Since life came into existence sometime in the Precambrian, there have been five mass extinctions in geological time scales, when more than 75 per cent of exiting life forms went extinct.
The environment on earth turned hostile and did not support existing life forms. As the environment of earth turned normal, new life forms were created, which flourished until the next mass extinction. Extinction of existing species appear to be linked with the changing environments on earth.
The 5th mass extinction happened about 66 million years ago and was probably caused by the impact of a huge meteorite with earth. This created a dooms day scenario and wiped out the existing life forms including the dinosaurs.
It is now widely believed that the sixth mass extinction has already begun and appears to be man-made. The human population has multiplied several times and now occupies most of the land and utilises almost all of the natural resources available around the globe. We are now using 1.7 times of the resources generated by the earth in one year (Global Foot-print Network, 2018). This has caused an ecological imbalance.
More than 300 types of mammals, birds, reptiles and amphibians, which existed earlier have gone extinct since 1500 AD.
The current rate is somewhere between 30-159 species, which disappear every day, caused largely due to human activities. Human activities appear to be the cause of 93 per cent bio-diversity loss. The rate of disappearance of species is about 100 times faster than the background rate normally seen between two mass extinctions. According to David Shultz (2016) we are in the midst of sixth mass extinction.
De-extinction of lost species: Several research projects are now in progress around the world to revive (de-extinct) some of the extinct species to restore the ecological balance damaged primarily by human beings and slow down climate change scenario.
Reviving the dinosaurs: The main problem for de-extinction is to get exact copies of the DNA of the extinct species or DNA of some related species to get an idea about the composition and structure of their DNA. The half-life of DNA is 521 years. Dinosaurs went extinct 66 million years ago. There are no existing species similar to them. It is therefore impossible to re-create the dinosaurs.
Current efforts now in progress therefore are to revive of some of the recently extinct species, whose partly damaged DNAs are available and they have some related species with similar DNA.
Most of the earlier efforts on de-extinction had failed due to use of inadequate technologies. The recent discovery in genomics, such as CRISPR/Cas9, which can be used to edit DNA offers new hopes.
The preserved specimens of de-extinct species collected from zoos around the world contain damaged DNA, since it degrades after death. The methods proposed to be used are to utilise the DNA of closely related species, which are still alive.
These can be used as a model and edited suitably using CRISPR/Cas9 to recreate the genome of extinct species. While it is difficult to predict the results, the recreated species are expected to be somewhat similar but may not be their exact replica.
The current researches as given below are still at various experimental stages.
Woolly Mammoth:A number of researchers from the Harvard University (2017) are involved in de-extinction of a large elephant like animal called Woolly Mammoth with huge tusks and long furs covering its body. It roamed in the frozen ice fields of Tundra during the ice age. The last living mammoth died about 4,000 years ago. However, a number of frozen bodies of these animals have been found trapped and preserved in the permafrost of the Tundra from about 4000-10,000 years ago.
While we have the intact frozen bodies of the animals, we do not have the whole copies of their DNA. They have however, a close relative, the Asian elephant. The Harvard team led by geneticist George Church, which started their research in 2017 propose to use the latest CRISPER/Cas9 technology to reconstruct the genome of woolly mammoth utilising the model of Asian elephant.
They have modified the Asian elephant genome by introducing genes that gave the woolly mammoth, its cold resistant blood and long hairs.
The next step is to take an egg cell of Asian elephant, remove its genome and replace it with the reconstructed genome of woolly mammoth.
Once the egg cell starts to divide, it will be placed in the womb of a female elephant, who will bear it and give birth to a woolly mammoth. The Harvard group propose to create a woolly mammoth by 2019. Only after its creation will it be known how far the de-extinct species resemble in its shape and behaviour with its ancestors.
If the experiment is successful, the de-extinct animals will be released to populate the tundra and taiga forests, their original abode in Siberia. Russian scientists are trying to create a ‘Pleistocene Park’ in Siberia to rehabilitate the woolly mammoth and other animal species, which once lived there.
Passenger Pigeon: The population of passenger pigeon in USA was estimated at 5 billion at the beginning of 19th century.
The early settlers in USA had seen their flock covering the entire sky when they were in flight. Indiscriminate hunting and loss of habitat wiped out the birds to extinction.
The last bird named as ‘Martha’ died in Cincinnati zoo in 1914 (Flannery, 2017). A young scientist Ben Novak has extracted DNA from 1500 museum specimens of passenger pigeon to revive this bird. He would use a close surviving relative called band tailed pigeon, whose DNA differs by 3 per cent from the passenger pigeon. He will edit its DNA using the CRISPR/Cas9 technology in the primordial germ cell to reconstruct the DNA of passenger pigeon. The germ cell so created will form the embryo of the first de-extinct passenger pigeon.
Tasmanian Tiger (Thylacine): With stripes of a tiger, body of dog and pouch of kangaroo, it was a unique combination of animals. Humans hunted it to extinction, the last one killed in 1936.
A taxidermy mount can be seen in the Australian museum in Sydney. Andrew Pask, an Associate Professor in University of Melbourne, is trying to revive the animal using modern gene editing technology CRISPR/Cas9. Earlier attempts of cloning the DNA from dead animals had failed.
Efforts are on to revive a number of different extinct species, such as Dodo, a large flightless bird found in Mauritius, which became extinct in 1640’s. It has a close surviving relative in Nicobar pigeon. Gastric breeding frog, Australia, with unique reproduction method went extinct in 1980s. The mother frog swallowed the egg and used its stomach as its womb. It stopped eating for six weeks and gave birth through ‘propulsive vomiting’. Heath hen, the bird was extremely common in north eastern USA, went extinct by 1932 due to overhunting. Wide availability of its DNA from museums makes it possible species for de-extinction.
Reviving the Neanderthals: Neanderthals were similar to human beings but had larger brains, stronger physique, simpler material culture but possibly lack of language. They diverged from human lineage about 500,000 years ago and lived in Europe.
They were probably driven to extinction by humans (Homo sapiens), who colonised Europe about 30,000 years ago. However, all modern humans except Africans carry 1-2 per cent Neanderthal genome probably due to interbreeding of humans with Neanderthals. The recovery of first Neanderthal genome was announced in 2013.
Technically it will be easier to revive a Neanderthals than possibly passenger pigeons. Should we do it? (Flannery, 2017). Will it be ethical to revive a human like species, which went extinct, 30,000 years ago? What will be its social impact on the existing human race? This needs to be answered before any scientist embarks on such a project.
Five renowned experts debated the topic of de-extinction in the annual Isaac Asimov Memorial debate at the American Museum of Natural History on March 29, 2017 (Isac Asimov [1920-1992] was a biochemist and a famous science fiction writer).
If humans go extinct and replaced by a more intelligent species, will they try to de-extinct some of the humans and put them in a zoo like condition to study their behaviour?
“Extremely frightening and scary,” said Greg Kaebnik, a research scholar, Hastings Centre of Bioethics, Garrison, New York. “Do we deserve to be brought back?,” he asked. So are the Neanderthals!
(Dr Mitra is a former Dean, Agriculture, a Professor and a researcher. gnmitra@gmail,com )