It is impossible for human beings fully to understand either themselves or their long prehuman history without knowing something of the process (or, rather, processes) by which our remarkable species became what it is. This is, as (almost) everybody knows, evolution. And although most of us have a vague idea of what evolution is all about, few realize quite how many factors have typically been involved in the evolutionary histories that gave rise to the diversity of today’s living world. For evolution is not, as we often believe, a simple, linear process; rather, it is an untidy affair involving many different causes and influences.
Evolutionary biology is a branch of science, and our perception of the nature of science itself is often flawed. Many of us look upon science as a rather absolutist system of belief. We have a vague notion that science strives to ‘‘prove’’ the correctness of this or that idea about nature and that scientists are aloof paragons of objectivity in white coats. But the idea that some beliefs are “scientifically proven’’ is in many ways an oxymoron. In reality, science does not actually set out to provide positive proof of anything. Rather, it is a constantly self-correcting means of understanding the world and the universe around us. To put it in a nutshell, the vital characteristic of any scientific idea is not that it can be proven to be true but that it can, at least potentially, be shown to be false (which is not the case for all kinds of proposition).
Science has made huge strides in the last three centuries or so, bringing humankind extraordinary material benefits. And it has advanced not only through a remarkable series of insights into how nature works but by the testing of those insights—or of aspects of them—and the rejection of those that ultimately cannot stand up to scrutiny. Science is thus inherently a system of provisional, rather than absolute, knowledge. Unlike religious knowledge, which is based on faith, scientific knowledge is grounded in doubt—which is why these two kinds of knowing are complementary rather than conflicting. Science and religion deal with two intrinsically different kinds of knowledge and address equally important but utterly different needs of the human psyche.
Clearly, then, to say disparagingly that ‘‘evolution is only a theory’’ is to dismiss the entire basis of the very science to which our unprecedented modern living standards and longevities owe so much. For evolution is a theory that is as well supported as any other theory in science. At the same time, though, it is a theory that is widely misunderstood. A common misperception of evolution is that it is a simple matter of change over time: a story of almost inexorable improvement over the ages, in which time and change are pretty much synonymous. But the real story is a lot more complicated—and a lot more interesting—than that.
In 1859, when the English naturalist Charles Darwin’s revolutionary book On the Origin of Species by Natural Selection was published, the notion of evolution was already in the air. Geologists and antiquarians were aware that both Earth and humankind had much longer histories than the 6,000 years derived from counting ‘‘begats’’ in the Old Testament; and as early as 1809, the French naturalist Jean-Baptiste de Lamarck had already discarded the notion of the fixed and unchanging nature of living species in favor of a view of the history of life that involved ancestral species giving rise to newer and different ones. Lamarck’s insight derived from careful studies of the fossils of mollusks, which he found he could arrange into series over time, one species gradually giving way to another. But Lamarck was even more daring than this. In an age when belief in the literal truth of the Bible reigned supreme, he was even willing to speculate that humans had arisen through a similar process, from apelike forerunners that had adopted upright posture.
These were brilliant perceptions, but Lamarck was too far ahead of his time for his insight to be appreciated by his contemporaries. What’s more, history has also treated him harshly, this time because of his explanation of how one species could transform into another. Lamarck believed that species had to be in harmony with their environments, yet from his paleontological studies he knew that environments were unstable over time. So species had to be able to change too. And this, Lamarck thought, must have been achieved through changes in their behaviors. Like many others of his time Lamarck believed that, during the lifetime of each individual, such new behaviors would elicit changes in its structure, and that these changes would be passed along from parent to offspring. It was such a process, he thought, that had given rise to the pattern of change he saw in the fossil record.
Most of Lamarck’s colleagues savagely (and justifiably) attacked this notion of the inheritance of acquired characteristics, with the result that the evolution baby was thrown out with the bathwater of a flawed mechanism of change. Yet Lamarck had dramatically opened a door that could never again be fully closed. Indeed, even before Lamarck went public with his ideas, the polymath Erasmus Darwin (Charles Darwin’s grandfather) had published a work that anticipated some elements of his grandson’s thinking, although they did not include the key idea of natural selection. And as early as 1844 the Scottish encyclopedist Robert Chambers argued (anonymously) that all species had developed according to natural laws, without recourse to a divine creator. By the time the 1850s rolled around, then, Western intellectuals were subliminally prepared for an explicit statement that all life forms had evolved from an ancient common ancestor.
Charles Darwin nurtured such a notion for two decades, more or less ever since returning in 1836 from a five-year round-the-world voyage (1831-36) on the British Navy brigantine Beagle. He was, however, reluctant to publish his ideas about evolution in a climate of opinion that was still dominated by biblical beliefs regarding the origins of the Earth and living things. It thus came as a shock to him when in 1858 he received from his younger contemporary Alfred Russel Wallace a manuscript entitled On the Tendency of Varieties to Depart Indefinitely from the Original Type, with a request for help in getting it published.
Wallace was an impoverished naturalist who made his living by collecting animal and plant specimens in exotic and uncomfortable places, and the ideas expressed in his manuscript had come to him during a bout of malarial fever endured on the remote Indonesian island of Ternate. These ideas were for all intents and purposes identical to those that had been maturing in Darwin’s mind for years. So who had priority on the notion of evolution? The moral dilemma was resolved by the simultaneous presentation to London’s Linnaean Society, in July 1858, of Wallace’s paper and of some older drafts written by Darwin. Darwin then began writing night and day; his great book was published a year later, and it sealed his popular identification with evolution by natural selection.
The central notion of both Wallace’s and Darwin’s contributions was that the diversity of life in the world today and in the past, and the pattern of resemblances among those life forms, are the results of branching descent from a single common ancestor. ‘‘Descent with modification’’ was Darwin’s succinct summary of the evolutionary process. And thus stated it is, indeed, the only explanation of the diversity of life that actually predicts what we observe in nature. It has never been validly disputed on scientific grounds (and only people with religious motivations have ever claimed to do so). Virtually all the subsequent vociferous scientific argument on the subject of evolution has been over its mechanisms, not over its power to explain what we see in the living world around us. Mechanisms, however, remain a vexing question.
Both Darwin and Wallace were highly experienced and perceptive observers of nature, fully appreciating the complexity of the interactions that occur among living organisms. And to both of them, natural selection (Darwin’s term) was the central evolutionary process. This is how it works. As both naturalists noted, every species consists of individuals that vary slightly among themselves. What is more, in each generation far more individuals are born than survive to reach maturity and reproduce. Those that succeed are the ones that are ‘‘fittest’’ in terms of the characteristics that ensure their survival and successful reproduction. If such characteristics are inherited, which most are, then the features that ensure greater fitness will be disproportionately represented in each succeeding generation, as the less fit lose out in the competition to reproduce. In this way, the appearance of every species will change over time, as each becomes better ‘‘adapted’’ to the environmental conditions in which the fitter individuals reproduce more successfully. Natural selection is thus no more than the combination of any and all factors in the environment that contribute to the differential reproductive success of individuals.
If you think about it a little, natural selection seems a logical inevitability as long as more individuals are born than survive and reproduce—which is always true. And there is thus no doubt that a process of natural sorting is continually happening within populations— even where it tends to trim away the extreme variations, rather than to move the average type in one direction or another. Nonetheless, in Victorian England it took natural selection a long time to catch on as an explanation of evolutionary change. In contrast, the idea that our species, Homo sapiens, is related by descent to ‘‘lower’’ forms of life became quite rapidly accepted—after an initial reaction of public shock and horror immortalized by the reported comment of a bishop’s wife: ‘‘Descended from an ape? My dear, let us hope it is not true. But if it is, let us pray that it does not become generally known.’’
Darwin and Wallace came up with their evolutionary formulations in the absence of any accurate idea of how inheritance is controlled. The observation—familiar to animal breeders since the dawn of time—that particular characteristics are passed on from parents to their offspring was enough for their purposes. It was only after the birth of the science of genetics at the turn of the twentieth century that explicit discussion of evolutionary mechanisms really took off; but in fact the first principles of genetics had been discovered as early as 1866 in what is now the Czech Republic by the abbot Gregor Mendel. However, Mendel’s article about this, printed in an obscure local publication, made no initial impact. His crucial insight—that inheritance is controlled from generation to generation by independent factors that do not blend— languished until 1900, when it was independently rediscovered by three different groups of scientists.
Before Mendel’s time it was generally believed that the parental characteristics of sexually reproducing organisms were somehow combined in their offspring and that it was the blend that was passed on to subsequent generations, between which it was blended again. Mendel saw, in contrast, that physical appearance was controlled by distinct elements—now known as genes—that did not lose their identity in the passage between generations. He recognized that each individual of a sexually reproducing species possesses two copies (now known as alleles) of each gene, one inherited from each parent. If one allele is dominant over the other, it will mask the latter’s effects in determining the physical characteristics of the offspring. But it has no greater a chance of being passed on to the next generation than its recessive companion has, and each of these factors is preserved independently from generation to generation.
We now know that the development of most physical characteristics is controlled by multiple genes and that a single gene may be involved in determining several characteristics. What’s more, we also now know that genes of different types may play very different roles in the developmental process. Mendel was exceedingly lucky in having chosen to study characters of sweet-pea plants that were simply controlled by single genes. Nonetheless, his principle holds: genes retain their identities when passing from one generation to the next—except when errors are made in the replicating process. Once in a while a gene is inaccurately copied from the parental original during the reproductive process. These changes, known as mutations, may have effects of various kinds and magnitudes (and most are decidedly disadvantageous), but they are the source of the new variants that make evolutionary change possible. The molecule of heredity is now known to be deoxyribonucleic acid (DNA).
Once the basic concepts of genetic change had been worked out early in the twentieth century, evolutionary biology buzzed with competing theories for how the evolutionary process proceeded. As you might expect, every possibility was being explored. All scientists agreed that lineages of organisms tended to show physical—and presumably genetic— change over time. But how? Some attributed the change to what they called mutation pressure—the rate at which mutations occur. Others favored the idea that new species were generated from sports—individuals that showed major changes relative to their parents. Yet another group of biologists argued that organisms had built-in tendencies toward change. Almost everyone was bothered to some extent by the obvious discontinuities that can be observed in nature, but at first only a minority opted for natural selection as the driving force of evolutionary change.
Two Views of the Evolutionary Process
There are two basic views of how evolution occurs. The arrows at the left represent the process of ‘‘phyletic gradualism,’’ whereby one species gradually transforms over time into another under the guiding hand of natural selection. In contrast, the notion of punctuated equilibria (right) sees change as episodic; species are essentially stable entities that give rise to new species in relatively short-term events. After Ian Tattersall, The Human Odyssey (1993).
By the 1920s and 1930s a consensus began to emerge from this busy process of exploration, as naturalists, geneticists, and paleontologists converged on a unifying theory of evolution known grandly as the evolutionary synthesis. Exponents of each discipline brought different offerings to the table. The geneticists brought their newfound understanding of the mechanisms by which genes interact in reproducing populations and of how they are passed along and occasionally modified between generations. The naturalists brought their expertise in the diversity of nature and in what species were and how new species might be formed. And the paleontologists brought the history of life: an eloquent demonstration via fossils of the paths along which life had evolved.
The geneticists had the upper hand in this convergence. Although some paleontologists and naturalists had initial misgivings, by midcentury the process of evolution was widely understood as being little more than the slow but inexorable action of natural selection in modifying the gene pools of species over vast spans of time. In this picture, species lost their individuality as they became no more than arbitrarily defined segments of steadily evolving lineages. Of course, the vast diversity of life argued strongly for the splitting of lineages too; but even this was seen as another gradual process that occurred as the ‘‘adaptive landscape’’ shifted beneath species’ feet when environments changed in different ways in different regions. Habitat changes and geographical factors such as mountain ranges rising or rivers changing course were seen as forces that divided single parental species into two or more descendant populations, diverting each into its own particular adaptive avenue. Eventually, each population would become different enough from its parent to qualify as a new species. Simple, eh? Too simple, maybe.
The grand edifice of the evolutionary synthesis was elegant in its simplicity, and it had all the appeal that simple elegance exerts. But, as the philosopher Thomas Kuhn gained well-deserved fame for pointing out, science progresses largely by occasionally overturning explanatory paradigms that no longer fit the accumulating facts. It was inevitable, then, that eventually somebody would notice that the synthesis conveniently ignored some of the complexities in nature that were becoming ever more evident. The first effective blow came from the direction of paleontology—the study of ancient life forms—a branch of evolutionary science that had taken something of a back seat to genetics in the formulation of the synthesis.
As Charles Darwin had been well aware, the fossil record does not in fact furnish the smooth flow of intermediate forms that would be expected under the notion of gradual evolution that he favored. But in Darwin’s day paleontology was in its infancy as a science, and it was still realistic to argue that although the expected intermediates had not yet been discovered, someday they would be. A century and more later, though, during which time untold numbers of fossils had been recovered, sorted, and analyzed, this argument was beginning to wear a bit thin. For the enlarged record still stubbornly refused to yield the expected series of intermediate forms. Instead, as the American paleontologists Niles Eldredge and Stephen Jay Gould argued in a paper published in 1972, the signal emerging from the fossil record was not one of gradual change but one of overall stability with short bursts of change (a pattern they called ‘‘punctuated equilibria’’). As a rule, they pointed out, fossil species have not generally shown evidence of slow change from one into another over the ages. Rather, they have tended to appear in the record quite suddenly, to persist relatively unchanged for periods of time that could stretch into the millions of years, and then to disappear with equal suddenness, to be replaced by other species, which might or might not have been their close relatives. The gaps in the fossil record, Eldredge and Gould suggested, might not simply reflect a lack of information but, rather, might actually be telling us something. More was going on than simple linear change under the guiding hand of natural selection.
The long, twisting DNA molecule is structured like a ladder with a chemical ''backbone’’ forming the legs and the rungs consisting of paired ''bases,’’ which may be of four kinds: A (adenine), G (guanine), C (cytosine), and T (thymine). A pairs only with T, and C only with G, so that each side of the ladder exactly specifies what the other side will be. When a cell divides, its DNA ''unzips,’’ and two identical ladders form where previously there was only one by adding the appropriate bases (available unassembled within the cell) to each unzipped side. In this way the genetic information encoded in the DNA strand is perfectly replicated (except in the case of copying errors—mutations—which form the basis for evolutionary novelty).
The missing ingredient turns out to be a very complex set of factors. Eldredge and Gould focused on speciation, the means by which a parent species gives rise to one or more descendant species. We only think that gradual evolution occurs, they pointed out, because Darwin told us so, very persuasively. But we know that the splitting of lineages (speciation) occurs, for otherwise life could never have diversified—giving us the pattern of groups-within-groups that we see in nature and that is predicted by an evolutionary pattern of ancestry and descent. They saw speciation as a short-term event (maybe, they hazarded, taking 5,000 to 50,000 years—in geological terms, the blink of an eye), rather than one involving gradual change over vast spans of time. They also suggested that most change was concentrated around the event of speciation itself.
The most persuasive evidence for gradual change would appear to be the undeniable indications in the fossil record of long-term evolutionary trends, such as the enlargement of the brain among members of our zoological family, Hominidae (members of Hominidae are hominids), over the past 2 million years or so. Yet, as Eldredge and Gould proposed, evolutionary trends could just as well be explained by competition among species as by processes taking place within species under natural selection. To take the hominid example, it is quite plausible to attribute the apparently rather steady hominid brain-size enlargement that we see in the fossil record to the relative success of larger-brained hominid species in the competition for life rather than to the competitive advantage of larger-brained individuals within each population. According to Eldredge and Gould’s theory, then, each species as a whole plays a part in the evolutionary process, as an actor in the evolutionary play. This idea revolutionized the way in which we perceive evolution.
At this point it’s probably necessary to say something about what species are, which is trickier than one might imagine. Back in 1864 the French biologist Pierre Tremaux wrote, ‘‘Of definitions of species, there are as many as there are naturalists.” Almost a century and a half later his words ring as true as ever. Species are the basic kinds of organisms, the fundamental units into which nature is packaged. Yet there is little agreement on what exactly species are and on how to recognize them. Of course, there are self-evident discontinuities in the living world, and it is generally acknowledged that members of the same species can interbreed successfully, whereas members of different species cannot.
An example of two closely related (yet distinctively different) species descended from the same common ancestor. Both are lemurs (lower primates) from Madagascar: Propithecus verreauxi (right) and Propithecus tattersalli (left). Courtesy of David Haring/Duke University Primate Center.
But when it comes to stating a precise definition, things are not so simple. Lack of successful interbreeding can be a result of lack of inclination, of incompatibilities of the reproductive apparatus, or of the inability of the progeny to develop or reproduce successfully. Each of these things expresses itself in a different way and will give rise to a different species definition. What’s more, members of different species tend to look different or to choose different habitats, and species definitions have been based on these criteria, too. Defining species becomes even more difficult when we are dealing with extinct species. For these are known only from their bones, and they exist in another dimension, time, that adds its own complexities.
Among mammals such as ourselves, fully individuated new species (and it is important to realize that each species is, in an sense, an individual entity) are derived from subpopulations of existing species that for some reason become isolated from the parent populations. If the isolated groups are small, novel characteristics that might appear within their number may become incorporated and passed down through generations. Small group size is apparently a prerequisite for significant evolutionary change of any kind, because large populations are simply too hard to modify. And it is in such populations that physical novelties must thus occur. But physical change itself has nothing itself to do with speciation, which is the development of reproductive isolation—that is, the separation of a new species. Moreover, we cannot even use the concept of ‘‘speciation’’ to help us in reaching a species definition. This is because speciation is not a mechanism but a result, one that may come about for a whole variety of different reasons. Thus, while it is clear that species are fundamental to the evolutionary process, it is also evident that species are to biologists much as pornography is to some U.S. Supreme Court justices, who cannot seem to define it even though they claim to know it when they see it.
The edifice of evolutionary theory is thus very much under construction, and it will continue to be tinkered with as long as there are scientists around to refine it. But despite a plethora of competing viewpoints, it is possible to discern the broad directions in which our understanding of evolution is likely to develop. Most importantly, adding the roles of species and populations to those of individuals in the evolutionary process helps to clarify how change may take place.
When the evolutionary synthesis was formulated, the individual was seen as the paramount entity in evolution. Some individuals were better adapted to prevailing circumstances than others; and it was the reproductive success of the well adapted, and the failure of the poorly adapted, that ultimately propelled populations—over vast periods of time—along the path of improved adaptation. All seemed as simple as that, and this view persuasively reduced complex and critically important phenomena such as the emergence of new species to little more than passive consequences of a basic process of sorting among individuals. Through this process a population could become better adapted to the same environment, it could mark time, or it could change to adapt to a new environment, and that was about all that was needed to make the whole thing work. An attractive formulation for the tidy-minded, perhaps; but, alas, nature turns out to be a rather untidy place.
For a start, let’s look at environmental change. Ever since Darwin’s day, everyone has agreed that shifting—sometimes dramatically shifting—climates have been marked features of Earth’s history and have also been major determinants of the evolutionary patterns we see in the fossil record. Certainly the period during which the human family, Hominidae, has been around has witnessed huge oscillations in environmental circumstances all over the globe. For instance, as recently as 20,000 years ago, parts of Europe that today are covered by oak forests lay under ice sheets a quarter-mile thick. But as this example suggests, such changes have tended to take place on relatively short time scales, much shorter than those that would be necessary for gradual transformation of species, generation by generation, under natural selection. And even in cases where adaptation to dramatically new environments might theoretically be possible, there are more plausible outcomes than adaptive change on the spot. For if a population is suddenly affected by major habitat change, migration to more congenial circumstances, or local or even total extinction, are much more likely to occur than is slow generation-by-generation change to another adaptive state—by which time circumstances might well have changed again.
And let’s look at adaptation, too. Adaptation is a process whereby members of a species fit into their environments in such a way that they can survive and flourish. Too often, though, we look upon adaptation as something that involves the optimization of particular features. We see it as a business of maximally improving the organism’s fit with its environment in every characteristic. Yet a moment’s thought should be enough to show that this cannot be the case. The process that governs adaptation within populations is natural selection, which operates by promoting or suppressing the reproductive success of individuals. Whole individuals, not their separate features. And every individual is an enormously complicated bundle of characteristics, most of which are controlled by many genes and are in turn linked genetically to other characters. There is, in short, no way in which the evolutionary fate of a particular characteristic can be determined without affecting the destinies of many other attributes as well.
Each organism succeeds or fails as the sum of its parts. And as far as the population is concerned, there is no way for particular characteristics to be singled out for promotion or elimination—although with enough imagination it is certainly possible to dream up situations in which a particular attribute might be crucial to success or failure, particularly among features directly related to reproduction. Yet we tend very easily to talk about the ‘‘evolution’’ of this or that aspect of an organism—the brain, say, or the gut, or the limbs, or whatever—without considering that none of these things could possibly have had an evolutionary history separate from that of the species in which they are embedded. In sum, it is unrealistic to look on evolution as a matter of fine-tuning organisms or their components over vast periods of time. What we actually see in the fossil record is the (dimly reflected) histories of species.
What seems to happen, then, is that any successful and reasonably widespread species tends to diversify, developing local variants in different parts of its range. We routinely see this among species of the order Primates, the grand group of living things to which we belong together with the apes, monkeys, and lemurs. Primate species often include recognizably distinct subspecies in different geographical areas. The basis of this phenomenon is doubtless natural selection, at least in part; but it is probable that entirely haphazard influences are also important, for regional variants are likely to differ among themselves at least partly for reasons of random sampling. Subspecies are local populations that differ from other such populations in identifiable features and occupy their own geographic ranges. And, for a while at least, they will be definable in terms of their physical characteristics.
On the other hand, subspecies remain potentially ephemeral, for they will lose their identities if they are reabsorbed within the general population by interbreeding with other subspecies. Speciation—the establishment of a reproductive barrier between groups—is thus necessary if new variant populations are to become true historical entities. And speciation is not at all the same thing as the development of anatomical novelties of the kind that allow us to recognize different subspecies. Indeed, like evolution itself, speciation is not a single process. Essentially, it is a result—the inability or failure of individuals of two groups to reproduce; and this may come about in many ways, through differences at the level of the genes, or of the chromosomes into which genes are grouped, or even of anatomy or of behavior.
The fact that the creation of new species does not equate directly with anatomical change is unpopular with paleontologists, for it often makes it difficult to identify species in the fossil record with any confidence. This is because morphology—an organism’s physical form—is essentially the only thing that paleontologists have to go by in making such judgments. The only other measurable attributes of fossils—their age and their geographic provenance—have an even more tenuous relationship to species identity than their physical form. In general, however, morphological differences between closely related species descended from the same parent species are not large, so the risk of not recognizing enough fossil species on the basis of anatomical differences will ordinarily be greater than that of recognizing too many.
In the end, though, despite the pivotal roles of speciation, competition, environmental change, and extinction in the evolutionary process, it remains true that evolution is also about the accumulation of inherited physical novelties over time in the packages we know as species. How does this happen? A new field, known by the nickname evo-devo (short for evolutionary developmental biology), is devoted to understanding how genetic innovations are related to patterns of physical change and has in recent years been making remarkable strides in this realm. While the evolutionary synthesis was being developed, the underlying assumption was that all genes acted in more or less the same ways, so the assumed gradual Darwinian evolution could be explained by averaging out the effects of several genes acting on each character. Now, however, developmental geneticists have discovered that not all genes are equal in determining physical outcomes. To be quite honest, it still is not entirely clear how genetic information is converted into living, adult beings; but it is known that although changes in most genes have small effects, those in some others may have dramatic effects on major developmental pathways.
Of particular interest here is a class of genes known as regulatory genes because they regulate development in the embryo by triggering (or suppressing) the activities of other sets of genes. The close similarity of many regulatory genes in organisms as disparate as insects, birds, and humans is a powerful argument for the evolutionary relatedness of these beings, as well as a reflection of the fundamental importance of such genes in the development of individual organisms. Genes of this kind are intricate in their workings, and their effects depend both on the interactions among genes and on the sequences in which they are switched on and off. Our increasing knowledge of regulatory genes has begun to shed light on how it is that organisms that appear to have radically different bodies can actually share common ancestry. What’s more, it points to ways in which new forms of bodily organization can arise, not in a series of minute steps over vast spans of time but simply from changes in when and in what combination genes are switched on and off during the developmental process.
This is not only of interest to those who study the evolutionary relationships among the great contrasting groups of living things, but it also has implications for major organizational changes within smaller, closely related groups. A good example of the latter is the transition among hominids from the so-called archaic early upright-walking forms with small bodies, short legs, longish arms, and somewhat curved hands and feet, to tall, striding bipeds resembling our own species. This change was evidently an abrupt one. There are no known intermediate forms between the archaic and modern body structures, so it would seem that the latter appeared on the scene rather suddenly. We don’t know exactly what genetic changes were involved in the shift from one body type to the other, but molecular and developmental geneticists are beginning to lift a corner of the curtain that lies over this mystery. And in the process they have provided a new set of reasons to revise our understandings of the evolutionary process as a slow, stately progression.
Every genetic novelty must, of course, arise in an individual. In his 1999 book Sudden Origins, University of Pittsburgh paleoanthropologist Jeffrey Schwartz broached the question of how such innovations can be transferred from the level of the individual in which they originate to that of the population to which that individual belongs. After all, if mutations do not make this move they will have no evolutionary future. Schwartz started from the observation that mutations that arise as dominant alleles tend to be bad for their possessors, and that successful—potentially advantageous—alleles hence tend to arise in the recessive state. Thus new recessive mutations could start to spread through the population—but invisibly, because they would not be expressed in the anatomies of heterozygous individuals (that is, those that possessed only one of the new alleles, along with a nonmutated allele).
In the early days of evolutionary theory, one idea proposed was that organisms of new kinds might arise as ‘‘hopeful monsters’’ resulting from a major mutation. This notion was roundly condemned on the grounds that such a ‘‘monster’’ would have no one to mate with. Under Schwartz’s theory, however, finding a mate would not be a problem. And anyway, once a critical mass of externally normal heterozygotes was reached, recessive homozygotes—individuals with two copies of the recessive allele, who would thus exhibit the corresponding novel physical feature— would begin to turn up regularly in the population. And at this point natural selection could start to act, favoring one kind of physical form over the other.
Advances such as these are allowing us to glimpse how evolutionary theory—always a work in progress—is likely to develop over the next few decades. But what do they mean for our understanding of human evolution today? For a start, our growing understanding of how evolutionary processes function on a variety of levels is leading us to revise our expectations of what we will find as the expanding fossil record reveals the story of human evolution in ever greater detail. What are those expectations?
More than 2,000 years ago the Greek philosopher Aristotle saw human beings as occupying the highest rung of a great ‘‘ladder of being’’ that ultimately linked them with the most ‘‘lowly’’ life forms—pond scum and so forth—at the bottom. In medieval times, this idea was resuscitated by scholars who sandwiched humans in between God and the angels on top, and the other Earthly forms, from primates on down, below them. Oddly, this persistent notion suited many early evolutionists as well, at least those who saw gradualist Darwinian concepts as an explanation for the progression they perceived in the complexity of life. Paleoanthropologists inherited this notion as they assumed responsibility for interpreting the human fossil record and ultimately found that it was congenial to them, too.
We tend to take what is familiar for what is natural or for what should be, and there is only one hominid species on Earth today: Homo sapiens. Once the evolutionary synthesis had become widely accepted, then, it seemed reasonable to many to assume that the evolutionary story of mankind had consisted of a steady progression from primitiveness toward perfection. Indeed, during the 1960s there arose a school of thought that held that there could, in principle, only ever have been one hominid species on Earth at one time. Over the next couple of decades, however, it became clear from the growing fossil record that this was not the case: a few blind alleys, at least, had been explored by hominid species that had ultimately gone extinct. But nonetheless the linear idea persisted, and some still today defend the notion that there is a ‘‘main line’’ of human descent along which a gradual succession of species can be followed. According to this viewpoint, hominid fossils form links in a continuous chain (admittedly with the occasional side chain) that joins Homo sapiens with its remotest precursors.
Darwin’s sketch of an evolutionary tree of related creatures, from his private ‘‘Notebook B’’ of 1837. This is arguably the first diagram of its kind ever drawn, long before the publication of Origin of Species in 1859. By permission of the Syndics of Cambridge University Library.
With the arrival of the idea of punctuated equilibria and the understanding that species are fully individuated entities, playing evolutionary roles that go beyond simply being intermediates between their ancestors and descendants, some paleoanthropologists began to see the need to rethink this form of received wisdom. Discoveries made during the last quarter of the twentieth century and the beginning of the twenty-first have only served to accentuate this need. It is becoming increasingly apparent that the evolutionary history of the hominid family has not been a straightforward story of the fine-tuning of a major central lineage over the eons. Instead, it has been a dynamic saga in which multiple hominid species have originated, done battle in the ecological arena, and, more often than not, gone extinct. It has been a story of evolutionary experimentation, of exploration of the many ways in which it is evidently possible to be a hominid.
In earlier years, when the notion of the continuous chain held sway, it was possible to view fossils as a succession through time of links in that chain. Thus, if you knew the age of a hominid fossil you pretty much knew what place it occupied in human evolution. In this view paleoanthropology was essentially a business of discovery: find enough links and you would know how and where the chain ran. Now, however, we are beginning to realize that the business of the paleoanthropologist is a lot more complex than that. If species are unique entities defined by reproductive boundaries, we need first to recognize them in the fossil record. And our first order of business after that is to sort out their relationships. We cannot do this by discovery alone, much as we clearly need more fossils! Relationships have to be revealed by careful analysis, an enterprise that paleoanthropologists have only recently begun to undertake. Still, it is already abundantly clear that we have to view ourselves as one twig on a giant branching tree of life, rather than as below the angels on the highest rung of the ladder of being.