by Ardea Skybreak
Revolutionary Worker #1170, October 13, 2002, posted at http://rwor.org
In earlier installments of this series we talked about how all life forms are the result of evolution, how evolution is still going on, and how the evidence of this is all around us. We also talked about what biologists today understand about the factors involved in the process of speciation --when, over the course of a number of generations, a new species splits or branches off from a different species which is its immediate ancestor. (See RWs #1163, Aug. 18, 2002 and #1164, Aug. 25, 2002.) It is through 3.5 billion years of such repeated series of speciation events that life evolved from primitive bacteria-like creatures into the great diversity of marine life and all the different species of amphibians, reptiles, birds, and mammals (including people) that live on land.
One of the reasons evolution is considered to be one of the most solid of all scientific theories is that the evidence comes from so many different directions and from so many different fields of science: molecular biologists point to evidence of evolution in the patterns of DNA, paleontologists point to evidence of evolution in the features and sequences of fossils, embryologists point to evidence of evolution in the patterns of development of embryos, and population geneticists and ecologists point to evidence of evolution in the characteristics, modes of interaction, and patterns of distributions of entire populations and communities of living organisms. Today there is so much documented evidence of evolution that you could literally read hundreds of scientific books and many thousands of scientific journal articles on the subject and you still wouldn't have finished reviewing all the evidence.
In this installment, we will briefly review some of the main categories of evidence which demonstrate conclusively that evolution is a fact.
The past leaves its marks on the present
In any process that has a history (whether in nature or society), the past leaves its marks on the present.So, for instance, you can study fossils of long-dead creatures and find in them features (bony parts, etc.) which belonged to even older ancestor species; or you can study living species and find in them features which clearly link them to each other, as well as to some pre-existing species from which they evolved.
The theory of evolution predicts that if the different plant and animal species did not, in fact, come out of "nothing," then we should be able to find within their bodies (as well as in their patterns of distribution across the planet) lots of concrete clues as to what it is they did come out of.
And that's exactly what we do find.
Direct evidence of evolution from both the fossil record and the molecular record
Earlier in this series we talked about how the fossil record is an important source of direct evidence about the past evolution of species. When fossils of a plant or animal lineage are arranged in the order of how old they are (as determined by a variety of scientific dating techniques), we can do point-by-point comparisons of their body structures and actually see many of the step-wise modifi- cations which have taken place between the "oldest" and "youngest" representatives of that line.
When a new species evolves, it can do so only on the basis of the inheritable variation which existed in populations of its immediate ancestors; when you examine any living species or fossil species of plants or animals, you will find some similarities which have carried over from its ancestors, as well as some differences which reflect the emergence of some new features which did not yet exist in the ancestor species. The similarities help you understand how different species are related ; the differences help you understand what makes each species unique .
Since life has already been evolving on this planet for some 3.5 billion years, there has been plenty enough time for all sorts of dramatic evolutionary modifications to take place in every single plant and animal line. So, for example, there are fossils of marine whales that are linked through a series of fossils of related species to a four-legged ancestor species that walked on land; and our own human species is connected to a whole series of past hominid ancestors--human-like species which walked upright like we do, but whose bodies still had much in common with those of tree-dwelling apes from which they had evolved. You can line up the fossils of the various upright-walking ancestor species which make up the human line according to how old they are , and you will literally see how the oldest ones were more ape-like and the more recent ones were more like modern humans. This can mean only one thing: humans are descended from previously existing, non-human, ape-like species. (We'll discuss human evolution more fully a little later in this series.)
Scientists have long been able to establish basic phylogenies (sequences of ancestors and descendants) for all plant and animal lines simply by comparing the morphological features (shapes of body parts) of living species and of fossils of different ages: It is in fact possible--as predicted by the theory of evolution--to reconstruct the basic tree of life simply by grouping species and lineages together according to the features of form and function that they have in common, and by separating them according to key features which they don't share. The more closely related species and lineages are to each other, the more features they have in common; and the more distantly related they are to each other, the more differences will have accumulated between them. It is from these kinds of comparisons that we can tell, for instance, that horses and zebras are much more closely related to each other than to wolves (they have many more features in common), and that, in turn, horses, zebras, and wolves are all more closely related to each other than they are to any birds. You can keep going like this, systematically grouping species into larger and larger groups, but always on the basis of features they really do have in common. In this way you will also find that, even though they are very different, horses, zebras and wolves all have at least some features in common with birds (for instance they are all warm-blooded, all have backbones. etc.); and that they all in turn share some of those features (like having a backbone) with some really old species of fish known from the fossil record (some lines of bony fishes which gave rise to the first creatures to crawl out onto the land). And, of course the horses, zebras, wolves, birds, bony fishes and all the other living and extinct vertebrates (animals having backbones) are, despite their many differences, more closely related to each other than to any of the soft-bodied invertebrate creatures which don't have backbones, such as sea sponges or clams, which represent a very different evolutionary pathway taken early in the history of life. As we will discuss more later, the very fact that all plant and animal species can be grouped into a succession of bigger and bigger groups solely based on their shared features (a pattern known as a "nested hierarchy") is itself proof that they are connected to each other by lines of ancestry and descent.*
And if all that weren't enough proof of ancestor-descendant relationships, now we have several new techniques from the field of molecular biology which even further corroborate (add more proof to) the evidence of evolution derived from comparing the anatomical and developmental features of living species, and from tracing the sequences of evolutionary modifications recorded in the fossil record. It turns out that the bodies of all living species on this planet contain many of the same biochemical molecules (such as DNA or blood proteins), which perform many of the same functions in those different bodies. In fact, one important piece of evidence that demonstrates that all organisms on this planet are descended from a series of common ancestors (and are related to each other to varying degrees) is that every single one of us (bacteria, pine trees, human beings, pigeons, etc., etc.) uses the very same system--the very same basic type of biochemical molecules (chains of nucleic acids known as DNA and RNA)--to store hereditary information (which serves as a blueprint for making different kinds of proteins in the body) and to transmit (pass on) this information from generation to generation.
Some of these biochemical molecules are extremely old and have hardly changed in millions of years.**
But biological molecules such as DNA, (as well as molecules such as blood proteins), do accumulate some changes over time. Some seem to change relatively faster than others. But it also seems to be the case that any given type of biochemical molecule tends to maintain a relatively steady overall rate of change when averaged out over long periods of time. This is what has made possible the modern technique of "molecular dating" which can help us to identify at roughly what point, in the past, two species still shared a common ancestor: the longer species have been separated from each other, the more differences will have had a chance to accumulate in their respective DNA and protein molecules; so by measuring the observable similarities and differences in one or more genes (DNA sequences) of any two species, and factoring in the average rate of change for that type of molecule, we can get a pretty good idea of how long it's been since these two species "split" and began to follow separate evolutionary paths.***
The new molecular dating techniques are always being improved but they have already been very helpful in fine-tuning evolutionary phylogenies ("family trees"): for instance, evolutionists have long-known from the fossil record and from the anatomical and behavioral evidence that raccoons, Red Pandas, Giant Pandas, and bears all share a common ancestor. A molecular analysis of the DNA of all these species has independently confirmed this, and has also supplied some additional information concerning the timing of the various evolutionary "splits." This has clarified, for instance, that, despite some similarities of form and behavior between so-called Red Pandas (or "lesser pandas") and Giant Pandas, Red Pandas are actually more closely related to raccoons than to Giant Pandas, which evolved as a later split from the bear line. This is just one of many examples where molecular biologists and evolutionary biologists have been able to work together to get a more complete and comprehensive picture of the evolutionary history of different plant and animal lines.
But again, even when it doesn't supply such additional precisions, the molecular evidence is invaluable because it provides important independent corroboration for the phylogenies (family trees) which scientists had already previously figured out on the basis of the fossil evidence and on the basis of the similarities and differences in form and function of living species.
The fact that there is such a tremendous "consilience of evidence" for evolution-- meaning that there are so many different types of evidence coming from so many different directions but all pointing to basically the same conclusions--is one of the main things I'd like readers to remember from this series. The great consilience of evidence from so many different sources is one of the reasons the vast majority of scientists consider the theory of evolution to be one of the most "robust" (rock solid) and most well-documented theories in the entire history of science.
ADDITIONAL CATEGORIES OF EVIDENCE
In addition to the direct evidence of evolution which can be found in both the fossil record and in the molecular record, there are many forms of indirect evidence of the evolution of species which can be inferred or drawn from the many features of living organisms (and of whole communities of living organisms) which just don't make any sense unless modern-day species are in fact evolutionary modifications of different pre-existing species. These features also wouldn't make any sense at all if a supernatural god or "intelligent designer" had consciously "de- signed" all the forms of life on this planet. Here are a few examples (drawing heavily on biologist Doug Futuyma's well-respected college textbook Evolutionary Biology,and his excellent book for general readership Science on Trial--The Case for Evolution , among other sources).
1) Evidence of the evolution of species from embryo development
Many species (including all the many vertebrate species, or animals having backbones, including people) produce eggs that develop into embryos containing features of past ancestors, even though these features are no longer of any use. For example, if you examine the embryos of reptiles, birds, and mammals (including human embryos) you will see that, early in their development, they all start off having not only tails , but also " gill slits " (also called "gill arches") just like the ones you find in fish embryos! In fish species, these gill slits eventually develop into the gills found on the sides of fish heads, which fish use to breathe in the water. The similar gill slits you can also clearly see in the early developmental stages of embryos of turtles, chickens, pigs, or humans end up disappearing a bit later in embryonic development, before turtles or chickens hatch or pigs or humans are born. But why were they there in the first place?And why do even human embryos start off with tails (which also eventually disappear in the course of development, except for that last little remaining piece we call the coccyx, or "tail-bone")? If a god or other "intelligent designer" had designed all the living creatures separately (as is said in the Bible), it wouldn't make any sense for us to temporarily have fish gills or tails! But it does makes perfect sense if--as we can plainly see from the fossil and molecular evidence--mammals evolved from a group of reptiles which at an earlier time had evolved from a group of fish. The embryos' gill slits and tails are just evolutionary remnants ("left-overs") from those earlier ancestors.
2) Evidence of the evolution of species from other vestigial ("remnant" or "left over") features
Even after birth, we can see that the individuals of many species often retain features that are not functional (of no use) or that in some cases are even maladaptive (worse than useless). For instance, in some species of plants which today have completely separate male and female flowers, there are still some small (and non-functional) remnants of female parts (pistils) on male flowers, and small (non-functional) remnants of male parts (stamens) on female flowers! Something this bizarre certainly wouldn't make any sense if a god had created living species in line with some kind of conscious and intelligent master plan, but it does make sense according to evolution: these non-functional remnants of male or female parts are just carry-overs from past ancestors which produced both male and female functional parts in one single type of flower (as many species still do today). Or how about the bodies of baleen whales? They still contain some very small, undeveloped and apparently completely useless pelvic bones (hip bones) which aren't even attached to other parts of their skeletons. These structures are irrelevant in streamlined bodies adapted to moving through water--they are just evolutionary left-overs from when the whales' ancestors lived on land and had legs connected to hip bones.
Or what about all the species of cave fishes and other cave-dwelling organisms that have eyes, even though they spend their entire lives in dark caves and can't see? Would a god design anything so nonsensical? The truth is that these cave organisms are descended from species which used to have functional eyes and which lived in more light-filled environments. Why do some insects have vestigial non-functional "wings" even though they don't fly? Simply because they are descended from ancestor species that had wings and could fly! And just look at people: we still have the remnants of a tail-bone; we have a backbone and abdominal muscles much like those of the four-legged mammals, but which leave us open to back pain and which barely hold in our vital organs (because they weren't "designed" to be carried around vertically!); and we have an appendix (a small remnant of a prior ancestor species' intestinal sack) which not only is of no use to us but which can sometimes kill us when it gets clogged up and infected! What kind of god or other "intelligent designer" would design organisms with such useless, imperfect, wasteful, and sometimes even harmful physical features?
The fact is that none of these vestigial structures (and there are many others) make any sense unless they are just evolutionary "carry-overs" from different pre-existing ancestors. It is very important to understand that the process of evolution itself is not some kind of "perfecting" mechanism: it never "starts from scratch," and it can't build perfect or "ideal" structures. It can only "work with what it's got," at each new generation: evolution can only build "new" structures out of the genetic variation which already exists in immediately preceding generations.
3) Evidence of evolution of species from homologous features
Nature is full of homologous features: parts of bodies that are made up of very similar structures even though they have somewhat different functions . For example, the hands of primates, the front legs of moles, the wings of birds, of bats and of flying dinosaurs, and the flippers of whales and penguins are all composed of the same basic bony parts (bones such as the humerus, the ulna, the radius and the carpal bones) even though the relative proportions of the parts are somewhat different and even though these parts are obviously used for somewhat different functions (such as grasping, digging, flying, swimming, etc.) Why are they made up of basically the same bony parts in the first place?A supernatural "intelligent designer" could certainly have designed completely unrelated and more specialized bones to make a "more perfect" hand, a more perfect wing, a more perfect flipper, etc.--yet none of the actually existing structures are "perfect" or ideal in terms of what is needed to accomplish their functions. But the imperfections in function and the similarities of structure found in all these parts do make sense if they are simply the result of evolutionary modifications of some pre-existing body parts which existed in earlier ancestor species. There really is no other logical way to explain this.
The same can be said about the fact that all the different life-forms on this planet (bacteria, plants, and animals, including people) make use of the same basic universal genetic code: all species use the very same types of nucleotides (the chemical components of DNA) to make the very same types of amino acids (chemicals used to build protein chains). Life could be organized on the basis of a different type of genetic code, but it isn't . And while protein chains could be built out of amino acids having either what's known as a "left-handed"' or "right-handed" chemical structure (basically chemical "mirror images" of each other) it turns out that all the proteins made by all the species on earth are always made up exclusively of "left-handed" amino acids. Once again, there is no absolute reason why life has to be organized this way: this universal pattern is something that makes sense only because all existing species evolved out of a series of common ancestors, going back to the very beginnings of life on this planet some 3.5 billion years ago. The very first forms of life evidently happened to use "left-handed" amino acids to make proteins, and all their many descendants have simply replicated the same pattern ever since.
4) Evidence of the evolution of species from Convergence
Convergent features are features that look similar and often perform similar functions, but which are made up of different underlying parts and are not derived from the same ancestral feature. For example, the eyes of vertebrates (animals having backbones) and the eyes of cephal- opods (a group of soft-bodied invertebrates, which includes squids and octopus) perform similar functions (seeing), but they came about through a different set of evolutionary modifications of some different underlying structures. Similarly, the fins of fish and the fins of whales might look to us somewhat alike, and they do perform some of the same functions (slicing through water) but they also are the result of the evolutionary modification of different anatomical structures which existed in their respective ancestors. Pandas can grasp and manipulate bamboo shoots with what looks (and functions) like a "thumb," but these so-called "thumbs" are not true finger bones--they turn out to be a modification of a wrist bone of one of their ancestor species. What all these kinds of examples show us is, first of all, that there is more than one way to evolve a particular functional capacity -- but also that the particular way it evolves (out of exactly what pre-existing structure) depends on what material happened to be available for modification in the population of immediately preceding ancestors.
Sometimes entire large communities of plants or animals show evidence of convergent evolution.
The great variety of life-forms on earth is testament to the fact that biological evolution is a very creative process and that natural selection is a very powerful mechanism for shaping change in all living creatures. A good illustration of this is the evolutionary convergence of entire communities of plants and animals which have evolved striking similarities of form and function, even though they live in completely different parts of the world and may not even be at all closely related. There are many examples of this in both plant and animal lines. For instance, certain cactus species living in North American deserts (such as the Organ Pipe cactus) look so much like some of the species in the Euphorb family of plants in Southern Africa that you could hardly tell them apart. And yet these two plant families are not at all closely related to each other--they represent two different splits from earlier lines of plant ancestors, and their current striking similarities of form and function evolved later (and independently ), simply as a result of natural selection producing similar adaptations in relation to similar environments (in this case, dry deserts). A similar example would be the four unrelated families of birds which independently evolved beak adaptations which allow them to suck nectar out of flowers: the hummingbirds of North and South America; the honeycreepers of Hawaii; the sunbirds of Africa; and the honey-eaters of Australia. They represent different evolutionary lines among the birds, but, over a very long period of evolutionary time, all four lines independently evolved extremely similar adaptations in relation to similar ecological opportunities.
The most famous example of biological convergence is that of the placental and marsupial mammals: Marsupials are the many mammals found mainly in Australia (like kangaroos or Tasmanian wolves) that give birth to immature young that have to finish developing inside a pouch on the outside of a mother's body. This makes them very different from the more common placental mammals (including people) found all around the world, which develop their young inside a uterus nourished by a placenta, and which don't have such an outside pouch. Marsupials diversified in Australia for millions of years before any placental mammals ever got to that continent (even today the only placental mammals in Australia are all relatively recent "imports," brought in by people). But what is really interesting about all this is that, in spite of millions of years of separate evolutionary tracks, many Australian marsupials have nearly identical counterparts among the placental mammals in other parts of the world -- counterparts which look like them, behave like them, obtain food in similar ways etc., etc. So, for instance, the marsupial Tasmanian wolf is a counterpart of the placental wolf, there is a marsupial "mouse" which is similar to the placental mouse, a marsupial mole and a placental mole, a marsupial flying squirrel and a placental flying phalanger, a marsupial ant-eater and a placental ant-eater, a marsupial "cat" and placental cats, etc.
How can this be explained? It seems that, at the time Australia broke off from other continents (more than 50 million years ago), it did not yet have any placental mammals, which were already evolving in other parts of the world. So marsupial mammals continued evolving separately--and in isolation from placental mammals--for tens of millions of years. Their different species ended up occupying many habitats and ecological "niches" which were instead occupied by placental mammals in other parts of the world. The fact that so many marsupials today are remarkably similar in appearance and behavior to their placental "counterparts" on distant continents is a reflection of what can happen when natural selection operates independently for very long periods of time on two different evolutionary branches, producing similar step-wise evolutionary modifications (adaptations) in variable populations of organisms that happened to encounter some similar environmental conditions. None of this would make sense if an "intelligent designer" had created all species all at once and as completely "separate kinds." What could reasonably explain why one sub-set of mammals, existing in one isolated part of the world (Australia), would end up having such strikingly similar counterparts among a completely different line of mammals (with a completely different form of reproduction), existing for millions of years in completely different parts of the world? As we have shown, biological evolution (combined with continental drift) is the only reasonable explanation for this phenomenon.
5) Evidence of evolution of species from "sub-optimal design": nature's quirks and imperfections
Once again, evolution is not a "perfecting mechanism" of any sort. This is very important to understand. Many people make the mistake of thinking that evolution means that all the species of plants and animals are always evolving in just one single direction, and that they are always becoming "better and better adapted" to their environments. This is not the case. Evolution is not a single, straight-line, forward march of "Progress" with a capital P. Yes, it is true that many species exhibit wonderfully tight adaptations--a sort of evolutionary fine-tuning which has led to a particularly "close fit" between some of their features and some aspects of their local environments (think of the evolution of "camouflage" features which allow some species to blend in with their background environments and thus better escape predators, to cite just one example). But it is not the case that all features of organisms are equally well "adapted" to their environments. And furthermore, local environments--which consist not only of physical features such as temperature and humidity, but also all the living species (including competitors and predators) that share the same local environment--are themselves always changing . So there tends to be a continual back and forth dynamic between organisms and their environment, and what might represent an "adaptation" in one particular environmental context can very well turn out not to be adaptive at all if the environment changes in significant ways.
Remember that evolution by natural selection simply means that any inheritable feature which happens to give an individual in a population a "reproductive edge" will automatically tend to get passed on and spread to more and more individuals over successive generations; but this will happen even if that "reproductively advantageous" feature also has a downside--is "maladaptive"--in some other respects. For instance, in quite a few species of animals, males have certain almost absurdly exaggerated physical features, such as the incredibly long and brightly colored tail feathers of male peacocks or the often gigantic "racks" of antlers on mature males in the deer, elk and moose family. It has been shown (including experimentally) that females often seem to preferentially mate with the "showier" males. This fact alone--a variant of natural selection known as sexual selection--would be sufficient to cause a disproportionate spread of the "showier" male features from generation to generation (since males having these features would, on average, get to produce more offspring) just as long as the showier features are truly inheritable (as is the case with the peacock's tail feathers). In other words, if the showier males simply get to mate more often because they are somehow more "attractive" to females, then it makes sense that those showier male features will spread from generation to generation and may even become increasingly "exaggerated" over time. This is in line with basic natural selection. But it would be difficult to argue that this evolutionary trend has led to peacocks being "better adapted" to their environment in any kind of more general or overall sense: for instance, the big flashy tails of male peacocks don't help them find food, or withstand temperature extremes, and they may even make it more difficult for them to get off the ground to escape predators. In short, there can often be "trade- offs" between the advantages and disadvantages of different features as populations evolve. And while there are many evolutionary changes which do represent adaptations to local environments, not all evolutionary trends and directions of change are adaptations.
The fact that many features of organisms are not "perfectly designed" for their particular functions makes perfect sense if they are the product of successive rounds of biological evolution and were never "designed" by any outside conscious force in the first place. But it certainly wouldn't make any sense for some all-knowing and all-powerful god to consciously design life-forms to incorporate what, from a "design" standpoint, would have to be considered flaws and imperfections. For instance, human beings are prone to back and hip pain and a number of other related ailments because the alignment of our skeletons is not "perfectly designed" for upright- walking. Which do you think is more likely: that some all-knowing and all-powerful god did downright shoddy work, or that our skeletons are "imperfect" simply because they were "built" (by well-known and observable evolutionary mechanisms such as natural selection) out of the skeletons of our ancestor species, going back to the ones that walked on all fours?
Human beings also have a dangerous tendency to choke on food: this has something to do with the fact that, in the back of our throats, the passage that air follows to get to the lungs actually crosses the path that food follows to get to the stomach. This would be an example of a really stupid (or perversely sadistic) design if a god had actually designed it that way! But this is not the result of anyone's conscious "design." This choking problem simply reflects our own past evolutionary history: the breathing channels of all land vertebrates also evolved in the distant past as modifications of pre-existing structures (in this case the "air bladders" of some bony fishes and lungfishes) which evolved into primitive lungs. This evolutionary "innovation" allowed the first land vertebrates to breathe out of water and to spread into many new habitats. But as much as it created all sorts of new opportunities and advantages, allowing new species to spread and diversify across the land, this evolutionary development came "packaged" with this little problem of air and food passages "crossing." And this problem seems to have been even further aggravated in human beings when the evolution of fully upright walking was accompanied by further changes in the relative positioning of the head and throat.
And, again, human beings weren't even "optimally designed" for upright walking, one of our most distinctive features. As much of an advantage as fully upright walking may have been for our species, it also created new stresses on the bones and muscles of the body. This is because we did not "come out of nothing," and our upright bodies could only evolve on the basis of what came before us--the preexisting bodies of our non -upright-walking ancestors. So, like every other species on earth, human beings have many body features which no doubt could have been much improved if any outside "intelligent designer" had been involved in creating us--unless that "designer" had a truly sick and twisted mind! Again, which do you think is more likely--that god designed people with throats that make it easy for them to choke and with chronic tendencies to back pain, or that the choke-prone configuration of human throats and the stresses on the back and knee joints, etc., is simply the not-so-perfect result of a series of body-plan realignments which took place when some of our not-fully-upright ape ancestors evolved into fully upright hominids (human ancestors)?
Concrete evidence of evolution can be found in all of nature's many odd quirks and imperfections . And it turns out that the specific ways in which evolution is not perfect can actually provide a lot of information about the actual sequence of previous evolutionary steps that went into any new evolutionary development. For instance, as mentioned earlier, if you line up the fossils of a dozen or so hominid species by age from oldest to most recent (spanning a few million years), you can see that the first of the upright-walking species of hominids did not immediately have all the features that we think of as fully human: what you see is that those first upright-walking species still had, among other things, very ape-like small skulls and brains and ape-like long arms and short legs, even though they were already very different from their ape ancestors in being able to stand and walk upright; and you also see that some of the later hominids, which were evolutionary derivations of those first hominids, eventually included species which had body shapes and proportions and brain sizes much more like those of modern humans. But all this didn't happen all at once, it happened over millions of years.
The evidence is clear: the evolution of life involves "imperfect" step-wise modifications of whatever pre-existing (and historically limited and constrained) living raw material happens to be at hand at a given time--not the handiwork of an infinitely wise and powerful deity.
6) Evidence from the patterns of geographic distribution of species around the earth
The patterns of distribution of many groups of plants and animals around the world make no sense unless they are descended from common ancestors. For example, the more ancient of the land- dwelling lineages--such as amphibians and reptiles--today are represented by relatively similar descendants (similar frogs, snakes, etc.) on all the different continents. This is not surprising, because the fossil record indicates that groups like amphibians and reptiles evolved and spread out to all corners of the world long before the continents split apart through continental drift. On the other hand, groups like the mammals, which we know evolved quite a bit later (they first appear in the fossil record close to the time when the continents started to break apart), ended up following much more distinct (separate and diverse) evolutionary paths on the different continents (again, the Australian marsupials are very different from placental mammals on other continents; and many African "Old World" mammals, including primates, are very different from "New World" mammals in the Americas). The science of evolution can explain such patterns on the basis of how long ago different lines split from each other and how much time has passed since they started evolving independently. But the biblical story of Creation cannot explain this.
Nor can the biblical story of Creation explain, for instance, why so many species that live on islands are much more similar to a species that lives on a nearby mainland than to species from much more far-flung corners of the globe. Many island species of birds, for instance, differ quite a bit from each other in some of their particular evolutionary adaptations, while still maintaining a lot of features in common with a bird species living on the nearby mainland. This makes perfect sense if island species are in fact modified descendants of individuals of a mainland stock that happened to migrate to the island at some point in the past, and then diversified (through repeated speciations in relation to a variety of environmental components) into a greater number of descendant species. But if, as portrayed in the Bible, a god had created all the different species of birds all around the world at the very same time and as completely unrelated, unchanging and separate "kinds," we shouldn't be finding such obvious evidence of relationship (shared features) between island species and nearby mainland species.
7) Evidence of evolution of species from the fact that the features of living organisms fit into a system of "nested" hierarchical classification
This sounds complicated but it really isn't. As previously mentioned, a "nested hierarchy" simply refers to the pattern of "group within group" classification (sort of like those Russian dolls that fit one inside the other) that all living species naturally fit into whenever you draw up phylogenies ("family trees") merely on the basis of a succession of shared characteristics. It turns out that only things that are genuinely related to each other by lines of historical descent (in which ancestors pass on some features to descendants, from one generation to the next) will consistently fit into a pattern of "nested hierarchy." If, on the other hand, you try to force things that are not genuinely connected by lines of actual descent and inheritance into such a pattern of nested hierarchy (things that have not actually evolved one out of the other over generations on the basis of some form of inheritable variation), you simply won't be able to do it: unrelated objects just can't be made to fit into such a consistent "group within group" pattern. For instance, you can make a simple list of the names and features of all the known chemical elements, or you can make a simple list of all the known minerals and their features. But what you can't do is connect the basic chemical elements to each other in any kind of "group within group" hierarchy on the basis of shared features, and you can't do that with the different kinds of mineral elements either. Why not? Because chemical elements didn't evolve out of pre-existing chemical elements, and minerals didn't evolve out of pre-existing minerals. There is therefore no way to establish family ties which could successfully group these objects into a nested hierarchy of bigger and bigger groups simply on the basis of sets of features they have in common.
Biological species, however, do fit a pattern of nested hierarchy, and even non-living things fit that pattern as long as they too really are related by consistent lines of descent. For instance, all the many thousands of human languages can be classified into a "group within group" hierarchy of "ancestor" and "descendant" languages, because each one came into being as a result of a process of "descent with modification" from a series of pre-existing ancestor languages, from which they "inherited" many features of vocabulary and syntax (rules of grammar, etc.). French, Spanish, and Italian, for example, are all closely related languages, but each represents a slightly different modification of a single older "ancestor" language (Latin). The simple fact that all current human languages (unlike minerals or chemical elements) fit into a nested hierarchy classification proves that they did not develop separately but rather "one out of the other." Similarly, the fact that biological species also fit such a pattern is just one more clear bit of evidence that life-forms really didn't appear all at once as separate unrelated "kinds" (as the Bible says) and that they have indeed evolved one out of the other, though a series of shared ancestors.
[See Box: "A Conscious 'Intelligent Designer' Wouldn't Design This Way!"]
I literally cannot think of any other scientific theory, in any field of science, that is as well supported by demonstrated fact and by so many different lines of mutually reinforcing evidence as the theory of biological evolution. How much more proof does anyone really need? Many scientists are getting pretty frustrated and angry that a bunch of fundamentalist Christian Creationists, bent on promoting know-nothingism in the service of a reactionary political agenda, are actually managing to get in the way of people getting a genuine science education and learning just how strong the evidence of evolution really is. These people will do anything in their power--distort the truth, spread outright lies, and even threaten and intimidate--to try to force people to accept and submit to a literal interpretation of the biblical story of Creation, even though all the scientific evidence which has accumulated over the past century and a half shows this doesn't hold water.
Paleontologists can show you all sorts of fossils, from all over the world, which clearly demonstrate sequences of evolutionary modifications and many of the successive "splitting" and branching events which characterize the different plant and animal evolutionary lines. Comparative anatomists and developmental biologists can also show you very concrete proof that all plants and animals are made up of parts that have retained significant features of species which preceded them in time, combined with new parts that are obviously the result of a modification of a corresponding part in an older species. Molecular biologists in just the last few decades have compared the DNA and other molecules of all sorts of species and have figured out the degree of relatedness of different species and at what times in the past different splits happened in their family trees--and while many molecular biologists don't know much about fossils or comparative anatomy (just as many paleontologists and anatomists don't know much about molecular biology) it is a fact that scientists in these very different fields (and in many other fields of science) have come up with the very same basic conclusions about the basic evolutionary tree of life which connects all living and extinct species.
Every working biologist can rattle off tons of examples of the features of organisms and communities of organisms (vestigial features, homologies, examples of convergent evolution, etc.) which can be explained quite simply through evolution but which cannot be rationally explained in any other way. And, as we discussed in earlier installments of this series, evolutionary biologists, population geneticists, and community ecologists have actually verified many of the predictions of the theory of evolution experimentally (both in the lab and in natural populations), conducting many thousands of studies and experiments which show evolution in action in all sorts of plant and animal lines and which have helped to decipher many of the mechanisms involved in evolutionary change.
None of these scientists, in any of those fields of science, have ever turned up a single piece of concrete evidence which disproves (or in any way fundamentally contradicts) the basic facts of evolution. So again, I ask you: how much more proof does anyone really need?
Next In This Series: The Evolution of Human Beings
* As we discussed earlier in this series, there are now various modern scientific techniques which allow us to date different fossils (determine their relative age). So scientists have been able to use these dating techniques to further verify and confirm the basic phylogenies (the basic sequences of ancestor and descendant species and lineages which make up the tree of life) that had already been established simply on the basis of comparing the similarities and differences in the anatomy and development of different species and larger groupings of plants and animals.
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** For instance, the basic structure of some genes, such as some of those involved in the regulation of aspects of growth and development of body parts, have apparently remained so constant over millions of years that they can be found, in essentially equivalent form, in organisms as different (and as evolutionarily distant) as bacteria and fruit flies! Similarly, the chemical structure of the hemoglobin protein in the blood of human beings and chimpanzees is almost exactly the same, even though the human line and the chimpanzee line split from a shared ancestor some 5 million years ago and have been evolving separately ever since.
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*** And, to take into account the fact that any one type of biochemical molecule might not always have changed at such a constant rate, results can be further cross-checked by repeating these calculations, using more than one type of biochemical molecule.
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