The Human Journey: Part 3

Below is the third and final part of the human evolution story. Parts 1 and 2 can be found in the previous two posts.

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Touching down once more in the time machine, we find ourselves surrounded by now familiar grassland expanses. The monotonous scene is punctuated only by a mud-lined waterhole, which at the moment is hosting a herd of Elephas recki, the same giant proboscideans from the second act. Standing in the shallows close by, seemingly oblivious to the elephants’ antics, a six-foot tall marabou stork fishes for whatever critters might reveal themselves. African waterholes in late afternoon attract a wide range of thirsty animals, and their predators. After a few hours spent watching the faunal procession, you guess correctly that all of the usual suspects are present—pigs, antelope, zebra, hyena, and the like.

Mesmerized by the setting sun and the building hum of insects, you’re surprised to see five hominins approaching, three individuals moving together in front and two trailing behind. As they get closer it becomes clear that these upright primates, all males, are wholly different than any seen previously. Compared to hobbit-like Ardi and Nutcracker Woman, these individuals are giants, approaching six feet in height. Much of this length is devoted to long, muscle-bound, striding legs. Gone are the elongate, gorilla-like arms, replaced by upper limbs proportioned like our own. Also absent is the profusion of body hair, exposing the naked skin beneath. Indeed, in the waning light of dusk, these figures look so familiar that for a moment you wonder if the time machine has returned you to the present day. But above the shoulders, that sense of familiarity diminishes. The skull dome looks bigger than that of an australopith, yet notably smaller than our own. A prominent nose protrudes from beneath thickened brow ridges. Among the trio in the lead, two are shouldering bulky antelope shanks still encased in mottled brown skin, while another totes some unidentifiable hunk of meat. Bringing up the rear are two younger males carrying stone tools. You elect to follow the group, who, after another a half-mile trek, join up with several females and children. The female adults, only slightly smaller than the males, are busy working with fist-sized stone tools to release nutrients from large tubers, while the four youngsters run about playing.

Welcome to Act III, the most recent of the human journey, extending from roughly 2 million years ago to the present day. We’ve arrived in the midst of the Pleistocene Epoch, about 1.5 million years ago, another landmark moment in our story. Increasing aridity has decimated the forests and driven the proliferation of grasslands. Several mammal lineages, herbivore and carnivore alike, have been traveling northward with the expanding grasslands, and some have recently broken the bonds of the African continent to venture into Asia and Europe. Accompanying these emigrants is a two-legged omnivore, Homo erectus, the very hominin you’ve been watching.

Whereas Act I hominins like Ardipithecus were devoted forest-dwellers, spending most of their time in trees; and Act II protagonists like Paranthropus and Australopithecus split their time between the trees and the ground; in the skeleton of erectus we see the first evidence of a permanent pact with terra firma. Tree-climbing adaptations were replaced by long, lean legs well suited for endurance walking. Brain sizes averaged close to 1000 cc, only about 25 percent less than our own. Regional populations, each with unique traits, developed in Europe and Asia, where they encountered new varieties of large mammals, from mammoth to cave bear. It is unclear whether erectus had made the transition from scavenger to predator by the time of its initial exodus from Africa. But it appears virtually certain that this close cousin of our’s was an effective big game hunter throughout most of its lengthy tenure. The hunting and gathering lifestyle that would eventually characterize the bulk of human history got its start in Homo erectus. The Old World had received its first taste of a carnivorous ape.

One of the perils of walking long distances over hot, arid terrain is water loss. This hazard is exacerbated by a cooling system that depends on sweat glands distributed over much of the body. Like chimps and gorillas, earlier hominins possessed a flat nose marked by a pair of forward-facing openings in the middle of the face. With erectus the bony margins of the nose openings became raised and conjoined, forming a distinctive, sloping bridge. Inside, expanded nasal cavities housed complex swirls of bone lined with spongy mucous. The newly remodeled nose offered a finely tuned water retention device, capturing moisture from the hot, humid air exhaled from the lungs. For similar reasons, although we can’t be certain, Homo erectus may have been the first hairless hominin. Body hair tends to hold onto heat, so erectus’ commitment to life on the hot savannah may well have triggered the loss of a “furry” covering. If so, we can thank Homo erectus for both our prominent noses and our nakedness.

The so-called “Acheulean” tradition of Homo erectus was dominated by a single stone implement—the hand ax. The Acheulean hand ax is the epitome of functionality, crafted by using some sort of “hammer”—often made of bone, antler, or wood—to remove flakes of rock from a “core.” Whereas Oldowan toolmakers were apparently most interested in the flakes, it’s the core that became the chief implement of erectus. By removing flakes symmetrically from both sides, the fist-sized hand ax offered a highly versatile tool that likely served for slicing, scraping, crushing, digging, and other uses. (Can’t you just imagine the late night television advertisement? “Brought to you by Erectus Industries Inc., the amazing tool that slices, dices, and juliennes!”) Homo erectus was the longest-lived hominin, thriving for over a million years. Although we see some evidence of a progressive improvement in tool technologies, the hand ax persisted throughout that impossibly lengthy duration. Confronted by the bewildering pace of technological change seen today—with computers outdated almost as soon as they reach the market—it’s inconceivable to imagine a particular tool being used for a century, let alone ten thousand centuries! What does this staggering monotony tell us of the mind of Homo erectus? On that matter there is much disagreement, but one thing’s for sure. We can safely assume that innovation was not a priority.

How are we to explain this suite of bodily and behavioral features, which pushed the hominin lineage, as one anthropologist put it, over to the human side of the “great divide?” Best we can tell, the eco-evolutionary chain of events went something like this. By 2 million years ago, the human line was devoted to a diet of high-quality, widely dispersed, and difficult-to-obtain foodstuffs, both plants and meat. The major bump in body size seen in erectus is plausibly linked to the dangers of carnivory. With pathetic footspeed and fewer trees to retreat into, hominins had no choice but to face down competing carnivores, among them lions, leopards, and sabertooths. Six-foot tall hominins would have had a great intimidation advantage over their four-foot forebears. With larger bodies came greater caloric needs, which in turn translated into more extensive home ranges (necessary to locate sufficient amounts of food). Longer legs would have been beneficial to travel the increased distances, and a larger brain would have come in handy to store mental maps of the surrounding terrain. Increased brainpower may also have been necessary to mimic Acheulean stone tool-making techniques.

The trend toward bushiness of the hominin family tree continued in Act III. In earlier scenes, Homo erectus shared the African continent with several Act II die-hards; Australopithecus sediba persevered until about 1.75 million years ago, Homo habilis until 1.4 million years ago, and Paranthropus boisei until shortly before 1 million years ago. Later in the record-breaking reign of Homo erectus, several additional species of Homo appeared, perhaps evolutionary offspring of the king himself. Examples included antecessor (Europe), heidelbergensis (Europe, Africa, and China), and neanderthalensis (Neanderthals, Europe).

Following the extinction of Homo erectus, several Homo species persisted [9]. Among the most successful were the Neanderthals, cold weather specialists that thrived in Ice Age Europe and Asia between about 200,000 and 28,000 years ago. Key adaptations of this clan, possibly our closest hominin cousins, included a brawny build and stocky stature (better for retaining heat), with males averaging about 5’ 5”. Dominating the Neanderthal face was a huge nose used for warming and humidifying cold, dry air. Other features included a long mid-face region, low forehead, and an elongate cranial vault housing a brain as large or larger than our own. Neanderthals controlled fire, lived in shelters, hunted big game, and buried their dead.

They made and wore clothing and developed a sophisticated tool kit. The highly successful Neanderthals may have displaced groups of Homo sapiens in certain regions when the climate turned extra frigid. The fate of Neanderthals remains a mystery, but recent genetic studies suggest that they interbred with humans, and that we still carry some of their genes[10].

Among several recently discovered hominins that lived late in Act III are a phantom and a hobbit. The phantom, informally dubbed the “Denisova hominin,” is known from a single fragmentary finger bone and an isolated tooth dating to 41,000 years old [11]. Recovered from Denisova Cave in Siberia, Russia, the single bone has yielded mitochondrial DNA suggesting that the finger’s owner belonged to a distinct species sharing a common ancestor with Neanderthals. Meanwhile, Homo floresiensis was a three and a half foot tall hominin that inhabited the Indonesian Island of Flores between 95,000 and 17,000 years ago [12].

Nicknamed “Hobbit,” this pint-sized, large-footed, small-brained, chinless wonder used stone tools to hunt pygmy elephants and giant Komodo dragons! Controversy still ensues about the closest relatives (and even the veracity) of this surprising member of our family tree, but the growing consensus is that Hobbit is descended from an ancient member of our Homo tribe who departed Africa about 2 million years ago.

Finally we come to our own species, Homo sapiens, a name that translates as “wise man.” Although various species of Homo had already spread over much of Asia and Europe, the first sapiens were birthed back in the womb of Africa some 200,000 years ago, perhaps evolving from erectus populations that had stayed behind. Compared to our first cousins, the Neanderthals, we possess a more lightly built skeleton, weaker jaws with smaller teeth, a flatter face, a thinner brow, a vertical forehand, and a vaulted cranium. Like erectus, neanderthalensis, and most other members of the Homo clan, the earliest sapiens gathered food and hunted animals. Indeed our ancestors were apparently ardent carnivores who took big game hunting to a new, more lethal level.

Around 60,000 years ago, Homo sapiens underwent what some refer to as the “Great Leap Forward.” The pace of cultural change suddenly accelerated. Bone artifacts such as fish hooks and needles appeared. Stone tools took on a strongly regional flavor, in some cases including sophisticated weaponry. Cave paintings, body ornaments, and sculptures appear, hinting at a fundamental shift of mind, and we see the first evidence of long-distance trade. Genetic evidence indicates that much of this revolution can be traced to a small group of sapiens that migrated out of Africa, mimicking the wave of erectus populations hundreds of thousands of years prior [13]. This time, however, the wave swelled to a tsunami that did not stop in Asia or Europe. The first tsunami wave of modern humans crossed the Red Sea into the Near East and then on to East Asia and Australia. The second moved northward into Europe, Asia and eventually the Americas. Along the way, these populations encountered and displaced other varieties of Homo, including Neanderthals, until we were the only surviving hominin species.

Homo sapiens, a late-arriving, globe-trotting bipedal ape, had somehow developed the capacity to live pretty much anywhere, from sweltering deserts to the frigid Arctic. (Among mammals, only the Norwegian rat [Rattus norvegicus] even approaches the geographic distribution of humans, occurring on every continent except Antarctica.) Beginning about 10,000 years ago, we began to give up our itinerant ways and settle down, likely in response to a shift toward more stable climates. The ability to stay put came largely from cultivating plants and animals. Over a relatively brief period, agriculture arose independently five times. With heightened food yields came much larger populations and, very quickly, civilizations. Civilizations have continued to expand ever since, swallowing all but a handful of foraging cultures and, in the last eyeblink of time, wreaking havoc with the Earth’s biosphere.

returning to the question that kicked off this discussion, why did humans, as opposed to chimpanzees, elephants, orcas, or some other creature, become such a dominant presence on Earth? The answer, it seems, invokes a unique concatenation of events within our lineage. Humanity’s runaway success required several essential elements—in particular, the physical, mental, and social capacity to develop advanced technologies. In Act 1, the most critical evolutionary acquisition was bipedalism. Walking upright freed the hands for other activities, including transport. It also enabled early hominins to expand their geographical ranges and encounter environments well beyond the experience of other apes. In Act II, manual dexterity was added to the repertoire, a capacity that transformed stones into tooth-mimicking tools. The making and transporting of stone tools opened up an entirely new niche—carnivory—sending our ancestors off down yet another evolutionary rabbit hole. Finally, in Act III (and the tail end of Act II), our newly acquired penchant for meat and nutrient-rich marrow sparked the evolution of larger, more sophisticated brains. Out on the open savannah, our ancestors quickly evolved traits that could help them tap into the large, scattered packages of plants and meat available there. The evolutionary results included new tool technologies, heightened imitative abilities, and a strong bias toward within-group cooperation.

But beware the fallacy of hindsight. Reflecting on the human journey, we must fight a powerful, largely unconscious bias to view early hominins as a succession of warm-up acts—“experiments in being human,” to use the preferred scientific phraseology—leading to our inevitable main event. Evolution has no foresight. It reacts only to present circumstances, fitting organisms to the habitats they live in. In hitting one rock against another to generate sharp-edged blades, Australopithecus garhi (or some near cousin) gained access to a new suite of foodstuffs, thereby enhancing its odds of survival. These little hairy hominins had no clue that stone tools might trigger a cascade of events that would ultimately send their distant descendants to the moon. They were concerned only with immediate circumstances, just as we are most of the time. In short, chimps don’t run the world today because the ecological circumstances encountered by their ancestors didn’t produce the necessary evolutionary steps (for example, bipedalism, manual dexterity, and bigger brains). Similarly, elephants and whales didn’t evolve key elements, such as the physical capacity to create a material culture.

Is that it then? Are we merely an evolutionary fluke, the result of a staggeringly improbable series of events? Or is there a larger arc to this narrative? What greater meaning, if any, are we to derive from the human journey? Answering these questions must be the topic of a future post.

Notes and References (continued from Part 2)

9. A key lesson emerging from the human journey is that our present day circumstance, with a single hominin species, is the exceedingly rare exception. Throughout most of the past several million years, multiple hominin species have co-occurred, with two or three frequently sharing the same habitat. Those who continue to make the claim that no evidence exists of fossil intermediates in the human family tree are either not paying attention, or (as is too often the case) purposefully twisting the data to match preconceived biases against evolution.

10. Richard E. Green et al. 2010. A Draft Sequence of the Neanderthal Genome. Science 328(5979): 710–72

11. Krause, J., Fu, Q., Good, J. M., Viola, B., Shunkov, M. V., Derevianko, A. P., and Pääbo, S. (2010). The complete mitochondrial DNA genome of an unknown hominin from southern Siberia. Nature, 464(7290):894–897

12. Brown P., Sutikna T., Morwood M., Soejono R. P., Jatmiko, Saptomo E.W. et al. 2004. A new small-bodied hominin from the late Pleistocene of Flores, Indonesia. Nature, 431:1055-61; Morwood M. et al. (2005): Further evidence for small-bodied hominins from the Late Pleistocene of Flores, Indonesia. Nature, 437:1012-1017.

13. Stringer, C. and McKie, R. 1998. African Exodus: The Origins of Modern Humanity. Holt, New York; Tattersall, I. 2009. Human origins: Out of Africa. Proc. Natl. Acad. Sci. USA, 106(38):16018-16021.

Image Sources (from top to bottom)

1. Homo erectus (www.monipol.de/photo/hominden-evolution.html

2. Homo erectus discovering fire. (www.djacobs.pbworks.com)

3. Neanderthal woman (Joe McNally, National Geographic)

4. Homo floresiensis (www.guardian.co.uk)

5. Homo sapiens sapiens (Jay Matternes, National Geographic)

The Human Journey: Part 2

Today's post continues the human evolution story begun in my last entry:

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NEXT STOP IS eastern Africa two and half million years ago—Act II of the human journey. Our arrival coincides with the dawn of the Pleistocene Epoch [5]. An initial scan of the landscape reveals another mosaic of grass and woodlands not so unlike Ardi’s homeland. The expected array of big mammals is here: monkeys, pigs, antelopes, horses, rhinos, hippos, elephants, and carnivores. Like the modern Serengeti, horned antelope are particularly plentiful and diverse. Wildebeest and impala roam the plains in great numbers. Gazelles bound in and out of woodland patches. Waterbuck frequent the thick scrub adorning lake margins. Yet slowly you realize that something is profoundly different. The proportion of grasslands has increased at the expense of forests, and many of the mammals are mega-sized. There’s Elephas recki, a giant elephant far exceeding the bulk of the present day African elephant (Loxodonta). Also present are ultra-big baboons, wildebeest, and pigs, among others. Together with extra body bulk, all of these plant-eating mammals possess beefed up jaw muscles and broad, high-crowned cheek teeth covered in thick enamel. This trio of traits—enlarged bodies, molars, and jaw musculature—represents a convergent evolutionary response to the same dietary challenge. Food in grassland settings is dominated by tough, fiber-filled, low quality offerings. Many animals here, like Elephas and wildebeest, are consumers of grass. For others, like pigs and baboons, the menu consists mostly of tough fruits, hard seeds, and underground tubers. Either way, oversized cheek teeth driven by big chewing muscles offer a first line of defense. Big bodies with enlarged guts are the second line, digesting high volumes of low-nutrition fodder.

In a nearby ravine, you spot an unfamiliar primate hunched over, intent on some task. Moving closer, you see that this is a hominin about four feet tall and less than 100 pounds—slightly bigger than Ardi, perhaps, yet smaller than an average chimp. This female is clearly digging with a stick, laboring to remove chunks of hardpan soil. Eventually, successful in her quest, the ape-like animal picks up a freshly unearthed tuber and walks upright to a nearby patch of shade. Like Ardipithecus, she possesses long arms, yet her gait appears much more like our own, with long, efficient strides. As she sits and begins to gnaw on the bulbous root, you note that her face is distinctive, with massive jaw muscles housed inside strongly flared cheek bones. The molars are oversized as well, mimicking the pattern described above. This is Paranthropus boisei, part of a group of robust hominins that lived over much of Africa between 2.7 million and just shy of 1 million years ago. Besides boisei, two other Paranthropus species are known (aethiopicus and robustus). All share the same body plan and elaborate chewing adaptations. The first discovered boisei specimen was nicknamed “Nutcracker Man” because of its giant chompers, heavy-duty jaws, and pronounced crest atop theskull for anchoring the thickened jaw-closing muscles. As with pigs and other local omnivores, this specialized eating

apparatus was well suited to a diet of coarse plants, tubers, and other hard foods. Although her appearance is strange, you can’t help but feel a certain kinship toward this bantam-sized creature.

Then, from far off in the distance, an alien sound arrives, carried by the whispering grasslands. Rising to investigate, you walk across several low hills, listening as the cacophony intensifies and spiraling vultures congregate overhead. Ascending the last rise, you find yourself on a ridgetop overlooking a stunning scene. At the center, under a large tree, is a dismembered wildebeest. Scattered around the carcass are about a dozen boisterous hominins. Other than being slightly leaner and longer-legged, this bunch closely resembles Nutcracker Woman a few hills over. These animals are also efficient bipeds, striding back and forth between the dead antelope and its various amputated body parts. The only notable physical differences occur in the face, which are considerably narrower and slightly more humanlike. Most astonishing of all is what this hominin mob is up to. Some individuals slice meat from the foul-smelling carcass with sharp stone blades. Others use larger, fist-sized stone tools to break bones and access the sweet marrow within. On the far side, three hooting, gesticulating males (about a third larger than the females) keep a large spotted hyena at bay, while a pair of jackals paces impatiently in the wings. Clearly frightened by the hyena,

several smaller hominins have retreated to the tree, using their long arms to move through the branches with great dexterity. The unfortunate wildebeest, perhaps the victim of a lion kill, has been dead for at least a day and apparently dragged to this location. The hominins are clearly excited to be feeding on meat, yet anxious to move on. In a world of lions, leopards, sabertooths, and hyenas, hit and run is the only viable strategy for a runty scavenger.

The scavenger in question is Australopithecus garhi [6]. Australopiths were a hyper-successful group of hominins that persisted throughout most of Africa for well over 2 million years. The oldest known example, from about 4 million years ago, is Australopithecus anamensis, a species that heralds the onset of Act II. Then comes afarensis (Lucy), africanus, garhi, and sediba, the last of which disappears about 1.75 million years ago. This sequence does not denote a straight line of ancestors and descendents. Several of these species overlapped in time. Some lived in southern Africa whereas others are known only from East Africa. If the pace of recent finds is any guide, additional kinds of australopiths await discovery in African sediments. The skeletons of Australopithecus and Paranthropus are closely similar in both size and shape— an arm bone or vertebra of one could easily be confused with that of the other. Their brains were similarly sized as well. But australopith skulls lack the swollen cheeks, massive molars, and large, muscled crests of Paranthropus. Early Australopithecus species such as Lucy’s tribe are thought to have consumed a mixed diet of fruit, seeds, and wild vegetables, though studies of microscopic tooth wear suggest that this diet was augmented with occasional hard foods.

Later australopiths, including our raucous toolmaker, Australopithecus garhi, appear to have developed a taste for meat. In fact, garhi is arguably the first hominin to embark on a technological adventure that would one day result in electron microscopes, 747s, and iPads. The earliest evidence of stone tools dates to about 2.6 million years ago. For a long time it was thought that these crude blades, scrapers, and hammers—referred to collectively as the “Oldowan” tradition—must have been made by early members of our own, big-brained genus: Homo. The oldest known representative, Homo habilis (“handy man”), had a respectable brain size of 600 cc, a substantial gain over the 450 cc of gray matter housed in the noggin of Australopithecus garhi. The embarrassing problem is that the habilis remains date back only as far as 2.4 million years ago, resulting in a 200,000-year hiatus between the first stone tools and the first Homo. Of course, we could simply be missing the earliest Homo fossils, but, by the same logic, we may be missing even earlier stone tools. Adding fuel to the controversy is the fact that garhi remains were found in close association with a 2.5 million-year old butchery site, in which antelope and horse bones show distinctive cut marks. Slices on a zebra jawbone demonstrate that at least one of the tool-users was going after the tongue. So perhaps big brains weren’t necessary to launch this technological revolution. My guess is that the stick- and hammer-wielding chimps in the present-day forests of West Africa wouldn't be surprised.

It’s difficult to overstate the evolutionary significance of Oldowan stone tools. Think of them as multi-use, replaceable teeth—external dentures perhaps—that gave hominins access to a range of previously inaccessible (or at least rarely acquired) foods, from hard-shelled nuts to meat and marrow. Whereas genetic evolution can require millions of years to craft a thick-crowned molar or slicing canine, our ancestors learned to modify quartz and lava into a modest array of tools. Hit one chunk of basalt against another and the slicing blade that flies off is likely to have a sharper edge than the most lethal canine. The larger rock with a piece missing can be used for crushing, chopping, grinding, or generating more blades. In effect, tooth-mimicking tools enabled hominins to escape the limits of their own dentitions and begin the process of digestion outside the body. Best of all, this newly acquired technological capacity could be passed, gene-like, from generation to generation.

Stone tools had cascading consequences. Most rocks make for lousy tools, so early Pleistocene hominins had to venture to lava outcrops or concentrations of stream cobbles to find the necessary resources. Studies of microscopic markings on the edges of Oldowan tools demonstrate that they were used for processing both plants and meat. Food plants tend to occur in predictable places from one season to the next. In contrast, the divergent distributions of meat and rock resources, heightened by the unpredictability of carcasses, raised an immediate conundrum. Since you can’t carry stone tools around all the time, and since predators aren’t always going to make their kills conveniently near your key rock sites, how do you ensure that you have a ready tool supply to take advantage of a carcass? To make matters worse, out there on the open savannah you could easily become a lion or leopard’s next meal, so scavenging requires quick action. The solution, which garhi and our early Homo ancestors apparently hit upon, is transport and storage. Collect armfuls of rocks suitable for tool making and carry them to scattered locations around the landscape. That way, as long as you can remember where the rocks are, you’ll always have a ready tool supply. When one freshly made blade dulls, others can be generated quickly. In addition to meat, tool making allowed access to a high nutrition food that had previously been the domain of hyenas--marrow. Some researchers think that marrow's

fatty acids fueled the initial brain expansion in our Homo lineage [7]. Coincidentally, it was within a few hundred thousand years of the earliest stone tools that the first major leap in brain size occurred.

Finally, stone tools conferred upon early hominins a large measure of flexibility in the face of change. Plants counter the efforts of plant-eaters through a variety of evolutionary “strategies,” from hard outer coverings to chemical toxins. When shifting African climates transformed plant communities in a given area, it would have required significant time and effort for resident hominins to determine which of the new varieties were palatable, and how their nutrients could be extracted. Yet, barring a major deterioration in ecological conditions, chances are that carcasses of large herbivores would still occur on the landscape. So as long as opportunistic scavengers had their stone toolkits handy, they could probably eke out a living.

Towards the end of Act II, evolution’s creativity generated a bevy of hominins. One of these was bigger brained Homo habilis, the aforementioned “handy man.” Arguably the first representative of our own genus, habilis was not much larger than an australopith. At present, Australopithecus garhi is a plausible candidate ancestor for Homo habilis, but any such conclusion is provisional at best [8]. Despite our continual emphasis on the importance of brain size, there’s no evidence that the initial appearance of a larger-brained hominin was an ecological “game-changer.” Homo habilis did not immediately utilize its heightened neuron-power to conquer all foes and emerge as the dominant “ape-man.” On the contrary, the waning scenes of Act II witnessed the greatest known florescence of hominins, with up to six co-existing species in Africa around 2 million years ago. Other than habilis, the hominin menagerie included two kinds of Australopithecus (sediba and africanus), two of Paranthropus (boisei and robustus), and a last minute walk-on—the thick-browed, lanky-limbed Homo erectus. As we shall see, erectus would turn out to have greater staying power than any other hominin, rising to become a major star of Act III.

Notes and References (continued from Part 1)

5. The Pleistocene Epoch is now recognized to extend from 2.6 million years ago until 12,000 years ago. Up until 2009, this time period was said to begin at 1.8 million years ago, so any sources older than this will regard the 800,000 year period from 2.6 million to 1.8 million as part of the preceding Pliocene Epoch.

6. Asfaw, B., White, T., Lovejoy, O., Latimer, B., Simpson, S., Suwa, G. 1999. Australopithecus garhi: a new species of early hominid from Ethiopia. Science, 284(5414):629–35; De Heinzelin, J., Clark, J. D. White, T., Hart, W., Renne, P., Woldegabriel, G., Beyene, Y., Vrba, E. 1999. Environment and behavior of 2.5-million-year-old Bouri hominids. Science, 284(5414): 625–9.

7. Cordain, L., Watkins, B. A., and Mann, N. J. 2001. Fatty acid composition and energy density of foods available to African hominids. Pp. 144-161 in A. P. Simopoulos, K. N. Pavlou (eds.), Nutrition and Fitness: Metabolic Studies in Health and Disease. World Review of Nutrition and Dietetics, Vol. 90. Karger, Basel.

8. Currently, Australopithecus garhi appears just prior to Homo habilis and makes a plausible ancestor to the genus Homo. However, given such a bushy hominin family tree, identifying ancestors and descendents has become a high-risk sport, with another imminent discovery likely to blow the latest hypothesis to smithereens. So paleontologists don’t put too much weight in such claims.

Images (listed from top down)

1. Australopithecus afarensis (John Gurche; smithsonianscience.org)

2. Paranthropus boisei (wired.com)

3. Australopithecus africanus (transformingcommunication.com)

4. Australopithecus garhi (iesribalta.net)

5. Homo habilis (rst.gsfc.nasa.gov)

The Human Journey: Part 1

WHEN I WAS pursuing a Masters degree in anthropology in the mid 1980’s, the story of human evolution differed greatly from the present version. Back then, the list of early hominins (the group of primates that includes humans and their upright relatives) contained a mere handful of names. The most famed of the bunch was Australopithecus afarensis, a small, apish creature discovered in Ethiopia only a decade prior. Recovered from 3.7 million-year-old rocks, afarensis laid claim to the title of oldest known hominin [1] and made a plausible human ancestor. The best-preserved specimen, nicknamed Lucy, possessed a human-like thighbone and a diminutive cranium,
clear evidence that upright walking preceded big brains. Other bipeds placed on the evolutionary “main line” heading for humanity included a succession of bigger-bodied, larger-brained, smaller-faced forms, including Australopithecus africanus, Homo habilis, and Homo erectus. This sequence also represented a geologic series from oldest to youngest. Put a chimp at one end and a human at the other, and there before you was the classic conga-line progression from stooped-over ape to upright human, with the man (it was always a man) striding boldly into the future. To be fair, we students also learned about a few hominin offshoots from the main line—among them a pair of super-robust australopiths in Africa and the Neanderthals in Europe—but these creatures were clearly evolutionary dead-ends. The story’s central plot was simple and straightforward, heading in a bee-line for the pinnacle of creation. Ah, those were the days.

Back in the 80’s, you could have fit almost all known early hominin fossils from Africa onto a large table or two. But times have changed. Thanks to the dogged efforts of insightful scientists and keen fossil-finders, the past quarter century has witnessed a startling series of discoveries and insights in paleoanthropology. The bounty of recently unearthed fossils, coupled with revelations from the world of genetics, have dramatically fleshed out the human evolutionary story, and greatly complicated it as well. Umpteen new characters have been added to the dramatis personae, with the total number of hominin species now varying somewhere between 15 and 25, depending on whose counting. Key metaphors have been swapped out too. The traditional evolutionary ladder with Homo sapiens sitting on the uppermost rung has morphed into an arborescent family tree, each branch occupied by a different long lost cousin.

If we are to understand our relationship with nature, we must know something of the human journey, which is but one chapter in a much longer cosmic journey. How we conceive of humanity’s emerging cannot help but shape our views of the natural world. In a series of three posts, I will offer a synopsis of this 6 million year epic drama in three acts, each spanning roughly 2 million years. For the beginning of the story—your story—I have chosen a pivotal event that took place sometime between 5 million and 7 million years ago: the splitting of the human and chimpanzee lineages. One of these lines would culminate in the present day with relict populations totaling about 100,000 animals, all restricted to a few patches of West African forest. The other would wind up tracing the rise of a globally dominant species numbering more than 7 billion. Had you been present to witness this consequential parting of ways, it’s highly doubtful that you would have discerned any hint of such divergent futures. Those signs would come later, as evolution gradually molded the raw materials of our ancestors into a new kind of animal. So what happened? Given our shared heritage, why didn’t chimps take over the world instead of us? Answering that question is our critical task.

AS THE TIME machine sets us down in Act 1, we find ourselves immersed in a savannah-like scene in northeast Africa during the early Pliocene Epoch, about four and half million years ago. From the vantage of a circling eagle, we see a patchwork of grasslands and forest. A river with thickly wooded borders snakes lazily through the landscape, emptying into a nearby lake where catfish congregate in the depths, hippos and crocodiles lounge near the shore, and an impala herd pauses to drink. Zooming down for a closer look, we find a rhino mother and calf browsing a woodland margin. A startled family of guineafowl scurries into the underbrush. Not far off, a kudu male with august, spiraling horns plucks leafy foliage, keeping a watchful eye on his “harem,” while a giant tortoise plods along with his own reptilian concerns. Stepping into a forest patch, we enter another world, full of greenery, shadow, and birdsong. Sweltering grasslands are exchanged for a cool canopy of hackberry, palm, and fig trees. Colorful parrots and peafowl abound, accompanied by doves, lovebirds, swifts, and an occasional barn owl (minus the barn, of course). The litter of the forest floor hosts a profusion of dung beetles, snails, millipedes, and other creepy crawlies. You catch brief glimpses of lizards and rodents. A loud roaring overhead marks the arrival of a colobus monkey troop, evidently pleased to take a hiatus from leaf-eating to consume some sumptuous figs. A nearby squirrel, unhappy about the primate interlopers, chatters his objections. Although the vast majority of species here are distinct from those living today, the characters seem very familiar.

But then the rowdy colobus begin a new call, one that sounds more like an alarm. You look up to see the monkeys abruptly abandon their meal and make a rapid arboreal retreat. Moments later another hooting primate arrives. Instead of monkeys, these agile, ape-like creatures are much larger, about the size of chimpanzees yet distinctly different. Two adults move through the tree canopy on all fours, running along branches with grasping hands and feet. Four more individuals, among them a pair of juveniles, arrive on foot—two feet to be precise. They are walking upright, but with a strange gait, waddling side to side like John Wayne inflicted with serious saddle sores. Hanging by their sides are exceptionally long arms terminating in oversized, flexible hands. Now at the base of the fig tree, the animals climb with powerful, deliberate movements, more like tree sloths than colobus monkeys. Finally, the group gathers high in the canopy to feast on the ripe fruit. Watching them eat, you note that the largest individual, a male, lacks the long, dagger-like canines typical of chimps and gorillas. What is this creature?

Meet Ardipithecus ramidus—“Ardi” for short—by far the best known resident (for the moment) at the base of the hominin family tree [2]. Ardi was recovered in the Aramis region of Ethiopia, entombed within 4.4 million year-old rocks that predate Lucy by a million years. She is a marvelous, surprising amalgam of primitive and specialized features,

seemingly with one foot planted in the ape camp and the other in the human camp. In fact, Ardi’s foot is one of the best examples of this “split personality,” with the big toe directed sideways, able to grasp tree branches like an ape, and the remainder of the foot bones stiff and forward facing, well suited (at least to the eyes of some experts) to upright walking. The pelvis of Ardipithecus is similarly schizophrenic, with a flared upper portion adapted for two-legged walking and a deep lower portion suggestive of powerful climbing muscles. The brain was slightly larger than a chimp’s, and about 20% the size of a human brain. The size and shape of the teeth point to an omnivorous diet of plants, fruits, nuts, and tubers, perhaps supplemented by such delicacies as small mammals, bird eggs, and insects. Ardi’s fragmentary fossils have even been used to infer social behavior. Whereas chimp males use their oversized canines to compete aggressively for females, Ardipithecus males had small canines equivalent to those of females, suggesting to some investigators that males exhibited much less aggression toward one another. Perhaps, they argue, Ardipithecus couples formed long-term pair bonds, as humans do, an innovation that would have cascading effects down the hominin line.

At first glance, Ardi seems to be the perfect “missing link,” part ape and part human. But the notion of missing links turns out to be outdated, misguided, and, well, just plain wrong. First, and most trivially, animals like Ardipithecus clearly are not missing, but found. Second, such transitional animals are not links, a metaphor of progress closely allied with ladder rungs. Instead, such creatures are better regarded as mid-level branches that help us fill out the human family tree. Finally, although chimpanzees may be our closest living relatives, Ardi makes it clear that our last common ancestor with them differed markedly from Jane Goodall’s most beloved primate.

Gorillas and chimps both walk on the knuckles of their hands, so it’s long been thought that we humans must have passed through a knuckle-walking phase on our way to becoming bipedal. Yet Ardi lacks the stiffened hands seen in knuckle-walkers, and evidently walked on her substantial palms instead. Similarly, Ardipithecus doesn’t have the specialized arm and shoulder anatomy of our closest living ape cousins, suggesting that tree-swinging evolved in the chimp lineage sometime after the split with our ancestors. The feet too are informative. Chimpanzees are adept tree climbers in part because they lack a specialized foot bone, allowing their flexible feet to grasp branches when aloft. But that same flexibility hinders their ability to walk upright. Like us, Ardi possesses the key foot bone, suggesting that this element was lost in the chimp line sometime after the “great branching event.” In short, we didn’t evolve from chimpanzees. Far from being “primitive” or “backward,” our closest ape cousins, with whom we share more than 98% of our genes, have been on their own journey since we last parted ways in some ancient African forest.

In all, Ardi was a unique creature with a switch-hitting lifestyle, splitting her time between the ground and the trees, but likely more at home in the latter. Bipedalism, rather than offering views over tall grass or freeing the hands for tool use, may have evolved initially as a way to get around in a mixed habitat, walking upright on the ground and moving on all fours while up in the trees. As befits any arbor-loving species, Ardipithecus appears to have inhabited woodland settings, an ecological interpretation bolstered by the abundance of forest-living organisms found in the same sediments (a list that includes, by they way, all the plants and animals mentioned in our time-traveling scenario). Grasslands had been expanding in Africa for millions of years in response to a long-term trend toward drier, more seasonal climates. Yet plenty of forest patches persisted, more than enough to sustain Ardi and her kin [3].

Ardipithecus is not the only fossil representative from this earliest phase of hominin evolution. Recent discoveries of two other fossil forms, Sahelanthropus tchadensis from Chad and Orrorin tugenensis from Kenya, offer additional tantalizing clues of the chimp-human split [4]. But Ardi provides the only substantial illumination of the human journey’s first act. My strong hunch is that we’ll ultimately discover a diverse tribe of Ardipithecus-like hominins that lived between 7 million and 5 million years ago. Why? Because so little is known from this time interval and, as we shall see in the next post, experience gleaned from explorations into subsequent acts of this drama reveal an African continent populated by unexpected clans of upright cousins. Stay tuned.

Notes and References

1. The terminology applied to our nearest evolutionary relatives has changed over time. Back in the 1980’s, the word “hominid” referred only to humans and their bipedal relatives. More recently, the close genetic relationship that we share with chimpanzees and other apes has prompted a shift in terms. Today, the word “hominid” refers to the larger family of primates that includes all great apes and humans, together with their last common ancestor and all extinct members of this group. In its place, the word “hominin” is now commonly used to denote humans and other bipedal apes, including Ardipithecus, Australopithecus, Paranthropus, and extinct species of Homo.

2. White, T, D. Asfaw, B., Beyene, Y., Haile-Selassie, Y., Lovejoy, C. O., Suwa, G., WoldeGabriel, G. 2009. "Ardipithecus ramidus and the paleobiology of early hominids. Science, 326 (5949): 75–86. doi:10.1126/science.1175802

3. Not all investigators are convinced that Ardipithecus inhabited a forest setting. A study by Thure Cerling (University of Utah) and colleagues presents evidence that forests were largely limited to the banks of rivers and that grasslands were abundant even then. Tim White (University of California, Berkeley) and his team have responded by arguing, convincingly I believe, that, although grasslands were certainly present, the weight of evidence points to a forest habitat for Ardi. Cerling, T. E., Levin, N. E., Quade, J., Wynn, J. G., Fox, D. L., Kingston, J. D., Klein, R. G., and Brown, F. H. 2010. Comment on the paleoenvronment of Ardipithecus ramidus. Science, 328(1105): doi: 10.1126/science.1185274; White, T. D., Ambrose, S. H., Suwa, G. and WoldeGabriel, G. 2010. Response to comment on the paleoenvironment of Ardipithecus ramidus. Science, 328(1105: doi: 10.1126/science.1185466.

4. Brunet M. et al. 2002. A new hominid from the Upper Miocene of Chad, Central Africa. Nature, 418:145-151; Senut, B., Pickford, M., Gommery, D., Mein, P., Cheboi, K., Coppens, Y. 2001. First hominid from the Miocene (Lukeino Formation, Kenya). Comptes Rendus de l'Academie des Sciences, Series IIA - Earth and Planetary Science 332(2):137-144.

Image Sources (from top to bottom)

1. http://www.dailymail.co.uk

2. http://www.theosophy-nw.org/theosnw/evol/ev-ibel2.htm

3-5. http://www.news.sciencemag.org

6. http://www.news.sciencenews.org

Backyard Dinosaurs

“Can we go see the barn owls one more time? Pleeeeeeeeeeeeease?” That was my daughter Jade a couple of Saturdays ago. “Ok, ok,” I gave in. “Let’s go!” We found the owl couple perched in the rafters of the barn just as we’d left them an hour before, with the tan-and-speckled female wedged up against the snowy-white male. Only the male cracked open his eyes as we snuck back in for another peek. Through binoculars, their striking, heart-shaped faces appeared unreal, almost otherworldly.

It was International Migratory Bird Day, and a nearby park, Muir Woods National Monument, had organized a bird walk in celebration. Jade and I joined a group of about 15 other kids and adults. Most of us carried binoculars. Our intrepid lead birder, Dave Mackenzie, toted an even more powerful spotting scope mounted on a tripod. As we met him in the beach parking lot, he pointed at the scope and said, “Check out the Pacific Loons. We’ve seen hundreds fly by this morning heading north to their Arctic breeding grounds.” It took Jade a minute to get the hang of looking through the eyepiece, but then her eyes went wide with amazement as she watched the black-throated fish-eaters zoom past one after the other.

“Turkey vultures,” someone yelled, and all heads tilted skyward. Jade and I are accustomed to seeing “TVs,” as they are affectionately known. But today, surrounded by this group of bird-lovers, the two-toned undersides of those giant, soaring wings took on new meaning. Ravens, Bonaparte’s Gulls, and an Anna’s Hummingbird were quickly added to the list before we set off for the nearby marsh. A host of Red-Winged Blackbirds perched on cattails was there to greet us, flashing their brilliant red epaulets while their piercing metallic voices rang out. A Snowy Egret attempting merely to fly over the marsh was immediately attacked by three of the dive-bombing blackbirds. The kids were enamored with two families of Mallard Ducks, laughing as the yellow, waddling chicks did their level best to keep up. Meanwhile I was staring through the scope at a male Northern Flicker, completely absorbed by his red “mustache” and black-speckled body. A huge highlight for both of us was climbing a nearby hill to get a “bird’s eye” view into the nest of a Red-tailed Hawk. The large stick nest, nestled atop a Monterey Cypress tree, held two gangly white chicks, while one of the parents kept a steadfast watch nearby.

The surprising truth of the matter is that every single one of these winged wonders is a dinosaur, members of the same family as T. rex. All living birds are the direct descendants of small, feathered dinosaurs that lived more than 150 million years ago. Thus, in a very real sense, those raucous Red-Winged Blackbirds are backyard dinosaurs, offering a vibrant window into the distant past. In recent years, fossils of more than a dozen different varieties of feathered (nonavian) dinosaurs have been unearthed in China. So similar are many of these animals to birds that it’s difficult to tell one from the other; indeed the two are often confused. Dinosaurs aren’t extinct, then. With over 10,000 different living species, they far outnumber mammals (closer to 6,000 species). The Mesozoic Era is often called “The Age of Dinosaurs,” and the succeeding Cenozoic Era, in which we live, “The Age of Mammals.” But no, it seems that we still live in the Age of Dinosaurs. (To be fair, whether your metric is number of individuals, variation, or total biomass, the past four billion years are best regarded as the “Age of Bacteria,” but we macro-sized creatures tend to overlook the microbial world.)

In previous posts, I have bemoaned the recent startling transformation in children’s leisure time. A child growing up today is likely to spend 90% less time outdoors than a child born just one generation ago. How can we possibly build sustainable communities if people don’t care about the places they live. And how are we to care if we don’t spend any time experiencing our local natural communities? Robert Michael Pyle has referred to this frightening state of affairs as, “the extinction of experience.” [1]

I grew up playing outside and fascinated by all things nature-oriented. As a parent, I am very conscious of exposing my daughter to the nonhuman world around our home. However, I’ve found that taking her on hikes is not always welcomed. “My legs hurt daddy,” is what I hear after the first half mile—in spite of the fact that she can run around all day long with her friends. So I’ve learned to replace the word “hike” with “adventure,” a tip shared by another frustrated parent. Adults tend to be goal-oriented; when out hiking, this means reaching a particular destination. Kids are more interested in playing. By making the outing more about the moment, and less about the goal, I’ve found it much easier to keep Jade engaged and happy.

But, to retain your parental credibility, you still need to deliver on the “adventure.” This is where the magic of birds (i.e., dinosaurs) comes in. Most kids think trees and other plants are pretty ho-hum. And the majority of animals out there can be tough to find, unless you focus on the creepy-crawlies beneath your feet. But, in addition to being beautiful, active, and diverse, birds are nearly ubiquitous. A set of binoculars and an identification book (the Sibley field guides are the birding gold-standard) are all you need to get started, though feeders are an excellent way to attract birds literally into your backyard. In addition to making identifications, it’s fun to watch what birds do. (While I was out walking a few months back, a large raven flew toward me about 20 feet off the deck and inverted itself just as it passed overhead. I just stood there stunned, suddenly reminded of Tom Cruise executing the same show-off aerobatics in Top Gun.)

Now, added to all this is the T. rex angle. As I know from direct experience in my role as Dr. Scott the Paleontologist on Dinosaur Train, most kids love dinosaurs. So what better way to entice youngsters to go outside than to offer the carrot of seeing living, breathing dinosaurs?! In short, we desperately need to connect kids to local nature—both for their sake and that of the local nature—and backyard dinosaurs are arguably the most powerful tool to make these connections happen. In my recent book [2], I called for a “backyard dinosaur revolution.” So how about it? Are you in?

Jade and I have decided to get more serious about birding and begin keeping a “life list,” a logbook of bird identifications and observations. So now our “hikes” have become “treasure hunts” as we look to add more feathered neighbors to our respective tallies. Mom is excited to join in too. Just yesterday Jade asked, for the umpteenth time, “Daddy, when can we go back to see the barn owls?”

References

Pyle, R. M. 1998. The Thunder Tree: Lessons from an Urban Wildland. Lyons Press, Guilford, CT.

Sampson, S. D. 2009. Dinosaur Odyssey: Fossil Threads in the Web of Life. University of California Press, Berkeley.

All images derived from National Geographic: http://photography.nationalgeographic.com/photography/