Will the woolly mammoth walk again?

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“Well it just so happens that your friend here is only mostly dead. There’s a big difference between mostly dead, and all dead.”

– Miracle Max (Billy Crystal), The Princess Bride, 1987

Beneath our feet, intermixed with soil and rock, lay the scattered bones of history. Clues captured in fossils tell us about the evolution and extinction of ancient life. Here, the word “extinct” carries a certain finality to it. Once a species goes extinct, that’s it—there’s no coming back. So went the mighty Tyrannosaurus rex, the towering woolly mammoth, and the famed passenger pigeon. But, then again, what if extinction isn’t so permanent? What if these creatures are only mostly extinct?

Of the many species now classified as extinct, few have captured the public’s attention like the woolly mammoth. For about 2 million years, these hairy giants roamed across northern Eurasia and America [1]. However, as the global climate changed and humans moved into their territory, the mammoths began to disappear. Archeological findings tell us that isolated populations survived until as recently as 4,000 years ago [1,2]. But when the last mammoth fell, they took with them millions of years of evolution.

Approximately 7 million years ago, the ancestors of all modern elephants were living in Africa [2,3]. There, they adapted to extreme temperatures by developing large ears and long tails that help release body heat. As time went on, some elephants migrated north to the Eurasian continent, and then further north to the Siberian tundra. With temperatures dipping to -50C, survival favored the few elephants who were better suited for cold weather—those with smaller ears, shorter tails, longer hair, and more fat deposits. Over millions of years, these traits became exaggerated in the northern elephant populations and eventually gave rise to what we now know as the woolly mammoth [1]. 

Thanks to DNA sequencing, it gained new life

Importantly, the evolution that led to the woolly mammoth was not a result of conscious decisions on the elephant’s part. Instead, changes in the elephant’s DNA that favored survival slowly built up and were passed from one generation to the next. Some of these changes altered how the elephant’s body stored fat or grew hair, while others created unique proteins that may have never existed before [1]. So the woolly mammoth, like all animals, was a collection of unique DNA sequences. And while the mammoth may be dead and gone, its DNA isn’t.

In the past decade, new technology and research practices have made it possible to sequence DNA extracted from ancient biological material, including mammoth remains. With this information, it’s possible to rebuild parts of the ancient giant. In 2010, researchers brought the woolly mammoth’s hemoglobin protein back from the dead [5]. Hemoglobin is a protein that transports oxygen through an animal’s body. Oxygen loosely latches onto the hemoglobin protein and is later released in regions of the body that need it.

How easily the oxygen molecule attaches to, or detaches from, the hemoglobin protein is influenced by temperature [5]. This is problematic for animals that evolved in warm climates, but now live in cold climates. After sequencing the woolly mammoth DNA, researchers were able to take the DNA sequence coding for the hemoglobin β/ẟ gene, and add it into the genome of a certain type of bacteria. Once there, the bacteria read the gene, and built the protein it coded for. Testing of the resurrected protein showed researchers that woolly mammoths had developed a mutation that helped their hemoglobin protein compensate for the cold temperatures and effectively transport oxygen [5]. This protein, which evolved in the woolly mammoth, hadn’t been made for more than 4,000 years. But thanks to DNA sequencing, it gained new life.

So, does this mean that extinct animals like woolly mammoths and dinosaurs could potentially come back from the dead? Not exactly. 

🤔Did you know?

Proteins can survive much longer than DNA. Currently, the oldest recorded DNA samples date to about 500,000-800,000 years ago. In contrast, scientists have found melanin protein—a brownish pigment responsible for skin, hair, and feather color—from ancient squid ink, and from dinosaur fossils that date back to almost 190 million years ago. This is part of the reason we know what some dinosaurs may have looked like in terms of skin or feather color [10].

 There are many barriers to bringing an animal back from extinction [6-9]. First and foremost, DNA carries instructions for how to build proteins, but there is much more to life than just making proteins. During embryonic development, parts of an animal’s DNA are selectively read, while others are selectively ignored. This process is extremely dynamic and requires precise spatial, temporal, and environmental cues—meaning it’s not enough to just have the animal’s DNA sequence, you also need to intimately understand these other factors before you can bring an animal back from extinction [6,7,9].

When an animal dies, the clock starts ticking for its DNA

One potential way around this is to modify the DNA of an existing species. The woolly mammoth’s closest living relatives are members of the Asian elephant species [7]. Relatively speaking, there are only a few differences between the woolly mammoth’s DNA and the Asian elephant’s. In theory, researchers could slowly introduce mutations into the DNA of some modern Asian elephants, and after several generations, it could lead to the rebirth of a woolly mammoth genome [7,9]. However, for better or worse, ethical and technical issues would first need to be resolved before this could happen [6,7,9].

To the question of bringing back dinosaurs, there’s an even bigger problem to overcome: the lifespan of DNA. When an animal dies, the clock starts ticking for its DNA. For a while, the DNA will remain stable in some of the animal’s sturdier structures (like bones). But as bacteria, temperature, and other factors act on the DNA, it breaks apart [9,10]. Currently, the oldest recorded DNA sample from an animal was recovered from a 700,000-year-old horse that had been frozen in permafrost—ideal conditions for storing DNA [10, 11]. This means that the DNA of dinosaurs who lived millions of years ago is unlikely to have survived to modern day.

We likely won’t see the woolly giants of history walk this earth any time soon. By most definitions, the woolly mammoth is extinct. But thanks to DNA sequencing, they may only be mostly extinct—and slightly alive.

References

1. Lynch, Vincent J., et al. “Elephantid Genomes Reveal the Molecular Bases of Woolly Mammoth Adaptations to the Arctic.” Cell Reports, vol. 12, no. 2, 2015, pp. 217–228., doi:10.1016/j.celrep.2015.06.027.

2. Roca, Alfred L. “Evolution: The Island of Misfit Mammoths.” Current Biology, vol. 25, no. 13, 2015, doi:10.1016/j.cub.2015.05.006.

3. Lipson, Mark, et al. “A Comprehensive Genomic History of Extinct and Living Elephants.” PNAS, National Academy of Sciences, 13 Mar. 2018, www.pnas.org/content/115/11/E2566.long.

4. Barnes, Ian, et al. “Genetic Structure and Extinction of the Woolly Mammoth, Mammuthus Primigenius.” Current Biology, vol. 17, no. 12, 2007, pp. 1072–1075., doi:10.1016/j.cub.2007.05.035.

5. Campbell, Kevin L, et al. “Substitutions in Woolly Mammoth Hemoglobin Confer Biochemical Properties Adaptive for Cold Tolerance.” Nature Genetics, vol. 42, no. 6, Feb. 2010, pp. 536–540., doi:10.1038/ng.574.

6. Shapiro, Beth. “Mammoth 2.0: Will Genome Engineering Resurrect Extinct Species?” Genome Biology 16 (2015): 228. PMC. Web. 18 Sept. 2018.

7. Sherkow, Jacob S., and Henry T. Greely. “What If Extinction Is Not Forever?” Science, American Association for the Advancement of Science, 5 Apr. 2013, science.sciencemag.org/content/340/6128/32.

8. Piña-Aguilar, Raul E., et al. “Revival of Extinct Species Using Nuclear Transfer: Hope for the Mammoth, True for the Pyrenean Ibex, But Is It Time for ‘Conservation Cloning’?” Cloning and Stem Cells, vol. 11, no. 3, 2009, pp. 341–346., doi:10.1089/clo.2009.0026.

9. Huynen, Leon, et al. “Resurrecting Ancient Animal Genomes: The Extinct Moa and More.” BioEssays, vol. 34, no. 8, June 2012, pp. 661–669., doi:10.1002/bies.201200040.

10. Briggs, Derek E. G., and Roger E. Summons. “Ancient Biomolecules: Their Origins, Fossilization, and Role in Revealing the History of Life.” BioEssays, vol. 36, no. 5, 2014, pp. 482–490., doi:10.1002/bies.201400010.

11. Orlando, Ludovic, et al. “Recalibrating Equus Evolution Using the Genome Sequence of an Early Middle Pleistocene Horse.” Nature, vol. 499, no. 7456, 2013, pp. 74–78., doi:10.1038/nature12323.