Taken largely from the Talk.Origins Archive web site, and is used here with the kind permission of its author,
According to the theory of evolution, the "descent with modification" road to humans (or any other group, for that matter) is paved with a sequence of transitional fossils, spaced out in a time sequence reflected in the ages of the fossils found. Since fossils of soft-bodied animals are relatively rare (they don't fossilize easily), the record is rather spotty prior to the first appearance of vertebrates (in the form of jawless fishes), so this lesson will focus only on the fossil record of vertebrates
As we study the growing number of fossils, we find that they usually fit nicely into one group or another, and most of those groups clearly show gradual change over time, even phasing into new and different groups along the way, adding changes upon changes. Nevertheless, many of those earlier groups apparently had populations which continued to exist with very little change, producing the modern day representatives of those surviving groups. As a result, the picture painted by the fossils reveals an ongoing coexistence, of older more primitive forms continuing to live alongside the growing diversity of animals which they produced.
However, most of those groups along the way apparently failed to survive in their original forms. They became extinct. But fortunately, some members of some of those groups were fossilized, and a few of those are found from time to time, giving us the hit-or-miss, very spotty record of fossils which has lead us to hypothesize that picture of a branched tree of being which we call evolution. New fossils are being found every day, helping to fill in some of the gaps, and those fossils continue to confirm and strengthen that picture of life through time with ever-increasing detail.
In this lesson, we will peek at a very small sampling of this fossil record, focusing mainly on the forms and times when various human traits first appeared. We will build a type of "family tree" called a "cladogram", which emphasizes the first appearances of traits which are also diagnostic for major animal groups living today. If you would like to see more of the transitional details, go to one of the documents on the well done web site of Talk.Origins Archives: "Transitional Vertebrate Fossils" (<http://www.talkorigins.org/faqs/faq-transitional.html>). Most of the following information was taken from that site. It was compiled and presented by Kathleen Hunt, a PhD candidate in zoology at the University of Washington in the mid 1990s. The references cited here can be found at the end of that 5-part document.
A. WHAT IS A TRANSITIONAL FOSSIL?
1. "General lineage":
2. "Species-to-species transition":
3. Transitions to New Higher Taxa
There are now several known cases of species-to-species transitions that resulted in the first members of new higher taxa.
4. An Example of a Transition Series: from Synapsid Reptiles
The list of some 27 species which best documents the transition from mammal-like reptiles to mammals starts with pelycosaurs (early synapsid reptiles; Dimetrodon is a popular, advanced, example) and continues with therapsids and cynodonts up to the first unarguable "mammal". This covered some 160 million years, from the early Pennsylvanian (315 ma) to the late Jurassic (155 ma), with a 30 million year gap in the late Triassic. Most of the changes in this transition involved elaborate repackaging of an expanded brain and special sense organs, remodeling of the jaws & teeth for more efficient eating, and changes in the limbs & vertebrae related to active, legs-under-the-body locomotion. What is most striking (here, as well as in most other transitional fossils) is a mosaic mixture (existing in each species along the way) of some earlier (more primitive) traits along with newer, more derived traits, with a gradual decrease in the primitive traits, an increase in the derived traits, and gradual changes in size of various features through time. Some differences observed:
(*) Fenestrae are holes in the sides of the skull
(**) The presence of a dentary-squamosal jaw joint has been arbitrarily selected as the defining trait of a mammal.
5. Two Examples of Species-to-Species Fossil Sequences
Rose & Bown (1984) analyzed over 600 specimens of primates collected from a 700-meter-thick sequence representing approximately 4 million years of the Eocene. They found smooth transitions between Teilhardina americana and Tetonoides tenuiculus, and also beween Tetonius homunculus and Pseudotetonius ambiguus. "In both lines transitions occurred not only continuously (rather than by abrupt appearance of new morphologies followed by stasis), but also in mosaic fashion, with greater variation in certain characters preceding a shift to another character state." The T. homunculus - P. ambiguus transition shows a dramatic change in dentition (loss of P2, dramatic shrinkage of P3 with loss of roots, shrinkage of C and I2, much enlarged I1) that occurs gradually and smoothly during the 4 million years. The authors conclude "...our data suggest that phyletic gradualism is not only more common than some would admit but also capable of producing significant adaptive modifications."
B. WHY DO GAPS EXIST (OR SEEM TO EXIST)?
Species-to-species transitions are even harder to document. To demonstrate anything about how a species arose, whether it arose gradually or suddenly, you need exceptionally complete strata, with many dead animals buried under constant, rapid sedimentation. This is rare for terrestrial animals. Even the famous Clark's Fork (Wyoming) site, known for its fine Eocene mammal transitions, only has about one fossil per lineage about every 27,000 years. Luckily, this is enough to record most episodes of evolutionary change (provided that they occurred at Clark's Fork Basin and not somewhere else), though it misses the rapidest evolutionary bursts. In general, in order to document transitions between species, you need specimens separated by only tens of thousands of years (e.g. every 20,000-80,000 years). If you have only one specimen for hundreds of thousands of years (e.g. every 500,000 years), you can usually determine the sequence of species, but not the transitions between species. If you have a specimen every million years, you can get the order of genera, but not which species were involved. And so on. These are rough estimates (from Gingerich, 1976, 1980) but should give an idea of the completeness required.
Note that fossils separated by more than about a hundred thousand years cannot show anything about how a species arose. Think about it: there could have been a smooth transition, or the species could have appeared suddenly, but either way, if there aren't enough fossils, we can't tell which way it happened.
2. Discovery of the fossils
Documenting a species-to-species transition is particularly grueling, as it requires collection and analysis of hundreds of specimens. Typically we must wait for some paleontologist to take on the job of studying a certain taxon in a certain site in detail. Almost nobody did this sort of work before the mid-1970's, and even now only a small subset of researchers do it. For example, Phillip Gingerich was one of the first scientists to study species-species transitions, and it took him ten years to produce the first detailed studies of just two lineages (primates and condylarths). In a (later) 1980 paper he said: "the detailed species level evolutionary patterns discussed here represent only six genera in an early Wasatchian fauna containing approximately 50 or more mammalian genera, most of which remain to be analyzed." [emphasis added]
3. Getting the word out
Why don't paleontologists bother to popularize the detailed lineages and species-to-species transitions? Because it is thought to be unnecessary detail. For instance, it takes an entire book to describe the horse fossils even partially (e.g. MacFadden's "Fossil Horses"), so most authors just collapse the horse sequence to a series of genera. Paleontologists clearly consider the occurrence of evolution to be a settled question, so obvious as to be beyond rational dispute, so, they think, why waste valuable textbook space on such tedious detail?
What is truly amazing, given the conditions described above, is that the fossil record shows as many contiguous sequences of fossils as it does. And furthermore, as new fossils are found (and these are many per year) they always fit nicely (or closely) into the sequences already documented, both in time and morphology, and occasionally fill one of the many gaps as well. Remember, particularly in view of the overwhelming number of transitional sequences, the lack of fossils here and there does nothing to weaken the overall picture of descent with modification; the process of evolution is very much a reality.
4. Overview of the Cenozoic
Ma = millions of years ago
C. WHAT IS "PUNCTUATED EQUILIBRIUM"?
There's been a heated debate about which of these modes of evolution is most common, and this debate has been largely misquoted by laypeople. Virtually all of the quotes of paleontologists saying things like "the gaps in the fossil record are real" are taken out of context from this ongoing debate about punctuated equilibrium. Actually, no paleontologist that I know of doubts that evolution has occurred, and most agree that at least sometimes it occurs gradually, and the fossil record clearly shows this. What they're arguing about is how often it occurs gradually. You can make up your own mind about that. (As a starting point, check out Gingerich, 1980, who found 24 gradual speciations and 14 sudden appearances in early Eocene mammals; MacFadden, 1985, who found 5 cases of gradual anagenesis, 5 cases of probable cladogenesis, and 6 sudden appearances in fossil horses; and the numerous papers in Chaline, 1983. Most studies seem to show between 1/4-2/3 of the speciations occurring fairly gradually.)
"Anagenesis", "phyletic evolution": Evolution in which an older species, as a whole, changes into a new descendent species, such that the ancestor is transformed into the descendant.
"Cladogenesis": Evolution in which a daughter species splits off from a population of the older species, after which both the old and the young species coexist together. Notice that this allows a descendant to coexist with its ancestor.
D. PREDICTIONS: EXPECTATIONS IN THE FOSSIL RECORD:
Predictions of evolutionary theory: Evolutionary theory predicts that fossils should appear in a progression through time, in a nested hierarchy of lineages, and that it should be possible to link modern animals to older, very different animals. In addition, the "punctuated equilibrium" model also predicts that new species should often appear "suddenly" (within 500,000 years or less) and then experience long periods of static equilibrium (little or no change). Where the record is exceptionally good, we should find a few local, rapid transitions between species. The "phyletic gradualism" model predicts that most species should change gradually throughout time, and that where the record is good, there should be many slow, smooth species-to-species transitions. These two models are not mutually exclusive -- in fact they are often viewed as two extremes of a continuum -- and both agree that at least some species-to-species transitions should be found.
Overview of the Transitional Vertebrate Fossil Record? The 35 page listing of transitional vertebrates offered in the TalkOrigins Archive, is a reasonably complete picture of the vertebrate record as it is now known. As extensive as it may seem, it is still just a crude summary, and some very large groups were, for convenience, left out. For instance, the list mostly includes transitional fossils that happened to lead to modern, familiar animals. This may unintentionally give the impression that fossil lineages proceed in a "straight line" from one fossil to the next. That's not so; generally at any one time there are a whole raft of successful species, only a few of which happened to leave modern descendents. The horse family is a good example; Merychippus (about 15 mya) gave rise to something like 19 new three - toed grazing horse species, which traveled all over the Old and New Worlds and were very successful at the time. Only one of these lines happened to lead to Equus, though, so that's the only line described in that listing. As they say, "Evolution is not a ladder, it's a branching bush."
A Bit Of Historical Background. When The Origin Of Species was first published, the fossil record was poorly known. At that time, the complaint about the lack of transitional fossils bridging the major vertebrate taxa was perfectly reasonable. Opponents of Darwin's theory of common descent (the theory that evolution has occurred; not to be confused with his separate theory that evolution occurs specifically by natural selection) were justifiably skeptical of such ideas as birds being related to reptiles. The discovery of Archeopteryx only two years after the publication of The Origin of Species was seen as a stunning triumph for Darwin's theory of common descent. Archeopteryx has been called the single most important natural history specimen ever found, "comparable to the Rosetta Stone" (Alan Feduccia, in "The Age Of Birds"). O.C. Marsh's groundbreaking study of the evolution of horses was another dramatic example of transitional fossils, this time demonstrating a whole sequence of transitions within a single family. Within a few decades after the Origin, these and other fossils, along with many other sources of evidence (such as developmental biology and biogeography) had convinced the majority of educated people that evolution had occurred, and that organisms are related to each other by common descent. (Today, modern techniques of paleontology and molecular biology further strengthen this conclusion.)
Since then, many more transitional fossils have been found, as sketched out in the listing. Typically, the only people who still demand to see transitional fossils are either unaware of the currently known fossil record (often due to shoddy and very dated arguments they may have read) or are unwilling to recognize it for some reason.
What Does The Fossil Record Show Us Now? The most noticeable aspects of the vertebrate fossil record, those which must be explained by any good model of the development of life on earth, are:
1. A remarkable temporal pattern of fossil morphology, with "an obvious tendency for successively higher and more recent fossil assemblages to resemble modern floras and faunas ever more closely" (Gingerich, 1985) and with animal groups appearing in a certain unmistakable order. For example, primitive fish appear first, amphibians later, then reptiles, then primitive mammals, then (for example) legged whales, then legless whales. This temporal- morphological correlation is very striking, and appears to point overwhelmingly toward an origin of all vertebrates from a common ancestor.
2. Numerous "chains of genera" that appear to link early, primitive genera with much more recent, radically different genera (e.g. reptile - mammal transition, hyenids, horses, elephants), and through which major morphological changes can be traced. Even for the spottiest gaps, there are a few isolated intermediates that show how two apparently very different groups could, in fact, be related to each other (ex. Archeopteryx, linking reptiles to birds).
3. Many known species-to-species transitions (primarily known for the relatively recent Cenozoic mammals), often crossing genus lines and occasionally family lines, and often resulting in substantial adaptive changes.
4. A large number of gaps. This is perhaps the aspect that is easiest to explain, since for stratigraphic reasons alone there must always be gaps. In fact, no current evolutionary model predicts or requires a complete fossil record, and no one expects that the fossil record will ever be even close to complete. Evolutionary biologists consider gaps as the inevitable result of chance fossilizations, chance discoveries, and immigration events.