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The ‘Heresy’ Act adds horseshoe crabs to the Arachnid . family tree

Horseshoe crabs are small armored vehicles with bright blue blood. For hundreds of millions of years, they have moved along the ocean floor. During that time, other mighty creatures have come and gone: dinosaurs, mammoths, terror birds, Neanderthals. Today, the humble horseshoe crab lived, today looking not much different from their ancestors in the Mesozoic Era.

Russell Bicknell, a paleontologist at the University of New England in Australia who studies crab evolution and development, said: “I find their fossils amazing, amazing and brilliant. “I really like that with such a small set of tools, they do so much.”

But while the horseshoe crab may seem eternal, it has been dragged into the midst of a scientific controversy.

In an article published last week in Journal of Molecular Biology and Evolution, Prashant Sharma, a biology professor at the University of Wisconsin-Madison, and his colleagues are challenging the idea that horseshoe crabs reside on a very specific and individual branch of the tree of life. Instead, they claim that the animals belong right in the middle of the arachnid family, the group that includes spiders and scorpions. If their analysis is correct, it would raise questions about the roots of arachnids, and show that spiders have a strange evolutionary history that is more complex than scientists had realized.

The new paper is the latest in the debate about the appearance of spiders. Traditionally, scientists have relied on detailed analysis of the bodies of living and extinct organisms to understand evolution. They examined the tips of the subarachnoid jaws, the position of the legs and other features, tracing the traits through evolutionary time. The tree they sketched shows a common ancestor of the group crawling ashore more than 600 million years ago. Since that time, almost all spiders are terrestrial (although they can fly up to your doorframe to catch bugs).

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

In recent years, with the proliferation of gene sequencing, another way to construct family trees has become possible. If anatomical comparisons are like studying the expression of passengers at Ellis Island to create a genealogy, this new technique is like the evolutionary 23andMe, which classifies organisms according to genetic similarities of them. It can provide a way to verify what previous methods have found and even generate new discoveries.

But, and this is where the debate begins, new trees don’t always agree with the old ones. Dr. Sharma and his colleagues found no consistent evidence for that common ancestor – the root of the traditional arachnid tree – when they built a genome-based tree for an article in 2014.

Instead, the tree suggests that spiders likely diverged even further in the past. And they are not a single, closely related group, but clusters of distinct species that have been grouped together by scientists. If that’s the case, then the horseshoe crab, which is said to be the spider’s neighbor, is actually a member of the clan.

The 2014 study by Dr. Sharma and his colleagues was small, but for the new study they drew on genetic data from more than 500 species as well as anatomical data. The result is the same: Arachnids do not cluster closely together. As a result, horseshoe crabs, nestled among them.

“Ultimately, we had to speak out about that heresy,” Dr. Sharma said.

If the common ancestor of arachnids is indeed much deeper in evolutionary history, then their ancestors may have crawled the land many times. There may have been many of those surprising transformations, with gills turning into lungs and limbs taking on new roles.

“We used to think that specific morphological features or ecological transitions, from land to sea or from sea to land,” said Dr Rokas, who is not the author of the paper. very rare,” said Dr Rokas, who was not the author of the paper. “But we really don’t know for any given lineage how difficult these transitions are.”

In other words, radical change can be less difficult than we think. In this alternate account of the early days of arachnids, horseshoe crabs remain comfortable in the water while their relationships are opportunistic on shore, at least two and perhaps three or four separate times compared to with eons. And if their bodies look similar, Dr Sharma and his co-authors suggest, perhaps this is because evolution has given them similar solutions to problems that came on dry, grinding land ruthlessly file them into effective forms.

The team’s 2014 paper was met with a lot of criticism by researchers who disagreed with their interpretation. A team of paleontologists and molecular biologists have been tracking The paper proposes ways that genetic information could build a tree that brings spiders back together.

The latest results, still largely based on genetic data, are difficult to match what is written in the fossil record, some paleontologists say. They imply a much more complex evolutionary path than the fossils suggest, Dr. Bicknell said.

Others describe the paper as a stop in science’s slow progress toward truth.

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

Hannah Wood, curator and research entomologist at the National Museum of Natural History, Smithsonian Institution, said the new results will inevitably lead to debate.

“But this is how the tool works,” she said, adding that another group could challenge their hypothesis. “I think we’ll eventually have an answer.”

Where does this leave horseshoe crabs? For now, says Dr Bicknell, the latest idea of ​​their history is another among many.

“They’re weird enough – just add fuel to that fire,” he said. “It was just a real case, in the family tree, when their weird branch popped off the main trunk. When did that happen? And why did it happen? It continues that discussion. ”

https://www.nytimes.com/2022/02/18/science/horseshoe-crabs-arachnids.html The ‘Heresy’ Act adds horseshoe crabs to the Arachnid . family tree

Fry Electronics Team

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