the Valdina Farms salamander, Eurycea troglodytes, is known only from a single cave in Medina County, Texas. It has eyes, but they are covered with skin. It is very closely related to the Texas salamander, Eurycea neotenes, which lives on the surface in springs, seeps, and small streams near the Balcones escarpment in central Texas. It is easy to see how, in times past, a small population of the Texas salamander became established in this one cave and became more and more unlike the parent stock, gradually losing its eyes and becoming paler in the process. The Tennessee cave salamander, Gyrinophilus pallecucus, seems to fit the same pattern. It is very closely related to forms that live outside caves (Gyrinophilus porphytiticus and Gyrinophilus danielsi), but it is paler and has much reduced eyes. It also has a much more limited distribution (Conant, 1975).
The Grotto salamander, Typhlotriton spelaeus, of the Ozark Mountains of Arkansas, Missouri, and Oklahoma, is a somewhat different case. In this instance, the salamander spends only its adult life in the deep part of caves. The larvae have working eyes and are well pigmented. The larvae first live outside the cave in springs and small streams; later they move into a cave, lose their pigment, and the eyelids grow shut. As one would expect, this is a much more widespread species than the obligatory troglobites which spend their entire life cycle in the subterranean environment (Conant, 1975).
What the creationists would like to say about these unique animals with their very special habitats is that God created these particular creatures for these special caves as a special act that was a part of his divine plan. But this scenario contradicts one of the creationists' major tenets. You will recall that creationists hold that the great sedimentary limestone formations of the world, chock full of fossils, are the debris of Noah's Flood. Creation was completed long before the Flood. So, where did the blind cave animals live before there were limestone solution caves for them to occupy? Almost none live in any other type of cave (a very few live in sea caves, but all of them are fish; none are salamanders, crayfish, or insects). Creationists can only reply that there must have been other caves before the Flood laid down sedimentary rocks so that caves could form for the troglobites to inhabit. Creationists cannot say that this special act took place after the Flood, because that would be long after the world was completely finished, according to the opening chapters of Genesis.
On the other hand, evolution has no difficulty explaining where troglobites lived before there were limestone solution caves. Troglobites simply didn't exist before there were caves for them to live in. According to evolution, the development of new life forms is a continuous process, ongoing and far from complete.
Blind cave animals provide some of the most clear and convincing examples of vestigial organs one can find anywhere. Vestigial organs are one of the main lines of evidence for evolution, but much that has been said about them seems farfetched. It has been suggested, for example, that the human veriform appendix is a vestigial intestine. This seems no more plausible than the absurd idea that since human males have undeveloped breasts there must have been a time when both males and females nursed the young—if not in humans, then in their ancestors. So far as I know, no one has ever suggested that! Many male mammals have undeveloped breasts, but nobody regards them as vestiges of a time when both sexes nursed. However, it is quite obvious that the undeveloped eyes on a cave salamander or crayfish are vestiges of a time when the ancestors of the species lived in the light, on the surface, and depended upon them for survival.
Some creationists are prepared to admit that "microevolution" exists. Small changes can be demonstrated under experimental conditions in a relatively few generations. Some creationists would probably not be very troubled by cases like blind cave crayfish, which very closely resemble their ancestors living just outside the cave, and would explain them as cases of microevolution. The argument for evolution is essentially based upon projecting microevolution over vast reaches of times. The blind cave salamanders, as a group, show a logical sequence of steps from species that closely resemble surface species to forms that are more and more unlike anything now existing on the surface. This gives us some insight into what microevolution can do, given time. Left to itself, micro becomes macro.
On the other hand, creationists might try to use the distribution of troglobites to support their position. There are species which have been found in widely separated locations, often with major barriers between them. For example, among the springtails, an order of wingless insects, Onychiurus schoette, is found in both Britain and Ireland. The same is true of Trechoblemus micros, a kind of beetle (Jefferson, 1976). This beetle is also found in many caves on the continent of Europe. A creationist might argue that this pattern of distribution is evidence against evolution, for, if the beetle evolved in caves and could not leave them, why is the same beetle in caves in widely separated areas? Cave beetles cannot swim across the English Channel or the Irish Sea.
This line of argument has initial plausibility, but it is weakened by the fact that adults of the species are sometimes, though rarely, found on the surface. Professor G. T. Jefferson, an expert on British cave fauna, believes that the beetle does sometimes breed outside caves (Jefferson, 1976). This explains its spotty distribution. There is good reason to believe that Britain was once connected to Ireland and the continent in the past, when sea levels were much lower than they are today and when much of the world's water was tied up in glaciers (Jefferson, 1976).
Compare this reasonable line of argument with what the creationist would have us suppose. If the English Channel and the Irish Sea form a barrier to troglobites, it would be interesting to know how they got from where the ark rested to the caves in England and Ireland where they now reside. (This argument would not apply to the numerous aquatic troglobites; presumably they could have continued in situ!)
In puzzling over a problem such as the distribution of cave animals, we begin to see what I consider to be perhaps the major difference between the creation and evolution accounts of the biological world. The evolution account presents genuine problems. There is evidence which is relevant: we have to try to find out if there are passages between caves through which animals could pass back and forth; we have to ask ourselves why a species of terrestrial troglobiont is almost always less widely distributed than an aquatic one. Is it because aquatic forms have a greater opportunity to move around in subterranean waters? Why did animals go into such an uninviting environment to begin with? Was it to escape climatic changes on the outside? Cooling? Drought? Or was it simply because the habitat provides opportunities for exploitation by some life form?
All these are interesting and exciting problems for scientific inquiry. On the other hand, the creationist presents exactly the same answer to all scientific questions: "God wanted it that way." One can continue with descriptive research, continue to collect and classify cave animals, and so forth, but only as a way of demonstrating the intricacy of God's divine plan. This was the motivation of many great scientists before Darwin and Wallace provided the great overarching theory that taught us how to make more meaningful and more fruitful inquiries.
Austin, Steven A. 1980. "Origin of Limestone Caves." ICR Impact series. Number 79. January.
Balch, Edwin Swift. 1970. Glacieres or Freezing Caverns. New York and London: Johnson Reprinting Corporation. (Reprint of original, first published in 1900.)
Barr, T. C., Jr. 1968. "Cave Ecology and the Evolution of Troglobites." Evolutionary Biology 2:35-102.
Brandon, R. A. 1971. "North American Troglobitic Salamanders: Some Aspects of Modifications in Cave Habitats with Special Reference to Gyrinophilus paleucus." National Speleological Society Bulletin. 33:1-21.
Conant, Roger. 1975. A Field Guide to Reptiles and Amphibians. Boston: Houghton Mifflin Company.
Halliday, William R. 1974. American Caves and Caving. New York: Harper and Row.
Hartzog, George B., Jr. 1987. Encyclopedia Americana. Volume 5. Hartzog is director of the National Park Service.
Jefferson, G. T. 1976. "Cave Faunas." In T. D. Ford and C. H. D. Cullingford (editors), The Science of Speleology (London, New York, and San Francisco: Academic Press).
Moore, George W., and Nicholas, Brother G. 1964. Speleology: The Study of Caves. Boston: D. C. Heath.
Morris, Henry M. 1974. Scientific Creationism. San Diego, CA: Creation Life Publishers.
Picknett, R. G.; Bray, L. G.; and Stenner, R. D. 1976. "The Chemistry of Cave Waters." In T. D. Ford and C. H. D. Cullingford, The Science of Speleology (London, New York, and San Francisco: Academic Press).
Tilden, Freeman. 1968. The National Parks. New York: Alfred Knopf.
Warwick, G. T. 1976. "Geomorphology and Caves." In T. D. Ford and C. H. D. Cullingford, The Science of Speleology (London, New York, San Francisco: Academic Press).
Whitcomb, John C., and Morris, Henry M. 1961. The Genesis Flood. Grand Rapids, MI: Baker Book House.
White, W. B. 1976. "Cave Minerals and Speleotherms." In T. D. Ford and C. H. D. Cullingford, The Science of Speleology (London, New York, and San Francisco: Academic Press).
Wood, C. 1976. "Caves in Rocks of Volcanic Origin." In T. D. Ford and C. H. D. Cullingford, The Science of Speleology (London, New York, and San Francisco: Academic Press).