Our aim on this page is not to provide a comprehensive survey of all that is inherent and implied in the term "Evolution" but mainly to provide an indication of how it is evidenced on Earth in the records of the past - namely, in the rocks that make up geologic history. To do this, we will examine the evolutionary changes associated with one faunal type - animals that fly. When the writer embarked on preparing this page, this task seemed fairly straightforward. But as material was downloaded off the Internet, the complexity of the topic became increasingly apparent and the writer remains unsure of parts of this page. There is still much controversy over the details and the conclusions among the scientists studying the fossil records.
The vast majority of scientists, as well as a substantial fraction of the general public, have now accepted the basic principles of Darwinian evolution (and most subsequent refinements) as valid and verified (by the so-called "scientific method" of seeking truth). The position adopted by the writer (NMS) is to likewise accept these concepts as a given. No attempt is made to reconcile Evolution with its antithesis, Creationism, or to any of the models which are offshoots of the "Young Earth" hypothesis.
To get a quick overview of Evolution, we strongly suggest you read through this Wikipedia website that provides an overview of Evolution, as used in modern science. Our treatment of Evolution will be very limited on this page, to just a few ideas, so to gain a fuller understanding this Wikipedia website is strongly recommended. There are approximately 160 million citations on Google when "Evolution" is the search word. One of the best sources of information and links is supplied by the University of California-Berkeley website on Evolution.
We will define "Evolution" by directly quoting the first sentence in the second Wikipedia entry: In biology, evolution is change in the genetic material of a population of organisms from one generation to the next (my words added: "in response to variations and shifts in environmental [adaptive] conditions"). The key words in this definition are italicized.
To the layman, the word "Evolution" is closely associated with one man, the Englishman Charles Darwin, who lived in the 19th Century. Nowadays, he is usually pictured as he appeared near the end of his life, complete with a scholarly white beard:
It behooves you to learn more about him by clicking on this Wikipedia website.
Darwin began to accrue the ideas underlying Evolution during what many believe to be the most famous scientific sea voyage in history, the sailing into the Pacific of the H.M.S. Beagle, a research vessel, from 1831 to 1836 with Darwin onboard as the ship's naturalist. In particular, the stopover at the Galapagos Islands west of Ecuador was to prove decisive in the eventual formulation of Evolution. Here he observed notable variations in the characteristics or traits in population among various animals from island to island. This led to the idea of adaptive radiation, best exemplified by interrelated finch (a bird) species. The most significant variable was the bird's beak, which was modified to optimize its use in the specific environment of a particular island. In time, Darwin realized that there were evolutionary lineages among these species. Here are two diagrams that speak to the changes involved:
Darwin began to devise his theory of Evolution during the voyage but, although he gave papers on the subject over the next 25 years, he did not publish the basic tenets of Natural Selection until 1859 (in part driven by the realization that a colleague, Alfred Russel Wallace, had reached similar conclusions). On November 24, 1859 (just over 150 years ago), the book that resulted, "The Origin of Species", became the most famous publication in the 19th Century. Its influence on mankind's - and especially scientists - thinking and philosophy cannot be underestimated. The Harvard biologist Ernst Mayr, possibly the greatest evolutionary theorist since Darwin, has stated: "It would be difficult to refute the claim that the Darwinian revolution was the greatest of all intellectual revolutions in the history of mankind." The Harvard paleontologist and historian of science Stephen Jay Gould called the theory of evolution one of the half dozen most important ideas in the entire history of Western thought. Below is a first edition copy (in green cover) of the book itself (this one sold for $110,000 to a collector) and a later edition needed because the book actually sold well in the 19th century.
The key concept espoused by Darwin is summed up in the phrase "survival of the fittest". Behind this deceptively simple phrase is this fundamental concept: A group of animals (or plants) represented by a species will have normal, natural variabilities in their physical properties, e.g, such diverse entities as hair color, lung capacity, ability to withstand cold. Some of these properties can be essential to each individual insofar as surviving any stress in the environment. As an example, choose the metabolic response to excessive heat. Some of the animals are less able than others to tolerate such heat. Many of these die during a hot spell. In so doing, they may have been unable or lacked the opportunity to mate and thus prolong their strain with offspring. They are "less fit" by virtue of this property (trait) weakness. Therefore, statistically they are less efficient in propagating the species, and are more likely to be eradicated, with this unfavorable property not being passed on to potential progeny. The survivors are those with better adapted critical properties (other situations can stress other properties as well, such as the ability to process available food, which some can do better than others). The overall effect is to improve the characteristics (properties) for those that do survive when the environment changes.
Natural Selection isn't the whole story. Biologists now know that chance mutuations of genes in chromosomes play a role too. This must occur within a population that is undergoing change over time. The effects are gradual, for many changing species a matter of thousands and even millions of years. Although the mutation of one species can be rapid (in terms of geologic time), as has been proved by scientific studies in the laboratory, it seems in many instances to be comparatively slow. Thus, when the history of a group of related animals is followed in the geologic record, new species often give the impression of appearing "suddenly", and many species disappear abruptly. This is in part illusory, owing to the fact that the record is commonly discontinuous, and conditions for the preservation of a species as a fossil are frequently not favorable. This gives rise to the "missing link" phenomenon - the gradual transition involved as one species evolves into another(s) is simply not preserved.
Thus, in Darwin's day, the evidence for evolution was largely macroscopic - that is, from observations of genera and species that occur in different strata of the sedimentary geologic record. But the change from one species into a new one is inferential rather than directly evident from a continuum. This raises the question: at what point in time is a new species said to first come into existence? How (and by what criteria) can this new species be fixed as really new, i.e., when in the transitional stage does one specify that a new species has emerged? (Stephen Gould's Punctuated Equilibrium model suggests that the changes can occur over relatively short time periods in which conditions, such as in local or regional environments, can undergo large changes that affect the resident populations forcing significant adaptations. Evolution proceeds therefore in "spurts".)
Darwinian evolution has been aided to some extent by the ideas put forth by the monk Gregory Mendel's hypothesis on heredity, as reviewed in this Wikipedia website. Mendel's ideas were based largely on observations. But in the 20th, and the present, Century genetics has placed the ideas of inheritance, and more broadly, those forming the foundation of Evolution on a firm molecular basis. Simply summarized, the variations in the genetic makeup of organisms, leading to the recognition of DNA and RNA as the prime factors in life functionality and reproduction (see the previous two pages), which have been determined from microbiology, comprise the essence of the mechanisms for Evolution as the process that has determined the diversity and comlexity of life on Earth.
The "proof" of Evolution was, prior to the DNA studies, usually cited as found in the fossil records. One of the best lines of evidence has been the pattern of changes observed from the early Cenozoic to the Present for the horse, a mammal whose bones have been preserved in sedimentary continental rock deposits. The record is abundant. This diagram summarizes the main representatives of horse evolution from the Eocene to modern times:
The horse, an animal with a vertebrate, is part of the phylum Chordata. Chordates can be traced back through the Ordovician, but their origin from invertebrates remains uncertain. One hypothesis treats them as evolving from the Echinoderms, but this "leap back" is controversial. Here is one family tree for the Chordata, which begins with the Echinoderms:
Once the general outlines of Evolution had become mainstream in scientific thought, various schemes relating the different Kingdoms and Phylla were developed (a large family tree type diagram was shown midway down page 20-12. Attempts have been made to simplify the classification by producing a broad phylogeny. Here is one version, similar to one that appeared earlier on page 20-12, which places life into three broad groups (ignore the individual genus names, given as examples), but is moot about a common ancestry:
The above paragraphs just skim through a few of the intrinsic concepts that underlie Evolution. Once again, we urge you to expand your knowledge, if need be, of this great discovery by Darwin (and Wallace) by reading through the Wikipedia entry on this subject. For the remainder of the page, we choose to dwell upon a very illuminating example of Evolution in practice by looking at the (evolutionary) history of animals that have developed the ability to fly.
Flight is one of the properties ascribed to many different "things". Birds, bats, tree seeds, and airplanes are all said to "fly". But they are quite different entities. This ability to perform the same function in grossly different entities is described by the term "analogous". But when the property responsible for some form of locomotion has a common origin or ancestry, the term "homologous" applies. The skeletal limbs of five common animals shown below are all homologous. In the diagram, three bones are shared in common: a)Humerus; b)Radius; c) Ulna.
Another example of a homologous appendage on dissimilar animals is the "wing". In this diagram, the commonality of bone structure in a reptile (pterodactyl), a bird, and two mammals - a bat and a man (people fly with hang gliders, and then there is Icarus in Greek mythology) - is evident.
The gross structures of a modern bird (here, a chicken), and ancient bird (Archaeopterix) and an ornithiscian dinosaur show broad similarities (are homologous):
The dinosaurs are a famed and diverse group of animals that include the progenitors of the birds and a related group of flying reptiles, the Pterosaurs, that we will consider as evolutionary representatives of flying creatures. Before taking up flight as such, it will be instructive to examine the dinosaurs and related animals that dominated the Mesozoic Era (250 to 65 million years ago). The first time the word "dinosaur" (from Greek words that mean "terrible" "lizard") was used dates to 1842, as introduced by Sir Richard Owen of England. At that time he published his studies of bones found by three separate groups. The first was Sir William Buckland who had described the bones of Megalosaurus (named later) found in an English quarry between 1815 and 1824 (part of a Megalosaurus femur was found in 1676 but was not recognized then as a dinosaur fragment) after being advised by the visiting French scientist Georges Cuvier that it was a giant "something". Here is the "type fossil" of Megalosaurus and a reconstruction of the animal's appearance:
Other bones accessible to Owen had been discovered by Gideon and Mary Ann Mantell near Lewes, England. These were eventually named Iguanadon. Here is one of their finds and a reconstruction:
Interest in dinosaurs and other similar life forms increased rapidly in the 19th Century. That interest has burgeoned ever since (witness the popularity of "Jurassic Park"). The Class Reptilia has proven to be quite diversified and is intimately tied to "flying animals". The reptiles are part of the Phylum Chordata which traces back to the Ordovician. Reptiles emerged from the Amphibians which first appeared in the Devonian. The dinosaurs dominated the land in the Mesozoic. As mentioned in Section 18, the dinosaurs disappeared abruptly at the close of the Cretaceous; their vanishing was probably due to a combination of events - climate change caused by volcanism in India and elsewhere, evolutionary "old age" as they grew too large to adapt, and the 'coup de grace', a major impact in Mexico and perhaps at other locations.
Although the script is not readable in this Internet version, this chart hints at the diversity of the Chordata (the Vertebrates).
These two diagrams concentrate on the classification of dinosaurs.
The dinosaurs are split into two main orders: the Ornithiscians and the Saurischians. Oddly, the Ornithiscians do NOT include the birds (studied by ornithologists); the flying members split from this group are the Pterosaurs. The birds have been evolved from small dinosaurs in the Saurischian order. The two orders are separated largely on the basis of hip structures:
This generalized diagram helps to distinguish the main groupings of Mesozoic reptiles:
The linear branches in this diagram are an example of a cladogram. The field of cladistics refers to the "genealogy" of various interrelated organisms, showing the sequence of ancestors and descendants. Cladograms have been prepared by Dr. Thomas Holtz of the Geology Dept. of the University of Maryland for use in his courses on the dinosaurs and other evolutionary animals. Visit his website at the University of Maryland. We have downloaded six of his cladograms that describe, in sequence, the details of the evolution of animals that include pterosaurs and birds, to be discussed later on this page. They are detailed; just try to get the overall picture:
The Amniota refer to a wide diversity of animals that all developed to live on the land (but also in the sea and the air). They include the mammals, the birds, and the reptiles. In common, they utilize the amniotic egg as a means of sexual reproduction.
The Archiosaurs are the late Permian to early Triassic progenitors to the dinosaurs, the pterosaurs, the crocodiles, and the birds.
The next four cladograms proceed through the general dinosaurs (note how the Ornithischia and Saurischia are split), the Theropods, and then the Coelurosaurids.
Finally, we reach the cladogram that includes the birds as well as the Velociraptors.:
Note that the Pterosaurs are include early on in the Archosauria cladogram but the birds (Aves) do not appear until the final Eumaniraptora cladogram
There are thus two broad, and rather distantly related, groupings of winged reptiles. They possess winglike membranes in common but do not possess feathers in common. The Pterosaurs were probably without feathers.Some dinosaurs are now known to have developed feathers but were not flight capable; Deinonychus, a velociraptor that lived about 115 million years ago, is an example:
Dinosaur feathers may be more common than once suspected. The first (oldest) dinosaur whose fossil remains show faint feathers is Sinosauropteryx. A later genus is Caudipteryx:
Another may be Anchiornus, who flight status is still conjectural.
By now you may have perceived a major distinction between the two groups of reptilian descendants that lived in the Mesozoic. The Pterosaurs are quite different from the Aves or primordial birds. Pterosaurs first evolved in the Triassic; birds first appeared in the Jurassic. We shall hereafter treat the evolutionary history of these two groups separately.
The Pterosaurs are an Order in the subclass Archosauria, part of the Class Reptilia. They have been further subdivided into the suborders Rhamphorhynoidea and Pterodactyloidea. The "Rhamphs" appeared first, in the late Triassic and died out at the end of the Jurassic. They are generally small and have long tails. The "Dacts" emerged in the Jurassic and died out along with the dinosaurs (the Pterosaurs are NOT themselves dinosaurs). Their size can range from small to very large; they have short tails. All Pterosaurs have membraneous wings but do not seem to have had feathers. They apparently glided rather than flapped and took off from perchs.
One of the first Rhamphs is Dimorphodon, which lived 213-203 m.y. ago.
Campylognathoides spanned an age from 200 to 186 m.y. ago; its wing span was less than 2 meters.
Rhamphorhynchus is the type example of the "Rhamphs". It had an average wing span of 1.75 m.
A recent entry into the evolution of the Pterosaurs is Darwinopteris. Twenty individual fossil remains have been found in China. Darwinopteris is considered to be transitional (it lived 160 million years ago) between the "Rhamphs" and the "Dacts". It has the tail of the former but the head of the latter.
The best known of the "Dacts" is Pterodactylus. Pterodactylus kochi is cited as the prototype, having been discovered in the famed fossil locality of the Solenhofen quarry of Bavaria in Germany. It is contemporaneous with the most famous of early birds, Archaeopteryx; both are 150 million years old.
Some "Dact" skeletons seem to have two sets of limbs, one functioning as legs, the other as claws. Dsungaripterus weii is sometimes depicted as having two sets of legs, to which membranes are attached to one. Consider these two reconstructions.
The next four artists' reconstructions show various Pterodactyls, given by their genus names. In the caption of each is the wing span and the age during which the pterosaur lived.Genus Tapejara:
Starting in the Jurassic, as we know for sure (but probably earlier), the first true birds appeared. These had feathers, well-developed wings beyond the membrane type, and gradually developed the ability to do more than just glide from tree to tree (attaining sustained free flight). The truly true first bird as such has not been identified as yet with any certainty. Various candidates have been proposed. One is Microraptor of the family Dromaeosauridae. Here is its skeleton and (a rather fanciful) reconstruction of it in flight.
Another candidate is Sinosauropteryx, shown earlier on this page. But, for the time being, paleontologists generally agree that the oldest bird whose identity as such is known for certain is the famous Archaeopterix, the first skeleton of which was found in the Solenhofen quarry in the mid 1800s. Its age is 150 million years. Here is the type skeleton and a reconstruction:
Here are some pros and cons concerning its nature as a true bird.
Since its discovery a number of younger Mesozoic bird remains have been found.
For the birds, the development of free flying, as is customary today, was gradual. This is one proposed scheme:
Various cladograms describing avian evolution in the Mesozoic into the Cenozoic have been put forth. Here is one example:
Two of the best known Mesozoic birds are Confuciusornis and Hesperonithiformes:
Birds proliferated and diversified throughout the Cenozoic, until today more than 60 billion individuals in about 15000 species exist. The Scissor-tailed Flycatcher, a common sight in Oklahoma, is a modern example that shows some similarities with the early birds.
The birds are considered by many evolutionists to be the modern survivors of the dinosaur lineage. The dinosaurs themselves are, of course, now extinct. Extinction is one of the hallmarks of the evolutionary record. Many species, families, orders, etc. disappear from the geologic record. Several times in the distant past there have been mass extinctions that kill off significant percentages of all life forms (plants and animals, including those that inhabit the oceans). The next two diagrams show this evolutionary history.
Before we close this page, we will just mention the Class Mammalia, which includes humans. The mammals first evolved at the end of the Paleozoic, thrived in the background in the Mesozoic, and emerged as the dominant animal life forms after the extinction of the dinosaurs. Their evolutionary history is too vast and complicated to warrant even cursory discussion on this page. A good overview is afforded by this Wikipedia" web site. You can gain an insight in the broadest terms into the cladistic history of the Mammals by looking at the diagram in that site listed under the heading of "The Ancestry of Mammals". Suffice to say here that progenitors of the Mammals extend back into the Late Carboniferous and true Mammals first appeared in the Triassic.
We will end the page with a summary diagram taken from the Internet. It is an attempt to display the phylogeny of the vertebrates, starting with amphibians, but without showing details of any cross-links between groups:
So, after perusing this page, where do YOU stand on this most controversial of science subjects? If you are an American, the chances are good that you reject the basic tenets of Evolution. The acceptance/rejection ratio varies with country; for example, most Europeans accept Evolution. Check this oft-cited survey, shown in bar-graphical format:
Evolution remains one of the most challenged and debated ideas ever put forth by intellectuals. Four groups of people in the United States (and to a similar extent everywhere else) have emerged in the 21st century with decided views about evolution; At one extreme 1) the Creationists; at the extreme opposite 2) the Scientists; a middle ground 3) the Intelligent Designers, and 4) a large segment of the population that just don't know enough to have formed a defensible position, and probably don't care to know.
There are those who favor much of the knowledge embodied in the meaning behind "Evolution" but who admit that the concepts involved have some problems and objections. One is, simply put, how life can form from inorganic chemicals. Another is the ultimate ancestor idea: do all the branches of life indeed work back to a few simple forms? A third dispute is the "missing link" argument: While many animals, and to a lesser extent, plants, seem to have some continuity in the fossil record but important transitions are often lacking. A fourth objection is how to explain the "sudden" (geologically short time) appearance of advanced forms in the Cambrian explosion of life. There are others, as you are encouraged to seek out on your own through the Internet.
But Evolution in its general form seems to have become accepted - with modifications, clarifications, and addition - by the scientific community. No doubt, both in the foreseeable future, and beyond centuries hence, its concepts and tenets will be refined. But to overthrow it would lead to the collapse of the scientific systems that themselves have evolved from Evolution.