Fossils allow scientists to sketch the divergent paths of evolution and to understand the fine lines of distinction between various closely related species. Not surprisingly, fossils of recent times are usually well preserved, enabling researchers to document evolution with some confidence. The older fossils are in poorer condition, often in pieces and sometimes hardly recognizable. Reassembling the pieces is much like working another of those “jigsaw puzzles” alluded to earlier. Trying to decipher where and when the reconstructed fossils fit into the evolutionary line of descent is another kind of puzzle.
The entire effort of unraveling human genealogy resembles the restoration of a gigantic mural painted over the course of millions of years. To the right, where the scene has been rendered recently by modern humans, the message is reasonably clear. In the middle, the mural is soiled, peeling, and generally deteriorating. Painted by our earlier ancestors, most of the central mural cannot be easily cleaned, nor can it be easily repaired to reveal the message once the dirt and grime have been removed. Toward the left, the oldest part of the painting is usually torn and tattered with perhaps pieces missing since it was sketched so long ago. Like any restoration, the discovery process is slow and painstaking, done very deliberately to avoid destroying the mural and thus the message.
In this section, we first concentrate on what the fossil record tells us about human development. This subject is known by the tongue-twisting name of paleoanthropology, the prefix paleo having been used in earlier epochs to denote the study of old, fossilized organisms (paleontology) and of ancient magnetism (paleomagnetism); here, paleoanthropology means the study of prehistoric humanity. Anthropology itself, deriving from the Greek anthropos, meaning man, is the scientific study of humans—their cultures, their development, their origins; it mixes the results of fossil discoveries with the essentials of animal behavior to decipher 21st-century men and women. In a later section, the behavior of advanced life forms—including the allied subjects of social anthropology and sociobiology—will help us refine the evolutionary paths implied by the fossils. Together, studies of fossilized life and of contemporary life should enable us to appreciate the essence of cultural evolution—the changes in the ways, means, actions, and ideas of society, especially among higher forms of life on Earth.
Anthropologists and archaeologists examining old fossils and artifacts need the virtue of patience in addition to an inquiring, unbiased mind. Very much like detective work, their research strives to tell a story based on scattered, uncertain data—not unlike our earlier efforts to understand stars and galaxies based on few hints and clues about their origin and evolution. The story here, however, in this CULTURAL EPOCH, means more than closing another crime case. It means learning about the origin and evolution of humanity itself, a goal that some find more satisfying, indeed more relevant, than deciphering the details of distant celestial objects.
Our Immediate Ancestors Knowledge of social and cultural advances has accumulated impressively during the past century—an exciting time for science, indeed an era full of startling revelations about our planet and ourselves. Field excursions and site excavations grew in popularity in the early 20th century as physical scientists began realizing that the ground beneath our feet held clues to the nature of Earth—and not just to the geological dirt and rocks but also to former life on Earth. Biological scientists ramped up their discoveries of that life, including a rich fossil record of many ancient creatures that well preceded our existence. And social scientists began uncovering old stone axheads among rough cutting and scraping implements along rivers and inside caves of western Europe. The latter were crude tools, but tools nonetheless—double-edged, teardrop-shaped rocks symmetrically crafted according to a preconceived plan and now termed Acheulean technology, after St. Acheul, a suburb of Amiens, in northern France, where the first examples were found. Subsequent radioactive dating showed many of them to be nearly 100,000 years old. The question arose: Who were the makers of those ancient tools?
Many of the first great fossil finds of prehumans occurred in the mid-19th century. (By prehumans, we mean ancestors of the hominid line, namely, humans and their extinct close relatives.) Around the time that Darwin published his seminal treatise on biological evolution, a primitive-looking skull was found in a cave in the Neander Valley in Germany. This stocky, flattened skull has a low, sloping forehead, a receding chin, and thick ridges over the eye sockets, though it still displays an overall “manlike” appearance. Given that the German word for valley is tal, Neandertal Man became the generic name attached to the original owner of this skull. Though a bit odd compared to today’s human skulls, there’s little doubt of its human origin. However, with only one such skull fossil then known, it was easy to classify it as a deformed specimen of modern man, which was exactly the approach taken at the time. Even as recently as 100 years ago, many biologists who embraced the evolution of plants and simpler animals were still unwilling to accept this skull’s evolutionary relevance to humans.
Throughout the 20th century, many more Neandertal-type fossils were found at dozens of sites in Europe and western Asia. Based on these many discoveries, Figure 7.1(b) is an artist's reconstruction of what Neandertal Man might have looked like. This figure also shows a similar reconstruction based on less primitive human-like skulls excavated from many places scattered across Eurasia, initially near the French village of Cro-Magnon. These newer skulls are among those belonging to Cro-Magnon Man, an entire subspecies of human ancestors Figure 7.1(c). Regardless of the designation, the important point is that many of these odd-looking, though clearly human-related, skulls were unearthed alongside the ancient Acheulean tools. Their proximity clearly implies that toolmaking humans of some sort resided in Europe ~100,000 ago. Since the Cro-Magnon skulls are more modern and their skeletons more slender than those of the muscular Neandertal variety, Neandertal Man is assumed by some to be the ancestor of Cro-Magnon Man; the two may have interbred and coevolved into modern humans. Other anthropologists demur, claiming that the Neandertals represent a divergent branch of hominids that died out (or were killed off) ~30,000 years ago; this majority view is supported by both recent genetic and anatomical studies. Whichever it was, and for whatever reason, the Cro-Magnons replaced the Neandertals ~300 centuries ago. A pivotal question then becomes: Who were the ancestors of Neandertal Man, whose fossils date back ~300,000 thousand years?
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FIGURE 7.1— Reconstructions of Java Man (a), Neandertal Man (b), and Cro-Magnon Man (c), each of them tool-using creatures known to have existed as long ago as ~1 million, ~300,000, and ~30,000 years ago, respectively. (Lola Chaisson) |
Rich fossil troves trace our roots much farther back in time. Human-like skulls and teeth were eventually discovered far from Europe, in an arid riverbed in Java, a large island in Indonesia. These remains—initially one skull cap, one thighbone, and one molar—date back nearly 1 million years and seem even more primitive than those of either the Neandertal or Cro-Magnon people. Yet, as shown in the artist’s reconstruction of Figure 7.1(a), the size, shape, and overall features of the Java Man bone fragments still resemble those of today’s humans. Moreover, the hole at the base of the skull, through which the spinal cord passes, is positioned in such as way that those creatures must have stood erect. They were bipedal, namely they walked on two legs, unlike most apes which need to use their arms as crutches.
Astonishing findings hardly more than 50 years ago, these old human-like fossils predictably drew a great deal of skepticism. It’s understandably hard for us to imagine that erect, humanesque creatures could have lived anywhere on Earth as long ago as ~1 million years before the present. That’s a terribly long time by human standards, equivalent to ~40,000 generations of human life. In fact, 1 million years is >100 times longer than all of recorded history. Put another way, >99% of humankind’s history is told almost exclusively by its fossils.
Confirmation of these startling results followed when many similar fossils were exhumed at widespread sites throughout the temperate zones of our planet. Diggers have now uncovered numerous Java Man skulls, as well as bones of Heidelberg Man in Germany, Peking Man in China, and a variety of other ancient though clearly human-like fossils in Hungary, France, Spain, and Africa. Most of these fossils are on the order of 0.5-million years old, though some are closer to a full million years and a few might even be older—such as a primitive jaw and partial skull found recently in the former Soviet republic of Georgia and estimated to be not quite 2 million years old. Significantly, these aren’t skulls of apes. Nor are they skulls of ape-men. They’re skulls of humans—erect men and women who lived an awfully long time ago.
Since human-like fossils dated to be less than a couple of million years old have skulls and teeth closely resembling those of modern humans, all of them are granted the designation Homo, a Latin word meaning “man.” And to distinguish these older fossils from contemporary bones, a suffix is often added. For example, Neandertals, Cro-Magnons, and fossils of other human-like creatures dated to have lived <200,000 years ago are collectively grouped by the name Homo sapiens, meaning “wise man.” This is the same biological species as modern men and women, though some researchers prefer to endow the most recent humans of recorded history (including ourselves) with yet a special appellation—Homo sapiens sapiens. No doubt another expression of human vanity, “very wise man” is a highly debatable label, especially given the plethora of global predicaments we’ve created for ourselves in the 21st-century.
In contrast, Java Man and other human-like fossils between 300,000 and 1 million (maybe twice that) years old are collectively referred to by the species name Homo erectus, meaning “erect man.” Definitely of human stock, these closely related creatures walked erect and displayed surprising manual dexterity, but their brain volume wasn’t as large and their tool use not as advanced as those of Homo sapiens. Several of these subspecies may have coexisted and even possibly interacted and competed. Apparently many different kinds of hominids lived simultaneously in Africa 1-2 million years ago.
More Ancient Ancestors The treasury of human-like skulls at least as old as a million years doesn’t really solve the central issue of human origins. It merely pushes back in time the key question: Who were the ancestors of Homo erectus? Data collected in recent years have helped flesh out more of the details.
Not until the last quarter of the 20th century—the generation of scientists still working today—has a reasonably clear line of descent emerged. Earlier anthropological expeditions, as far back as the 1920s, provided some of the first clues, mainly rare fossilized skulls having simultaneously human and ape characteristics. More recently, many more hybrid skulls such as the one in Figure 7.2 have been found throughout warm climate regions, notably on the African continent. After each skull was carefully dug up, dusted off, and reassembled from pieces, analysis showed them to have the following curious blend of ape and human traits: an interior skull volume (brain capacity) larger than an ape’s, though smaller than a human’s; a jaw larger than a human’s, though smaller than an ape’s; a forehead resembling an ape’s more than a human’s; canine teeth more like those of a human than those of an ape; a skull aperture through which the brain stem (upper spinal cord) passed, implying that this creature had walked upright, or nearly so.
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FIGURE 7.2 — An Australopithecus africanus skull fossil. This one was found in the East African Rift Valley and dates back ~2 million years. (Smithsonian) |
Such a mixed bag of bone qualities strongly implies that this creature belongs, in both place and time, near the threshold of humanity. Fossils of this hybrid ape/human kind were subsequently given the jaw-breaking Latin name of Australopithecus, meaning “southern ape.” Unfortunately, the earliest findings in the sandy soil of southern Africa couldn’t be dated; sand isn’t radioactive and tends to shift with time. But newer discoveries, with firmer dates, have focused modern paleoanthropological research on the African continent, where it has been ever since.
Excavations during the past few decades have revealed many additional australopithecine skull and tooth fragments. Some of these findings have been made in the same southern African area where the mobile soil hampers dating. Numerous similar fossils were also gathered all along the East African Rift Valley, a giant crack produced by the disjointed drift of that large continent, and there the ordered layers of volcanic rock can be accurately dated. For example, an Australopithecus fossil was noticed protruding from volcanic ash along a dried riverbed at Olduvai Gorge, Tanzania, an area shown in Figure 7.3, including its ordered layers of volcanic rock that can be radioactively dated. Thus decades after its original discovery, the australopithecine fossils could finally be set in time. That date is ~2 million years ago, an age estimate since verified by more recent findings. Clearly these protohumans, alias ape-men, inhabited our planet a good long time ago.
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FIGURE 7.3 — Shown here are several sites where important fossil discoveries have been made in recent decades: Olduvai Gorge in Tanzania (a), Omo in Ethiopia (b), and Koobi Fora along the eastern shore of Lake Turkana in Kenya (c). (Smithsonian) |
The official naming of the 2-million-year-old skull remains found at Olduvai Gorge is not without controversy. The discovering Leakey family of Britain and Kenya argued that these are the skulls of a species related to but distinct from the australopithecines. In particular, the co-discovery of primitive stone tools prompted them to propose a new species designation, Homo habilis, or “handy man”; and the utensils they used to be products of the earliest known technology, namely, the pre-Stone-Age Oldowan period after the gorge where they were found. However, the rock chips and bone flakes that the Leakeys consider tools are very primitive indeed—well simpler than the crude Acheulean implements noted above—making it hard to assess just how handy these creatures really were. Perhaps the H. habilis fossils are merely those of advanced australopithecines and not ones deserving of the humanesque status of the genus Homo.
This and other controversies have fueled competing theories for the origin of modern humans. One, the multi-regional hypothesis, based mostly on fossils, holds that humans arose in several parts of the world as long ago as 2 million years, thereafter spreading, evolving, and culturally exchanging as a single species. When descendants of H. erectus later left Africa, they interbred with hominids already and locally sapienized in Europe and Asia, including Neandertals, giving rise to the ethnic and racial diversity seen in today’s populations. By contrast, the out-of-Africa (or uniregional) hypothesis, based mostly on genetic analyses, posits a much more recent African origin for modern humans, perhaps hardly more than 150,000 years ago—a new species, distinct from Neandertals and other ancient humans, whom they later replaced without any interbreeding. In support of the former idea, recent DNA samples recovered from Neandertal fossils have provided some evidence that modern Europeans do carry Neandertal genes, if only a very small percentage. Thus, it is currently unclear if the molecular (gene) clock is once again clashing with the dates of many fossilized bones, and many more data will be needed to resolve this issue. Like other “either-or” issues in our cosmic-evolutionary story, the truth lay most probably somewhere in between—in this case several migrations across Eurasia, each perhaps interbreeding with or otherwise displacing those pre-humans and humans who went before.
Sketched by an artist in Figure 7.4, the 2-million-year-old adult ancestors were <150 cm tall, weighing ~50 kilograms, or roughly 5 feet and ~100 pounds. Although they were surely smarter than any other life forms with which they shared the open plains away from the forests, their brains were probably not large enough to have managed speech; rather, they more likely communicated using a repertoire of grunts, groans, arm gestures, and other body movements. The more talented members surely possessed dexterous hands, nimble fingers, and keen eyesight—not as good as ours, but good enough to fashion simple stone tools. The eye-hand-brain combination was again at work, surely to their advantage. Whatever their full attributes, these creatures seem to have adapted well to their changing environments, for adaptation above all else is a key to survival.
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FIGURE 7.4 — An artist's reconstruction of A. africanus emerging from the forest several million years ago. (Lola Chaisson) |
Recent fieldwork reveals that at least 2, and maybe 4 or 5, types of hominid creatures might have coexisted throughout Africa several million years ago. Hundreds of australopithecine fossils have now been classified into at least two distinct species of prehumans. One of these is characterized by a huge jaw and large grinding teeth, suggestive of a species that enjoyed a diet of mostly coarse vegetation, much like that eaten by modern gorillas. This more robust type is often called Australopithecus boisei, or A. boisei and sometimes A. robustus for short. Paleoanthropologists are notorious for inventing tongue-twisting names, and whether these are different species, subspecies, or variations within a given species, no one yet knows. The truth still lies well-interred as buried bones, for the fossils are too old for genes to help much here. The other type, Australopithecus africanus, or A. africanus, or even A. gracile, is of the originally discovered southern African variety. This species is typified by a more slender jaw and smaller molars, implying a gentler anatomy and thus a class of prehumans that probably feasted on meat. Those implications are just that—suggestions and not conclusions—but they do represent the prevailing view among anthropologists today.
Given the duality of these findings, we might naturally wonder if the observed differences in the australopithecine fossils could simply be variations of the same species. After all, today’s humans display slight, yet obvious, differences; contrast thin sprinters against husky weight lifters, or jockeys against wrestlers. This interpretation doesn’t seem tenable, however, since all the 2-million-year-old fossils of prehuman creatures fall into two distinct categories: Either skulls and teeth are clearly big and oversized, or they are small and graceful. How about male and female? Could these two types of australopithecine fossils correspond to sexuality? Again, this interpretation seems improbable because the two classes of fossils are hardly ever found at the same place within sedimentary rock; boisei is linked to east Africa, gracile to south Africa. Unless highly peculiar behavior among prehuman cultures kept tribes of males separated from females, it would seem impossible that these classes correspond to sexual differences. Besides, males and females could hardly reproduce their species while segregated.
Thus, at least two species of protohumans, and quite possibly more, apparently shared the same niche on Earth a few million years ago. Presumably, only one of these species is our true ancestor. Further field work tentatively shows which one.
More Recent Field Work Expeditions throughout the East African Rift Valley have revealed much new information during the past few decades. Figure 7.5 maps the sites of several important discoveries, as well as some places where major groups of researchers are now operating. Besides the rich lode at Olduvai Gorge in Tanzania, several groups are continuing to trace the thread of our origins by examining fossils found along the shores of Lake Turkana (formerly Lake Rudolf) in Kenya. And, before the (human) “guerrilla” war in the 1980s prevented further digging, particularly well-preserved fossils were found in easily datable volcanic rock at Omo, Ethiopia. (Consult Figure 7.3.)
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FIGURE 7.5 — Locations of some famous anthropological digs that have produced rich lodes of protohuman fossils are shown here by the drawn circles. Most of them cluster along the East African Rift Valley, a region of rapid environmental (tectonic) change during the past several million years, and continuing today. (Lola Chaisson) |
Among the recent discoveries at several of these sites, the most interesting is perhaps what's missing: No A. boisei fossils are <1 million years old. This more muscular prehuman species rather abruptly disappears from the fossil record, implying rapid and unorthodox extinction. The most popular explanation contends that competition between A. boisei and A. africanus was inevitable. Each biological niche can ultimately be filled by only one species, yet here were two prehuman species trying to make a go of it simultaneously. Surprising at first thought, it then becomes clear why the bigger, more robust species was the loser.
Despite their larger physique, A. boisei found vegetation plentiful, allowing a rather comfortable way of life. Such easy living, however, isn’t necessarily conducive to rapid evolution toward something more complex—such as an intelligent technological society. The smaller species was almost surely more versatile, quicker, and perhaps smarter. Only basic intelligence could help A. africanus capture the less abundant meat needed for survival. As a result, natural selection worked to help generations of A. africanus expand their brains, their capabilities, and their niche, all the while apparently crowding A. boisei right off the face of the Earth. This idea is supported not only by the documented demise of A. boisei, but also by the finding that alongside A. africanus are often found stone tools, however primitive. Whether these tools are a measure of A. africanus‘ proclivity for manual dexterity and gradual brain development, or whether they might have been used more directly as weapons to accelerate A.boisei‘s extinction, is unknown.
Differing Viewpoints As for many evolutionary scenarios, the details have yet to be worked out. Many of those details haven’t yet even been excavated and in any case new fossils still likely lurk below the surface that will push dates further back. Accordingly, several alternative views invite revisions in the evolutionary picture outlined here, some of which are sketched in Figure 7.6.
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FIGURE 7.6 — Many evolutionary schemes are consistent with the sketchy data concerning human origins. The simplest one is shown in (a), alongside two other popular but slightly different versions (b and c). The lines of descent are drawn here literally with a pencil because they change so often—with new data, new interpretations, and new controversies. |
Figure 7.6a is the line of descent described above, the simplest scenario consistent with the data. By contrast, Figure 7.6b shows the view preferred by scientists who feel that the tool-using creatures of ~2 million years ago should be labeled H. habilis, not A. africanus. But, again, other workers assert that these oldest “tools” aren’t tools at all. The controversies mostly amount to semantics yet typify the wide range of interpretation among experts. Still other researchers contend that Homo dates back hardly more than 1 million years, while a few argue that some species of Homo existed at least 2, and perhaps 3, million years ago (Figure 7.6c).
Recent fossils do seem to push our lineage farther back, as skull, tooth, and bone fragments discovered in the Afar lowlands of Ethiopia argue for human-like creatures nearly 4 million years ago. Figure 7.7 shows some of the data backing this claim. Similar skulls having smaller brains and larger canine teeth than ours were found alongside footprints preserved in the hardened radioactive ash, near Laetoli, Tanzania, implying that these awfully ancient creatures—the most famous of which is “Lucy,” a partially complete skeleton of a teenaged female—must have stood erect. Yet she was also an ape-like tree-climber, based on study of her elbow and shoulder joints. Accordingly, these fossils probably comprise the best evidence for the remote ancestor midway between apes and humans—a missing link of sorts. A whole new species, A. afarensis, has been proposed as this common ancestor of H. sapiens as well as the extinct A. boisei. Opponents argue that these Ethiopian fossils simply belong to the A. africanus species, but acknowledge that that already distant ancestor must be pushed even farther back in time. Still others claim these fossils imply a more primitive version of H. habilis. Whichever evolutionary viewpoint is correct, these oldest human-like fossils virtually prove that our ancestors walked erect before their brain enlarged appreciably.
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FIGURE 7.7 — The jaw structure (a) of the A. afarensis species (center) has qualities midway between apes (left) and humans (right). The jaw’s protrusion beyond the face plane, the extent of curvature of the tooth pattern, and the canine teeth themselves are all intermediate for the A. afarensis fossil. Frame (b) shows an artist's reconstruction of A. afarensis—a proposed link between the great apes and erect prehumans—based on a particularly rich trove of ~4-million-year-old fossil remains. (Smithsonian; Lola Chaisson) |
So many different evolutionary paths are consistent with all the fossil data that a cynic might remark that there seem nearly as many possible paths as there are paleoanthropologists. More than a dozen different, yet often coexisting, hominid ancestors have been proffered by researchers, many of whom tend to assign new fossil finds to novel rather than established species. Such was the case recently when archaeologists announced the discovery of skeletal remains of extinct, diminutive humans, termed Homo floresiensis, who lived on the tropical Indonesian island of Flores only 18,000 years ago. Each new hominid bone often seems more like a monkey wrench upsetting experts’ cherished ideas of our human lineage, and the resulting subjectivity helps to reinforce this social study’s reputation as a “soft science.” Pirating of data, poaching of fossils, acrimonious debate, even law suits have plagued work in this field as fiercely contentious investigators vie to decipher one of the biggest puzzles in modern science. The real problem here is that the current picture of human evolution is based on hardly more than a roomful of partially crushed skulls and broken teeth, most of them found scattered across East Africa, Asia, and central Europe. The whole lot thus far unearthed doesn’t contain enough parts to reconstruct a single skeleton of an australopithecine; the oldest complete hominid skeleton is that of a ~60,000-year-old Neandertal.
We need not overly confuse the issue here. Most anthropologists do agree upon a general evolutionary trend over the past several million years: Australopithecus --> Homo, or near man --> true man. What’s more, nearly everyone concurs that hominids arose in Africa, stayed there for a few million years, and then began colonizing the globe—migrating first into Asia, then Europe, and thereafter the Americas. The controversies, frequently shedding more heat than light, essentially concern details—specific dates, emergence of new species, coexistence of many species, invention of tools, cooperation in hunting, development of language, among many other humanist milestones. The emotion is genuine, for at stake is our own origins. But until more fossils are unearthed and genes begin to weigh more heavily in the diagnosis, multiple viewpoints will continue to flourish. All of which is fine and even useful, since each viewpoint is a slightly different idea to be tested experimentally with new fossils and new geonomes. This is the way the scientific method really works—warts and all—indeed the way science progresses, somewhat subjectively in the short term and often more objectively in the long term.
Shifting Environments Ample evidence does exist that environmental change has made its mark—right up to the present and continuing. As with all changes, whether physical, biological or cultural, evolution among our ancestral species is an accumulation of adaptive responses to changing environments: Nearby supernovae can trigger the infall of galactic clouds, causing some of their contents to form stars, all the while hopelessly scattering others. Flash floods or geological faulting can speciate organisms caught in the midst of change so rapid they can no longer interbreed, allowing some to adapt and survive while others go extinct. Climate change can create long-term hardships for previously thriving hominids, forcing some of them to come to grips with new venues, whereas others aren’t so lucky—or resourceful.
In Africa, the birthplace of humanity by virtually all accounts, the Rift Valley (Figure 7.5) cuts across the eastern part of the continent from north to south, causing climate and vegetation to vary dramatically on each side. Today, we find wet western woods giving way to dry eastern grasslands, the whole valley comprising a rapidly changing ecology. As tectonic events and widespread draughts began affecting the valley landscape ~8 million years ago, environmental change naturally separated our distant ancestors into two groups. Those in the western, tropically forested part of the valley became our closest cousins, the chimpanzee. Those in the eastern, drier, open savanna evolved differently, ultimately becoming human. To be sure, today’s chimps live only in the wet and woody western part of the valley, whereas hominid fossils are found mostly to its east.
Closer in time, ~3 million years ago, changing environments once again speeded evolution. Climatologists know that global climate was changing rapidly and often, and that the whole Earth was then cooler as ice sheets advanced over parts of North America and northern Europe. Eastern Africa, in particular, became yet drier, shifting the vegetation from plants adapted to humidity to those capable of thriving in more arid lands. It was in those open plains that the earliest hominids had to become more mobile, adept at long-distance hunting and skilled at opportunistic scavenging, for it was meat they were then after—and with it presumably came the rudiments of bipedalism. Our remote ancestors, also subject to those climatic fluctuations, had evolved more or less in turn: afarensis --> africanus --> habilis --> erectus --> sapiens. Despite the diversity of fossil finds suggestive of a messy evolutionary tree or bush, some researchers contend that they probably came forth in a reasonably linear parade within a single lineage of hominids over the past several million years.
Not surprisingly, some researchers in the field disagree. Instead of a steady procession of hominids, others prefer a bushy lineage tree with different hominids hanging from different branches at the same time, making it difficult to draw clear lines of descent. More than most sciences, paleoanthropology has a greater share of controversy among its “splitters” and “lumpers.” The splitters tend to interpret skeletons with pronounced shape differences as belonging to separate species; the lumpers often regard such disparities as anatomical variations within a single species. Usually, it’s a matter of perspective, with subjectivity challenging objectivity in the absence of much hard data—yet the scales will tip eventually, with objectivity ultimately trumping as new findings accumulate.
Oldest Human-like Fossils The prevailing view that the A. africanus and afarensis species were on the evolutionary path linking modern humans and whatever ancestry we share with the apes is certainly instructive but, even if valid in every respect, it simply pushes the basic question at hand still farther back in time: Who were the ancestors of the australopithecines? And here the answer becomes much more vague because the fossils are older, scarcer, and less well preserved.
Few discoveries have been made of hominid fossils predating the ~4-million-year-old specimens of the Afar lowlands. Just recently, however, an international team of paleoanthropologists working in Ethiopia has apparently pushed back our ancient kin yet again in time. As shown in Figure 7.8, a handful of bones, a partial jaw, and several teeth from at least a half-dozen individuals of the species Ardipithecus (a name meaning “root ape” in local dialect) have been dated to be ~5 million years old; and a hominid cranium (called Sahelanthropus, or Toumai after a region in the West African Sahara) from Chad is perhaps as old as 6 million years. Though these bones are comparable in size to those of modern chimpanzees, their dental features resemble other hominids more than either fossilized or living apes. These fossil finds now overlap in time when DNA studies suggest that a common ancestor of humans and chimps lived in Africa 5-7 million years ago—a common ancestor who was chimp-like, forest-dwelling, knuckle-walking, mainly arboreal and fruit-eating.
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FIGURE 7.8 — These ~5-million-year-old tooth and jaw fragments are among the very few protohuman fossils ever found within the time interval 4-10 million years ago. The fossilized jaw (foreground in left frame) closely resembles a human jaw placed behind it for comparison. However, the same fossil (at right in right frame) is clearly more robust than a human jaw (at left in right frame) which has been cut to show its thinner cross section. (Harvard) |
Creatures having some human qualities, then, possibly resided on Earth even >5 million years back, but it’s frustrating that so few prehuman fossils have yet been found at the base of the hominid tree for the period 5-10 million years ago. Plenty of fossils from that time span—known to some researchers as “anthropology’s black hole”—lie scattered about in Earth’s soil, but these are usually the remains of animals unrelated to humans. It was during this period that East African habitats changed dramatically, causing both widespread extinctions and the rapid rise of ancestors of animals such as giraffes, rhinoceroses, and antelopes—but not leaving much by way of early hominid remains. Some additional exceptions are fossils of an arm bone and some jaw fragments found near Lake Turkana and embedded in rock dating back ~5 million years. Most workers concur that these remains belong to an australopithecine, or whatever preceded that species, though no one can be sure on the basis of one arm bone and a partially crushed jaw. And a ~6-million-year-old thickly enameled molar tooth has been uncovered at a nearby location. While a single tooth can hardly be used to trace hominid ancestry with any degree of assurance, it’s enough to know that hominids were there then.
Tooth and jaw fragments of the oldest known creatures having any resemblance to humans or prehumans were discovered in India and later at several other places in Africa, Asia, and Europe. Figure 7.9 shows a representative sample of these fossils. Radioactive dating of the rocky dirt in which they were buried implies that these fossils are 8-12 million years old. Despite this old age, the jaws in particular still seem to have a mix of ape-like and human-like qualities. The creature’s brain capacity and anatomical posture are unknown, however, since a complete skull has never been found. Only a few good fossils exist, none of them well preserved. Some anthropologists, examining mainly bone shapes, contend that this fox-sized fossil-ape, originally called Ramapithecus in honor of the Indian god Ram, is possibly the ancestor of the australopithecines—a sort of protoaustralopithecine. Again, this is only conjecture based on meager data currently available, but if correct this creature must have ventured in and out of jungles at the time, living partly in the forests and partly on the plains. Other researchers—mostly biologists examining molecules trapped in the bones—insist that Ramapithecus (now called Sivapithecus) is more likely the direct ancestor of our great-ape cousin, the orangutan, and not part of any direct lineage toward humankind. Instead, they nominate yet another fossil primate, Dryopithecus found mostly in Europe and having powerful grasping capabilities for hanging and swinging below branches, as the more likely forebear of humans. Much more fieldwork and critical analysis are needed to sketch a reliable portrait of our distant relatives who roamed Earth ~10 million years ago.
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FIGURE 7.9 — Sketches of scarce fossils (a) enable only a rough portrait of the ~10-million-year-old Sivapithecus creature (b). (Lola Chaisson) |
Earlier than that, and to connect with the prosimians of the BIOLOGICAL EPOCH who came down from the trees ~30 million years ago, anthropologists have found a key transition species between the primate apes, which at the time resembled monkeys, and the living great apes of the present—the gibbons and orangutans of Asia as well as the chimpanzees and gorillas of Africa. Kenyapithecus, now extinct and dating back ~14 million years, is taken by many (though not all) workers to be the ancestor of today’s apes. At the least, all agree that this species was part of a wholesale migration of apes out of Africa and toward Europe and Asia. However, this interpretation hangs on hardly more than a handful of teeth found in western Kenya several decades ago. Other extinct species, such as the prehistoric ape Proconsul found in Africa and dated to be ~18 million years old or a partially complete 13-million-year-old fossil skeleton named Pierolapithecus and discovered recently in Spain, have been proposed as more reasonable candidates for the last common ancestor of both modern men and great apes. The fossil record is indeed sparse, the hominid evolutionary picture murky. Some doubt whether the fossilized bones of extinct apes will ever offer enough clues to fill in the branches of the ape family tree reliably.
Figure 7.10 summarizes the simplest version of current paleoanthropological thinking regarding the relatively recent path of "descent with modification" toward humans. This figure is an enhancement of the evolutionary path labeled "humans" at the very bottom of Figure 6.24.
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FIGURE 7.10 — Relatively recent line of decent toward humans. While extremely simplified, such a sketch does impart an overview of the size, scale, and timing of the evolutionary changes that led specifically to humankind. |
Precisely when and where one species changed into another cannot be pinned down much better than described here, for one life form slowly transformed into another over the course of history. The fossil record will never document an ape-like mother giving birth to a distinctly human infant, or A. africanus parents raising an H. erectus baby. Evolution just doesn’t operate that way. Changes of this sort are usually so gradual as to be imperceptible, occurring over long, long periods of time.
Genetic Insight Paleoanthropologists aren’t the only scientists tracing the evolution of hominids. As noted in the BIOLOGICAL EPOCH, geneticists can scan the DNA of living primates and tally the number of mutations that have occurred over comparable durations of time. Most studies to date contend that humans and our closest relatives, the chimps, last shared a common ancestor 5-7 million years ago—in quite close agreement with recent studies of fossilized bones. However, such a molecular clock relates little about the specific lines of descent among the more recent hominids and their ancestors during the past few million years—namely, what separates modern humans from our extinct forebears. And it doesn’t resolve the heated controversy regarding the number of coexisting hominids in Africa and elsewhere, or the times of hominid migration and species divergence in Europe and Asia, or the reasons for the extinction of all but one of them—us. Even so, the history of the human species must be faithfully recorded in the genes of people still alive today—and therein lies the origin of humankind, one of the great prizes of the CULTURAL EPOCH.
Part of the problem in untangling the genetic routes that led to humanity is that not all molecular clocks are calibrated in the same way. And some of them may tick at different rates, if, owing to variable lifespans and generational turnover, mutations occur less frequently in humans than in other primates and mammals. Discord regarding timescales often arises between bone experts and molecular biologists, thereby prompting us to ask: Do genes or fossils give the best results when mapping evolutionary lineages? One might think that, in principle, careful laboratory counting and sequencing of the nucleotide bases within genes ought to deliver accurate, objective answers, especially when the unearthed bones are often in such terrible shape, their interpretations laden with subjective opinion. But geneticists must still rely on the fossil record to calibrate their molecular clocks, that is, to calculate the number of nucleotide changes that have likely occurred per million years of evolutionary time. The calibration point currently used by most researchers for primate studies is pegged to the ~20-million-year-old split between apes and monkeys. But if this pivotal date is wrong—and it’s uncertain by at least 20%—then the clock’s calibration is faulty, and so are the results derived from it.
Of course, if biologists could extract DNA samples from the fossil bones themselves (and such experimental methods are now being pioneered in a few laboratories), then most of the uncertainty would cease thereby allowing precise tracking of the evolutionary paths among the hominids. For example, how many genetic changes were needed for the onset of Homo sapiens sapiens, say, compared to the Neandertals? Were they our immediate ancestors or an evolutionary dead end of a coexisting type of hominid? Usable DNA is easily extracted even from single hairs of people deceased for a few hundred years, yet this formidable task has been successfully performed on only a few Neandertal skeletons dating back ~50,000 years. (DNA begins to degrade from the moment of death as water, oxygen, and microbes attack it; all claims of DNA extracted from much older dinosaur bones or insects stuck in amber have been discredited.) The tentative conclusion is that Neandertal DNA was sufficiently different from ours and thus their species was a side branch of the human family tree that diverged from ours ~0.5-million years ago, ending in extinction ~30,000 years ago. However, the genetic trail quickly fades away farther back in time. Fossils older than ~100,000 years do not yield measurable DNA samples, thus do not currently bring order to the confusion of bones and stones among the hominids dating back millions of years. Though genes and molecular clocks hold great promise to grant coherence to a very contentious subject—ultimately identifying the specific changes that made us human—they have thus far contributed marginally to this great origins enterprise.