Why Darwin Matters Page 3
By way of example, once when my young daughter asked how evolution works, I used the polar bear as an example of a “transitional species” between land mammals and marine mammals, because although they are land mammals they spend so much time in the water that they have acquired many adaptations to an aquatic life. But this is not correct. It implies that polar bears are on their way (in transition) to becoming marine mammals. They aren’t. Polar bears are not “becoming” anything. Polar bears are well adapted for their lifestyle. That’s all. If global warming continues, perhaps polar bears will adapt to a full-time aquatic existence, or perhaps they will move south and become smaller brown bears, or perhaps they will go extinct. Who knows? No one.
Where Are All the Fossils?
Evolution is a historical science, and historical data—fossils—are often the evidence most cited for and against it. In the creationist textbook, Of Pandas and People—one of the bones of contention in the 2005 Intelligent Design trial of Kitzmiller et al. v. Dover Area School District, in Dover, Pennsylvania—the authors state: “Design theories suggest that various forms of life began with their distinctive features already intact: fish with fins and scales, birds with feathers and wings, mammals with fur and mammary glands. . . . Might not gaps exist . . . not because large numbers of transitional forms mysteriously failed to fossilize, but because they never existed?”16
Darwin himself commented on this lack of transitional fossils, asking, “Why then is not every geological formation and every stratum full of such intermediate links?” In contemplating the answer, he turned to the data and noted that “geology assuredly does not reveal any such finely graduated organic chain; and this, perhaps, is the gravest objection which can be urged against my theory.”17 So where are all the fossils?
One answer to Darwin’s dilemma is the exceptionally low probability of any dead animal’s escaping the jaws and stomachs of predators, scavengers, and detritus feeders, reaching the stage of fossilization, and then somehow finding its way back to the surface through geological forces and unpredictable events to be discovered millions of years later by the handful of paleontologists looking for its traces. Given this reality, it is remarkable that we have as many fossils as we do.
There is another explanation for the missing fossils. Ernst Mayr outlines the most common way that a species gives rise to a new species: when a small group (the “founder” population) breaks away and becomes geographically—and thus reproductively—isolated from its ancestral group. As long as it remains small and detached, the founder group can experience fairly rapid genetic changes, especially relative to large populations, which tend to sustain their genetic homogeneity through diverse interbreeding. Mayr’s theory, called allopatric speciation, helps to explain why so few fossils would exist for these animals.
The evolutionary theorists Niles Eldredge and Stephen Jay Gould took Mayr’s observations about how new species emerge and applied them to the fossil record, finding that gaps in the fossil record are not missing evidence of gradual changes; they are extant evidence of punctuated changes. They called this theory punctuated equilibrium.18 Species are so static and enduring that they leave plenty of fossils in the strata while they are in their stable state (equilibrium). The change from one species to another, however, happens relatively quickly on a geological time scale, and in these smaller, geographically isolated population groups (punctuated). In fact, species change happens so rapidly that few “transitional” carcasses create fossils to record the change. Eldredge and Gould conclude that “breaks in the fossil record are real; they express the way in which evolution occurs, not the fragments of an imperfect record.”19 Of course, the small group will also be reproducing, following the geometric increases that are observed in all species, and will eventually form a relatively large population of individuals that retain their phenotype for a considerable time—and leave behind many well-preserved fossils. Millions of years later this process results in a fossil record that records mostly the equilibrium. The punctuation is in the blanks.
The Evidence of Evolution
In August 1996, NASA announced that it discovered life on Mars. The evidence was the Allan Hills 84001 rock, believed to have been ejected out of Mars by a meteor impact millions of years ago, which then fell into an orbit that brought it to Earth.20 On the panel of NASA experts was paleobiologist William Schopf, a specialist in ancient microbial life. Schopf was skeptical of NASA’s claim because, he said, the four “lines of evidence” claimed to support the find did not converge toward a single conclusion. Instead, they pointed to several possible conclusions.21
Schopf’s analysis of “lines of evidence” reflects a method of science first described by the nineteenth-century philosopher of science William Whewell. To prove a theory, Whewell believed, one must have more than one induction, more than a single generalization drawn from specific facts. One must have multiple inductions that converge upon one another, independently but in conjunction. Whewell said that if these inductions “jump together” it strengthens the plausibility of a theory: “Accordingly the cases in which inductions from classes of facts altogether different have thus jumped together, belong only to the best established theories which the history of science contains. And, as I shall have occasion to refer to this particular feature in their evidence, I will take the liberty of describing it by a particular phrase; and will term it the Consilience of Inductions.”22 I call it a convergence of evidence.
Just as detectives employ the convergence of evidence technique to deduce who most likely committed a crime, scientists employ the method to deduce the likeliest explanation for a particular phenomenon. Cosmologists reconstruct the history of the universe through a convergence of evidence from astronomy, planetary geology, and physics. Geologists reconstruct the history of the planet through a convergence of evidence from geology, physics, and chemistry. Archaeologists piece together the history of civilization through a convergence of evidence from biology (pollen grains), chemistry (kitchen middens), physics (potsherds, tools), history (works of art, written sources), and other site-specific artifacts.
As a historical science, evolution is confirmed by the fact that so many independent lines of evidence converge to its single conclusion. Independent sets of data from geology, paleontology, botany, zoology, herpetology, entomology, biogeography, comparative anatomy and physiology, genetics and population genetics, and many other sciences each point to the conclusion that life evolved. This is a convergence of evidence. Creationists can demand “just one fossil transitional form” that shows evolution. But evolution is not proved through a single fossil. It is proved through a convergence of fossils, along with a convergence of genetic comparisons between species, and a convergence of anatomical and physiological comparisons between species, and many other lines of inquiry. For creationists to disprove evolution, they need to unravel all these independent lines of evidence, as well as construct a rival theory that can explain them better than the theory of evolution. They have yet to do so.
The Tests of Evolution
Creationists like to argue that evolution is not a science because no one was there to observe it and there are no experiments to run today to test it. The inability to observe past events or set up controlled experiments is no obstacle to a sound science of cosmology, geology, or archaeology, so why should it be for a sound science of evolution? The key is the ability to test one’s hypothesis. There are a number of ways to do so, starting with the broadest method of how we know evolution happened.
Consider the evolution of our best friend, the dog. With so many breeds of dogs popular for so many thousands of years, one would think that there would be an abundance of transitional fossils providing paleontologists with copious data from which to reconstruct their evolutionary ancestry. Not so. In fact, according to Jennifer A. Leonard of the National Museum of Natural History in Washington, D.C., “the fossil record from wolves to dogs is pretty sparse.”23 Then how do we know the origin of dogs? In a 200
2 issue of Science, Leonard and her colleagues report that mitochrondrial DNA (mtDNA) data from early dog remains “strongly support the hypothesis that ancient American and Eurasian domestic dogs share a common origin from Old World gray wolves.” In the same issue of Science, Peter Savolainen from the Royal Institute of Technology in Stockholm and his colleagues note that the fossil record is problematic “because of the difficulty in discriminating between small wolves and domestic dogs,” but their study of mtDNA sequence variation among 654 domestic dogs from around the world “points to an origin of the domestic dog in East Asia ~ 15,000 yr B.P.” from a single gene pool of wolves. Finally, Brian Hare from Harvard and his colleagues describe the results of their study in which they found that domestic dogs are more skillful than wolves at using human communicative signals indicating the location of hidden food, but that “dogs and wolves do not perform differently in a non-social memory task, ruling out the possibility that dogs outperform wolves in all human-guided tasks.” Therefore, “dogs’ social-communicative skills with humans were acquired during the process of domestication.”24 Although no single fossil proves that dogs came from wolves, the convergence of evidence from archaeological, morphological, genetic, and behavioral “fossils” reveals the ancestor of all dogs to be the East Asian wolf.
The tale of human evolution is revealed in a similar manner (although here we do have an abundance of transitional fossil riches), as it is for all ancestors in the history of life. One of the finest compilations of evolutionary convergence is Richard Dawkins’s magnum opus, The Ancestor’s Tale, 673 pages of convergent science recounted with literary elegance. Dawkins traces innumerable “transitional fossils” (what he calls “concestors”—the “point of rendezvous” of the last common ancestor shared by a set of species) from Homo sapiens back four billion years to the origin of replicating molecules and the emergence of evolution. No one concestor proves that evolution happened, but together they reveal a majestic story of a process over time.25 We know human evolution happened because innumerable bits of data from myriad fields of science conjoin to paint a rich portrait of life’s pilgrimage.
But the convergence of evidence is just the start. The comparative method allows us to infer evolutionary relationships using data from a wide variety of fields. Luigi Luca Cavalli-Sforza and his colleagues, for example, compared fifty years of data from population genetics, geography, ecology, archaeology, physical anthropology, and linguistics to trace the evolution of the human races. Using both the convergence and comparative methods led them to conclude that “the major stereotypes, all based on skin color, hair color and form, and facial traits, reflect superficial differences that are not confirmed by deeper analysis with more reliable genetic traits.” By comparing surface (physical) traits—the phenotype of individuals—with genetic traits—the genotype—they teased out the relationship between different groups of people. Most interesting, they found that the genetic traits disclosed “recent evolution mostly under the effect of climate and perhaps sexual selection.” For example, they discovered that Australian aborigines are genetically more closely related to southeast Asians than they are to African blacks, which makes sense from the perspective of the evolutionary timeline: The migration pattern of humans out of Africa would have led them first to Asia and then to Australia.26
Dating techniques provide evidence of the timeline of evolution. The dating of fossils, along with the earth, moon, sun, solar system, and universe, are all tests of evolutionary theory, and so far they have passed all the tests. We know that the earth is approximately 4.6 billion years old because of the convergence of evidence from several methods of dating rocks: Uranium Lead, Rubidium Strontium, and Carbon-14. Further, the age of the earth, the age of the moon, the age of the sun, the age of the solar system, and the age of the universe are consistent, maintaining yet another consilience. If, say, the earth was dated at 4.6 billion years old but the solar system was dated at one million years old, the theory of evolution would be in trouble. But Uranium Lead, Rubidium Strontium, and Carbon-14 have not provided any good news for the so-called Young Earth creationists.
Better yet, the fossils and organisms speak for themselves. Fossils do show intermediate stages, despite their rarity. For example, there are now at least eight intermediate fossil stages identified in the evolution of whales. In human evolution, there are at least a dozen known intermediate fossil stages since hominids branched off from the great apes six million years ago. And geological strata consistently reveal the same sequence of fossils. A quick and simple way to debunk the theory of evolution would be to find a fossil horse in the same geological stratum as a trilobite. According to evolutionary theory, trilobites and mammals are separated by hundreds of millions of years. If such a fossil juxtaposition occurred, and it was not the product of some geological anomaly (such as uplifted, broken, bent, or even flipped strata—all of which occur but are traceable), it would mean that there was something seriously wrong with the theory of evolution.
Evolution also posits that modern organisms should show a variety of structures from simple to complex, reflecting an evolutionary history rather than an instantaneous creation. The human eye, for example, is the result of a long and complex pathway that goes back hundreds of millions of years. Initially a simple eyespot with a handful of light-sensitive cells that provided information to the organism about an important source of the light, it developed into a recessed eyespot, where a small surface indentation filled with light-sensitive cells provided additional data on the direction of light; then into a deep recession eyespot, where additional cells at greater depth provide more accurate information about the environment; then into a pinhole camera eye that is able to focus an image on the back of a deeply recessed layer of light-sensitive cells; then into a pinhole lens eye that is able to focus the image; then into a complex eye found in such modern mammals as humans. All of these structures are expressed in modern eyes.
Further, biological structures show signs of natural design. The anatomy of the human eye, in fact, shows anything but “intelligence” in its design. It is built upside down and backwards, requiring photons of light to travel through the cornea, lens, aqueous fluid, blood vessels, ganglion cells, amacrine cells, horizontal cells, and bipolar cells before they reach the light-sensitive rods and cones that transduce the light signal into neural impulses—which are then sent to the visual cortex at the back of the brain for processing into meaningful patterns. For optimal vision, why would an intelligent designer have built an eye upside down and backwards? This “design” makes sense only if natural selection built eyes from available materials, and in the particular configuration of the ancestral organism’s pre-existing organic structures. The eye shows the pathways of evolutionary history, not of intelligent design.
Additionally, vestigial structures stand as evidence of the mistakes, the misstarts, and, especially, the leftover traces of evolutionary history. The cretaceous snake Pachyrhachis problematicus, for example, had small hind limbs used for locomotion that it inherited from its quadrupedal ancestors, gone in today’s snakes. Modern whales retain a tiny pelvis for hind legs that existed in their land mammal ancestors but have disappeared today. Likewise, there are wings on flightless birds, and of course humans are replete with useless vestigial structures, a distinctive sign of our evolutionary ancestry. A short list of just ten vestigial structures in humans leaves one musing: Why would an Intelligent Designer have created these?
1. Male nipples. Men have nipples because females need them, and the overall architecture of the human body is more efficiently developed in the uterus from a single developmental structure.
2. Male uterus. Men have the remnant of an undeveloped female reproductive organ that hangs off the prostate gland for the same reason.
3. Thirteenth rib. Most modern humans have twelve sets of ribs, but 8 percent of us have a thirteenth set, just like chimpanzees and gorillas. This is a remnant of our primate ancestry: We share common ancestors with chi
mps and gorillas, and the thirteenth set of ribs has been retained from when our lineage branched off six million years ago.
4. Coccyx. The human tailbone is all that remains from our common ancestors’ tails, which were used for grasping branches and maintaining balance.
5. Wisdom teeth. Before stone tools, weapons, and fire, hominids were primarily vegetarians, and as such we chewed a lot of plants, requiring an extra set of grinding molars. Many people still have them, despite the smaller size of our modern jaws.
6. Appendix. This muscular tube connected to the large intestine was once used for digesting cellulose in our largely vegetarian diet before we became meat eaters.
7. Body hair. We are sometimes called “the naked ape”; however, most humans have a layer of fine body hair, again left over from our evolutionary ancestry from thick-haired apes and hominids.
8. Goose bumps. Our body hair ancestry can also be inferred from the fact that we retain the ability of our ancestors to puff up their fur for heat insulation, or as a threat gesture to potential predators. Erector pili—“goose bumps”—are a telltale sign of our evolutionary ancestry.
9. Extrinsic ear muscles. If you can wiggle your ears you can thank our primate ancestors, who evolved the ability to move their ears independently of their heads as a more efficient means of discriminating precise sound directionality and location.