The Accidental Species: Misunderstandings of Human Evolution

by Henry Gee

  If just one manuscript of Beowulf survives, one can hardly imagine the numbers of other works in Old English that once existed but that have been lost.

  The facts of the manuscript speak for themselves. Apart from showing signs of fire damage, the Beowulf manuscript (you can see it on permanent display at the British Library in central London) is certainly a copy. It was made sometime in the eleventh century, presumably from another copy. The date of the composition of the original is not known—the poem might have been in existence for two or three hundred years before the single surviving copy was written. This suggests that the poem started as oral tradition; that there must have been a number of earlier written versions, all now lost; and that there might not have been a single, definitive, “official” version.

  The single copy also shows signs of having been bowdlerized. The setting of the story is pagan, and concerns pagan values, but the copy we have was written many centuries after England had been Christianized. It is possible that the several references to Christianity in the poem are later additions, either in the manuscript we have—or in earlier versions, all now lost.

  That tales once existed in Old English of which we now have no knowledge is illustrated by the use in Beowulf of words found nowhere else in the surviving corpus of medieval literature, but which are unlikely to have been neologisms created specially for the occasion; and obscure references to stories, whether of fact or fancy, that the contemporary audience would have found familiar, but which have since been lost and so mean nothing to us. Proof in the breach comes with an episode in Beowulf concerning a battle between two warlords, Finn and Hengest—an account of which same incident subsequently turned up in another fragmentary manuscript.9

  What would our view of the past be like had no copies of Beowulf survived? And what of the alternatives? For example, if we think that the library of Old English is thin, of the native literature of England before the Anglo-Saxon invasion we know even less. What would our ideas of the languages, literature, and customs of the Dark Ages have been like had the single remaining manuscript of Beowulf been destroyed in that fire in 1731? What would our ideas have been like had we found instead an epic poem of King Arthur written in medieval Welsh? This is not idle fancy—the existence of such lost works was hinted at by Geoffrey of Monmouth in his History of the Kings of Britain, written in Latin in the twelfth century. Or what, perhaps, of tales in an otherwise obscure language such as Pictish, whose scant relics remain completely undecipherable?

  The point of this is to show not only that history turns on a hair (the outcome of events is “contingent,” as Stephen Jay Gould put it in Wonderful Life) but also that our present-day view of history is sensitively conditioned by those few and arbitrarily sampled fragments that have survived the ravages of time. I call this the “Beowulf effect.”

  As with fragile, unique handwritten scrolls from a thousand years ago, the chances of any living creature becoming a fossil are extremely remote. What’s more, the fossils we have document an almost infinitesimally tiny, entirely arbitrary, and almost certainly unrepresentative selection of the range of living creatures that once existed, the preservation of any one depending very largely on luck. The fossil record shows the Beowulf effect in action.

  The word “fossil” derives ultimately from the Latin verb fodere, which means “to dig.” Baldly, fossils are things that are dug up. More specifically, fossils are physical signs of the presence of creatures that lived long ago, and that were buried. Fossils might constitute the actual physical remains of the creature—its bones, its shell, or even its DNA—but this is exceptional. More commonly, they are what happens when the tissues of a dead creature are replaced by minerals that percolate into the buried remains through the groundwater, creating a stony representation of the shape of the creature. The fossils of sea urchins that my family and I find on Cromer beach now and again aren’t made of the actual material from which sea-urchin tests are made, but from chalk, or flint, a rock that forms when silica-laden groundwater percolates into chalk.

  In some cases, especially when the creatures have become buried in an oxygen-poor environment such as the mud at the bottom of a stagnant lake, the bacteria responsible for breaking down the corpse will leave very detailed impressions of that corpse in the form of the deposits of their own mineral waste products. In other cases, fossils are the petrified impressions that a creature leaves in sand or mud—the cast of a shell or, more evocatively, signs of past activity, such as a trail of footprints, a bite mark, or a burrow.

  What can’t be emphasized strongly enough is that the chances of a creature leaving any trace at all in the geological record are vanishingly small. In the wild, many organisms—perhaps most of them—are eaten by predators. Should animals or plants live long enough to die without their bodies having been consumed by a predator, their remains are almost always recycled within days. Their soft tissues are soon eaten by scavengers, and any remnants are broken down to nothing by fungi and bacteria. The hard parts—whether bones or shells—are pulled apart and dispersed, and time eventually grinds them to powder. To stand any chance of fossilization—to become a recognizable memorial to an otherwise evanescent existence—the body of a creature must remain sufficiently intact until it becomes buried or otherwise put beyond reach of the normal agents of dispersal and decay. A fossil is therefore a sign of some rather unusual circumstances in which the normal course of events has been cheated.

  Fossilization, if it occurs at all, almost always happens underwater, and to those parts of a creature that are most resistant to physical breakdown. This explains why the fossils we have are overwhelmingly those of the hard shells of animals that spent their lives in water, are therefore likely to die in it, and so stand a chance of becoming buried in the sediment at the bottom of the sea or in a lake. It is no coincidence that the collections of most amateur rock hounds contain fossils of sea creatures—clams, ammonites (the shells of creatures related to squid), belemnites (ditto), sea urchins, trilobites (marine creatures that looked rather like pill bugs), perhaps a fish or two, and the occasional bone of a marine reptile such as an ichthyosaur (whose bobbin-shaped vertebrae make excellent ash trays, I am told), but rarely the remains of ancient land animals such as dinosaurs. This is because land animals tend—of course—to die on land, and their remains disperse quickly before they can be buried. Fossils of land animals are usually what are left once the uneaten scraps of some hapless corpse get washed into a watercourse after everything else has finished with it. Transfer to water and subsequent burial break up the remains even further. This is why fossils of land animals are rarer than those of aquatic ones, and why even the best of those that survive long enough to be recognizable as the remains of living things are in general fragmented and in very poor condition. The majority of fossils of land vertebrates consist of teeth—this is because enamel is very much harder and more resilient than any other tissue, and is the last to be broken down.

  Land animals that lived in dry conditions but close to water (a somewhat conflicting set of circumstances) are the least unlikely candidates for fossilization. Dinosaurs sometimes fall into this category, as do hominins. Creatures close to water sometimes fall in, or are pushed. There are, very occasionally, mass-death assemblages of dry-land creatures that have been overwhelmed by floodwater and quickly buried. The bodies of creatures that live in hot, damp tropical forests are almost always decomposed by other creatures and hardly ever fossilize; the bodies of animals that live at high altitude are broken up and decomposed long before they can be interred underwater in any recognizable state. Ancient hominins—at least, the ones we know about—lived in the lowlands, often near water (or in caves—another location that ups the odds of fossilization), so their fossils, while meager, are sufficient to mark their having existed. Chimpanzees are forest creatures, and although they have been evolving for precisely the same length of time as hominins, their fossil record consists, so far as is known, of just a few half-million-year-
old teeth.10 Gorillas, like chimps, live in tropical forests, sometimes at high altitude, and have been going their own evolutionary way for much longer than either chimps or hominins, but their fossil record is completely blank. Hundreds of thousands of generations of gorillas have come and gone, but apart from the creatures alive today or whose skeletons are preserved in museums, there is not one single sign, not even a scrap of half a tooth, to betray their lineage having existed, as it surely has done, for the past 7 or 8 million years.

  Such is the process of fossilization: scrappy, chancy, biased, uncertain, and threadbare. There are, however, episodes of fossilization—remarkable because very rare indeed—in which living creatures are interred, often quite suddenly, to leave remains that are far better preserved than in the normal run, and which shed fractionally more than the usual murky half-light on vanished worlds.

  The early birds and feathered dinosaurs of the Cretaceous of northeastern China, for example, were preserved in great abundance at the bottom of extensive lakes in association with volcanic ashfalls.11 This has allowed preservation in such detail that the entire early history of birds and their relationship with dinosaurs has been completely revolutionized. Most of what we know about feathered dinosaurs comes from these deposits. These deposits are also rich in the fossils of early mammals. Isolated teeth or jawbones are usually all the traces that mammals leave as fossils, but the mammals from the Cretaceous of northeastern China are often preserved entire, complete with their furry coats.

  The famous Burgess Shales of British Columbia, made more famous still by Stephen Jay Gould in Wonderful Life, preserve in exquisite detail an entire ecosystem of soft-bodied creatures from the Cambrian period, some 505 million years ago, creatures that just happened to have been suddenly buried in a submarine mudslide and preserved as shiny impressions on black shale. Sometimes the fossils are very hard to see unless immersed, when their gorgeous detail and strangeness emerges as if by magic.12 Although fossils of marine creatures like those found in the Burgess Shales have since been found in strata in other parts of the world, some of them deposited since the end of the Cambrian itself, such fossils tell us far more about ancient marine life than can be revealed by regular garden-variety Cambrian fossils such as trilobites.

  In another example, a freak sandstorm engulfed dinosaurs and other animals at a place called Ukhaa Tolgod in what is now Mongolia in the Late Cretaceous, some 70–80 million years ago,13 burying them alive by a kind of three-dimensional instant photography—reminiscent of the preservation of the unfortunate citizens of Pompeii, overcome by the hot ashes and dust from the eruption of Vesuvius in AD 79.

  There are several other instances of sudden, unusual kinds of preservation up and down the fossil record.14 These cases are highly localized and possibly unrepresentative, but because they have the potential to yield so much more information on past worlds than fossilization does in general, their effect on our knowledge is disproportionately great. Had the freak mudslide that buried the Burgess Shales creatures not happened, how would our knowledge of ancient marine life have differed? What would we know had the mudslide entombed a completely different set of creatures? How would our knowledge of the relationship between birds and dinosaurs been affected had the conditions for preservation in those Chinese lakes been different, so that corpses that landed in them rotted more quickly, rather than less, or didn’t preserve the feathers? Some rather smelly experiments on how bodies of fishes decompose have shown how our ideas of what ancient creatures looked like are very sensitive to their state of preservation.15 On such tiny chances does the edifice of knowledge turn.

  To return to my Beowulf analogy, consider these questions: How representative is Beowulf of Old English poetry? Did poets of that vanished age regularly write about manly heroes and horrible monsters, or was this exceptional? Did they, perhaps, tend more toward kitchen-sink or sitcom? Did they always write in alliterative verse, or did they occasionally stray into rhyming couplets? Indeed, can we say anything reliable about the totality of the Old English literary tradition, given the few examples that now survive? That we should be practical and do what we can with the evidence we have is no doubt the prudent answer—but we should never be lulled into thinking that any reconstruction we might build about the everyday repertoire of the scops and bards of yore is much more than educated fancy.

  All the above presupposes that we have at least some Old English poetry—at least some fossils—to discuss, never mind how paltry the remains.

  There are creatures in the modern world that are barely known as fossils. You’ll remember how in the last chapter I made a big deal about parasites, and how their existence cocked a snook at the idea that evolution was necessarily a force for improvement and increased complexity. I mentioned that parasitism is very common, and that most creatures are infested with parasites. Many kinds of nematode worms (roundworms) are parasitic, living inside the tissues of most animals and plants. Nematodes also live freely in the soil, and even in rocks deep underground,16 where they hunt for bacteria on which to graze. A student of nematode worms once remarked:

  In short, if all the matter in the universe except the nematodes were swept away, our world would still be dimly recognizable, and if, as disembodied spirits, we could then investigate it, we should find its mountains, hills, vales, rivers, lakes, and oceans represented by a film of nematodes. The location of towns would be decipherable, since for every massing of human beings there would be a corresponding massing of certain nematodes. Trees would still stand in ghostly rows representing our streets and highways. The location of the various plants and animals would still be decipherable, and, had we sufficient knowledge, in many cases even their species could be determined by an examination of their erstwhile nematode parasites.17

  Nematodes are ubiquitous, and have probably been so for hundreds of millions of years, yet their fossil record is almost nonexistent. That “almost” is a big word, however—fossil nematodes have been found, preserved in amber, another unusual and chancy location for fossilization that yields spectacularly well-preserved fossils.18 They have also been found in coprolites—fossilized feces—of dinosaurs.19 But such occurrences only serve to underline my point. Nematodes are everywhere, and in everything, and (presumably) have been so for hundreds of millions of years. But their prehistory is betrayed only by a very few examples of fossils formed in rather peculiar circumstances.

  Tapeworms, however, are another matter. They are completely unrelated to nematodes. All tapeworms are parasites, and are likely to have had a very long history and relationship with the animals (including humans)20 they infest—but they have no fossil record at all. None.

  Now, imagine that tapeworms existed for half a billion years, leaving no fossils at all, and became extinct. We would have no knowledge of their ever having existed at all.

  From the above it makes sense that there must have been many kinds of creatures that once existed but that have vanished without trace. Until the Burgess Shales had been uncovered, we couldn’t have known of creatures such as Opabinia, a swimming shrimp-like creature with five eyes on stalks and a flexible proboscis furnished with serrated jaws, a creature of a kind that nobody had even imagined existed.21 It is possible that entire groups of unimaginably strange creatures have lived on this planet for millions of years but died out leaving no trace at all in the fossil record. If we had never found Opabinia, what other strange creatures might we have found instead? And how might the stories of life that we tell one another have been affected?

  All this having been said, attempts have been made to quantify the degree of our ignorance, to assess the incompleteness of the fossil record as it applies to various groups of organisms.22

  Completeness is relatively easy to assess on a small scale, though I use the word “relatively” with due caution. Let’s say that you’re digging in a quarry with the aim of finding a representative sample of all the kinds of fossil that might be present at that location. On the first day you find, s
ay, five different kinds of fossil clam. The next day you might find two or three more, but after a while you just find more of the same kinds. As time goes on, the likelihood of your finding a kind of clam you hadn’t seen before dwindles to almost zero (though never to zero itself). For all practical purposes, you could say that you’d excavated every kind of clam from that quarry that happened to be preserved there, as a fossil.

  As we have seen, however, a number of factors influence the repertoire of the animals and plants that once existed in a locality that get preserved as fossils. Had the soil in which the dead creatures were buried been more or less acid, or more or less oxygenated; had the winds and currents been blowing this way or that; had the ambient temperature been somewhat higher or lower; had the groundwater been infiltrated by one mineral rather than another—all such things and more might have influenced the kinds of creatures more likely to have been entombed as fossils.

  You might, in your quarry, have found every kind of clam that once existed there in the remote past, but try as you might—and completely without your knowledge—you’d never find a single momewrath, because momewraths would not have fossilized well in the conditions that entombed the clams so faithfully. You would have no way of knowing that in the ecosystem whose only vestiges are found in that quarry—that tangled bank—momewraths were the most abundant and dominant creatures, outnumbering even borogoves, both creatures being the prey of the utterly frumious bandersnatch. Clams were always something of a sideshow. But clams are all that’s left, and momewraths, borogoves, and frumious bandersnatches have disappeared from the earth without leaving a trace. If this sounds fanciful, here is a real example. In the Doushantuo phosphorites of China you can find fossils of 600-million-year-old creatures preserved so beautifully that you can pick out individual cells. The fossils are all microscopic—anything larger than a pinprick is absent. Nobody really knows why.23 Neither does anyone know what these creatures were.