We will never know the precise reasons why the first group of humans left Africa. But there are clues in our genome concerning those who stayed, and they could suggest why the others left. As we reinvaded Africa, we would have encroached on our forebears and likely displaced them in many places. I suspect, though, that a recent study by Stephen C. Schuster and Webb Miller from the Pennsylvania State University Center for Comparative Genomics and Bioinformatics with forty-seven other authors from sixteen different institutions sheds some light on this question. The forty-nine authors generated the complete genome sequences of five African group identities, including a Khoisan (!Gubi) and Bantu (Archbishop Desmond Tutu) from southern Africa. The Khoisan (also known as San or Bushmen), they conclude, are “genetically different from other humans” and, furthermore, “more different from each other than, for example, a European and an Asian.” The Bushmen are apparently near or at the root of the human genetic tree whose branches extend now to the most recent ones of the natives in Tierra del Fuego at the southern tip of South America, where the diversity of human DNA is lowest. The Bushmen’s genetic difference is related to their more ancient history. Their mutations were retained in the population, whereas small bands of humans who left Africa would have had only a select sampling from the variety of the genome, so that the resulting splinter population would be more genetically homogeneous. I believe, however, that it is significant that the Bushmen as a whole did not leave their home in Africa and retained their uniqueness. Why?
Elizabeth Marshall Thomas, in her writings on the life of the Bushmen, provides clues: “They are a roaming people and therefore seem to be homeless . . . each group of them has a very specific territory which that group alone may use, and they respect their boundaries rigidly . . . the people who live there know every bush in it where a certain kind of food may grow—where there is a patch of tall arrow grass or a bee tree. . . .” In other words, they are precisely the opposite of homeless. Thomas writes further that the Bushmen’s “sleeping hollows were like the soft grass nests of pheasants, hidden in the leaves,” and that they were “at home” where few would venture. The Bushmen’s home is the land they live on, and their campfire is their hearth and home. There was little need to enclose it with a structure; they were free. Living in southern Africa, the Khoisan were thousands of kilometers southward of fierce pastoralist tribes, which could not penetrate their homeland. Being locked in place with no place to go, they had to manage their populations in a land that was marginal to others. As an alternative option to emigrating or escaping, they were so at home that to them everything they needed was at hand, but few others or nobody else wanted their land and so they were not driven out.
Ironically, we are becoming like the vast shiftless herd that becomes threatened by its own adaptations. Our cities “evolved” in the Neolithic for some of the same reasons that large pigeon roosts did, in part for defense against raiders. At the same time, cities became filled with riches, and thus also became targets of predation, in this case from our own kind. Ever-bigger cities meant the mustering of armies and ever-better defenses but at the same time engendered improvements in offenses. As with trees in a forest in their life-or-death competition to grab the light, human civilization is subject to limits in growth, lest it become vulnerable to collapse from its own weight.
It could be debated whether we “migrated” all over the globe by push or by pull, although I’m not sure the difference matters. It’s not a great difference if we’re pushed from a place we’ve degraded or pulled to a better one that is replete with resources. One thing is sure, though: we have almost no innate homing navigation mechanisms. Compared to an albatross, a loggerhead turtle, or a monarch butterfly, we can hardly walk in a straight line except between landmarks. Our lack of innate navigational ability, aside from the use of local landmarks, is almost as good as proof that we evolved as homebodies.
Nomadism is a strategy that reduces internal conflict, but at a price. In India, one of the most overpopulated countries in the world, nomadism is a way of life for many people. There are groups that have adapted to it and become specialists at it. However, despite that learned lifestyle, those practicing nomadism would gladly give it up. The nomads live out of their carts for the simple reason that they have no choice. “I will be the most happy person in the world,” one Lohar woman told journalist John Lancaster, “if I get some land and a house.”
We may argue that since we are highly social animals, we are not subject to such brutal territoriality as loons. But it doesn’t wash. Why did people inhabit “all” of the far-flung islands in the Pacific Ocean? Why and how did people get to Easter Island, over three thousand kilometers from the mainland and sixteen hundred kilometers from the nearest inhabited island? By chance and blown off course no doubt, but why chance it? And how many did not win the lottery? Would the Anasazi have “nested” on high ledges where they used retractable ladders to climb up, and made their tiny chambers on cliff edges, if it were not to escape human marauders? Why would others retreat into the far north, where they had to live in bitter cold and darkness for half the year? It is not in the least remarkable that humans were in Tierra del Fuego just a few thousand years after crossing the Bering Strait. How could they not! They were being dispersed and trying to find a good place to make a home. Like the aforementioned loons in northern lakes, and the Canada geese in my bog where at least a dozen fight viciously every spring for possession of the only safe spot to nest on—a beaver lodge—and where only one pair stays and the others must go far to find any place at all, it’s all about the matter of the costs and benefits of leaving versus staying and fighting. But if and when people find a home to stay in, the place grows on them because they have invested in it, and once adapted to it they have their families there, one after another, as long as they can.
Wherever we were, though, we were still at home with our “campfires.” Regardless of their new forms, we are at least indirectly still huddled around our fire as though we had never left it, and indeed without it, most of the Northern Hemisphere would now be unpopulated by our species.
Homing to the Herd
Someone said that Brecht wanted everybody to think alike. I want everybody to think alike. But Brecht wanted to do it through Communism, in a way. Russia is doing it under government. It’s happening here all by itself without being under a strict government; so if it’s working without trying, why can’t it work without being Communist? Everybody looks alike and acts alike, and we’re getting more and more that way. I think everybody should be a machine. I think everybody should like everybody.
—Andy Warhol, in an interview with Gene Swenson for Art News
The beginning of morality is to see the world as it is.
—Carl Sagan
THE CHOICE BEHAVIOR OF ANIMALS IN THE ENVIRONMENT that they live in, their home, is the result of biological evolution that individuals have experienced over long periods of geological time. In some so-called highly successful species that inundated and altered their ancient environment, the extreme numbers created a new set of selective pressures, favoring different physiology and behaviors. In a huge crowd, the nuances relating to specific individuals become irrelevant. The crowd itself becomes the dominant feature of the environment, and so it (like money for art) becomes a new stimulus. This stimulus is as real as the roar of a storm, the smell of the ocean, or the sight of a forest of pine trees. Oceanic fishes in the open seas feeding on nearly inexhaustible plankton live in immense schools, and there is little for them to home in on except themselves.
Extreme gregariousness is not just a provenance of some oceanic fishes. Certain insects, birds, and mammals also home primarily to the crowd consisting of each other, and not always with satisfactory results, as the French naturalist J. Henri Fabre famously showed in his observations and experiments with the pine processionary caterpillars, which follow silk trails between their communal home, where they are sheltered at night and in the cold, and their feeding areas
out on the tree branches. Fabre determined that they followed each other, normally traveling out in the morning and returning at night. But one group of his caterpillars by accident got onto the top rim of an urn, where they continued to circle for seven days straight, making approximately 335 circuits. None abandoned their evolved behavior pattern where the others, and not the biologically relevant environment, had become the reference. This is not to say that others are not biologically relevant—because in the vast majority of cases they are their lifeline; temporary encampments of largely anonymous individuals commonly yield benefits to the individuals in them. Crowds of starlings, grackles, red-winged blackbirds, crows, ravens, various species of finches, swifts, and swallows sometimes sleep in huge communal nocturnal roosts. Not only do the roosts provide a pooling of experience and hence an aggregate knowledge of the environment (such as of suitable chimneys or rock crevices for swifts, and of safe places in cities, away from owls, for blackbirds and crows), but the crowd itself dilutes risk both from predation and also by acting as an information center for finding patchy, widely distributed food. However, in most cases, the gregariousness is a seasonal phenomenon, not a permanent state of being.
The most notable for their extreme gregariousness in North America were the Rocky Mountain grasshopper, Melanoplus spretus; the passenger pigeon, Ectopistes migratorius; the Eskimo curlew; the Carolina parakeet; and the bison. All prospered to become dominant members of their ecosystem, but recently five of them “suddenly” became extinct, and the sixth nearly disappeared but has been resurrected to a token presence through our conscious effort, thanks to intervention at the national level in the nick of time. Since aggregating can be and often is highly adaptive, the extreme examples of animals homing to each other rather than to a place and of that resulting in the ultimate of maladaptiveness—extinction—are of interest.
First, the enduring mystery of the Rocky Mountain grasshopper. This species formed truly massive sky-darkening swarms of sometimes billions; it is only one species of about ten grasshoppers that aggregate, out of the ten thousand other species of the Acrididae that don’t form swarms. Large population size is generally considered to be a solid buffer against extinction. What went wrong? There is debate about what happened to the Rocky Mountain grasshoppers; they were gone before we could study them live, although two other species with similar habits are still extant in the Old World, and from them we can get insights into the swarming habit.
The previously mentioned desert “locust,” Schistocerca migratoria, another of those ten migratory or aggregating hoppers, is probably the best known and best studied of the group. It is of Old Testament fame for coming in clouds and descending on the land to devour every green thing and then moving on, leaving devastation and famine in the land. A second species, the brown locust, Locustana pardalina, of southern Africa, has the same pattern: it has an aggregating migratory phase, and a sedentary one of another appearance and behavior. The two phases of the same species were the cause of confusion; it was long thought that the migrating locust was a different species.
The scale of size and movement of the “locust” swarms is hard to imagine, given the few grasshopper species that we know locally, which seldom fly and which, when they do, rise up for a few meters only before settling back down onto the ground in some patch of sunshine. Grasshoppers are a favorite food of birds, and most have amazingly cryptic coloration that helps them blend into the background environment in which they live. Most species don’t show the slightest sign of being attracted to another. But the migratory desert “locusts” orient to each other, and that is what helps to produce the massive wandering swarms; and because they wander and sweep up all those in their path, the swarms become ever bigger.
A surprising discovery having to do with the desert locusts’ aggregating and wandering responses is that these actions are facultative. That is, when populations are sparse, the grasshoppers actively avoid each other. It is only when they are dense that the hoppers become attracted to each other. At low population densities, the hoppers are colored green so that they are cryptic in the vegetation, which helps them to avoid detection by predators. They then also have relatively short wings. On the other hand, at high population densities when they start to eat plants with poisons, they start to become less palatable to predators, and they are bright orange and yellow, which warns potential predators to avoid eating them. That is, their diet, appearance, shape, and behavior converge to produce a different adaptation, and each of these two adaptations is suited to stationary versus vagrant life strategies. The major take-home point here is that the behaviors are responses to a specific environmental cue, namely, each other, beyond a certain population density. Many other insects also have specific adaptations that aid them to disperse at some point in their life cycles, but in those cases the responses are to stimuli such as food and signals from the physical environment.
The desert locust swarms were, because of their massive size, for a long time relatively immune to destruction by humans despite massive efforts to try to fight and even eradicate them. The main reason for this failure was that the efforts were applied on the swarms. They are too large to be overwhelmed by any potential predator, which was the selective pressure for their evolution in the first place.
The developmental switch that turns the normally solitary cryptic and sedentary grasshoppers into forms that are no longer cryptic and that swarm is of obvious practical interest. We now know that the transition is activated by tactile stimuli. The solitary-phase grasshoppers have one-third more sensory receptors on their hind femurs than do vagrant grasshoppers, and the aggregation behavior can be induced in solitary locusts if the nerve that innervates their hind legs is artificially stimulated. While it could lead to interesting experimental results to immobilize the legs of grasshoppers so that they could not make the developmental transition to the aggregation phase, it won’t yield a practical application. Spraying them in their nurseries so that their populations remain low enough not to trigger aggregation and the wandering phase may be.
The Old World desert locust, a species we have now battled with our whole arsenal of science for a century, still exists. But the last live specimen of the American species, the Rocky Mountain grasshopper, was collected in 1902, long before we understood anything about them and long before we could have mounted any deliberate and scientifically effective attack on them. This grasshopper, the only American species known as a “locust,” was perhaps one of the most gregarious and destructive grasshoppers of all. It was the most abundant insect on the Great Plains, occurring from Canada to Texas. As documented in the recent book by Jeffrey A. Lockwood, an entomologist from the University of Wyoming who has spent a lifetime delving into this species’ past in North America, swarms contained billions of individuals and in terms of overall biomass approached that of the bison herds. Egg numbers per square kilometer of soil on one Minnesota farm were near four million, and there were nearly as many on one in Utah. When a swarm came in the mid-1800s, it was likened to the approach of a cyclone that blackened the skies. The locusts ate everything in their path, reputedly including the clothing on people, and they were considered one of the prime impediments to human colonization of the Great Plains. Observers recorded seeing the adults of a swarm pickled in the brine of Great Salt Lake “by the hundreds of thousands of tons” as the dead formed great walls along the lakeshore.
In Lockwood’s fascinating account of modern-day attempts to unravel the mystery of their extinction, one of the long-standing theories needed to be examined first, namely, that Rocky Mountain locusts are not really extinct at all. Do they exist to the present day, mistaken for some other grasshopper that, like the desert and the brown locusts, have different solitary and migratory phases? Maybe conditions recently have not been right to induce the migratory phase; past outbreaks occurred during droughts. The main posit for a possible present existence of Melanoplus spretus was that they were in cryptic form, namely, the still-common sol
itary M. sanguinipes. But how could one know? The answer came from studies of their genitalia, and global warming.
Grasshopper males may jump on anything resembling another grasshopper to try to mate. But what they lack in behavioral discrimination to maintain mating fidelity within the species is made up by mechanics. Grasshoppers, like many other insects, have species-specific genitalia. Male and female genitalia of any one species have a unique lock-and-key mechanism that prevents mating with the wrong species.
Global warming is melting glaciers worldwide, and those in the Rocky Mountains are spilling out tons of dead grasshoppers. A hundred years ago Montana had 150 glaciers, but since 1966 eleven of them have completely melted due to the indirect effects of our own growing numbers. In 2012 only twenty-five of those glaciers remained, and Montana’s Glacier National Park is estimated to be glacier-free by 2030.
Piles of grasshoppers have been dropping out of the ice at the foot of melting glaciers in the Beartooth Mountains near Yellowstone Park since the 1930s, and Lockwood and his team have sifted through tons of decaying grasshoppers looking for the hard parts, mainly genitalia and mandibles. The parts that they retrieved matched those of whole identifiable Rocky Mountain grasshoppers of (very rare) museum specimens, and so they concluded that the swarms, some of them dating to about four hundred years ago, had indeed been the migratory species. Comparisons with other present-day grasshoppers found no match. Indeed, a recent study of mitochondrial DNA also disputes the idea that the Rocky Mountain locust still exists as M. sanguinipes in disguise. The conclusion: the Rocky Mountain locust is extinct.
But why precisely did this grasshopper go extinct, whereas most of the other grasshopper species have not? This mystery is not yet solved, although Lockwood and DeBrey conclude that the advent of farming and perhaps other changes in land use after the demise of the bison are prime suspects. These are excellent reasons for the lack of outbreaks of this grasshopper for over a century. But, after the facts are digested and the known causes of mortality assessed, we still can’t truly explain the extinction. Unlike the case of the passenger pigeon, which went extinct in part because it was hunted, this species was never hunted. Why didn’t it just become rare? It is highly unlikely that there is not a tiny bit of habitat suitable for its existence remaining somewhere. What we know for sure is that what went missing is the locust crowd, and then not a single individual of the species that once turned the skies to cyclonelike blackness remained.
The Homing Instinct Page 27
