by Henry Gee
That the shapes of birds seem to have been subsumed to the needs of flight is testament to the enormous energetic cost of this habit. Therefore it should be no surprise to you—having read this far—that birds conspire to lose it at every opportunity. All birds alive today are thought to have descended from flighted ancestors, so it is remarkable to see how many are flightless. The power of flight has been completely lost in two entire orders of bird—the ratites (ostriches and their relatives) and the penguins, and many other bird groups have representatives that are flightless. Birds that find themselves on remote islands free from land-living predators routinely give up flight as an expensive luxury. The Galápagos Islands have their flightless cormorant, and the extinct dodo of Mauritius was a gigantic, flightless pigeon. The kakapo of New Zealand is a large, flightless parrot. Some birds that retain the power of flight aren’t actually very good at it: the world has yet to see chickens migrate.
Many flightless and other ground-nesting birds, particularly on remote islands, have been particularly vulnerable to extinction from the depredations of human colonists and their retinues of cats and rats. One thinks not just of the dodo but the moas of New Zealand and the gigantic elephant birds of Madagascar, and whole hosts of birds endemic to remote island groups such as Hawaii. But even before people and their domestic animals and pests turned up to spoil things, birds gave up flight wherever they could. Not long after the dinosaurs became extinct, the role of top land predator was taken by gigantic, carnivorous, flightless birds, the phorusrhacids, relatives of modern cranes and rails. Even further back in time, in the Cretaceous period, when dinosaurs were still running round and bumping into one another, the flightless seabird Hesperornis ducked and dived in the seaway that once bisected North America from north to south.
When I said, a few lines above, that the whole frame of birds seems to have been adapted to the habit of flight—well, that was the view until relatively recently. In the past twenty years or so it has become generally accepted that birds are the closest living relatives of dinosaurs, and it so happens that many of the features that we see in birds, and that were generally thought to have been unique to birds and specific adaptations for flight, also turn up in dinosaurs, many of which were large, heavy, and as aerodynamic as a sack of spanners. The hollow bones of birds, combined with the loss or fusion of many bones, especially in the limbs, are found in dinosaurs, even quite large ones, and I think I’ll have done seen ’bout everything, when I see a Brachiosaurus fly. There is good evidence that the bodies of dinosaurs were full of air sacs connected to the lungs; that some of them folded their forearms, with the hands backward, just as birds fold their wings; that dinosaurs such as Velociraptor had wishbones, a distinctive feature otherwise only seen in birds; and that some dinosaurs incubated their eggs just as hens do.28 But the most dramatic evidence among many features once thought distinctive of flying birds is the presence of feathers in many dinosaur species.29
Much research over the past few years has shown that the origin of birds lies somewhere among a group of dinosaurs called theropods, specifically small theropods collectively known as Paraves (near-birds). This group includes Velociraptor and the fearsomely clawed Deinonychus as well as the remarkable “four-winged” gliding dinosaur Microraptor and the feathered hunter Sinornithosaurus.30 It is among the Paraves (and their close relatives the oviraptorosaurs, such as Oviraptor and the enormous Gigantoraptor)31 that one finds the greatest concentration of feathered dinosaurs. Somewhere in this group lies one of the most iconic fossil species ever discovered—Archaeopteryx. So iconic, that knowledge of its significance has permeated society at large.
My elder daughter, then age three, was a very frustrating kindergarten student. When all the other children were paying attention and behaving themselves, Gee Minor would whizz around the playground, arms outstretched, shouting “I’m Archaeopteryx! The first bird!” When Mrs. Gee or myself would come and collect her, we would suffer remonstration from her teacher. “Your daughter is not an Archaeopteryx,” we’d be told: “she’s a little girl.” That the status of Archaeopteryx should be known and appreciated by small children should be a guide to its importance and the place it holds in the general consciousness of evolution.32
Archaeopteryx first came to light as a single fossil feather in 1861, soon followed by skeletons of entire animals, each with a halo of feathers, impressions on the very finely grained limestone in which these creatures had been entombed.33 Only a handful of Archaeopteryx specimens have since been found, all from the same area of southern Germany. Apart from the feathers, arranged as beautifully on the wings as on any pigeon, Archaeopteryx looked very reptilian. Where modern birds have a short, stubby tail (the “parson’s nose” of your Christmas or Thanksgiving roast), in life surmounted by a fan of feathers, the tail of Archaeopteryx was long and bony. Where modern birds have a toothless beak, Archaeopteryx had jaws full of teeth. Archaeopteryx lacked the large keeled breastbone for the attachment of flight muscles that is typical of modern birds. And so the list of differences goes on. But there were similarities, too—studies of the skull of Archaeopteryx show that its brain was similar to that of birds in many ways,34 and it also had the hollow bones typical of birds today.
At the time of its discovery, and for a century or more after, Archaeopteryx was seen as a transitional fossil—a missing link—between reptiles and birds, a wonderful vindication of Darwin’s ideas only two years after the publication of the Origin. It is no surprise, therefore, that Archaeopteryx gathered the soubriquet of the first bird, and that in it was seen a tendency—a trend—toward the progressive loss of reptilian features (teeth, long tail) and the gain of more birdlike ones (feathers, hollow bones, keeled breastbone) seen in modern birds.
In retrospect, the days of Archaeopteryx holding its place in the hearts of small children and paleontologists as the first bird were numbered with the first account of a feathered dinosaur, Sinosauropteryx, in 1998.35 Many of the reptilian features of Archaeopteryx were dinosaurian ones—and, more remarkably, so were those of its features once thought typical of modern birds, such as feathers and hollow bones.
In fact, the latest research suggests that Archaeopteryx was not especially closely related to modern birds, being more closely related to dinosaurs such as Velociraptor and Deinonychus.36 Small children in playgrounds the world over will now have to shout “I’m Archaeopteryx! Just another feathered dinosaur!”
But here’s the killer. Even if Archaeopteryx was only a first cousin of birds, it was still a flyer. So was it a representative of a tendency among ground-living dinosaurs to get airborne—to strain, perhaps to yearn, in a suitably nature-philosophical manner, for the wide cerulean welkin?
Well, actually, no.
The latest research shows that the more feathery, flight-inclined members of the group to which Velociraptor belonged also tended to be the earliest and more primitive members of the group—creatures such as Archaeopteryx (and others such as Microraptor and the less familiar Xiaotingia). It looks very much as if this group of dinosaurs started off with flying and gliding animals that tended to lose this capacity, rather than improve on it.
Archaeopteryx was, therefore, not a stage in the acquisition of flight in birds, but in its loss among a related but different group of dinosaurs whose later members, while feathered, did not fly. Archaeopteryx was not a harbinger of things to come, but a one-way ticket to extinction. As far as we can tell at the time of writing, the closest relatives of birds among dinosaurs were small, very peculiar, and nonflying feathered dinosaurs called scansoriopterygids.37
The latest version of the story of Archaeopteryx (which, it has to be said, remains highly controversial) turns the original on its head, and shows that the tendency to lose the habit of flight runs deep into the dinosaurian roots of birds—and that the phenomenon of loss, more generally, pervades evolution even to the extent of knocking one of our most treasured missing links off its perch.
Once one realizes the
extent to which loss has shaped evolution, one starts to see it everywhere.
Among the tetrapods—the group of four-legged animals that includes most creatures with which we are familiar (including, as it happens, birds and human beings)—we see the loss of some or all the limbs in many different lineages, such as snakes, several kinds of lizards and amphibians, and whales: four-legged animals, therefore, that have lost most or all of their legs. A recent discovery and the cause of much hilarity in the press corps is that human beings count among their unique attributes the loss of spines on the penis, a feature found more generally in other animals.38 Loss is pervasive.
The take-home message of this chapter is that it is very hard, objectively, to decide which features of organisms are primitive and which advanced, especially if one is wedded to the view that the function of natural selection is to produce ever greater refinement and complexity. If a creature gains more mates, more resources, more short-term advantage by losing a structure rather than gaining one, then it will do so, and posterity can look after itself. The example of Archaeopteryx shows that the very idea that there can be such things as “missing links” represents a fundamental misunderstanding of how evolution works.
The term “missing link” should be expunged ruthlessly from our vocabulary. Journalists who use it should be subject to some embarrassing sanction, such as that in the probably apocryphal story that when the staff of the outgoing President Clinton left the White House to make way for the entourage of the incoming George W. Bush, they removed all the W keys from all the keyboards in the building.
This sanction should apply not just to the description of fossils, but of syndromes in modern humans in which patients appear to exhibit atavisms—throwbacks to some earlier stage of evolution. Much play, for example, was given to a family in Turkey whose members had a tendency to walk around on all fours.39 Another better attested example concerns a gene called FOXP2, which seems rather different in modern humans compared with its form in other animals, and whose mutation is associated with a condition in which patients have great difficulty speaking and forming words.40 Is FOXP2 a “language” gene? Caution should be exercised in both cases. Such pathologies are the results of mutations in modern humans, and we have no way of knowing if they can tell us anything much about evolutionary history. In other words, they might be very revealing of how things are, but not how they got that way. Such examples perhaps say more about our own prejudice toward a “progressive” view of evolution.
The only direct evidence we have about the past comes from fossils. But fossils are mute. It is we who tell their stories for them, and these stories are likely to flatter our prejudices as much as reveal what is really there.
To make matters worse, fossils are so scarce it’s a wonder we can use them to say very much at all—any patterns we’re likely to learn from fossils are likely to be as provisional as our interpretations of the fossils themselves. This is the subject of the next chapter.
4: The Beowulf Effect
Charles Darwin was much exercised by what he called the “imperfection” of the fossil record, and viewed it as one of the chief difficulties of his theory. He was, perhaps, overdoing it—as I have discussed, subsequent research on the similarities between extant creatures, down to the molecular level, provides dramatic evidence for the community of all life. Darwin would have been safe had no fossils ever been discovered. It remains the case, however, that fossils provide direct evidence of evolutionary change in the past, and reveal how creatures have adopted many strange shapes not seen among organisms today. Without fossils, we’d be ignorant of Archaeopteryx and Homo floresiensis. The problem with fossils, though, is that no matter how strange they seem, we are overly inclined to see them as way stations in the canonical pattern of evolution we assume is there, the one that leads inexorably from primitive to advanced. It is all too easy to assume that Archaeopteryx is a “missing link” between reptiles and birds, and to dismiss Homo floresiensis as a genuine species because it doesn’t fit in with deeply ingrained views about how the evolution of humanity “ought” to have happened.
Darwin was right, however, to have pointed out the imperfection of the fossil record. The fossil record is indeed imperfect, and in many ways more imperfect than we can imagine. In this chapter I shall show that it’s so imperfect that one can never simply use what we’ve found to bolster preexisting notions of progress. More than that, the fossil record is so scanty that we cannot in all conscience ignore the lives and times of all those creatures that lived and died without leaving any trace of their existence. Such creatures probably constitute the vast majority of all creatures ever to have evolved. To ignore them would be as irresponsible as astrophysicists ignoring the majority of mass in the universe that appears to consist of “dark” matter, the nature of which is still unknown.
Finding fossils is a chancy business. We are therefore entitled to ask the following questions: What would our ideas about evolution have been like had none of the fossils we’ve discovered been found—but an entirely different collection unearthed instead? Would we still use this collection to justify a view of evolution based on progressive increases in complexity, culminating in Man?
I shall consider this by analogy with something we all know—the English language, so rich and strange, but something we take for granted. English is a first language for many, and a second for many more. English is among the top five most influential languages in the world,1 and so ubiquitous that, as an English speaker, even the most remote parts of the world don’t seem so far from home.
It didn’t have to be that way. English is subtle and flexible, to be sure, but there is no particular reason, inherent in the language itself, why it should have achieved its present dominance over Latin, say, or Portuguese, or Malay, or, come to that, many of the other languages among the 6 or 7,000 spoken today.
The current success of English can be put down to historical accident, determined by two things. First, by its spread between the seventeenth and early twentieth centuries as the language of the British Empire, the most populous and geographically the most extensive commercial concern in history.2 Second, by the fact that it just happened to be the language of those former British colonies that, as the United States of America, grew to eclipse their progenitor in influence and power.3
Neither the growth of the British Empire nor of the United States was inevitable. Had Wolfe failed in Quebec in 1759, say, and the Royal Navy lost to the French at the battle of Quiberon Bay in the same year,4 I might be writing this book in French: or Spanish, perhaps, had the Armada not been blown off course in 1588. Or German, had Hitler pressed his advantage at Dunkirk. These examples might seem playful, but they are meant to be serious. Things that we take for granted, and assume to be the way they are through some inherent superiority or the inexorable machinations of destiny, might so easily have turned out differently.
Historians are now quite used to considering the might-have-beens as well as the documentary facts, and reconstructing “counterfactuals,” scenarios of how things might have turned out had events gone slightly differently. These are more than merely speculative exercises. For example, documents still survive showing how the Nazis planned to govern the Soviet Union, had they managed to conquer it.5 Pioneering Americans were brought up to believe in “manifest destiny,” the doctrine that the United States would spread from coast to coast. That it did so might to some extent have been self-fulfilling prophecy. However, historian D. W. Meinig has challenged this, showing that the United States might easily have been much larger than it is—or much smaller—had certain policies been followed that were instead ignored, or put aside instead of being pursued.6
And so for the English language. Despite its modern currency, English began as a language—we call it Old English—that is as unintelligible to an untutored modern English speaker as, say, Swedish. It was spoken in England and lowland Scotland for about six centuries, from the invasions of Britain by the Angles and Saxons
in the fifth century, until the Norman Conquest drove it to the brink of extinction. The few fragments of Old English literature that have come down to us from that remote yet immense period have survived thanks only to blind chance. For example, 30,000 lines of Old English poetry are known to us—all that’s left of more than six hundred years of poetry and song. For comparison, Shakespeare’s plays total some 150,000 lines, written over a period of twenty-four years. What’s more, almost all Old English verse is found in just four surviving manuscripts, all written in the West Saxon dialect of Old English around the year 1000—which does not mean that we knew who originally composed them, nor in what language.7
Perhaps the best-known example of Old English that survives today is Beowulf. This is a long poem written in alliterative verse (a style characteristic of the period) concerning the adventures of the eponymous hero and his battles against a succession of monsters. The fact that Beowulf is a staple of the school and college curriculum, and can be found in the proverbial All Good Bookstores,8 in the original Old English and in Modern English translation, inures us against the revelation that there is only one known manuscript of the poem—and that narrowly avoided being destroyed in a fire in 1731.
We are so used to the mass dissemination of information that it’s hard for us to imagine a time before the invention of printing, when books were fabulously rare and expensive custom-made products, copied from an original (or from other copies), with great labor, and by hand. The fact that literature before the age of print could be reproduced only very slowly had an important consequence for knowledge. That is, it was once very much more fragile than it is now, much more prone to extinction. Given the prevalence today of print and electronic data storage, it would be very difficult, nowadays, to completely expunge all traces of Hamlet, say, or Middlemarch. Before printing, however, to put just one monastery library to the torch would have been to consign hundreds of unique manuscripts to total oblivion, irreparable and irretrievable.