The idea to live among animals long extinct is very attractive and has been imaginatively described in many books, including “The Lost World” of Sir Arthur Conan Doyle. However, as we cannot move back in time, why not move extinct species to our modern era, as imagined in “Jurassic Park”?
While the development of new methods has made working with the ancient DNA and genomes of more and more extinct species possible, ecological and ethical questions concerning resurrection or de-extinction of extinct species have arisen. However, the first question to consider is: is it principally possible to resurrect a species based on its DNA?
Initially, all hopes for species de-extinction were placed in cloning techniques, the aim of which is to produce genetically identical individuals. For some organisms like bacteria and plants cloning is the natural process of reproduction. In contrast, complex multicellular organisms like vertebrates, which are the usual prime targets for de-extinction, cannot be so easily cloned. In classical cloning via somatic cell nuclear transfer (SCNT), the nucleus from a somatic cell is transferred to an unfertilized oocyte whose original nucleus was removed. Ideally, such an oocyte gives rise to an embryo that is then implanted in a surrogate mother. As a successful example, Sooam Biotech in South Korea provides a service for cloning dogs for owners who want to compensate the loss of their loved pets by their genetically exact copies. Cloning extinct species presents a number of factors complicating this procedure. The SCNT method is effective if the somatic nuclear source comes from a living animal and is immediately frozen to avoid DNA degradation. Such material is not available from most extinct species. Even the mammoth material found in frozen soil does not contain completely preserved cells. Furthermore, the interaction between donated nuclear DNA from the extinct species and the oocyte cell environment is crucial for proper embryonic development. This means, only very closely related species can be used as oocyte donors. It is to note that not many extinct species are lucky enough to have such living close cousins. Thus, the SCNT technique might help when frozen material is available, encouraging the cryo-preservation of endangered species tissues and the creation of frozen zoos, but it is not promising for resurrecting already extinct species.
However, there is an alternative way that avoids some of the pitfalls of SCNT – genome editing. Using specific DNA regions of extinct species as a template we can modify the genome of the closest living relative using CRISPR genetic engineering. In this case we do not actually resurrect an individual genetically identical to the extinct one, but rather create an animal that has the key features of the extinct species. George Church and his team at Harvard University are now working to generate such a “mammophant” hybrid, an Asian elephant with the small ears, subcutaneous fat, long shaggy hair, and cold-adapted blood of the mammoth. The team has already made some progress editing the Asian elephant genome but has not yet crossed the boundary of creating “mammophant” embryos. In a next step scientists have to grow the created embryos. An additional difficulty is that, in contrast to lab mice, experiments on endangered elephants are from aspects of biodiversity conservation restricted. Implanting embryos to living female elephants may threaten the elephant reproductive system. To avoid this, the scientists plan to raise the embryos in artificial wombs that make the procedure even more complex. Thus, it is unlikely that we will see a mammoth-like elephant in the near future.
Another candidate for de-extinction that attracts public and scientific attention is the Tasmanian tiger. This largest marsupial predator of modern times inhabited Australia and New Guinea, but 2000 years ago was reduced to the area of Tasmania where the last individuals were killed by humans in the early 20th century. For a long time, de-extinction of the Tasmanian tiger was only a dream because, despite its recent extinction, no good non-degraded DNA was available. A year ago an amazingly good, ethanol-preserved, 109-year-old pouch young specimen from Museums Victoria, Australia was found to contain high quality genomic DNA. This enabled a consortium led by Andrew Pask from Melbourne University to sequence the complete nuclear genome of the Tasmanian tiger and provided new hope for its resurrection. However, even though the Tasmanian tiger went extinct much more recently than the mammoth, its de-extinction presents an even more complicated task. The Tasmanian tiger’s ancestors evolved away from other marsupials around 30 million years ago. Their closest living relatives today are Tasmanian devils and numbats, whose morphological and behavioral characteristics are, nevertheless, both significantly different from those of the Tasmanian tiger. Furthermore, we still do not know which genes should be modified or specifically regulated so that the Tasmanian devil becomes twice as tall with stripes on its back or so that the numbat forgets about termites and starts hunting small kangaroos instead. Thus, we are still far from creating viable representatives of extinct species.
Even if some extinct animals are resurrected in the future, there is still a huge gap between a single de-extinct individual and viable populations with their indispensable genetic diversity and required ecological conditions that might also be “co-extinct”. Thus, species that went extinct millions or even just thousands of years ago will most probably remain extinct forever. But developing “de-extinction” technologies may in future be useful for conservation purposes focusing on species that went extinct just yesterday or are in the process of becoming extinct today.
By Liliya Doronina
Institute of Experimental Pathology, ZMBE, University of Münster
For more articles on extinct animals check out: The Cave: Neanderthals and Humans
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