The humans knew it was coming, but there was nothing they could do. Four thousand light years away, a collapsing star sent a giant gamma ray bursting across the galaxy in a massive wave of energy. When it arrived, it wiped out the ozone layer, created monsoons of acid rain and froze the planet. It extinguished all life on Earth.
Well, not quite all.
Just as the extinction of the dinosaurs helped pave the way for the rise of mammals (and subsequently humans), the gamma ray made way for something else. If you'd been part of an expedition sent by an alien civilization to see whether any life survived the gamma ray, imagine guiding your spacecraft down through the omnipresent storm clouds until you catch sight of the shattered, empty cities with their toppling husks of skyscrapers, their flooded subways, their corpse-strewn sidewalks. Imagine skimming the funereal suburbs, the ruined farms, the dead forests and assuming that nothing could have possibly remained alive in the face of such devastation.
But soon enough you would see us. Many billions of us, in fact. Who are we? The humans called us tardigrades. We call ourselves top dogs. That's because there aren't any dogs left, metaphorical or otherwise. With all those other lifeforms gone, we've been free to evolve from microscopic, eight-legged, indestructible, panda-like creatures into giant eight-legged, indestructible, panda-like creatures.
Once, human scientists called us cute. Starting with their discovery of us in 1773, they began poking, prodding, dehydrating, freezing, baking alive and generally torturing us in every twisted way imaginable. Where are they now, those supercilious white-coated demons with their soft, vulnerable flesh and blood? They're the ashes we tread underfoot as we go about ruling this planet in fulfillment of our destiny!
For starters, a tardigrade is an animal. A very, very small animal. One of its many nicknames is "water bear" because, as mentioned earlier, some people say it resembles a panda bear (if a panda bear were microscopic and had eight legs). It's also been called a moss piglet, a pygmy rhinoceros and a pygmy armadillo. It has no backbone, but this does nothing to detract from its good looks. Currently, it appears to be the leading candidate for the title of "cutest invertebrate ever."
A lot of the tardigrade's fellow mini-invertebrates tend to ping-pong around like kids on a sugar high. Tardigrades, by contrast, amble about in a slow, charming manner that has endeared them to generations of biologists. In fact, the name tardigrade is derived from Latin and means "slow walker."
They're about a hundredth of an inch in length (500 micrometers) and can actually be seen with the naked eye if you have excellent eyesight and the light is just right [source: Miller]. However, they're also see-through, which makes an already difficult task even harder. A microscope is your best bet.
They have short little legs, each one outfitted with a set of claws. In a unique formation, their two rear legs face backward, which gives them added agility when climbing ponderously over moss, lichen and fallen leaves. At least, that's what the land-roving species of tardigrades does. There are also aquatic and marine variations, which hang out in both fresh and salt water. In other words, taken as a whole, tardigrades can be found in just about every environment we've got.
Like snakes or lobsters, tardigrades sport a tough exterior cuticle that they shed and re-grow as they mature. Inside, they've got the basics: a brain, nervous system, esophagus, stomach, intestines and anus. But they lack frills like a heart, lungs or veins because their body cavity is what's called "open hemocoel," which means that gas and nutrition can move in, out and around efficiently without complex systems [source: Miller].
The animal kingdom (Animalia) has about 36 phyla in it — one of those is phylum Tardigrada [source: Miller]. That's right, they've got their own phylum, whereas we humans belong to the Chordata phylum, a classification we share with every other vertebrate on Earth, including fish and the aforementioned skin-shedding snakes.
Tardigrades. They think they're sooo special. And they're right.
- Number of species: 1,000+ and counting (300 marine, 100 freshwater, 600 terrestrial)
- Reproduction method: They lay one to 30 eggs, which are usually fertilized through sex, although some species are even able to reproduce without mating, and still others can self-fertilize — no wonder they’re so ubiquitous!
- Diet: Fluids of plants (such as moss and lichen) and animals (such as rotifers and even other tardigrades!)
- Lifespan: A few months, although some researchers believe they can live for as long as a century in the “tun” state (find out what the heck that means on the next page)
- Kryptonite: It’s actually extremely easy to kill a tardigrade if it’s not in its tun state. If it’s a tun, good luck! [sources: Lindahl and Balser, Herkewitz]
So, yes, tardigrades have a phylum to themselves, backward-facing rear legs and are cute in their weird, boneless way, but as we indicated earlier, that's not why they're famous. What makes water bears really special is that they're extremophiles. This means they can survive just about anything we throw at them (or throw them in). They even put cockroaches to shame.
Scientists have frozen tardigrades down to "functional" absolute zero (0.05 kelvins, -272.95 degrees Celsius, -459.31 Fahrenheit) for 20 hours. When thawed out, the little water bears went right back to business as usual. They've also been put on ice for nearly two years at -200 Celsius (-328 Fahrenheit) and brought back to life. Researchers have boiled them alive, tried to crush them with 40,000 kilopascals (5,801 psi) of pressure, attempted to suffocate them with (variously) carbon monoxide, carbon dioxide, nitrogen and sulfur dioxide, and even shot them into space to see if they could endure the intense ultraviolet radiation from the sun (they could and did).
All this has won them the status of absolutely, hands down, the toughest creatures in existence. But how the #&¡?!! do they do it? In two non-simple words: anoxybiosis and cryptobiosis. These are two of the three states in which tardigrades can exist. The other one is "active," or what we commonly call "life." That's the state in which they crawl around adorably and eat, sleep, dream, wake, have sex, get in fights, etc. But if, for some reason, their oxygen supply is suddenly lowered, they can blow themselves up like the Stay Puft Marshmallow Man and just hang out in this anoxybiosis state until they can breathe again.
Then there's cryptobiosis. Tardigrades like or, to be more precise, need water. But, incredibly, if the water dries up, they can shed 97 percent of their own moisture, wither to one-third their normal size and stop metabolizing. It's basically a form of suspended animation. This dehydrated, non-metabolizing version of a tardigrade is called a "tun," and a tun is, for all intents and purposes, indestructible. Add a bit of a water, and a tun quickly transforms back into a cuddly water bear and trundles off as though nothing happened.
And just to add yet another layer of ridiculous invincibility, in their tun state, water bears produce huge quantities of antioxidants that essentially neutralize the ill effects of intense radiation. That's why they were able to survive outer space [source: Herkewitz].
These amazing strategies are not shared by all tardigrades, but only by the terrestrial species who live in small films of water on leaves, moss and lichen. The ones who live in ponds or oceans have fairly stable environments, so they haven't needed to be equipped with quite as many superpowers. But if you're a water bear who likes to hang out in a little micro-puddle on a maple leaf, you've got to be ready for anything. When the sun comes out and dries up that water, you have to follow suit [source: Miller]. And at that point, you should probably be prepared for quite a journey if a gust of wind hits; stiff breezes are the tardigrade's main mode of long-distance travel.
So now we know what tardigrades can withstand and why they developed such extreme adaptive strategies. The next question is — how? How did they manage to evolve such unique characteristics?
Lateral Gene Transfer?
In December 2015 a research team from the University of North Carolina at Chapel Hill published the startling results of their efforts to sequence the genome of a tardigrade species known as Hypsibius dujardini. The team, headed by Thomas Boothby, had been astonished to discover that the water bear's DNA was full of genes from other sources, especially bacteria. In fact, it appeared that more than 17 percent of the tardigrade's genes were from bacteria, microbes and fungi [source: Yong]. Nobody had ever found anything like this before.
It's not unusual for an organism to have a few foreign genes — aphids get their colors from fungi, ticks acquired bacterial genes that can produce antibiotics and some wasps have weaponized viruses for personal use. But in almost all such cases, the alien genes contribute no more than 1 percent of the organism's total genome.
At the level of the single cell, beings can trade genes back and forth "horizontally" with no regard for propriety or hygiene. So it's not at all uncommon to see bacterial DNA stuffed full of foreign genes they picked up from their neighbors. But when you get into the multicellular world (i.e., the world of living things you can see with the naked eye) most of us get our genes the traditional way — from our parents and our parents' parents and so on in a "vertical" ancestral lineage.
It seems that multicellular organisms typically keep their reproductive genomes carefully protected from foreign genes, preventing the kind of "horizontal gene transfer" seen in bacteria. So why would tardigrades, with their 1,000 cells, be so open to exotic genetic information? Boothby and his team theorized that the secret lay in the cryptobiotic process.
Apparently, when organisms dry out, their DNA breaks up into pieces. Luckily, tardigrades have a particular talent for stitching those pieces back together the moment they get wet again. But, Boothby et al. postulated, in the process of mending their genomes, it could be that tardigrades accidentally sew in a few genes from other nearby dehydrated sources (like bacteria or fungi), soaking them up like a sponge. By doing this over and over again, Boothby said, they'd basically seasoned their genome with thousands of genes from foreign sources [sources: Richards and Monier, Yong].
Boothby and his team wondered if this unusual genomic profile might contribute to the now-legendary durability of tardigrades. More research would have to be done.
And it was.
Ah, science. Science is fun, science is fascinating, but science also plays hardball. It's not a profession for the faint of heart or for those who get sentimentally attached to a research result. Boothby and his team knew very well that their tardigrade study had surprising findings, and they also knew that all those foreign genes might be the result of contamination (i.e., some bits of fungi and bacteria might've managed to get mixed in with the tardigrade sample by accident).
They knew that because contamination happens all the time. It's all too easy for bits of floating bacteria and fungi to get mixed in with a sample. So the team double and triple checked their results with increasingly sophisticated technology. It seems you can't be too careful when it comes to sequencing genomes.
The digital ink was barely dry on all the excited press coverage of the Boothby team's findings before a careful review of the evidence by other scientists found that there was, in fact, a ton of contamination involved. After reanalyzing the sequencing, it appeared that instead of a whopping 17.5 percent foreign gene count, the tardigrade species in question had only about 0.4 percent of the stuff. And that, it turns out, is a pretty standard amount [source: Richards and Monier]. In other words, there is evidence of horizontal gene transfer in tardigrades, as there is in all organisms (including us), just not very much of it — which is, as it turns out, the usual amount.
So it turns out that tardigrades aren't a perfect metaphor for the durability of multicultural societies, but they're still cool nonetheless. They've been around for 500 million years, so they can surely teach us a thing or two, like how to survive in outer space, how to repair our DNA and how to get our own phylum.
Best of all, you can keep them as pets! All you need is a little container, some moss, a microscope and a really small leash (you'll have to special-order it). The only downside is that you can't really give them names — or you can, but since they're all nearly identical, it'll be tough telling Charles from Charlene and Charles Jr.
Author's Note: How Tardigrades Works
Much of the scientific literature on tardigrades makes mention of how "cute" they are. I think this must be a matter of context. If you spend a lot of time staring through a microscope at weird-looking creatures, your standards for cuteness must necessarily be lowered. Personally, I find tardigrades grotesque. I mean, I greatly respect their invincibility, but they look like a cross between a mole and an overstuffed pool raft. But beauty is as beauty does.
More Great Links
- American Museum of Natural History. "Tardigrades as Environmental Bio-Indicators." (Dec. 22, 2016) http://www.amnh.org/learn-teach/young-naturalist-awards/winning-essays2/selected-winning-essays-1998-20032/tardigrades-as-environmental-bio-indicators/
- Boothby, Thomas C. et al. "Evidence for extensive horizontal gene transfer from the draft genome of a tardigrade." Proceedings of the National Academy of Sciences of the U.S.A. Vol. 112, No. 52. Pp. 15976-15981. Dec. 29, 2015. (Jan. 6, 2016) http://www.pnas.org/content/112/52/15976.abstract
- Brennand, Emma. "Tardigrades: Water bears in space." BBC. May 17, 2011. (Dec. 22, 2016) http://www.bbc.co.uk/nature/12855775
- Dvorsky, George. "Scientists Finally Figured Out Why Tardigrades Are So Indestructible." Gizmodo. Sept. 20, 2016. (Dec. 22, 2016) http://gizmodo.com/genes-hold-the-key-to-the-water-bears-indestructibility-1786814698
- Fox-Skelly, Jasmin. "Tardigrades return from the dead." BBC. March 13, 2015. (Dec. 22, 2016) http://www.bbc.com/earth/story/20150313-the-toughest-animals-on-earth
- Gray, Michael W. "Mitochondrial Evolution." Cold Spring Harbor Perspectives in Biology. Vol. 4, No. 9. September 2012. (Dec. 21, 2016) https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3428767/
- Herkewitz, William. "Secrets of the Water Bear, the Only Animal That Can Survive in Space." Popular Mechanics. Aug. 7, 2014. (Jan. 13, 2016) http://www.popularmechanics.com/space/a11137/secrets-of-the-water-bear-the-only-animal-that-can-survive-in-space-17069978/
- Keeling, Patrick J. and Jeffrey D. Palmer. "Horizontal gene transfer in eukaryotic evolution." Nature Reviews Genetic. Vol. 9. Pp. 605-618. August 2008. (Dec. 22, 2016) http://www.nature.com/nrg/journal/v9/n8/full/nrg2386.html
- Lindahl, Karen and Susie Balser. "Tardigrade Facts." Illinois Wesleyan University Species Distribution Project. October 1999. (Jan. 13, 2016) https://sun.iwu.edu/~tardisdp/tardigrade_facts.html
- Miller, William R. "Tardigrades." American Scientist. September-October 2011. Vol. 99, No. 5. Pg. 384. (Dec. 23, 2016) http://www.americanscientist.org/issues/feature/tardigrades/1
- Minard, Anne. "Gamma-Ray Burst Caused Mass Extinction?" National Geographic. April 3, 2009. (Dec. 23, 2016) http://news.nationalgeographic.com/news/2009/04/090403-gamma-ray-extinction.html
- Ochman, Howard et al. "Lateral gene transfer and the nature of bacterial innovation." Nature. Vol. 405. May 18, 2000. (Dec. 22, 2016) http://www.stat.rice.edu/~mathbio/Ochman2000.pdf
- O'Malley, Maureen A. "Endosymbiosis and its implications for evolutionary theory." Proceedings of the National Academy of Sciences of the U.S.A. Aug. 18, 2015. Vol. 112, No. 33. Pp. 10270-10277. (Dec. 21, 2017) https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4547300/
- Richards, Thomas A. and Adam Monier. "A tale of two tardigrades." Proceedings of the National Academy of Sciences of the USA. Vol. 113, No. 18. Pp. 4892-4894. (Dec. 22, 2016) http://www.pnas.org/content/113/18/4892.full
- Stromberg, Joseph. "How Does the Tiny Waterbear Survive in Outer Space?" Smithsonian. Sept. 11, 2012. (Dec. 22, 2016) http://www.smithsonianmag.com/science-nature/how-does-the-tiny-waterbear-survive-in-outer-space-30891298/
- Understanding Evolution. "From prokaryotes to eukaryotes." UC Berkeley. (Dec. 21, 2016) http://evolution.berkeley.edu/evolibrary/article/_0/endosymbiosis_03
- WikiHow. "How to Find and Care for a Pet Tardigrade (Water Bear)." (Dec. 22, 2016) http://www.wikihow.com/Find-and-Care-for-a-Pet-Tardigrade-(Water-Bear)
- Yong, Ed. "Inside the Bizarre Genome of the World's Toughest Animal." The Atlantic. Nov. 23, 2015. (Dec. 21, 2016) http://www.theatlantic.com/science/archive/2015/11/tardigrades-worlds-toughest-animals-borrowed-a-sixth-of-their-dna-from-microbes/417243/