Introduction
Both creationist and intelligent design writers assert that there are large gaps in the fossil record, and that the conventional scientific picture of a gradual evolution is a myth. To a certain extent, this is true: the fossil record includes transitions that appear abrupt. In 1972, paleontologists Niles Edgredge and Stephen Jay Gould published a paper arguing that this is the rule, rather than the exception, and named their notion the “punctuated equilibrium” theory [Eldredge1972]. As Gould later elaborated [Gould1993]:
Before Niles Eldredge and I proposed the theory of punctuated equilibrium in 1972, the stasis, or nonchange, of most fossil species during their lengthy geological lifespans was tacitly acknowledged by all paleontologists, but almost never studied explicitly because prevailing theory treated stasis as uninteresting nonevidence for nonevolution. Evolution was defined as gradual transformation in extended fossil sequences, and the overwhelming prevalence of stasis became an embarrassing feature of the fossil record, best left ignored as a manifestation of nothing (that is, nonevolution).
To this day, creationists and intelligent design writers make great hay of the Eldredge-Gould “punctuated equilibrium” concept in general, and highlight published excerpts such as the above, claiming that this is a bona fide acknowledgement that the scientific picture of gradual evolution is simply wrong [Menton2017].
Time and again the creationist-intelligent design writers are guilty of “quote-mining,” taking a passage such as the above out of context, and greatly distorting the author’s intended meaning, all in an attempt to claim that evolution didn’t really happen as scientists claim. See the discussion in Prothero’s book for details [Prothero2007, pg. 81-85].
But the issue remains: Are there major gaps in the fossil record and are they truly unbridgeable, as these writers claim, or not? Can evolution proceed slowly for the most part, but very rapidly during times of rapid environmental or ecological change?
Gaps in the fossil record
In discussing the issue of gaps, one first must carefully define the context. Does one mean a “gap” that had been identified in Darwin’s time, or one that was identified in the mid-twentieth century, or one that exists now? After all, at least one if not more transitional fossils have been found for virtually all gaps thought to exist in Darwin’s day. Along this line, if one transitional fossil is found for a given gap, does that mean that two more gaps have suddenly appeared and must be filled (one on each side of the new fossil)? This is obviously a game that no amount of scientific research can ever fully satisfy.
It must be kept in mind that the discovery of a fossil is a highly fortuitous event. Almost all biological organisms that have ever lived were either eaten by predators or otherwise destroyed soon after death, leaving no trace. Most that persisted in some form (e.g., as skeletons) were later destroyed by chemical effects, or were part of a geological layer that subsequently disappeared into the Earth’s molten mantle. Almost all fossils that have survived these and numerous other perils are far beneath Earth’s surface and will never be found by humans. Only a tiny fraction of fossils are so fortunate as to be within a few meters of the surface, at a location where geologists and paleontologists can find them [Prothero2007, pg. 51-52].
Some notable transitional fossils
Nonetheless, many significant transitional fossils have have been found over the past few decades, nicely filling “gaps” that once were thought by some to be well-nigh unbridgeable. In general, the paleontological (fossil) history of the Earth is now much better understood today than even a decade or two ago. Here are a few examples. See Fossils for additional examples and discussion.
- Creationists have long listed ichthyosaurs, namely dolphin-like reptiles that lived in the ocean during the age of dinosaurs, as a leading counter-example to evolutionary theory. Ichthyosaurs are hypothesized to have evolved from earlier terrestrial creatures that subsequently re-entered the water, yet until recently no transitional fossils were known. But in November 2014, a team of researchers from the U.S., Italy and China announced that they have discovered a perfect intermediate fossil, a 50 cm-long specimen that lived 248 million years ago. Unlike ichthyosaurs, this species had large flippers to facilitate walking on land like a seal [Feltman2014]. Here is a photo of their specimen [Feltman2014]:
- Marine-land mammal transition. Both Duane Gish (a leading creationist scholar) and Michael Behe (a leading intelligent design scholar) have questioned whether scientists would ever discover fossils linking the hypothesized transition between land mammals and marine mammals. Gish, for example, has highlighted his presentations on this topic by displaying a cartoon of “Bossie” the cow, “Blowhole” the whale and a ridiculous intermediate creature between a cow and whale [Gish1985, pg. 78-79; Prothero2008, pg. 318]. Behe, for his part, wrote “if random evolution is true, there must have been a large number of transitional forms between the Mesonychid [an ancient land-based mammal] and the ancient whale. Where are they? It seems like quite a coincidence that of all the intermediate species that must have existed between the Mesonychid and whale, only species that are very similar to the end species have been found.” [Behe1994]. Yet at least 30 distinct intermediate fossil species are now known within this “gap,” with exactly the expected combination of terrestrial and aquatic features, comprising six known phyla, according to a recent compilation [Thewissen2002; Zimmer2001, pg. 138]. A summary of these intermediate fossils is presented at Fossils. For a fully detailed version, see [Thewissen2002; Zimmer2001, pg. 138].
- Fish-tetrapod transition. In another striking recent development, scientists discovered a long-sought transition between bony fish and tetrapods (four-legged animals including reptiles, birds, mammals, humans). Named “Tiktaalik,” it was discovered on a remote island in the Canadian Arctic Ocean, in sedimentary rock 375 million years old. Although this creature had many features of fish, it also had several traits characteristic of early tetrapods, including fins that contained bones, forming a limb-like appendage that the animal could use for locomotion on land. This fulfilled a key prediction from more than 100 years ago [NAS2008, pg. 1-3; Prothero2007, pg. 228-229]. A fascinating account of this discovery, by the scientist himself, is given in [Shubin2008, pg. 1-27]. A sketch of how Tiktaalik fits into known intermediate forms is shown in the following graphic [Myers2006]:
- Elephants. Twenty-two distinct species of elephants have now been identified during just the past six million years, generously filling the “gap” between ancient forms and three modern species (the two currently existing elephant species and the recently extinct wooly mammoth). One recently compiled family tree of these species is shown at Fossils.
- Horses. Contrary to numerous claims by creationists and intelligent design writers (see [Prothero2007, pg. 300-305] for discussion), the evolution of horses over the past 55 million years is particularly well understood, due to recent discoveries, with no indication of any significant “gap.” Here is a graphic showing the evolution of horses over the past 55 million years, which exhibits the “bushy” form that often characterizes such well-understood family trees (note however that this graphic is now 14 years old; other fossil species have since been discovered) [MacFadden2005]:
- Spiny-finned fish. Recently an international team of researchers presented research on the evolution and proliferation, over the past 150 million years, of spiny-rayed fish (also known as spiny-finned fish, but more formally as acanthomorphs), which constitute nearly one-third of all currently living vertebrate species on Earth. According to the authors, this group of fish species has long been “the last frontier” in reconstructing the family tree of modern vertebrates. The team’s research was based on DNA-genetic analysis, but they also correlated their results with known events in paleontological (fossil) history, such as the Cretaceous-Paleogene mass extinction event 66 million years ago. A reduced-resolution graphic is available at Fossils, while the full-sized graphic is available from the PNAS site. Details of the research are available at [Frazer2013; Near2013].
- Dinosaur-bird transition. Since the discovery of the original archaeopteryx fossil in 1860, numerous fossils have been found confirming a hypothesis originally made by Thomas Huxley that birds are modern descendants of dinosaur-like creatures. With regards to the archaeopteryx fossils, the current consensus is that although they were feathered dinosaurs capable of flight, most likely they were not ancestral to modern birds [ODonoghue2011]. However, other recent fossil finds have illuminated the transition between dinosaurs and birds. In 2008 Chinese paleontologist Xu Xing and his colleagues documented what appears to be the simplest type of “protofeather,” which was seen in a therizinosaur fossil. Other fossils show feathers gradually became more complex, turning into the veined feathers suitable for flight [ODonoghue2010c]. Another remarkable development in this area was the 2010 discovery by Xu Xing’s team of a 160-million-year-old dinosaur fossil in northwestern China that had a bird-like keel-shaped chest and a long beak, yet also had three toes on each leg like a dinosaur. Kevin Padian, a noted paleontologist and dinosaur expert at U.C. Berkeley, said, “This is another salvo that blows apart the claims of the ‘birds are not dinosaur’ crew.” [Perlman2010]. As a result of these and other findings, the overwhelming majority of paleontologists are now convinced that modern birds are descendants of dinosaurs. A nice overview of recent research in this field is given in a Quanta article by Emily Singer [Singer2015].
- Prehuman species. At least 20 distinct prehuman species have now been identified during the past seven million years. A recent version of the family tree of prehuman species is shown in
Prehuman fossils.
Again, the sheer number of distinct species, even if some are later shown not to be in the direct path of humans (which is likely), clearly makes a mockery of the popular notion that there is an unbridgeable “gap” between apes and humans in the fossil record.
Some additional examples are listed at Fossils. Additionally, several hundred transitional fossils are listed in [Hunt1997], yet even this is only a partial collection and is now more than 20 years out of date. For instance, the Tiktaalik fossil, among many others, was not known at the time this list was prepared and thus is not included. Mark Issak’s book lists numerous examples of transitional fossils [Isaak2007, pg. 113-118]. At the present time, perhaps the best single reference for fossils, and counters to creationist-intelligent design claims about fossils, is Donald Prothero’s book Evolution: What the Fossils Say and Why It Matters. It includes a very nice recent treatment of this general topic, including mention of numerous transitional fossils with many lovely illustrations, some in color [Prothero2007]. One other good recent reference is Daniel Fairbank’s book Evolving: The Human Question and Why It Matters, which describes in some detail how many details of human anatomy, ranging from our eyes to our arms and feet, have their precursors in the fossil record [Fairbanks2012]. A good summary of some ongoing research in this arena is available in a February 2013 New Scientist article [Hecht2013].
How fast can evolution operate?
Closely connected to the issue of gaps in the fossil record is the question of how fast can evolution operate. As mentioned above, implicit in the Eldredge-Gould theory of punctuated equilibrium is the notion that evolution can proceed very slowly most of the time, but quite rapidly during times of environmental or ecological stress. Creationist and intelligent design writers argue that evolution cannot occur on short time scales, so that existing evolutionary theory cannot explain the sudden emergence of new features, new species, or other abrupt changes that are observed in the geological record. They also often dismiss instances of present-day evolution [Ham2017].
Examples of evolution in short time scales
But such arguments are utterly defeated by a growing list of instances of evolution in action, where nature generates novel, beneficial features within just a few years or decades. Indeed, biologists now recognize that evolution can happen significantly faster than previously thought possible [Schilthuizen2016]. Here are a few recent developments. See Fast evolution for additional examples and discussion.
- Antiobiotic-resistant diseases. Perhaps the best-known examples are the recent evolution of new strains of tuberculosis that are resistant to all known anti-TB drugs. By analyzing DNA sequences, researchers have identified at least six different families of tuberculosis, at least one of which appears to be evolving on an unexpected and potentially very dangerous path [Lehrman2013]. Another example is the announcement on 27 May 2016 that a Pennsylvania woman, who was diagnosed with a urinary tract infection, was in fact infected with a strain of bacteria that was resistant to colistin, an antibiotic “of last resort” — used only when all other antibiotics have failed. This was the first instance of this strain in the U.S. More are feared [Moyer2016].
- Tibetan high-altitude genes. In 2010, researchers at the University of Utah and Qinghai University in China have found that natives of the Tibetan highlands have evolved ten unique genes that permit them to live well at very high altitudes. Because of these genes, Tibetans have more efficient metabolisms, do not overproduce red blood cells in response to thin air, and have higher levels of nitric oxide, which helps get oxygen to tissue [SD2010b]. A even more recent study found a total of 30 genes that were distinct in the Tibetan population, and concluded that this change constitutes the fastest documented case of human evolution [Wade2010b].
- Hawaiian crickets. In the 1800s, a species of cricket was introduced to the Hawaiian Islands, where they became quite common in grassy areas. Males attract mates with their chirps, and females select males based on their songs. However, unlike their counterparts in other Pacific islands, the Hawaiian crickets have a fearsome predator — dive-bombing flies that target chirping crickets, then implant their larvae in them. In the 1990s, researchers noted that a field in Kauai that previously was the home to many crickets now seemed silent. However, a nighttime search found that in fact there were lots of crickets there, but very few of the males now chirped — in just five years, or roughly 20 generations, a mutation had arisen in a single gene that inhibited many of the males from chirping [Zuk2013, pg. 81-82].
- Stickleback fish in Alaska. In 1990, biologist Michael Bell found to his surprise that marine stickleback fish had recolonized Loberg Lake in Alaska, after being exterminated in 1982. What’s more, a few of these fish had developed features more typical of fresh water stickleback species, such as loss of body armor and changes in feeding structures in the throat. He and his team have returned to the lake each year since then and have documented a steady increase in the percentage of fresh-water-typical features. For instance, Bell and his graduate student Windsor Aguirre have observed that the lake fish population has changed from mostly 30 or more armor plates per side to mostly between five and eight plates per side. As Bell notes, “it has become clear that populations can evolve substantially on contemporary time scales and that the magnitude of evolutionary divergence between ancestral and descendant populations can be comparable to differences among related species” [Bell 2011; LePage2011]. Here are photos of a typical marine stickleback (top) and two typical Loberg Lake stickleback (bottom), courtesy Michael Bell:
- Stickleback fish in Switzerland. In yet another example of speciation of stickleback fish, researchers at the University of Bern have found that a population of these fish in Switzerland’s Lake Constance, which was only introduced roughly 150 years ago, appears to have split into two species — one lives in the main lake, and the other lives in streams flowing into the lake. The lake fish are generally larger, and have longer spines and tougher armor — see photos below (on the New Scientist website, copyright David Marques). The researchers confirmed that it was speciation, not just lifestyle, by noting clear genetic differences between the two populations [LePage2016].
- Tasmanian devils. Tasmanian devils, a rodent-like creature with impressive fangs, have been seriously threatened by an unusual cancer, namely “facial tumor disease,” which is typically transmitted when one individual bites an infected animal. The condition, first observed in 1996, has caused the devil population to plummet by roughly 80% overall, 95% in certain locations. Now genomic analysis has confirmed that a gene has arisen that confers partial immunity, and in only ten generation has spread very widely in the population [Klein2016].
Some additional examples are given at Fast evolution.
Summary
In summary, while creationist and intelligent design writers have made great hay over the Eldredge-Gould “punctuated equilibrium” concept, claiming that this is a bona fide acknowledgement that the scientific picture of gradual evolution is simply wrong, the true picture is considerably more nuanced.
Yes, there are gaps in the fossil record. But the fossil record, as has been studied by paleontologists to date, is clearly incomplete. Almost all creatures that ever lived left no trace whatsoever, since the formation of a fossil requires a fortuitous combination of rare events, and almost all fossils that have been formed have either been subducted into the Earth’s mantle or else are far underground and will never be seen by humans. In spite of these limitations, hundreds of “gaps” thought to exist in earlier decades have recently been filled rather well by the discovery of transitional fossils, and there is no indication that the remaining gaps are in any way fundamentally different than those that have already been filled. So it is not clear that at this point in time that the “gap” argument against evolution has any remaining substance.
Similarly, it is no mystery why the fossil record shows species with long periods of stasis, interrupted by brief periods of rapid change, in some cases too rapid for the intermediate forms to be seen in the fossil record. This is because a species that is well-suited to a relatively stable environment has no need to change, and in fact does not change beyond some gradual drift. But when a species is under some environmental or ecological stress, it can change rather rapidly indeed. This is seen in numerous examples of species that have either changed or split into separate species, all within a few years or decades of human observation. Such rapid changes would never appear in the geological record, which can seldom resolve below a few thousand years.
In short, the creationist-intelligent design argument, namely that the Eldredge-Gould “punctuated equilibrium” concept is a prima facie acknowledgement that evolution is “wrong,” is deeply mistaken. It is countered by hundreds of examples of transitional fossils and fast-evolving species, and has no currency among professional research evolutionary biologists.