Hardly a century ago, the concept of biological evolution was intellectually and morally shocking. Few people embraced it; even many scientists of the late-19th century rejected it. The problem wasn’t really the idea of evolution. Surely evolution occurs; that much was known >100 years ago. Fossils were already abundant then, and agriculturists had for centuries bred crops and livestock in a successful effort to develop healthy, disease-resistant foodstuffs.
The real problem was that many people were disturbed to hear that humans have anything in common with a bunch of hairy apes. When ideas involve ourselves, vanity seems to surface like an irrepressible force. Largely owing to this conceit—often a dogmatic desire by some to put humankind on an anthropocentric pedestal—minor segments of 21st-century society still refuse to accept the reality of biological evolution.
Bioscientists now combine fossil discoveries, genetic assays, and behavioral studies to virtually prove that, of all non-extinct species of life now on Earth, the chimpanzee and gorilla are our closest relatives (Figure 6.22). Humans haven’t descended from these apes, a common misunderstanding. Rather, modern science demonstrates that apes and humans share so many attributes that they likely have a common ancestor. We shouldn’t be able to identify that ancestor from among any of the presently living creatures on Earth, for genes and environments change over the course of millions of years. But such an ancestor should be part of the fossil record. Our common ancestor more likely resides in a museum than in a zoo.
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FIGURE 6.22 — Chimpanzees (a) and gorillas (b), both members of the ape family, are known to be among the closest relatives of humans (c). |
Sketching Lines of Descent To discern our most recent ancestors and thereby trace the ways and means of relatively recent biological evolution, paleontologists rely heavily on the fossil record. Fossils of recent times are generally well preserved, enabling researchers to document evolution with reasonable accuracy. Not surprisingly, older fossils are in poorer condition, often in pieces and hardly recognizable. Much as with stars and stellar remnants of different ages and states, reassembling the pieces of decayed and broken fossils back together is very much like solving a jigsaw puzzle. Trying to understand where and when the reconstructed organisms fit into the evolutionary line of descent is often an even trickier task.
Teeth and skull bones account for the majority of fossil finds ever since people began digging around for artifacts in Earth’s rubble a few centuries ago. Teeth are the most enduring part of any life form because of their very hard enamel. Skulls are the most recognizable part, largely because they are more noticeable than arm bones or leg joints among the sticks and stones of other ground litter. Careful study of these and other bone fragments has enabled researchers to arrive at a consensus for the lines of descent eventuating in human-like creatures. Consider a few examples.
Figure 6.23 illustrates three types of teeth. The molar at the top is that of a monkey (which is not a member of the ape family); it has two pairs of cusps or grinding bumps, for a total of four cusps. The teeth at the center and bottom of the figure are those of an ape and a human; most of their molars have five cusps, and none of the cusps are paired very well, forming instead a Y-shaped pattern. This same Y-pattern characterizing our teeth is found in almost all mammals, now living or extinct. It’s the more primitive of these two tooth patterns. But the line of descent culminating in the modern monkey doesn’t share this characteristic. At some point along the way, an ancestor of the monkey speciated, and moved along an evolutionary path different from that of ancestral apes and humans. Why this genetic change occurred no one knows. "Why" questions here, as elsewhere, are nearly impossible to answer scientifically. At any rate, detailed studies of numerous fossils prove that this change did occur. Furthermore, radioactive dating of the fossil surrounds imply that it probably happened some time ~40 million years ago.
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FIGURE 6.23 — Slight differences in hard, enameled teeth can be used to sketch lines of descent among relatively recent animals. Monkey molars (top) have four cusps, usually paired, as marked by the small green rectangles. Ape molars (middle frame) and human molars (bottom) have five cusps, each oriented around a Y-shaped pattern (orange). (Smithsonian) |
This is one way we can trace various paths of evolution. As suggested by the diagram in Figure 6.24, it's a method of distinguishing the line of descent that eventually led to modern monkeys from those that led to other species, such as modern apes and humans. Even so, analyses of individual teeth cannot distinguish between apes and humans. Another method is needed to document when or how this further speciation occurred.
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FIGURE 6.24 — A rough sketch of recent evolutionary paths now agreed on by most researchers. Minor refinements may be needed as new fossils are unearthed and species’ genes are mapped. |
Figure 6.25 illustrates one of the methods used by researchers to distinguish the line of descent leading to modern apes from that leading to modern humans. Shown there are two jaws, that of a typical gorilla (an ape) at the left, and that of a typical woman (a human) at the right. Careful study demonstrates the following basic differences: Humans have smaller jaws than apes when compared against the total skull size of each; humans have an arched mouth roof (that can be felt with your tongue), whereas an ape's is flat; humans have a hyperbolic tooth pattern versus a rectangular one for apes, as can be clearly noted in the figure; and while both apes and humans have canine teeth, apes have greatly oversized ones that fit snugly into gaps within the opposite row of teeth when their jaws are shut. These are the kinds of subtle differences that help to determine where and when the human line of descent deviated from that of the apes. As suggested by Figure 6.24, this speciation seems to have occurred nearly 20 million years ago.
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FIGURE 6.25 — Differences can also be seen among arrays of teeth, thus helping to distinguish evolutionary paths. Here, an ape's jaw (left) differs distinctly from that of a human (right). (Smithsonian) |
These are just two examples of the many techniques used to decipher relatively recent evolutionary paths. The specific map of where, when, and how all species originated and flourished on Earth is still a work in progress, its details messy and uncertain, although the overall picture now seems resolved after a century of research. In what follows, we chronicle a consensus for the lines of descent culminating in human-like creatures.
Recent Evolutionary Highlights Early in the Cenozoic Age, ~60 million years ago, squirrel-like mammals apparently took advantage of opportunities to increase their chances of survival within a rather meager environment. Depicted in Figure 6.26, these were insect-eating creatures living mainly off the land that had recently undergone severe change, the probable result of an asteroid impact that nearly extinguished life itself. The dinosaurs had vanished by that time, but life at ground level was still a challenge for these ancient mammals. The fossil record implies the existence of somewhat larger reptiles that no doubt survived at the expense of the smaller mammals. Fortunately, sporadic mutations and changing conditions granted some of the mammals opportunities to alter their life styles.
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FIGURE 6.26 — The insect-eating, squirrel-like mammal sketched here is widely regarded as the ancestor to humans, apes, monkeys, and a large number of creatures currently inhabiting Earth. These land-dwelling prosimians, now extinct, lived ~60 million years ago. |
At about this time, many mammalian species invaded the trees. We know this again because of dated fossils found buried in the dirt. Small, furry, with large eyes and grasping hands, they were undoubtedly searching for more food (especially fruit) while trying to escape fierce competition prevalent on the ground. Some of those species found the trees even rougher going and thus became extinct. Others discovered the trees to be to their liking, surviving famously. A few, like the tree shrews of Southeast Asia of Figure 6.27, still thrive today, their traits just-in-time adapted to life aloft. In fact, the trees became a whole new niche, helping to transform these creatures from ground-dwelling, insect-eating mammals to tree-dwelling, banana-eating prosimians. These protomonkeys are the least advanced members of the order of primates, a zoological category to which both apes and humans belong.
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FIGURE 6.27 — The tree shrew, now living in many parts of Southeast Asia, resembles our ancestral tree-dwelling prosimians of some 50-60 million years ago. |
Fossils reveal untold refinements in life forms, lifestyles, and life-and-death issues, as many successful prosimians adjusted to the best available niches within constantly changing environments. Generation after generation of natural selection gradually changed paws into hands. Stubby claws eventually became flexible fingers. And the opposable thumb took shape as a superior tool for maneuvering among the branches. These were not anatomical changes that occurred as individuals grew during single lifetimes. Rather, they were genetic changes endured over the course of millions of years as the environment promoted some traits, rejected many others, and generally made life miserable for most. Favorable mutations gave those few prosimians having good balance, keen eyesight, and dexterous hands and fingers a naturally increased opportunity for survival within their newly found tree-based venue. Some creatures more than others excelled in jumping, leaping, swinging, clinging, grasping, and scavenging. Those better adapted to the lofty environment not only reproduced more efficiently, but also passed their favorable traits on to future generations of offspring. The result, implied by the fossils and bolstered by the genes (see next subsection below), was widespread speciation causing the creation of legions of novel tree-dwelling life forms.
The evolution of accurate sight was a particularly important development, as suggested by Figure 6.28. Trees are, after all, 3-dimensional, unlike the flat 2-dimensional ground. The advantageous trait of smelling on the ground gave way to that of seeing in the trees. Fossils show how, during millions of years and tens of thousands of generations, mutations gradually brought the eyes of some of these tree-dwellers around toward the front of the head, thereby gaining binocular vision and some real value-added benefit. With eyes at the sides of the head, two independent fields of view result, much like the flat perception when placing our nose and forehead against the edge of an open door. Better yet, try catching a baseball or hammering a nail with one eye closed; it’s not so easy without a way to gauge depth.
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FIGURE 6.28 — The head of some tree-dwellers (left) gradually transformed into that of monkey-like creatures (right). Genetic changes, followed by environmental adaptation, shortened the snout and brought the eyes around toward the front of the head, granting the more advanced creatures binocular vision and thus a distinct advantage ~50 million years ago. The shaded areas outline the field of vision. (Lola Chaisson) |
The gradual shortening of the snout and the slow displacement of the eyes toward the front granted some early prosimians an overlapping field of view—called “orbital convergence” of the eye sockets—and thus more sophisticated, stereoscopic vision. Depth perception, in particular, enabled them to estimate distances more accurately among the branches. Clearly, these versatile ancestors of ~50 million years ago had acquired distinct advantages in the struggle for survival. They had become monkeylike creatures—small, four-legged, yet highly mobile. As sketched in Figure 6.24, a major new evolutionary path had originated.
Fossils also disclose that some species of monkeys gradually became larger. Again, a single generation of a given species didn’t suddenly balloon in size because food was plentiful and life in the trees sedentary. Rather, sporadic mutations in their DNA molecules, spread over scores of generations, gave larger monkeys advantages in the competition for survival—not the least being that larger, more aggressive males have clear superiority over smaller ones in the sexual competition for females. Also, bulky bodies usually provide additional protection from predators. On the other hand, large size isn’t a wholesale advantage. Sheer mass brings some problems too. Bigger monkeys, for instance, find it harder to hide and they also need more food to survive. Both advantages and disadvantages accompany most genetic changes. Only when the advantages outweigh the disadvantages are there enhanced opportunities for living.
The ability to grasp a branch securely while simultaneously extending an arm to secure food also provided an important advantage at the time. Figure 6.29 suggests how manipulative fingers and opposable thumbs were already favored by natural selection for some tree-dwellers. In fact, fossil evidence implies that advances in grasping ability preceded the evolution of binocular eyesight (much as we shall note later in the next CULTURAL EPOCH that manual dexterity among early hominids preceded the enlargement of the human brain). Those members of a species unable to cling well enough to hold on, plunged, died, and became extinct. Those without long enough arms to reach the food starved, died, and also became extinct. The obvious advantage was had by those prosimians able to coordinate clinging and grasping simultaneously. Of course, being smart enough to repel attacks from an array of enemies didn’t hurt—the paramount issue of rising intelligence to be examined shortly.
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FIGURE 6.29 — Manipulative fingers and an opposable thumb gave some tree-dwellers another distinct advantage in the constant struggle for survival. Here, a monkey is shown grasping a branch while simultaneously reaching for food. This type of monkey is thought not to have changed much in the past 40 million years, and thus best resembles the advanced tree-dwellers of long ago. (Lola Chaisson) |
The elevated tree environment of ~40 million years ago thus became a fairly comfortable niche for some species of monkeys. These well-adapted creatures could have probably remained on high indefinitely if a problem hadn’t arisen. Fortunately for us, change stirred yet again. Otherwise, we wouldn’t be here.
Leisurely life in the trees thenceforth became troublesome. The ancestral monkeys of ~40 million years ago were so cozily accustomed to their tree-dwelling environment that they multiplied faster than many other species stuck in harsher environments. Time not used trying to survive can be most agreeably spent trying to reproduce. The sexual urge seems an innate biological tendency dating back literally hundreds of millions of years. The result was probably a population explosion, the type of crisis often inevitably followed by a food shortage. Consequently, the prosimians likely survived only by using their limited ingenuity to find new sources of food. And for some, that meant leaving the trees and returning once again to the ground.
Some monkey species elected to stay in the trees; their genes changed unappreciably. Most of those species eventually became extinct, though some still survive in altered form today. Baboons, gibbons, orangutans, and many other modern tree-dwelling creatures are descendants of the well-adapted monkeys that remained in the trees. By contrast, those prosimians that successfully abandoned the trees embarked on a whole new evolutionary path—a path leading to the progressive primates, including humankind itself.
Aegyptopithecus, a species whose fossils were first discovered decades ago near Cairo, is widely considered to be a good candidate for the ancestral creature common to the old-world monkeys and the lineage that led to both apes and humans. The now-extinct aegyptopithecines of ~30 million years ago were as big as large cats or small dogs, with long tails and moderate snouts, yet stood apart from their predecessors by virtue of their notably developed cerebral volume equaling ~5% of our present human brain. Resembling today’s lemurs and reconstructed in Figure 6.30, they dwelt in forests, not venturing too far from the protection afforded by the trees, yet they were also likely venturesome, exploratory individuals. They foraged for food mainly on the ground, all the while gradually evolving larger brains and rudimentary bipedalism, as well as the roots of social and communicative skills needed for the onset of culture nearly 30,000 millennia later.
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FIGURE 6.30 — Aegyptopithecus, depicted in an artist’s conception (at upper right) on the basis of 30-million-year-old fossils, is thought by many researchers to be the common ancestor of apes and humans. (Smithsonian) |
At first notice, it seems foolish for some of the mammals to have taken up residence in the trees ~60 million years ago if some of their descendants were just going to have to come down out of those trees ~30 million years later. But a critically important change occurred while they were aloft: Evolution played out in that much more challenging 3-dimensional environment. When the tree-dwellers returned to live at ground level, they were equipped with qualities that probably would not have been naturally selected had they stayed grounded. With their manual dexterity and binocular vision, among other assets, they were far more advanced than any other type of life then on the ground. In short, had our prosimian ancestors not taken refuge in the trees, they might never have developed several exquisite qualities, many of which we now use, for example, while building and using Web sites like this one. The 30-million-year detour into the trees was well worth the time and effort—so say we humans who have roundly benefited from it.
By ~20 million years ago, the ground-based prosimians had become the dominant life on the planet. Our ancestors of the relatively recent past had become more agile, versatile, and smart. They faced few roadblocks in their rush along evolutionary paths that eventually led to a variety of peculiar animals now inhabiting our planet, not least street-wise human beings.
Genome Mapping Fossils, in principle, ought to tell what and how evolution happened among all life forms. Additional fossil finds will eventually fill the transitional gaps now hampering a full understanding of evolution, all the while the fossil record grows more comprehensive. But, in practice and at present, that record is surely incomplete and will likely remain so indefinitely. For how would we know if or when our knowledge of the fossils is complete? More new fossils might always be found, yet currently lurk in the rubble awaiting discovery.
Genes should also be able to determine the ways and means of evolution. In fact, many geneticists feel so confident about molecular biology now underway in biotech laboratories that they someday expect to put paleontologists out of business. Those researchers who work with atoms, molecules, and genes often claim a decided advantage over those who work with bones, stones, and whole organisms. Some geneticists even regard as irrelevant the past many decades of paleontological studies of the bumps, grooves, and uncertainties of ancient fossils. This ongoing competition between evolutionary and molecular biologists has raged for half a century since DNA’s double-helical code was broken and much of the mystery of life solved.
Just as DNA samples have become the evidence-of-choice in criminal courts—even more so today than fingerprint identification—genetic studies promise to bring clarity and objectivity to a field often mired in subjective interpretation of withering bones and ancient artifacts. With genetic mapping, there’s no digging for fossils, no reconstruction of skulls or skeletons, just the need to collect small samples of blood. Since variations among DNA nucleotide sequences in a modern population is a summary of events of the remote past, comparison of those sequences should enable biologists to construct an evolutionary tree of sorts. In other words, since genes change naturally with time, researchers can use genetic differences among species to test how long ago those species’ ancestors split apart—provided that genes mutate at a relatively constant rate, which is tantamount to saying that we know how fast the molecular clock runs. Metaphorically, each mutation represents a “tick” of the molecular clock, and therefore the greater the difference between two species, the more time has passed since they diverged from a common ancestor. Although genetically mapping current diversity among living things is straightforward, extracting information about long-dead life forms is more challenging—and this alone will likely keep the rival paleontologists in business for the foreseeable future. In fact, most paleontologists counter the geneticists, claiming that molecular clocks are inaccurate since some of them ticked with different rates in different lineages and at different times earlier in history.
Huge databanks have now been created as part of the Human Genome Project, an international collaboration to decipher the number and sequence of nucleotides among all of humankind’s ~22,300 genes—in all, some 3 billion nucleotide bases, or enough A, T, C, and G’s to fill several hundred telephone books. (By contrast, typical bacteria each have ~1000 genes and typically a few million bases.) For some scientists, this project—the last major science goal of the 20th century—was as spectacular a triumph as the landing of men on the Moon. Its size and scale (and expense) were unprecedented for biology, indeed its achievement changed much of the way biologists now ply their trade at the start of the new millennium. We have likely exited the “century of physics” and entered the “century of biology,” with major implications for medicine and society. Alas, though an average human genome might be a thorough description of our DNA, it’s by no means a complete explanation of what makes us human. Furthermore, there’s no “the” human genome, as everyone’s genome differs slightly lest we all look and act identically. The book of life needs to be read, interpreted, and comprehended for a full understanding of who we really are, individually.
Genomes have been constructed for dozens of other living species as well, with thousands more to come in the next few years, altogether granting enormous promise for appreciating the coevolution of life and Earth. Preliminary studies already suggest that gene sequences shared by different groups of organisms can be used to infer evolutionary relationships—hence the new interdisciplinary field of phylogenetics, which during the past decade has begun to challenge widely held notions about the history and evolution of life on Earth. As noted earlier in this BIOLOGICAL EPOCH, these are the sequence-based methods that have caused biologists to revamp recently the way all of life is classified—namely, the three domains of bacteria, archaea, and eukarya, the first two of these being prokaryotic and the last eukaryotic thus including all known animals.
DNA sequence data are also questioning the conventional wisdom of evolutionary branching, especially those times when speciation occurred in the ancestral lineages of many groups of higher organisms. Whether attempting to determine the past time of common ancestry of hippopotamuses and whales (~60 million years ago), or perhaps that of giraffes and antelopes (~30 million years ago), or even that of humans and great apes (~10 million years ago), geneticists are striving to better decipher the mode and especially the tempo of evolution. That said, their molecular techniques are new and their results often at odds with the traditional methods of paleontology, hence they are currently causing more discord between what should be complementary avenues of historical enquiry.
Take chimps, for example. Studies of selected genes from our closest relative have consistently found that ~98% of human DNA is identical to that of chimpanzees (and >99.9% identical to the person next to you). That means there are fewer genetic differences between chimps and humans than between horses and zebras or between dolphins and porpoises. Only a small part of the human genome is responsible for the traits that make us human—including our ability to walk, talk, write, build intricate things, and enact moral imperatives. Even so, chimps look and act like us only to an extent; their anatomy and behavior do distinctly differ from ours. (Admittedly, a 2% difference among an estimated billion base pairs does still allow for millions of variations among strings of nucleotides that govern protein manufacture.) So, what comprises those 2% biochemical differences and can we trace them back to their genetic origins—to look back in time to infer evolutionary insight? If recent studies are correct, it’s not only the number of gene differences that’s telling, but also the relative activity (or “expression” by which they produce proteins) of certain genes. Evidently, gene expression in human brains differs greatly from that in chimps, implying faster rates of neural evolution for our ancestors while on the road to humanity.
Likewise consider living apes and monkeys. Differences in DNA sequences between these two contemporary life forms can help locate in time when they last shared a common ancestor. First, the gene differences themselves indicate how closely or distantly the primates’ lineages are likely to be on the evolutionary tree. Second, using a molecular clock that specifies the rate of genetic mutations, the past time when those lineages split apart can be estimated. The answer is ~25 million years ago, in this case in good agreement with the fossils of archaic apes and monkeys.
By contrast, other phylogenetic studies disagree with those of paleontology, some greatly. Most notably, for the Cambrian explosion when life greatly diversified among many major animal types, the fossils suggest that animals appeared abruptly ~550 million years ago, whereas the genes imply origins roughly twice that old and a good deal more gradually. Could tiny and squishy animalistic creatures have existed for hundreds of millions of years before leaving any hard evidence, or is the fossil record merely incomplete? Generally, molecular data imply older ages than fossil data for many species, including a potential origin of life itself dating back 6-7 billion years—which is either nonsense given Earth’s younger age, or highly significant if life did arrive from space already in tact. Such molecular clocks, admittedly subject to assumptions and uncertainties regarding mutation rates—assumptions ironically that rely on the fossil record for calibration—nonetheless promise to provide evolutionary information where the fossil record is fragmentary or missing.
In the end, both genetics and paleontology will surely be needed to build an elaborate evolutionary tree, or bush, detailing all the many varied paths from life’s origin to the present. Much as astronomers often find useful both optical and radio observations of, say, galactic nebulae or newborn stars, both these subjects—genetics dissecting biology reductionistically from the bottom-up and paleontology treating it more holistically from the top-down—will together yield insights into the ways and means that Nature mixed chance and necessity to give rise to novel, complex life forms. Only then shall we know our deep roots in deep time, based on a deep understanding provided by both microscopic and macroscopic studies of our ancestral origins.