The Accidental Species: Misunderstandings of Human Evolution (13 page)

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.
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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.
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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 pre
serve 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.
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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.
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They have also been found in coprolites—fossilized feces—of dinosaurs.
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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.
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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, say, 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.
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Neither does anyone know what these creatures were.

You might contend that the case of the Doushantuo phosphorites seems like special pleading. They represent, it is true, a rather specific set of circumstances somewhat different from the usual run of fossilization. I invite you, therefore, to consider the conodonts. These are fossils of small but elaborately constructed arrangements of toothlike elements of such abundance and variety that many rock strata are
known by the species of conodonts they contain. The problem is, nobody had any idea about the kinds of creatures to which these toothlike fossils belonged.

Many different candidates were offered, more or less bizarre, but the case could not be settled because no fossils had been found that preserved conodonts and any associated animal in any convincing way.
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One of the most peculiar candidates (in a pretty weird bunch) was a 320-million-year-old fossil called
Typhloesus wellsi
, found with conodonts in its insides.
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Critics argued very reasonably that
Typhloesus wellsi
wasn’t the conodont-bearing animal, but a predator that ate conodont-bearing animals, leaving only the conodonts to fossiliferous posterity. To this day, nobody knows what kind of animal
Typhloesus wellsi
was. All we had was a tiny glimpse of momewraths being eaten by bandersnatches of dubious frumiosity. Considering the conodonts as a whole, it was as if we human beings and all our works vanished utterly, all except for our dentures.

Eventually, fossils of soft-bodied, eel-like animals were found in which conodont elements were found arranged at one end, like teeth. A few more turned up just to show that this wasn’t a fluke, and a consensus was reached that conodont animals were akin to fishes, although representing an entirely separate evolutionary experiment in aquatic vertebrate life.
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It has to be said that not everyone agrees with this view,
²⁷
but the fact remains that conodonts are so common as fossils that the oceans must, at one time, have seethed with these creatures—all now gone. All, that is, except for their enigmatic smiles, like so many million Cheshire cats.

Measuring completeness on a larger scale is even more problematic.

One of the most important repositories of paleontological information is the catalogue of fossil diversity first assembled by the late J. John “Jack” Sepkoski of the University of Chicago.
²⁸
Sepkoski tracked down every report of every kind of marine invertebrate fossil ever found, charting their first and last occurrences in the geological record, and their ranges in time. Using Sepkoski’s magnum opus, other paleontologists have sketched broad outlines of the history of life, noting—for example—epochs in which life seemed more or less abundant, in which entire “guilds” of creature replaced one another over geological time, and episodes of “mass extinction” in which life seemed almost to wink out altogether. Databases such as this have allowed paleontologists to approach the completeness of the fossil record in an altogether
more scientific, quantitative way, applying statistics to the known unknown.
²⁹

Let’s go back to that quarry where you’ve been collecting fossil clams. Imagine you are interested in the fossil record of just one species. You know that fossils of this species have been collected over a range of 20 million years, based on the first and last known occurrences and reports from perhaps a dozen localities in between (of which your quarry is one). Now, ask yourself this question: how “complete” is the fossil record of this species?

In one sense the answer is “not at all,” given that you know of only a few fossils of this species, representing a spread of at least 20 million years. Perhaps billions of individual clams of this species lived and died during this period, in which its fossil record is infinitesimal. However, the record is just good enough to show that the species existed and survived for a span of time, so one can get a measure of whether this range of 20 million years bears any relationship to reality.