by Brian Switek
The expansion of our species from Africa, through Eurasia, and onto other continents appears to fit the pattern of extinction. Whereas extinctions were not as severe in Africa and Eurasia, where different species of humans had lived alongside large mammals for thousands to millions of years, the extinctions in Australia, North America, and South America appear to have occured when humans took up residence on those continents. Waves of migration might have pushed a few humans to these places during earlier times, as suggested by an over 14,000-year-old stone tool found in an Oregon cave discovered in 2009, but it was not until larger populations of humans became established in North and South America by around 11,000 years ago that most of the large mammals disappeared.
The questions are whether this correlation signals causation and by what means humans could have eliminated the megamammals. Hunting has been the most commonly considered factor, especially since the bones of extinct elephants and stone tools have regularly been found together at archaeological sites. Humans may have hunted large mammals into extinction, and those they did not kill directly may have been extirpated by other human influences such as disease, pests, and parasites that traveled with them to new places.
This idea certainly has popular appeal that conflicts with the notion that early humans lived in harmony with nature, but as with the rapid climate change model, the “overkill hypothesis” has some major problems. Among them is the fact that definitive signs that humans hunted large mammals, such as a series of hunting spears lodged in the bones of a mammoth, are exceedingly rare. Relatively more common, but still hard to come by, are bones showing signs that humans butchered the carcasses of some prehistoric elephants for meat. Faced with this association, however, we cannot be entirely certain that the humans hunted and killed that animal or how often they did so.
Looking at the associations of fossil elephant bones and human stone tools in Eurasia, especially, human butchering of mammoth carcasses is rare and usually seen in solitary skeletons. Nor can it be determined for certain whether humans hunted the animals actively or scavenged their carcasses. The somewhat surprising lack of signs of cut marks on mammoth bones, even at sites where multiple mammoth skeletons are preserved, suggests that perhaps mammoth meat was not nearly as important to some humans as that of other, easier-to-hunt animals like deer and bison.
There are also signs that humans butchered mammoth carcasses in North America, but it cannot be taken for granted that these associations indicate hunting on the part of humans, much less a level of hunting so severe as to wipe out the species. It is also strange that, if humans were the main cause for the extinctions of mammoths, the extinction of the mammoths was relatively slow, with several populations that persisted for thousands of years after the 10,000 year mark.70 In fact, the signs that humans actively hunted and killed many other large mammals that went extinct are either extremely rare or nonexistent. The pattern of human occupation and large mammal extinction seems to fit, but this correlation might not fully explain the ecological catastrophe.
Though arguments over climate change versus overkill can still become fierce very quickly, a more nuanced approach is beginning to emerge. Rapid climate change surely would have affected large mammals, especially those adapted to cold grassland habitats, and as this change was occurring our species was spreading across the planet. The habits of our prehistoric relatives, especially if they regularly hunted large mammals, may have pushed some of the largest mammals over the edge into extinction. The loss of these keystone species might have sent through already destabilized habitats a cascade that precipitated further extinctions.
But what if there was a way to bring back the Pleistocene? One such proposal, formally outlined in a 2005 article in the journal Nature, is called “Pleistocene Rewilding.” Conceived and supported by those who believe that humans are largely to blame for the extinction of large mammals in North America, if not the rest of the world, this project would establish fenced-in parks or reserves in which large mammals would be allowed to roam free. African and Asian elephants, lions, cheetahs, horses, and other animals would be used as proxies for extinct species and mixed with modern bears, pronghorn, and other endemic animals.
This proposal has generated a lot of controversy but relatively little understanding. While its supporters are not seeking to unleash elephants into the suburbs of the American West, it contains some fundamental conceptual flaws. For example, some extinct mammals may have relatively close living relatives; others, such as giant ground sloths, do not. Without the latter a late Pleistocene wilderness would not be complete. Likewise, as a result of climate change (both natural and human caused), some of the habitats—such as the widespread steppe favored by mammoths—the lost animals would have occupied are now lost. Neither does evolution stop to wait for extinct animals to catch up. Organisms and ecosystems have continued to evolve, and there is no certainty that an artificial collection of mammals would interact with modern environments as their ancestors did. The main motivation behind “Pleistocene Rewilding” is penance for what our Pleistocene relatives did; while it might be an interesting ecological experiment, the project cannot turn back evolution to a time before many of the large mammal species were lost.71
But what if some of the extinct species could be resurrected with the help of preserved DNA? In an April 1984 issue of MIT’s Technology Review, a curious article appeared entitled “Retrobreeding the Mammoth,” which claimed that, with the help of MIT scientist James Creak, the Siberian veterinarian Sverbighooze Nikhiphorovitch Yasmilov had managed to rejuvenate some eggs recovered from a frozen female mammoth, inseminate them with Asian elephant sperm, and bring two of the embryos to term in female Asian elephants. The resulting offspring, indistinguishable from woolly mammoths, were to be called Elephas pseudotherias, and Yasmilov had big plans to breed them to help rescue stranded Siberians workers or help repair the Trans-Siberian oil pipeline.
Newspapers ate it up, but most neglected to check the date of the article or check with the Technology Review about the story. It had been an April Fool’s joke. Nevertheless, the idea of cloning a mammoth has remained popular. After all, scientists have found a number of well preserved woolly mammoths frozen in the ice of Siberia over the years. If we have the intact hair, skin, and organs of the animals how hard could they be to clone?
The novel Jurassic Park and its film adaptation, in which dinosaurs are cloned from blood preserved inside prehistoric mosquitoes trapped in amber, have made the idea of bringing an extinct creature back to life seem like a piece of cake. All you need is some preserved DNA and—bingo!—the past comes alive. Anyone who was paying close attention to the sci-fi story will have noticed that the fictional scientists glossed over some crucial parts of the process, however. It is not so easy to go from a string of ancient DNA to a living organism.
Extracting DNA from mammoth skin, hair, or teeth would just be the initial step in a very complicated process, made all the more difficult by the fact that DNA degrades over time. Scientists cannot simply pluck a cell from a frozen mammoth and map its entire genome. The entire complement of DNA would have to be carefully identified and put together by looking at multiple specimens. To date, scientists have identified the complete sets of mitochondrial DNA for woolly mammoths and the American mastodon, or the unique DNA found in that sausageshaped organelle inside the cell, but we are still a long way from mapping the entire set of DNA that would be contained in the cell nucleus, the necessary genetic material for the cloning of an animal.
FIGURE 77 - An evolutionary tree of living elephants and woolly mammoths based upon genetic data. Asian elephants are more closely related to woolly mammoths than either is to African elephants.
For the sake of argument, however, let’s assume that we presently have the complete set of nuclear DNA from a woolly mammoth. How do we get from that to a cute, fuzzy newborn mammoth? The fact that genomes might be compared to blueprints or recipes does not explain how they get translated into a living orga
nism.
What we would need to do is create a fertilized egg that can grow inside the womb of a mother animal. This would involve removing the DNA from the unfertilized egg of an Asian elephant, fertilizing that with revitalized mammoth sperm (the DNA inside of which would probably be degraded), and getting that to grow to term inside a female Asian elephant. The embryo would require such an environment in which to develop and grow; it cannot be replicated outside the womb. All of this, of course, is an oversimplification. This process would be unimaginably intricate and require an knowledge of DNA, ancient DNA no less, that we do not as yet possess. And even if it could be accomplished, we must ask whether the end result would be a true mammoth, especially if input from the Asian elephant, the woolly mammoth’s closest living relative, was required at every single step.
Perhaps there will come a day when cloning a mammoth will be possible, but for now it seems that it would be easier to breed the hairiest Asian elephants together over many generations until you had individuals with long, heavy coats. It would not be a mammoth, but it would be about as close to seeing one again as we could get. Despite the low likelihood that we will be able to clone a mammoth (even cloning living animals is still fraught with problems) attempts at doing so could still teach us much about the recovery and study of prehistoric DNA. Perhaps that is not as glamorous as producing a species that has been extinct for thousands of years, but it would help us better understand how life has evolved and continues to do so.
On a Last Leg
“It has become evident that, so far as our present knowledge extends, the history of the horse-type is exactly and precisely that which could have been predicted from a knowledge of the principles of evolution.”
—THOMAS HENRY HUXLEY, American Addresses, 1877
On February 9, 1857, Emily Gosse passed away. Suffering from a breast cancer discovered only a few months before, her health had ebbed bit by bit until she succumbed to the disease. It was a devastating loss for her son, Edmund, and her husband, the celebrated naturalist Philip Henry Gosse.
Several months prior to Emily’s death, Gosse seemed to be at the high point of his career. His highly regarded 1854 book on marine biology, The Aquarium, had inspired a frenzy of aquatic-themed English decoration, and led to his election as a Fellow of the Royal Society in 1856. Emily’s death made this academic victory seem sour. Gosse’s despair deepened when his hastily written biography of his wife, Memorial of the Last Days on Earth of Emily Gosse, was not well received by his friends and family. The now widowed and middle-aged Gosse felt isolated, and the departure of his wife required that he ensure the Christian salvation of his son at a time when the ruminations of evolutionists were becoming increasingly prominent. The incessant hammering of geologists and the macabre activities of comparative anatomists were threatening the ideals Gosse believed in most fervently.
FIGURE 78 - Philip Henry Gosse and his son, Edmund, photographed in 1857.
His next two publications were direct attacks on evolution, a threat he perceived as remaining partially concealed from the public. The first book, which appeared in 1857, was a collection of essays he had written for the magazine Excelsior, entitled Life in its Lower, Intermediate, and Higher Forms: or, Manifestations of the Divine Wisdom in the Natural History of Animals. As the title suggested, Gosse had a hierarchical vision of the natural world, in which God fitted each organism to a purpose. His son would later admit in a biography that Gosse’s approach in this volume was more heavy-handed than usual:These essays were slight, and the religious element was quite unduly prominent, as if vague forebodings of the coming theory of evolution had determined the writer to insist with peculiar intensity on the need of rejecting all views inconsistent with the notion of a creative design. This book entirely failed to please the public, who had now for so many years been such faithful clients to him; with the scientific class it passed almost unnoticed.
In Gosse’s next book, Omphalos, published the same year, he proposed to once and for all reconcile geology with Genesis. While many before him had tried, Gosse believed that he had the ultimate answer to the conundrum. He declared that the entire expanse of geological time, from the days mammoths trumpeted on the Siberian steppe to the era dominated by the fearsome dinosaurs and beyond, was just an illusion.
If there was any conflict between science and the Bible, Gosse argued, science was in the wrong. The world had been created with the appearance of age. According to Gosse, the strata of the earth were analogous to the rings inside the mature trees that God had created. God knew that each living thing had a life cycle and he merely chose the adult stage to start with, thus creating the world and the creatures within it according to the foreordained trajectories of development.
Theologians and naturalists alike were dumbfounded by Gosse’s thesis. To naturalists, Omphalos appeared as an anachronistic attempt to shoehorn all of nature into the confines of Gosse’s interpretation of Genesis; to the faithful it cast God as a trickster. Even those who were sympathetic to Gosse’s aims, if not his arguments, could not accept his views. In a personal letter to Gosse, his clergyman friend Charles Kingsley charged that, in proposing the world to have been created with the impression of great antiquity, “you make God tell a lie.”
Gosse was crestfallen, his views dismissed and forgotten by the time Charles Darwin’s On the Origin of Species was published in November of 1859. Nature would be understood on its own terms. Lamarck’s hypothesis had been sneered at, Robert Chambers’s anonymous tract had been dismissed, and the work of Wallace had been pushed aside. Yet while social conservatives such as Richard Owen and Adam Sedgwick bridled at Darwin’s evolutionary vision, On the Origin of Species brought the debate over the evolution of life to a higher level.
Despite the massive amount of data Darwin mustered in defense of his ideas, however, the lack of transitional fossils remained the most often-repeated objection to his hypothesis (perhaps because, in his honest treatment of the subject, Darwin identified it himself). Among those who doubted Darwin’s mechanism was the French paleontologist Albert Gaudry.
Between 1855 and 1860, Gaudry participated in the excavation of a Miocene fossil site in Pikermi, Greece, that contained the bones of mammals of an age intermediate between those already known from the older Eocene and the much younger Pleistocene. If Darwin was correct, then the Pikermi strata would contain mammals intermediate in form between those of the earlier and later periods, and that is precisely what Gaudry found. There were fossil monkeys, giraffes, rhinos, elephants, saber-toothed cats, and more, many of which appeared to fill in a gap in prehistory. As Gaudry would later write:Thanks to the palaeontological researches which are everywhere conducted, beings of which we did not understand the place in the economy of the organic world are revealed to us as links in chains which themselves are connected; one finds transitions from order to order, from family to family, from genus to genus, from species to species.
These transitions fit within the evolutionary framework Darwin had predicted. When On the Origin of Species was published, it contained only one illustration, a “tree of life” that revealed evolution as a gradual, branching process. Darwin’s tree was only hypothetical, but Gaudry was one of the first to take this general concept and plug in actual organisms in order to trace their “filiation” (or relatedness to each other). One such tree, published in 1866, represented the evolution of horses. At the time, horses were recognized perissodactyls, or ungulates with an odd number of toes, but how they were related to the rest of the group was difficult to determine. Even though it had first been described years earlier, for Gaudry the equid Hipparion, discovered at Pikermi, provided both an anatomical and a temporal intermediate between the more recent horses and older, more generalized perissodactyls. Hipparion differed from modern horses in that it had small toes on either side of the hoof that did not touch the ground, thus connecting the one-toed members of Equus with other perissodactyls, such as tapirs and rhinos, that bore more toes on their fee
t. This, of course, still left the problem of what animal Hipparion itself arose from, but it was a start in filling out the history of horses.
The fossils of Hipparion found at Pikermi were important for another reason related to natural selection, as well. There were enough of them to study the variation of the genus. Just like there are differences between individuals of any one population or species living today, so there is variation between the remains of fossil animals, and variation is the raw material that natural selection works on to produce evolutionary change. In fact, the remains of Hipparion exhibited so much variation that Gaudry noted that they may have been considered distinct species had they not all been found together, which in turn suggests that gaps between gaps will closed up as more fossil material is recovered.
Yet Gaudry did not believe that natural selection could account for the evolutionary patterns he saw. Much as Gaudry appreciated Darwin’s making biological transformation a legitimate question, he dismissed natural selection because he abhorred the idea that evolution could involve an element of chance:If we recognise that organised beings have little by little been transformed, we shall regard them as plastic substances which an artist has been pleased to knead during the immense course of ages, lengthening here, broadening or diminishing there, as the sculptor, with a piece of clay, produces a thousand forms, following the impulse of his genius. But we shall not doubt that the artist was the Creator himself, for each transformation has borne a reflection of his infinite beauty.
Regardless of his disagreements with Darwin, however, Gaudry’s work on the placement of Hipparion as an intermediate and possible ancestor to the modern horse was only a relatively early example of the importance of horses in illustrating evolutionary change.