Are Tardigrades "a Head" of Arthropods?
“Water bears” (phylum Tardigrada) are fascinating little creatures. Only about half a millimeter long, they look like aliens when magnified (see this BBC photo). They act like them, too; they are some of the toughest animals on Earth or in outer space. According to Wikipedia:
Tardigrades are notable for being perhaps the most durable of known organisms; they are able to survive extreme conditions that would be rapidly fatal to nearly all other known life forms. They can withstand temperature ranges from 1 K (−458 °F; −272 °C) to about 420 K (300 °F; 150 °C), pressures about six times greater than those found in the deepest ocean trenches, ionizing radiation at doses hundreds of times higher than the lethal dose for a human, and the vacuum of outer space. They can go without food or water for more than 10 years, drying out to the point where they are 3% or less water, only to rehydrate, forage, and reproduce.
Consequently, they thrive everywhere. “They have been sighted from mountaintops to the deep sea, from tropical rain forests to the Antarctic” although they prefer lichens and mosses. They are found in hot springs and under layers of ice. By any measure, they are extremely successful colonizers of our planet, even though the name “tardigrade” means “slow walker.” About 1,150 species have been described. See how they move in a short video from NPR’s Science Friday and in beautiful color video clips at National Geographic, showing smooth, coordinated motion of their eight legs. Ecologically, they play a role as herbivores, consuming plant material and bacteria. You might be able to find some in your back yard with a magnifying glass.
Where do they come from? Fossils of tardigrades have been found in middle Cambrian strata from Siberia, 530 million years old. Stephen Meyer lists the phylum as a participant in the Cambrian explosion (Darwin’s Doubt, p. 32). They may be related to the Burgess Shale animal Opabinia (animated in the film Darwin’s Dilemma). Because they have body segments with legs, tardigrades are lumped with arthropods and onycophorans in a super-phylum called Panarthropoda, but there has been dispute about those relationships. Tardigrades seem like unique animals disconnected from others.
Now researchers from the University of North Carolina at Chapel Hill have a new origin story: tardigrades are degenerate arthropods. Their paper in Current Biology announces, “The Compact Body Plan of Tardigrades Evolved by the Loss of a Large Body Region,” including the thorax and most of the abdomen. What remains is mostly an arthropod-like head with eight little legs.
The superphylum Panarthropoda (Arthropoda, Onychophora, and Tardigrada) exhibits a remarkable diversity of segment morphologies, enabling these animals to occupy diverse ecological niches. The molecular identities of these segments are specified by Hox genes and other axis patterning genes during development. Comparisons of molecular segment identities between arthropod and onychophoran species have yielded important insights into the origins and diversification of their body plans. However, the relationship of the segments of tardigrades to those of arthropods and onychophorans has remained enigmatic, limiting our understanding of early panarthropod body plan diversification. Here, we reveal molecular identities for all of the segments of a tardigrade. Based on our analysis, we conclude that tardigrades have lost a large intermediate region of the body axis–a region corresponding to the entire thorax and most of the abdomen of insects–and that they have lost the Hox genes that originally specified this region. Our data suggest that nearly the entire tardigrade body axis is homologous to just the head region of arthropods. Based on our results, we reconstruct a last common ancestor of Panarthropoda that had a relatively elongate body plan like most arthropods and onychophorans, rather than a compact, tardigrade-like body plan. These results demonstrate that the body plan of an animal phylum can originate by the loss of a large part of the body.
That’s one way to get a head in life, but is it evolution? Darwin’s tree wouldn’t get very far by chopping off parts of animal body plans below the neck. They are suggesting that the original panarthropod had an elongate body, from which arthropods and onycophorans descended. Tardigrades are truncated remnants with just the head region. This is evolution in reverse!
If the authors had a more pro-Darwinian conclusion, they surely would have offered one, because their opening sentence sympathizes with Darwin’s doubt:
Understanding the origin of animal body plans has been a longstanding issue in evolutionary biology, ever since Darwin struggled to reconcile his theory with the early fossil record of animals.
Yet they only offer a theory of evolution by reduction:
Our model of segment homologies suggests that the tardigrade body axis is reduced, relative to other panarthropods, comprising mostly anterior identity, in line with an earlier hypothesis based on nervous system anatomy. How did this evolve?…. The expression pattern of the posterior marker Hd-Abd-B1 suggests that posterior identity is retained in the posterior of the tardigrade body axis, which indicates that simple truncation is not the answer. Expression of the posterior marker caudal (cad) in a posterior region of the fourth leg-bearing segment (Figures 3H and 3I) confirms the retention of posterior identity.
What they are left with is removal of intermediate segments, leaving a head attached to a few posterior segments with legs. Did this come about through loss of Hox genes? No, they believe, “because Hox genes typically specify segment identities rather than regulate segment production.” Instead, the Hox genes “might become dispensable when the segments they once specified are lost, leading to loss of such Hox genes through neutral processes.” But panarthropods develop by adding segments onto the end. How does evolution chop out the middle of an animal? Things get complicated in their speculation:
We speculate that reduction, and ultimately loss, of terminal addition accounts for the loss of intermediate segments in the tardigrade lineage. In this view, most of the tardigrade body axis represents the short germband of other panarthropods, i.e., the few anterior segments that appear simultaneously during development before terminal segment addition commences. This model would require that the posterior region of the short germband be respecified as the posterior of the body axis, since segments with posterior-most identity are normally the last segments to emerge through terminal addition, and posterior identity is retained in tardigrades. Diversity in segment number in other panarthropods emerges in the body region that is produced through terminal addition. We speculate that the loss of terminal addition in the tardigrade lineage explains the invariant segment number of this phylum.
Basically, the first tardigrade was a head, and the back end of the head grew legs. It’s a wild idea. We’ll have to see if other evolutionists can swallow it. It seems odd that this would happen in the Cambrian and persist 530 million years till now. Wikipedia noted a tardigrade found in Cretaceous amber with parts that look identical to living ones. That’s stasis, not evolution.
What researchers should be focusing on is the amazing design of these tiny animals. They have stubby legs with claws. They have a mouth and eyes. They lay eggs. They molt periodically. They have a digestive tract and sexual organs. They have muscles and nerves. That’s a lot of specialized tissue to pack into half a millimeter! And to think that these are among the most durable animals on Earth, able to survive in habitats beyond all necessity for a Cambrian marine organism, including outer space — that should challenge all notions of unguided evolution. Organisms should only adapt to their immediate circumstances, not to distant unknown habitats they might encounter some future day, or never.
Tardigrades are hard-core survivalists. The BBC News photo caption says, “Boil them, deep-freeze them, crush them, dry them out or blast them into space: tardigrades will survive it all and come back for more.” The article struggles with Darwinian explanations for these superpowers.
How do these seemingly insignificant creatures survive in such extreme conditions, and why have they evolved these superpowers?….
They are truly ancient. Fossils of tardigrades have been dated to the Cambrian period over 500 million years ago, when the first complex animals were evolving. And ever since they were discovered, it has been clear that they are special….
Yet despite their rather tedious lifestyle, they have evolved to cope with environments so extreme, they don’t even exist on Earth….
Evolutionists are stumped trying to answer such questions. The new theory of evolution by subtraction is likely to leave them falling further and further behind in explaining the wonders of life. Tardigrades give evidence of design perfection in miniature, and should be celebrated as such.
Image by Schokraie E, Warnken U, Hotz-Wagenblatt A, Grohme MA, Hengherr S, et al. (2012) [CC BY 2.5], via Wikimedia Commons.