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Life and extreme conditions:

Exobiology - tardigrades, aliens on Earth?

Tardigrades also called "water bears", "space bears", or "moss piglets" are water-dwelling, eight-legged, segmented micro-animals.

Tardigrades are about 0.5 mm (0.02 in) long when they are fully grown. They are short and plump with four pairs of legs, each with four to eight claws also known as "disks". The first three pairs of legs are directed ventrolaterally and are the primary means of locomotion, while the fourth pair is directed posteriorly on the terminal segment of the trunk and is used primarily for grasping the substrate. Tardigrades are prevalent in mosses and lichens and feed on plant cells, algae, and small invertebrates. When collected, they may be viewed under a very low-power microscope, making them accessible to students and amateur scientists.

Tardigrades form the phylum Tardigrada, part of the superphylum Ecdysozoa. It is an ancient group, with fossils dating from 530 million years ago, in the Cambrian period. About 1,150 species of tardigrades have been described.

Most tardigrades are plant eaters or bacteria eaters, but some are carnivorous to the extent of eating other smaller species of tardigrades. Others are cannibalistic to their own species.

Discovery:

They were first discovered by German zoologist Johann August Ephraim Goeze in 1773. The name Tardigrada means "slow stepper" and was given three years later by Italian biologist Lazzaro Spallanzani.

Locations:

Tardigrades were found everywhere from mountaintops to the deep sea, mud volcanoes, tropical rain forests, and the Antarctic.

Aliens?

The interesting fact is that they can survive extreme conditions that would be rapidly fatal to nearly all other known life forms:

They are not considered extremophilic by definition because they are not adapted to exploit these conditions. This means that their chances of dying increase the longer they are exposed to the extreme environments, whereas "true" extremophiles thrive in a physically or geochemically extreme environment that would harm most other organisms.

The genome of Ramazzottius varieornatus, one of the most stress-tolerant species of Tardigrades, was sequenced by a team of researchers from the University of Tokyo in 2015. Analysis revealed less than 1.2% of its genes were the result of horizontal gene transfer. They also found evidence of a loss of gene pathways that are known to promote damage due to stress. This study also found a high expression of novel Tardigrade-unique proteins, including Damage suppressor (Dsup), which was shown to protect against DNA damage from X-ray radiation. The same team applied the Dsup protein to human cultured cells and found that it suppressed X-ray damage to the human cells by ~40%.

Because they survive conditions that are not commonly encountered at all on Earth, some people suggested they must be alien, i.e., originating from another planet.

IT is indeed puzzling that an animal should have evolved to resist so many harsh conditions when these conditions do not exist; Darwinian evolution theory would suggest they would have lost the useless resistance they show.

(This actually looks like my own reasoning I published in September 2002 about the Deinococcus Radiodurance bacteria, another case of lifeform resistant to conditions not met on earth.)

However, at first sight, there is no big chance that these animals come from another planet:

Recent analyses indicate that tardigrades are a sister group of Lobopodia, the lineage consisting of arthropods and Onychophora. In other words, they "fit in" quite reasonably with other species from Earth.

Questions:

I do not yet know exactly wat to think.

Dryness conditions exist on Earth, so tardigrades resistant to dry conditions is not something that would prove it comes from another planet (Mars for example). What about the tremendous resistance to UV? It is, indeed, useless on Earth now. They may have been more UV on Earth in the past, but there isn't now, so this resistance should have disappeared long ago.

But on the other hand, we are told that tardigrades "repair" their genes when they are mutated by some exernal conditions such as UV. Then, how could they ever evolve?

The fossil record of tardigrades is pretty sparse, but it is known to extend at least to the Middle Cambrian ~520 million years ago. But there is no clear picture yet as to how they evolved. for examplem one fossil found in Siberia showed only three pairs of "legs" and it is still debated whether it is the "same" species or a different one or even a very "young" tardigrade.

As one science stud paper said:

"One difficulty that may be encountered by molecular biologists is the verification of tardigrade species. Specimens obtained from Carolina Biological Supply Company (North Carolina, USA) labeled “Milnesium and other species” were a mixture of Milnesium and Macrobiotus (both eutardigrades, but one in the order Apochela and one in Parachela) as well as the heterotardigrade Echiniscus. Specimens marketed as “Hypsibius sp.” by Ward's, Inc. (Massachusetts, USA) were actually Thulinia stephaniae. In addition to misidentifying specimens, commercial suppliers may change their sources, resulting in different species present in the samples."

And:

"Despite their overall abundance and cosmopolitan distribution, the Tardigrada have been relatively neglected by invertebrate zoologists. Because of difficulties in collecting and culturing the organisms and their apparent lack of economic importance to humans, our knowledge of tardigrades has lagged that of other groups. However, their importance in elucidating the phylogeny of the Metazoa, particularly the arthropods, has recently increased interest in this group. In addition, their development and ecology are poorly understood, and proper training of taxonomists skilled in identifying tardigrade species is essential for systematic, ecological, and molecular analyses."

Tardigrades' genome is still a matter of investigation. For example, one science article in 2015 said:

"In December 2015, another team of researchers in Scotland also sequenced the tardigrade genome, and found at most 500 genes from foreign species. They concluded that the original researchers might have inadvertently sequenced DNA from bacteria living alongside the tardigrades. Further research is needed to confirm just how foreign the water bear's genome really is."

So, this looks like more studies should be conducted before anything very certain can be said...

But definitely, some scientists are thinking a lot about a panspermia origin (tardigrades carried from one planet to another of meteorites) for tardigrades. I found a 2017 science paper in the Astrobiology science journal in which the authors test tardigrades for resitance to acceleration!

The abstract reads:

"Tardigrades are microscopic organisms renowned for their ability to survive extreme environmental conditions. Tardigrade extreme-tolerance research has centered on the ability to withstand desiccation, low and high temperatures, and high hydrostatic pressure and radiation levels. Tardigrade tolerance to hypergravity, however, has yet to be described. We used the eutardigrade species Hypsibius dujardini to investigate short-term tolerance to g-equivalent accelerations (i.e., mimicking g-forces). Data obtained from specimens centrifuged between 3421g and 16,060g for 1?min inclusively reveal tolerance in an acceleration-dependent relation, with lower survivorship and egg production at higher accelerations. This is the first study to demonstrate tardigrade potential for tolerance to hypergravity and describe expected effects on tardigrade survival and reproduction. These findings will prove to be useful in lithopanspermia research (i.e., viable spread in meteoritic rocks). Key Words: Astrobiology—Extreme tolerance—Hypergravity—Tardigrade. Astrobiology 17, 55–60."

References:

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This page was last updated on June 2, 2017.