Considering whether tardigrade physiology could signal the existence of extra-terrestrial life
By Courtney Kousser
The Earth has experienced five major mass extinction events in its history, and humans are inherently fragile, vulnerable creatures. Wars, climate change, and diseases could all eliminate humanity. However, when life as we know it is eradicated, one lonely organism may remain: the tardigrade. These tiny, plump animals have baffled scientists for years and have proven to be the world’s most unnecessarily resilient animal. Tardigrades, also adorably known as water bears or moss piglets, reside in almost every corner of the planet; from the peaks of the oxygen-deprived Himalayas, to the high-pressure abyss of the deepest ocean, to the frozen tundra of Antarctica. They live in mundane back gardens, busy rainforests, and everywhere in-between.
When put to the test, tardigrades appear miraculously invincible. For example, water is essential for all animals, and humans can only last up to 100 hours without it. However, some species of tardigrade can survive for decades without water. Experts believe that this is due to the animal’s ability to enter a ‘tun state’ whereby they shrivel into a ball, reducing their water content by 97%. In this condition, they are technically dead, with their biochemical and metabolic processes being suspended until they are rehydrated.
Dwelling deep in oceans, tardigrades can withstand incredible levels of pressure. Their endurance reaches further than this; they can cope with over 6,000 times the pressure exerted at the bottom of the Marianas Trench. On the other hand, tardigrades are the first animals able to withstand the extremely low pressure within the vacuum of low-orbit space. In 2014, tardigrades frozen in Antarctic moss for over 30 years were resuscitated and even able to produce viable eggs. Beyond this, they can live after several minutes at temperatures almost reaching absolute zero (-273.15 degrees Celsius), whereas the coldest recorded temperature on Earth is around -90 degrees Celsius. Conversely, they can withstand being boiled at temperatures up to 150 degrees Celsius. Tardigrades can also survive impossible amounts of radiation, up to 5,000 times stronger than any animal on Earth can tolerate. This is due to a tardigrade-unique protein that shields DNA from damage and deactivates reactive oxygen species.
Darwin’s theory of natural selection dictates that most adaptive attributes make sense in the context of their environment. Giraffes evolved long necks, allowing them to reach the leaves of tall trees. Camels gained the stereotypical humps on their backs as a fat reserve, which reduces fat throughout the body, keeping them cooler and providing emergency fuel in the desert. . Zebras developed their stripes because they confuse predators in long grass, and polar bears evolved white fur, as it helps them hide from their prey. However, the tiny tardigrade, with their abundance of adaptations to extreme conditions not present on Earth, challenge Darwinian evolution. These facts have led to the hypothesis that tardigrades evolved elsewhere in the Universe.
This has provided support for the theory of panspermia; the idea that life on Earth originated extra-terrestrially. While some scientists argue that evidence places tardigrades’ origins firmly on Earth, others propose fascinating theories about tardigrades coming to Earth by hitchhiking on comets, or as passengers on passing stardust. While we may never know if life on Earth originated from another planet, the existence of tardigrades provides comfort in knowing that life is able to exist in the most inhospitable environments. Tardigrades could be considered as evidence that humans are not alone, and that somewhere in the vast void of the Universe resides another world in which animals such as these could still thrive.
From Issue 18