Figure 11. CRISPR/Cas9. Long strands of DNA that match the region of the genome to be edited are synthesized and used to construct CRISPR-RNAs (the dark strands of DNA). These are delivered into the cell with Cas9. Once inside the cell, Cas9 picks up a CRISPR-RNA, which guides the entire complex to the correct place in the genome (the light strands of DNA), and the cut is made.
The mammoth revivalists’ de-extinction plans are relatively subdued for now. No Asian elephant cells were available when they got started, so they are editing African elephant genomes in African elephant cells. Also, for now, they are working with a type of skin cells—fibroblasts—and not stem cells, again because that was the only type of cell that was available. They have a separate line of ongoing research to try to make stem cells from the elephant fibroblasts, which has had limited success thus far. Once they have successfully created stem cells, they intend to use these to create different types of cells that can then be used to test whether or not their edits have been successful. No one is talking just yet about actually making these cells into a living mammoth. For now, the goal is to edit the genome and grow cells containing the edited genome in tiny plastic dishes in the lab.
The team hopes to edit the African elephant genome in a way that produces two specific phenotypic changes. First, they will make all four changes in the hemoglobin genes that are known to differ between elephants and mammoths. This should produce cells capable of making mammoth-like hemoglobin. If they can make these changes in hematopoietic stem cells—stem cells that differentiate into different types of blood cells—they will be able to measure directly the oxygen-carrying potential of the resulting red blood cells, which should tell them whether their experiment was a success. They’re also hoping to create cells capable of growing what George calls “the thickest, most luxurious, mammoth-like hair.” This is a tougher task, though, because no one is certain which (or how many) genes are involved with making thick, luxurious mammoth hair. For the moment, George is content to make an educated guess, based on which genes are believed to be associated with hair phenotypes in other species.
This, of course, is just the beginning. Once it is clear that extinct phenotypes can be genetically engineered into the cells of living species, de-extinction will be off and running. But what precisely will the resulting animal be? How many changes do we have to make in order to call an elephant a mammoth? Is it possible to make every single change that might differ between the two genomes? If not, what should we change?
CHAPTER 7
RECONSTRUCT PART OF THE GENOME
Here is my prediction: Within the next couple of years, George Church and the mammoth revivalists will succeed in transferring at least one mammoth gene into an elephant stem cell. They will use that stem cell to produce cells that express the newly inserted mammoth gene. They will carefully measure their success by designing a smart experiment to show that the gene is now producing mammoth proteins rather than elephant proteins. When they see positive results, indicating that they have, indeed, engineered a mammoth gene into an elephant cell, they will announce this success with deserved pride. It will be an astonishing achievement.
No elephants will have been harmed in the process. No elephants will have been involved in the process, other than to donate blood during a routine veterinary visit. No female elephants will have been subjected to any experimental manipulation whatsoever. No one will have performed nuclear transfer on an elephant. No baby elephant whose genome contains mammoth genes will be gestating anywhere.
The press will not hear any of the above caveats, however. The headlines will read: “The Mammoth Is Back.” “Extinction Is No Longer Forever.” “Scientists Create Woolly Mammoth in Test Tube.” It will be the biggest, most exciting, scariest, most wonderful, most terrible thing to happen in recent memory. There will likely be widespread announcements of dire consequences as well as excitement and some hysteria.
But there is no need, really, to speculate on how people will react. We can simply look to recent history.
THE MAMMONTELEPHASE OF DE-EXTINCTION
On April 23, 1984, the following appeared in the Chicago Tribune, tucked neatly away among the inside pages. The headline read: “A shaggy elephant story.” The full article is reproduced here, with permission:
When a species becomes extinct, we expect it to stay that way. Scientists in America and the Soviet Union have upset that seemingly safe assumption by “retro-breeding” a hybrid animal that is half elephant and half woolly mammoth, The story starts in Russia, where Dr. Sverbighooze Yasmilov of the University of Irkutsk was able to extract the nuclei from egg cells taken from a young mammoth that was found frozen in Siberia. Technology Review reports that he sent the material to the Massachusetts Institute of Technology, where Dr. James Creak mixed the DNA from the cells with elephant DNA. Woolly mammoths, which roamed Europe until they died out 10,000 years ago, have 56 chromosomes; elephants, their near-relations, have 58. Based on Creak’s success, Yasmilov decided to try to fuse the nuclei from the mammoth’s egg cells with sperm from an Asian elephant. The experiment produced eight fertilized eggs, which were implanted in Indian elephants. Six miscarried, but two hybrid animals—males that are probably sterile—were born. The hybrids, which some call “mammontelephases,” are covered with yellow-brown hair and have jaws that are similar to the mammoths.
The tiny story was picked up and distributed by the Chicago Tribune’s news service, and versions of it appeared in more than 350 newspapers within the following days. It even appeared in a nationally circulated Sunday supplement, where it no doubt received the widest potential readership.
Not one of the newspapers that picked up and ran the story bothered to check the facts. If anyone had bothered to contact the author of the report mentioned in the Technology Review, for example, or had tried to talk to any of the scientists involved in the research, they would have made a startling discovery: the whole thing was a joke. The scientists did not exist. The project did not exist. The story was meant to be a parody of science, written by a talented undergraduate student to fulfill a science writing assignment. The story was published in the Technology Review in celebration of All Fools’ Day. The article, which is on page 85 of the April 1984 edition of the Technology Review, concludes with the name of the student author—Diana ben-Aaron—and the date—April 1, 1984.
Perhaps those at the Tribune and the many other newspapers that decided to run the story were simply too excited about the possibility of mammoth de-extinction to notice the date or to question the authenticity of the report (including the unlikely collaboration between Soviet and American scientists at the peak of the Cold War). Or perhaps they didn’t get the joke.
The fictional piece by ben-Aaron was prescient in many ways. She predicted, for example, the poor success rate of nuclear transfer, despite writing the article more than twelve years before of the birth of Dolly at the Roslin Institute. She predicted that the Asian elephant would be used as a surrogate, although it would be more than two decades before we knew with certainty that the Asian elephant is more closely related to the mammoth than the African elephant is. She also anticipated and attempted to defuse some of what the public would fear about de-extinction. For example, she foresaw that containing these new creatures—not allowing them to escape and breed with the wild elephant population—would be a key concern. As Michael Crichton would do six years later, she invented a mechanism by which breeding of the cloned animals would not be possible without human intervention. While Crichton’s dinosaurs were all female and therefore unable to reproduce, ben-Aaron’s mammontele-phases were all infertile males. She made them infertile by giving them an uneven number of chromosomes. With mismatched numbers of chromosomes, they, like mules, would be sterile.1
The reaction to the press coverage of ben-Aaron’s fictional article was fast, heated, and mixed. Some people celebrated, either because they were amused by the display of obviously poor journalism, or by the parody itself, or because they didn’t
know it was fake and simply were excited that a mammoth had been brought back from the dead. Others were angry, either because they felt that the parody was improper or unfair or because they didn’t know it was fake and were really annoyed that scientists would do such a terrible thing as bring a mammoth back from the dead.
The reaction was, in fact, much like the reaction I anticipate when the mammoth revivalists publish the first evidence that their genome-engineering project has been a success and that edited elephant cells may—at some point in the future—be used to make edited elephants. Of course, the 1984 scenario was entirely fabricated. The hypothetical headlines of our future will reflect actual science going on in an actual cutting-edge research lab at one of the most respected research institutions in the world.
In 1984, those who read and believed the story in the Chicago Tribune or elsewhere came away with one message: a mammoth had been brought back to life. That was, however, not what the article said.
The headlines to come when the mammoth revivalists produce the first mammoth-flavored elephant cell are likely to be more spectacular than the subdued title of the Tribune piece. Careful journalists are unlikely to omit the fact that very little of the elephant genome is actually changed; however, this fact will be conveniently brushed aside to make way for impassioned and melodramatic commentary reflecting on the central message of the piece: a mammoth will have been brought back to life.
Except it still won’t be true.
IF IT LOOKS LIKE A MAMMOTH AND ACTS LIKE A MAMMOTH, IS IT A MAMMOTH?
Let’s return to the work that is going on in the present day. It is feasible today to use genome-engineering technologies to directly edit DNA sequences within a living cell. George Church’s lab is using this technology to edit elephant cells so that the genome within them looks more mammoth-like than elephant-like. For now, this work is limited to editing only one or a few genes in somatic cells. However, we have somatic cells that contain genomes in which some genes have had the elephant version removed and replaced with the mammoth version. This is the status quo of mammoth de-extinction.
If the somatic cells edited by the mammoth revivalists are used to create a baby elephant, that baby elephant would have only a very tiny amount of mammoth DNA. The mammoth revivalists’ goal is to engineer an elephant so that it can survive better in the cold. Let’s imagine that they achieve this by replacing the elephant version of something in the range of five to ten genes with the mammoth version of those genes. In this scenario, the phenotype of the hypothetical baby elephant hopefully would change, but more than 99.99 percent of its DNA would still be elephant DNA.
In the fictional scenario published in 1984, the babies that were born were first-generation hybrids, created by fusion of DNA preserved in a mammoth egg and DNA from elephant sperm. The hybrid creature’s DNA was 50 percent elephant and 50 percent mammoth, but ben-Aaron never went so far as to call them mammoths. In fact, she provided an entirely new scientific name—Elaphas pseudotherias—which places the hybrid mammontelephase in the same genus as the Asian elephant, but gives it an entirely new, and fictional, species name. Perhaps her goal was to be scientifically precise about what she created. Perhaps it was to avoid confusion. Whatever her motivation, the piece provides an excellent opportunity to observe the public’s reaction to the creation of a (fabricated) hybrid species.
The public did not care that it was a hybrid. The media called it a mammoth, and so it was a mammoth. Perhaps what was most important was how it was described, but even this was absolutely minimal in the media reports: the hybrid had yellow-brown hair and mammoth-like jaws. Clearly, even a tiny bit mammoth-like was good enough for the public. It was a mammoth.
This is great news for those in favor of de-extinction, because it provides an enormous amount of wiggle room for determining when de-extinction is a success. A mammoth will not have to be pure in order to be received as a mammoth. This is a relief, because—as we’ve discussed—while 100 percent mammoth is out of the question, 1 percent mammoth may not be.
This provides an opportunity to redefine de-extinction, shifting away from a species-centric view. Genetically pure mammoths, or genetically pure versions of any extinct species, are likely not possible. However, we do not need genetic purity to benefit from de-extinction technology. If we select wisely which 1 percent of the genome to change, we may be able to resurrect those characteristics that distinguish a mammoth from an elephant. More importantly, we may be able to resurrect those characteristics that allow the elephant to live where the mammoth once lived. Once released into the wild, the hybrid elephant could stomp around, knocking down shrubs and consuming vast quantities of vegetation. It could disperse seeds and insects and distribute nutrients. This new hybrid animal could replicate the mammoth, without necessarily being a mammoth, with vast potential benefits to the arctic ecosystem.
Most people who are seriously considering either de-extinction or back-breeding are doing so because they believe that bringing these species back would provide an upper hand in present-day struggles to preserve biodiversity and maintain healthy ecosystems. Extinctions at any level—whether of prey species or predator species or species that distributed seeds or species that consumed shrubs and trees so as to preserve open spaces—can have cascading effects across an entire ecosystem.
The project to breed back the auroch in mainland Europe aims to create giant herbivores that will graze wild, open land and thereby keep the shrubs and trees at bay. The result, the team hopes, will be a restored habitat that can be used by both large and small mammals and at the same time increase the diversity of plant species on the landscape. The auroch is the target phenotype of their back-breeding experiments. However, the team’s intention is not to bring an auroch back to life but to resurrect a phenotype that can do in that environment what the auroch used to do. They hope to replace the auroch with something similar in function but not necessarily identical in form.
In my mind, it is this ecological resurrection, and not species resurrection, that is the real value of de-extinction. We should think of de-extinction not in terms of which life form we will bring back, but what ecological interactions we would like to see restored. We should ask what is missing from the existing ecosystem that could be recovered. De-extinction is perhaps better imagined as an elaborate bioengineering project in which the biological end product is modeled on something that evolution created but that has unfortunately been lost.
WHAT PARTS OF THE GENOME SHOULD WE ENGINEER?
Genome engineering, and not cloning by nuclear transfer or back-breeding, seems to be the most likely avenue to resurrect extinct traits and—depending on how loosely we care to define a species—extinct species. But where do we begin? The answer to this question is likely to be different for each de-extinction project.
If our goal is to create an elephant that is capable of surviving a Siberian winter, then we have to change this tropically adapted species into something that fares well in the bitter cold. Longer, thicker hair will definitely help, as will hemoglobin with higher efficiency in carrying oxygen at low temperatures. But what other traits should we try to engineer? Are there other ways to make an elephant more efficient at maintaining its internal body temperature? Are there energetic requirements to living in the Arctic that we haven’t yet considered? Are there adaptations to the digestive system that will be necessary to allow an elephant to eat the plants that grow in Siberia? Do we need to engineer morphological changes that make the elephant capable of digging plants out of the snow? Will we need to engineer the elephant’s immune system so that it can evade pathogens that are not present in the tropics? These are all good questions to which we do not yet have any answers, much less a target gene or suite of genes that we could sequence and look for mammoth-specific changes that we will want to engineer.
The scientific world is unlikely to prioritize elephant genomics in the near future, which means that we won’t know anytime soon where all the genes are in the elephant ge
nome, what these genes do, or how they interact with each other. This information, however, is all crucial if we really want to genetically engineer a mammoth in a piecemeal fashion. Given that so much is unknown, one solution might be to change every nucleotide in the genome where the mammoth differs from the elephant. In doing so, we would be less apt to overlook any important difference or interaction between genes. This would, however, require making a lot of changes: if we assume that the Asian elephant and the mammoth diverged from their common ancestor around four million years ago and that the rate of divergence is similar to that in other mammals, we can expect something like 70 million genetic differences between the two species (on the same order as the number of genetic differences that separate humans and chimpanzees). Less than 2 percent of the elephant genome would need to be edited, but 70 million changes is a lot of changes to make.
How to Clone a Mammoth Page 14