Standing in front of a small tank of mudskippers in the special "Water" exhibition at the American Museum of Natural History, I heard a gentleman next to me comment to his friend "You know, if evolution is true, it's really amazing how many different kinds of animal there are." I have to admit that the first thought to pop into my mind was "If?" but after my twinge of arrogance passed I had to agree; it really is fantastic that evolution has produced such diverse forms of life.
Present diversity is only half of the evolutionary equation, though. Without an understanding of common descent we have to push and pull to get many animals to fit into inaccurate artificial systems, the "aberrant" always creating problems. In fact, prior to the evolution idea coming to fruition with Darwin & Wallace, various systematic schemes took the "grade" view and tried to discover the "natural order" among extant organisms. The Great Chain of Being is perhaps the most (in)famous attempt at ordering nature, but diversity ultimately caused the Chain to break; how are we to rank thousands of species of beetles, bacteria, or birds in an ascending order without special pleading? It could not convincingly be accomplished, and by 1799 the Chain was effectively rusted away.
While Linne gave us binomial nomenclature and Cuvier made comparative anatomy a powerful scientific discipline, the various scientific techniques and tools used to try to understand the relatedness of organisms suffered from a lack of evolutionary understanding. The coming of age of paleontology further complicated matters. Initially debates about fossils centered on "easy" specimens like bivalve shells, the mode of creation of these fossils being the biggest question about them. Were they really the remains of creatures that had once lived or were they merely "replicas" of living creatures created by some plastic force in the earth? How could a seashell find its way onto a mountaintop? Several naturalists arrived at the conclusion that fossils were organic remains of ancient creatures, and even though it might be fashionable to look down our nose at their ideas that some of these fossils were laid down by the Noachian Deluge or within the time frame of 6,000 years, this was still a major breakthrough in understanding the history of life on earth.
Once the matter of the origin of fossils was settled more "difficult" remains could be effectively recognized. The bones of strange, extinct creatures gave rise to many legends involving gods, saints, heroes, and monsters throughout history, but by the first half of the 19th century (with the antiquity of the earth, the organic nature of fossils, and the method of comparative anatomy established) fossils of previously unknown creatures could no longer be said to be the remains of human giants or remains of ancient battles. Even so, "bursting the limits of time" presented some new taxonomic problems. Many of the animals being recovered were unlike anything living today, the fossil record only adding to the diversity of life on earth. The ancient creatures surely shared resemblances with some living animals that allowed them to be related to extant forms, but this was still something of a forced fit; of course dinosaurs were reptiles, but they were nothing like any kind of living reptile. The overall taxonomic system that was then in place was not destroyed or shaken, but the discovery of creatures like pterosaurs and plesiosaurs posed new questions about why ancient life was so different from recent life.
Despite the growing menagerie of "extinct monsters", however, the fossil record that was available for scrutiny during the time that Darwin was writing his abstract On the Origin of Species by Means of Natural Selection was still scanty. Darwin had to spend considerable time explaining why some of the transitions his theory predicted had not yet been discovered, but even if the record was fragmented it still provided enough examples to help the naturalist illustrate the connection between living and extinct creatures. In the 1st edition of his most famous work Darwin wrote;
Some writers have objected to any extinct species or group of species being considered as intermediate between living species or groups. If by this term it is meant that an extinct form is directly intermediate in all its characters between two living forms, the objection is probably valid. But I apprehend that in a perfectly natural classification many fossil species would have to stand between living species, and some extinct genera between living genera, even between genera belonging to distinct families. The most common case, especially with respect to very distinct groups, such as fish and reptiles, seems to be, that supposing them to be distinguished at the present day from each other by a dozen characters, the ancient members of the same two groups would be distinguished by a somewhat lesser number of characters, so that the two groups, though formerly quite distinct, at that period made some small approach to each other.
Discovering a "half fish/half reptile" akin to something out of a modern creationist tract was not a reasonable expectation. Instead, living members of two groups that seem related to each other would be more divergent in character than older representatives of those lines, the two lineages converging on very similar forms as we move backwards through geologic time. This is precisely what has been found over and over again in the fossil record. Evidence of direct ancestry may often be elusive, but there is ample evidence that representatives of related lineages "approach" the a common form as we trace their relationships backwards.
That such relationships are seen in the fossil record is a fact, but by itself it does not explain why such relationships should be seen. Darwin's concept of evolution by natural selection provides the primary mechanism that describes how the changes seen in the geologic record took place. Without evolutionary theory there is no way to explain why dinosaurs have feathers or why some "fish" have limbs, and the use of evolutionary theory to predict the existence of creatures like Tiktaalik speaks to the power of Darwin's idea.
Finding the proper visualization for the evolutionary paths organisms have taken is something of a difficulty, however. The most common image is the evolutionary "tree" or "bush," but if we were to create an image of the evolutionary history of all life through time it wouldn't look like any tree or bush any of us would have ever seen. (This led Stephen Jay Gould to propose that a lawn in which some blades of grass are cut and others are missed would be a better analogy in his essay on Hallucigenia in Eight Little Piggies, but this analogy doesn't fully capture the essence of the thing, either.) It might more closely resemble abstract art, flurries of twigs and arms branching off only to be cut off entirely, others remaining entirely intact over the course of more than 3.5 billion years.
Even if we were to trace the intact limbs, however, the creatures represented at the terminating (yet ever-growing) branches would look nothing like those further down. While every species alive today has a 3.5+ billion year ancestry, the gulf between the form of ancestors and living descendants widens as we move down through the evolutionary canopy, and while this idea should not be shocking given our understanding of evolution it still can be difficult to visualize. Think of our own species, for instance. Right now we call ourselves Homo sapiens, but if we were able to create a massive pedigree where we had access to every generation for the whole of the history of our lineage we might feel compelled to attach different names to some of our ancestors (and perhaps we have).
Three million years ago we might call our ancestors Australopithecus, and about 65 million years ago our ancestors probably would have looked something like Purgatorius. Jumping back even further, the 315 million-year-old Hylonomus might approximate the appearance of our ancestors during the Carboniferous, and the ~380 million-year-old Panderichthys may well represent one form during the time the first tetrapods were evolving. We could even go back further, way back through the first jawed fish and chordates and multicellular animals and so own to the first life, our privileged temporal position allowing us to make these connections even though at any point our own lineage could have been unceremoniously lopped off by some historical contingency. Yes, we did not evolve "from monkeys" that are alive today, but it cannot be denied that through the course of deep time our ancestors have come in a variety of ever-changing forms, and we have more more in common with the rest of life on earth than we might think.
Such realizations make paleontology a far-more personal experience than is often recognized. During my visit to the AMNH this past weekend, the train I was on lost power, stranding me in the dirty coach for an hour while the other passengers and I waited for another train to come pick us up. Fortunately I had a book with me to help pass the time, Martin Rudwick's The Meaning of Fossils (a volume that I certainly recommend to anyone who has the time to sit down and read it). Thinking about the material presented in the book as I roamed the fossil halls on the fourth floor of the AMNH, I realized that fossils truly do have different meanings to whoever looks at them.
To some visitors they are the testimony of a great Flood, creatures that God saw fit to create and then erase for reasons that no one is privy to (why save the mosquito but doom the gomphotheres to extinction?). For others the bones are the remains of monsters, creatures that lived long ago in a nightmarish time of horns, teeth, and claws, a world that we can be thankful no longer exists. For those that acknowledge the oddly familiar smile of an Australopithecus skull or the conformation of limb bones in a mastodon, though, fossils take on a more visceral meaning. The skeletons are no longer static things that only belong in a particular place and time that is out of our reach; they are snapshots or snippets from a grand narrative, players moving on & offstage and changing costumes as time rolls on. Recognizing that the old bones once gave support to a creature that actually lived and followed its own trajectory from birth to death, it becomes possible to start tracing ancestry backwards again, an exercise that makes it impossible not to recognize the connection of life.
This particular view of life doesn't always come easy, and it often raises more questions than definitive answers. Without an understanding of time & evolution, picking out the similarities between my skeleton and that of a chalicothere or a sauropod seems absurd; how could we share common ancestry and yet take on such different forms? This is the sort of question that could easily induce sleepless nights and years spent in academia, but it's worthwhile to ponder it. When we look at the fossil record we are not just looking at times that can be packaged up as the "Age of Fishes" or "Age of Reptiles"; our own ancestors and relatives were present during those times, too, and it's difficult to learn about the history of life and not learn anything about ourselves. Indeed, the more we know about ancient life to more we know about present life, and to reverse the old uniformitarian saw, the past is the key to the present.
I've been kind of obsessed with the evolution/creation wars thingie for a few years now. This is one of the best summaries of some major points that I have read.
A very cogent and well-written essay, Brian!
Well done, Brian. I have to say, looking at fossils and that great Evolution book remind me that we're all connected. We all have two eyes and two ears and two nostrils and pelvises and shoulderblades and whatever else. We're all the same, and that's a wonderful feeling.
Earmark this one for Open Lab 2008.
This is a wonderful essay. The sense of connection to all other forms of life, past and present, that you so beautifully describe here is what I prize the most about studying biology. It's also why I have such a hard time understanding why some people see science as cold or as something that strips the wonder out of the world. What could be more wondrous than knowing how you're related to three and half billion years of life and history?
I've seen various estimates of how many of the total number of species which have ever lived are present at the moment. I recall these % estimates as being in single digit or less. So the vast majority of species which have ever lived are extinct, and poorly represented by a scanty fossil record. Incidentally, I exclude prokaryotes. I wonder if a bacterial species, for example, has ever gone extinct.
Excellent post, Brian.
Fossils are fascinating, and evolution gives us a whole new way of understanding them. I found this article on the folklore surrounding fossils rather interesting.
Great post! I'd definitely recommend it to anyone interested in those things.
However, the paragraph on the quinary system contains some inaccuracies. You seem to be describing Swainson's version, but he didn't place the typical group in the center of the circle and the aberrant ones at the periphery. See the wikipedia article http://en.wikipedia.org/wiki/Quinarian_system
for Swainson's diagram of the classification of birds. The "typical" group is the circle on the upper right, the "subtypical" on the upper left, the other three circles are the "aberrant" ones. In Vigors's diagram (also in the wiki article) there is no distinction between typical/subtypical/aberrant groups. Your description rather sounds like Edward Newman's system, as described in his book "The System of Nature" (online at Google Books: http://tinyurl.com/3mf7m3 ), but that was septenary, not quinary. This was the guy who claimed pterosaurs were marsupial bats. Of course much shoehorning had to be done to get to a fixed number everywhere, and there was really no room for fossils that didn't fit in the usual categories, as was recognized by A. R. Wallace and others already in the 1840s.
Great post; thanks.
Incidentally, I exclude prokaryotes. I wonder if a bacterial species, for example, has ever gone extinct.
Why not? Lots of specialized gut symbionts and pathogens must have gone extinct.
The real question is which species concept you are using. Many, perhaps most, are not applicable to bacteria or archaea.
Nice post, Brian.
Your opening reminded me of this:
"There is grandeur in this view of life, with its several powers, having been originally breathed by the Creator into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being evolved." -- Charles Darwin, ON THE ORIGIN OF SPECIES, 6th ed., p. 429
Discovering a "half fish/half reptile" akin to something out of a modern creationist tract was not a reasonable expectation.