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‘Heresy’ Law Adds Horseshoe Crabs to Arachnid Family Tree


Horseshoe crabs are small armored vehicles. bright blue blood. They’ve been rolling on the ocean floor for hundreds of millions of years. Other powerful creatures have come and gone in all this time: dinosaurs, mammoths, terror birds, Neanderthals. The humble horseshoe crab lived on nowadays, looking not much different from its ancestors in the Mesozoic Era.

“I find the fossil record amazing, fantastic and brilliant,” said Russell Bicknell, a paleontologist who studies the evolution and development of crabs at the University of New England in Australia. “Realistically, I love how they manage to do so much with such a small set of tools.”

However, although the horseshoe crab may seem endless, it has been drawn into the middle of a scientific debate.

In an article published last week Journal of Molecular Biology and EvolutionPrashant Sharma, a professor of biology at the University of Wisconsin-Madison, and colleagues challenge the idea that horseshoe crabs are on very specific and individual branches of the tree of life. Rather, they claim the animals belong right in the middle of the family tree of arachnids, the group that includes spiders and scorpions. If his analysis is correct, he questions the roots of the arachnid tree and suggests that arachnids have a weirder, more complex evolutionary history than scientists realize.

The new article is the latest salvo in a debate about the emergence of arachnids. Scientists have relied on detailed analysis of the bodies of traditionally living and extinct creatures to understand evolution. They studied the tips of the arachnids’ jaws, the placement of the legs, and other features, tracing the features through evolutionary time. The tree they drew showed a common ancestor of the entire group that landed 600 million years ago. Since then, nearly all spiders have lived on land (though they may climb up on your door frame to catch insects flying in a web).

But it’s hard to know what really happened on the shores of a younger Earth 600 million years ago. Antonis Rokas, professor of evolutionary biology at Vanderbilt University, said that the ancestors of the Arachnids very suddenly morphed into a new group of species, and which groups split first and it was always difficult to form their own branches on the tree of life.

In recent years, with the rise of genome sequencing, another way of constructing family trees has become possible. If comparing anatomy is like examining passenger manifests on Ellis Island to build a genealogy, then this new technique is like evolutionary 23 and Me, which classifies organisms based on similarities in their genetics. It can provide a way to verify what previous methods have found and even make new discoveries.

But this is where the debate begins, new trees don’t always agree with old ones. Dr. Sharma and colleagues found no consistent evidence for this common ancestor (the root of the traditional spider tree). A paper from 2014.

Instead, the tree suggested, the arachnids were more likely to have drifted further apart in the past. And these were not a single closely related group, but rather separate clusters of species put together by scientists. If that’s the case, then horseshoe crabs, thought to be mere neighbors of the arachnids, were actually members of the clan.

Dr. Sharma and colleagues’ 2014 study was small, but the new study drew on genetic data from more than 500 species in addition to anatomical data. The results were the same: Arachnids did not cluster tightly. As a result, horseshoe crabs nest between them.

Dr. “We finally had to say this heresy out loud,” Sharma said.

If the common ancestor of arachnids was actually much deeper in evolutionary history, their ancestors may have crawled ashore more than once. There may be more than one wave of this surprising transition, where gills become lungs and limbs take on new roles.

The non-author of the article, Dr. “We used to think that certain morphological features or ecological transitions from land to sea or from sea to land were very rare,” said Rokas. “But we don’t really know how difficult these transitions are for any particular lineage.”

In other words, perhaps radical change is less difficult than we think. In this alternate account of the early days of the arachnids, horseshoe crabs have remained comfortable in the water, while their relatives have tried their luck on the shore at least two and perhaps three or four separate times over the eons. And if their bodies look similar, says Dr. Perhaps this is because evolution has provided them with similar solutions to the landfall problem and has transformed them into ruthlessly functioning forms, Sharma and his co-authors said.

The team’s 2014 paper was met critically by researchers who disagreed with their comments. A group of paleontologists and molecular biologists, Paper that proposes ways that genetic information can build a tree that reassembles arachnids.

Some paleontologists say it’s hard to reconcile the latest results, which still rely primarily on genetic data, with what’s written in the fossil record. Dr. These imply a much more circuitous evolutionary path for arachnids than the fossils suggest, Bicknell said.

Others describe this article as a way station in science’s slow progress towards truth.

“Personally, I think it’s an interesting finding,” said Jeffrey Shultz, professor of entomology who studies arthropod evolution at the University of Maryland, “but experience shows that when the same data is analyzed by different workers, the results can change when new data are mixed into the mix. are added or new insights into genomic evolution come to light.”

Hannah Wood, curator and research entomologist at the Smithsonian Institution’s National Museum of Natural History, said the new results will certainly spark controversy.

“But that’s how things work,” he said, adding that it might challenge another group’s hypotheses. “I think we’ll finally get an answer.”

Where does that leave the horseshoe crab? For now, Dr. This last idea of ​​their history is another oddity among many, Bicknell said.

“They’re weird enough anyway—just spray that fire with more fuel,” he said. “It’s really a case of the family tree, with weird branches sticking out of the main trunk. When did it happen? And why did it happen? That keeps the debate going.”



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