by Henry Gee
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.24 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.25 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.26 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.
The first thing to appreciate is that the first occurrence of a fossil almost certainly does not represent the earliest existence of the species in life. Given that fossilization is rare, the species presumably existed for an unknown measure of time before one individual chanced to have been preserved. Neither does the last known occurrence of a fossil species necessarily record the last ever individual of that species before it became extinct.30 How close can the fossil record get to reality, if fossilization is such an unlikely event? The answer lies in the density of the sampling between the two extremes. If, in that period of 20 million years, records of your species of interest are very sparse, then it is likely that the species tended not to fossilize well, which suggests that it lived long before its first record as a fossil, and long after its last. However, if a fossil species appears quite suddenly, is found pretty much everywhere in large quantities in closely spaced intervals of time, and then disappears without recurrence, we can be more confident that the time interval of 20 million years is a good reflection of reality. By the same token, one can predict that recurrence of the same species after a long gap would be unlikely.
Such recurrences do happen, however, and the reason is to do with geology and the circumstances of fossilization. Let’s say that your clam, in life, preferred to live in shallow seas. Its extinction after 20 million years could be real—or it could simply reflect the fact that geological deposits representing shallow-marine habitats became rare, being replaced by deposits indicative of dry land. The discovery, perhaps, of a younger stratum representing shallow seas would be accompanied by more fossils of your favorite clam. Species that appear to become extinct but miraculously come back to life later on are known as “Lazarus taxa” after the biblical character whom Jesus raised from the dead (John 11:1–44). This phenomenon tells us something very important about fossils, and builds into my entire argument about the problems of building a narrative based on fossil evidence. That is, to become a fossil, a creature has to be found in the right kind of rocks. Rocks and rock types vary over time as much as the creatures whose remains are buried within them.
Recent work has shown that our measures of past diversity are quite sensitively affected by the amount of rock available in which fossils might be found.31 The problem is that rocks (and the fossils they might contain) do not simply accumulate over time. An unknowable (and unknowably large) quantity of rock, created during the earth’s long history, has itself disappeared—eroded, transformed, or sucked down into the ocean floor in the process of continental drift—taking its load of fossils with it to oblivion.
This sounds rather obvious, in hindsight. After all, you can’t go looking for fossils in rocks that don’t exist. However, it makes attempts to reconstruct past life—and account for its variation—rather tricky. It means that any trends we see in the history of life as reconstructed from the entries and exits of fossils might say very little about the history of life, but much more of the history of the rocks in which fossils are found. This implies that a great deal of life’s story happened offstage, without report—and that we might be completely unaware of entire groups of creatures that once existed but have disappeared without trace. It could even mean that some creatures have undergone a kind of double extinction—that even after all representatives died, the few that remained as fossils were also expunged as the rocks in which they were entombed also perished. The stories we tell ourselves—of the rise of amphibians from fishes; of the domination of the earth by dinosaurs; of the subsequent rise of mammals, culminating in the proverbially zenithal apotheosis that is our own species—might very well be a sideshow, a tale that would not be supported were we made aware of the totality of all life that once existed on this planet.
I’ll end this chapter with a discussion on how very close even the known fossil record is to being unknown—how close many species are to the lone copy of Beowulf that we can study and treasure. Sepkoski’s compendium of fossils was based on marine invertebrates for the reason I have discussed above—that marine invertebrates have the best chance of all organisms of becoming fossils. The fossil record of animals that lived on land is much sparser.
Many species of dinosaur, for example, are known from just one or two specimens—if these specimens had not been found, the e
xistence of that dinosaur would not have been reported. In such cases, chance effects such as rock type, and even whether a paleontologist happens to be there as a fossil erodes out of a cliff, before it is destroyed, have large effects—as large as the fact of the rescue of the single manuscript of Beowulf from that fire in 1731.
My favorite case of the Beowulf effect in action concerns a fossil of which you probably haven’t heard. It is called Palaeospondylus, and on its own it’s not much to look at—a tiny fish, between five and sixty millimeters in length. Quite a few specimens of Palaeospondylus are known, but almost all come from a single quarry at a place called Achanarras in northeast Scotland, in Devonian rocks that are around 380 million years old.32 What can one do in such a situation? Estimating the geological range of a species is impossible if all one has is a single point, just one datum, so no one knows when Palaeospondylus first appeared, or when it went extinct. Paleontologists have debated the nature and identity of Palaeospondylus ever since its discovery in 1890, and have yet to reach agreement more than a hundred years later.33 There have been suggestions that it was a larval form that would have grown up into one of the many other, larger fishes known from that part of Scotland of the same age. This idea makes a kind of ecological sense, given that so many specimens of Palaeospondylus are found together in a single place. It might be a snapshot in time of some kind of nursery, a nest or pond in which adult fishes sequestered their brood. There are problems, however—the fossils look far too bony to be larvae, at least of any fish we now know. But if the fishes are adults, we are left with a species without descent or antecedent, a species lost in time. Many, many species in the fossil record are like Palaeospondylus—known from just one locality, in which, perhaps, conditions for preservation just happened to be exactly right; in which the rock was not itself ground into powder with the fossils it contained; which just happened to have been unearthed by geologists and paleontologists who knew what they were looking at: fossils that had just that one, slim chance of making it into the realms of the known. We have no way of knowing the toll of species that were not so favored.
It’s now time to apply this new, pragmatic, if rather chilly, view of the relics of evolution to the fossils that tell of the evolution of ourselves.
5: Shadows of the Past
With a working knowledge of evolution in our pocket, together with an appreciation that any trends or strivings toward perfection we see in evolution or the fossil record are readings we humans have made, after the fact, we’re now ready to delve into the problem of human exceptionalism—the tendency to see ourselves as special products of creation, the result of an inevitable and predictable trend toward improvement and complexity.
In this chapter I’ll pay particular attention to the extreme scarcity of hominins in the fossil record. Despite this scarcity, scientists still apply models of human evolution that are progressive and directed. Each new discovery of a fossil hominin is greeted by the press as a “missing link,” when closer inspection shows that newfound fossils challenge our preconceptions at every turn. When this happens, scientists sometimes fight a rearguard action—the new fossil can’t be new and different, but is really something known in another guise, such as a deformed human or an ape. We’ve seen this tendency at work in the discussion of Homo floresiensis, and it’s not a new phenomenon.
Whatever happened in the past, everyone agrees that there are lots of human beings now. Not long before I started to draft this chapter, the world welcomed its 7 billionth human being. Although nobody could agree precisely which new baby was the 7 billionth, everyone agrees that 7 billion is an awful lot of people, and governments are beginning to wonder how many people the earth can realistically support.1 Wherever you look, the world seems awash with people and the signs of their activities. This is of course entirely obvious in the world’s teeming cities, but it is evident in the countryside, too, and even in apparently pristine wilderness. Much of the earth’s surface has been changed to accommodate human needs for food and water. Human activity has started to change the earth’s climate, affecting remote regions such as Antarctica where relatively few humans have trod. Beachcombers on the earth’s most remote islands, uninhabited and far from the usual shipping lanes, find appreciable quantities of human refuse.2
When you look more closely, we human beings are a varied lot. Our most obvious feature—our skin tone—varies from deeply pigmented to virtually colorless, but humans vary in many other ways, both obvious and subtle, ranging from details of our anatomy to a whole host of differences in our body chemistries. We are not, however, as varied as we sometimes like to think. Compared with many other species, the genetic variation within Homo sapiens—the single species to which we all belong—is rather small. It is smaller, for example, than the genetic variation between the several isolated groups of chimpanzees scattered through central and west Africa—despite the fact that there are 7 billion of us and only a few hundred of them.3
It is easy to cast chimpanzees in the role of Our Ancestors. It is, however, only that, a role. Chimpanzees have been evolving away from our common ancestor for precisely as long as we have. However, chimpanzee variation does give us an insight into what human genetic variation might have been like for most of our evolutionary history. Humans might have been much scarcer—and much more varied.
The earth’s current burden of humanity is an anomaly, for population has surged only relatively recently. When I was a boy, in the 1960s, there were only half as many humans as there are now. Before the invention of agriculture 10 to 12,000 years ago, there were probably no more than a million people on the planet at any one time.4 This meant that population densities were very low, on average about one person for every fifty-seven square miles.
For most of the past few million years, humans and other hominins lived, like chimpanzees, as small, scattered groups, meeting one another only rarely. This meant that genetic variation between different groups was probably higher than it is today, tempered by the occasional exchange of mates. In most primates, it is usual for males to stay in the group in which they were born and raised, and for females to join other groups. This is true for humans, too—and is believed to have been true for early hominins such as Australopithecus.5 On the whole, though, genetic variation in fossil hominins was probably higher than it is in modern humans, even though there might have been fewer individuals.
Rare species living in small groups are prone to becoming extinct by accident. The human genome shows that many human populations, especially outside Africa, were founded by small populations of individuals. It is this effect that probably accounts for humanity’s rather low degree of genetic variation today.6
Several things follow from these arguments. The origin of new species requires genetic isolation between groups that might otherwise interbreed. Therefore, if groups were scattered and genetic variation high, it is likely that some of these groups diverged from one another to the extent that they would be considered as different species, at least when compared with the differences one might find between any two members of Homo sapiens today. There were probably many more species of hominin on Earth at any one time in the past 6 million years or so than Homo sapiens or its immediate ancestors. Second, and following from the arguments I’ve laid out in earlier chapters, the fossil evidence for such hominins will be meager. Third—and rather more controversially—there might be nonhuman species of hominin still around today, or which perished in historical times.
Let’s look at these points in turn.
As we’ve seen, the preservation of creatures in the fossil record is vanishingly unlikely, particularly for those that lived on land. Hominins would have left a sparser fossil record than most, because they were always rare to start with. For all that, between around fifteen and twenty different species of hominin are known to have evolved since the hominin lineage split from that of chimpanzees. The number is inexact, partly because most of the fossils are very fragmentary, and also because scientists ca
nnot always agree on the identification of any particular one, whether it belongs to a new species or is a member of a species that is already known.
Having read this far, it probably won’t surprise you to learn that until recently the “picture” of human ancestry often relied on the assumption that members of one fossil species were directly ancestral to members of other species. This view still persists here and there, but there is one particular doctrine I wish to examine here—that is, the view that only one species of hominin could have lived on Earth at any one time.7 The usual reason given for this idea was that the global ecology could only ever have had room for one species of hominin at a time. As soon as a new species of hominin appeared, the old one was inevitably driven to extinction. It follows from this view—which I’d call the this-town-ain’t-big-enough-for-the-both-of-us hypothesis, except it’s too unwieldy for everyday—that species of hominin had to be directly ancestral to one another, rather than cousins who shared a common ancestor in the past—there would have been no other source for a new species, other than the old one, already in existence. The scenario sprang, I think, from the conventional view of evolution as linear and progressive (the idea of ecological exclusivity being a scientifically dressed-up excuse for this, made after the fact) and from our own experience.
After all, every human being we know belongs to a single species, Homo sapiens. (There were once attempts to classify members of different human races as different species, but such work has long since been discredited and shown to be false.) As far as we know, no other hominin survives on the planet. From this it might be easy to assume that this situation was always so, yet the present era appears to be exceptional. As recently as 50,000 years ago, Homo sapiens shared the earth with at least five others—and these are only the ones we know about. Further back in time, when hominins were probably restricted to Africa, Australopithecus of various species coexisted with at least two species of early Homo. The idea that only one species of hominin lived on Earth at any one time was easy to accept when the fossil record of hominins was even worse than it is now. By 1976, however, the fossil record from east Africa, showing early Homo living alongside australopiths—could no longer be discounted. The human family tree was not a single line, but a bush with many branches, all but one leading to extinction.8
