Biology of Extinction and Ways to Overcome It

Introduction

Extinction is when a species ceases to exist. Extinction is thought to be permanent and forever, but what if it wasn’t? De-extinction, also known as resurrection science or species revivalism, creates an organism that closely resembles or is a member of an extinct species (Martinelli et al. 2014). Thanks to recent advances in genetic engineering technology, specifically CRISPR (clustered regularly interspaced short palindromic repeats), scientists are considering it to be more obtainable. This paper will explore the three approaches, past and current studies, and debates of de-extinction.

Three Approaches to De-extinction

The three approaches to de-extinction are cloning, genetic engineering, and back breeding (Martinelli et al. 2014). The famous movie, Jurassic Park, is a common notion that pops into many heads when thinking of de-extinction, but one main component for this process to work is missing from dinosaurs…DNA. Dinosaur DNA is far too degraded. However, not all extinct species have such degraded DNA. The time that has passed since extinction and preservation methods directly impact the quality of DNA.

Cloning

The first and most basic of three processes is cloning. Cloning made a major breakthrough when the first mammal was successfully cloned in 1997 (McKinnell & Di Berardino 1999). This, of course, was Dolly the sheep. Since then, many other mammal species have been cloned. This includes endangered and even extinct species.

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The following are needed for cloning extinct species: extinct species’ intact DNA, a closely related living species to serve as an egg donor, and a surrogate mother. Since the DNA must be intact, the cloning method works best with species that are recently extinct. Ideally, DNA would be collected before the last individual dies. De-extinction cloning follows the same process in which ‘Dolly’ was derived. Somatic (non-gamete) cells are needed from the individual that is going to be cloned. Many somatic cells are skin tissue samples. The DNA in the nucleus of the donor egg is extracted. The somatic cells are introduced to the newly denucleated egg. Electric pulses stimulated the somatic cells to fuse with the denucleated egg (McKinnell & Di Berardino 1999). Next, cell fusion then cell division will occur. This early stage embryo is then implanted into a surrogate mother. If all is successful, the surrogate mother will become pregnant and a cloned individual will be produced.

Genetic Engineering

Genetic engineering is employed when cloned embryos fail or the intact DNA is not available. This process can work for species that have been extinct for a longer period, such as the woolly mammoth (Mammuthus primigenius). Genetic engineering is when extinct DNA is inserted into the genome of a living relative. This is made possible with CRISPR-cas9. CRISPR has been around since 1987, but CRISPR-cas9 was introduced in 2012 and 2013 (Doudna & Charpentier 2014). It is a powerful gene-editing tool that allows scientist to change the genetic structure at site-specific locations on the genome sequence. Cas9 is the enzyme that works similar to scissors cutting the DNA at the targeted locations.

For this process, the following is needed: fragments of DNA from an extinct species, the genome sequence of the living related and extinct species, a donor egg, and a surrogate mother. Fragments of DNA from an extinct species can come from frozen specimens, museum specimens, bones, and other preserved tissues (Ogden 2014). Some of the best samples of ancient DNA have been preserved in permafrost. To begin the genetic engineering process for de-extinction, the extinct and closely related species genome must be sequenced to reveal genetic coding. Utilizing CRISPR-cas9, selected genes are then replaced with sequences from extinct genes. For example, woolly mammoths are adapted to live in cold climates. Scientists will capture the warm climate adapted sequence of the Asian elephant’s (Elephas maximus) DNA and replace with the cold sequenced DNA of the woolly mammoth. The modified DNA is then combined with a denucleated donor egg and shocked with an electric pulse to trigger fusion. After this, the following steps are the same as the cloning process. Cell fusion and division will occur resulting in an embryo that is implanted in the surrogate other. This surrogate mother should be closely related to the extinct species. A hybrid species will be produced.

Back Breeding

Back breeding is selective breeding to exemplify extinct traits. This process creates a new species that will resemble the extinct species of interest. Desired traits (most similar to those of the extinct species) are identified in the closest living relative. Researchers use artificial breeding to breed sequential generations until the desired traits are shown and the resulting population looks like the extinct ancestor (Driessen & Lorimer 2016). Essentially, certain phenotypical traits are selected for and all others are breed out. Back breeding creates populations and has less direct manipulation of DNA cells. This process is similar to how humans have bred dogs from wolves but in reverse.

Current and Past Studies

De-extinction has already occurred. An extinct species was in fact brought back. The first extinct subspecies was a clone of the bucardo or Pyrenean ibex (Capra pyrenaica pyrenaica). The Pyrenean ibex was one of the four subspecies of the Spanish ibex (Capra pyrenaica). The last living Pyrenean ibex was a female. Scientists captured her in 1999 and took an ear tissue sample that was then frozen in liquid nitrogen. The female was found crushed by a fallen branch thus causing this subspecies to be listed as extinct by IUCN (International Union for the Conservation of Nature) Red List in 2000 (Folch et al. 2009). The Pyrenean ibex was heavily hunted and had a low genetic diversity which contributed to its demise. In 2003, scientist collected somatic cells from the frozen skin biopsy that occurred in 1999. They fused this cell with a denucleated egg of a domestic goat. Out of the 57 embryos that were implanted into subspecies of the Spanish ibex and/or hybrid goat (domestic goat and Spanish ibex) surrogates, only 7 became pregnant and only one went to full term (Folch et al. 2009). The clone survived seven minutes before becoming ‘re-extinct’. The cause of death was respiratory failure. This was due to the development of an extra lobe in the lung.

Similarly, endangered species have been cloned. The first cloned endangered species was the Gaur (Bos gaurus) in 2001. A male was born but died 48 hours later due to dysentery (Folch et al. 2009). In 2001, the first cloned endangered animal to survive past infancy was a mouflon sheep (Ovis orientalis musimon) (Ptak et al. 2002). The process of cloning for this study was duplicated in the cloning of the endangered Isfahan mouflon (Ovis orientalis isphahanica) in 2011 (Hajian M. et al. 2011) and the Pyrenean ibex in 2003. Overall, there are many issues with cloning. Due to the invasive tendencies of manipulating the egg cells, clones can exhibit deformities, premature aging, and compromised immune systems (McKinnell & Di Berardino 1999). Cloning is costly and overall not very effective.

As of today, no extinct species have yet been produced from genetic engineering. Currently, the “Revive and Restore” project are focusing on bringing back the woolly mammoth and passenger pigeon (Ectopistes migratorius) (Martinelli et al. 2014). The woolly mammoth DNA sequence is now complete and researchers are hopeful they will be able to one day bring this species back (Yamagata et al. 2019). The Asian elephant is closely related to the woolly mammoth. In theory, scientists will modify the Asian elephant’s genome sequence and replace certain genes with that of the woolly mammoth. Genes that allow for the mammoth to survive in cold climates are particularly of interest for managers at Pleistocene Park in northern Siberia. They hope to reintroduce a hybrid woolly mammoth to help with climate change (Yamagata et al. 2019). Some find genetic engineering to be controversial and see it as “playing god”. It can take years to sequence out extinct DNA. There must be enough sources of DNA to create the entire sequence. Some sequences can have well over a million base pairs.

Back breeding is currently being applied to the extinct quagga (Equus quagga quagga), aurochs (Bos primigenius), and Galapagos tortoise (Chelonoidis nigra) species. Currently, the Taurus project is attempting to back breed the aurochs. This was previously attempted by the Heck brothers in 1920, but their cattle were completely genetically different from the original aurochs. Aurochs went extinct in 1627 and are considered to be the ancestor of all domestic cattle (Driessen & Lorimer 2016). Humans began to domesticate this species over 8,000 years ago. The Taurus Project, in Europe, is breeding the most closely related and resembling European cattle in their back-breeding program in hopes to bring back the auroch (Driessen & Lorimer 2016). They estimate in 20-25 years they will achieve a hybrid that closely resembles the auroch species. Back breeding is a slow process but unlike the other approaches to de-extinction, it will create populations.

Similar to back breeding, backcross breeding was applied to the endangered American Chestnut (Castanea dentata). The American chestnut has been devastated by blight, an invasive fungus (Cryphonectria parasitica) that infects and eventually kills the trees. Researchers turned to the Chinese chestnut (Castanea mollissima) for help. The Chinese chestnut is resistant to this blight. The Chinese and American chestnut were bred to create a hybrid. Then, the goal was to outbreed all genes/traits of the Chinese tree to have an almost ‘pure’ American chestnut that is resistant to the blight. After many attempts, researchers decided to try gene transfer of the resistant gene (Friese & Marris 2014). This was proven to be more successful than backcrossing.

Debates on De-extinction

The question is not can we bring species back. We know it is possible but should we? Many scientists argue that it would be beneficial to restore extinct species while others say it would be disastrous. One major organization is taking this concept seriously. The IUCN has published guidelines for de-extinction and established de-extinction task force (Valdez et al. 2019). The SSC Guiding Principles on Creating Proxies of Extinct Species for Conservation Benefit, published in 2016, focuses on vertebrates, particularly birds and mammals (IUCN SSC 2016).

Holocene extinction, anthropogenic extinction, sixth mass extinction, no matter how you word it, this extinction event is currently happening and is mainly driven by human activity. Paleontologists define mass extinctions as periods when Earth undergoes a loss of more than three-quarters of its species within in a time geological time period (Barnosky et al.). There have been five known mass extinctions thus far: Ordovician, Devonian, Permian, Triassic, and Cretaceous. The current mass extinction is occurring at an extremely high rate. Humans are known to accelerate and cause the extinction of species. Our impacts on species can be traced all the way back to the Pleistocene (12,000 years ago). Due to our destruction, it is our moral responsibility to do everything we can to protect and restore species. Wouldn’t that include bringing them back?

De-extinction would restore keystone species and therefore ecosystems. Essentially, de-extinct species would assist with ecological restoration. For example, the woolly mammoth would, theoretically, restore the tundra and boreal forests. Soil compaction and restoration of tundra grassland would keep the permafrost from melting as fast, an issue we are currently dealing with. By bringing back extinct species, biodiversity would increase. Introducing new genes into a population would increase genetic diversity.

On the other hand, many legal issues and questions develop alongside the idea of de-extinction. De-extinction results in an entirely new species. What kind of protection would this warrant? Would it qualify as an endangered species? Would it even be considered a product of nature? If not, this species can be patented in the United States (Sherkow & Greely 2013). Beyond the IUCN guidelines, how would de-extinction be regulated? Some argue the Endangered Species Act (ESA) would no longer be needed if extinction is taken off the table.

Many conservationists are concerned with the implications that could develop with de-extinction. Conservation is critically underfunded. Scientists are worried money will be spent on bringing back extinct species instead of spending it to save the ones we have left. Ultimately, we are unsure of the impacts these de-extinct species will have on the environment. They will need habitat. Does their previous ecosystem even exist? Will species be able to adapt to the ecosystems that are available? It would be cruel and pointless to bring back a few individuals of a social species (e.g., the passenger pigeon). How would we provide husbandry efforts for animals we don’t fully understand? Many ancient species were exposed to various ancient diseases. Would these diseases be awoken along with their genes and spread? De-extinction is costly and not very effective. It takes a lot of time and effort to bring back just one individual. Bringing back a few individuals would result in low genetic diversity and all the efforts will be for nothing.

De-extinction is indeed no longer a fantasy and could very well be a reality in the future. A recent report by the United Nations states that one million of the planet’s species are at risk for extinction. If humans do not make the proper changes, maybe de-extinction will one day be the answer.

Citations

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