Horseshoe crabs are small armored vehicles with bright blue blood. For hundreds of millions of years, they have sauntered across the ocean floor. In all that time, other mighty creatures have come and gone: dinosaurs, mammoths, terror birds, Neanderthals. The humble horseshoe crab has lived on and doesn’t look all that different today from their Mesozoic ancestors.
“I find their fossil record astonishing, fantastic and brilliant,” said Russell Bicknell, a paleontologist at the University of New England in Australia who studies the evolution and development of the crabs. “I just love that with, realistically, such a small toolkit, they’ve managed to do so much.”
But while the horseshoe crab may seem eternal, it has found itself in the midst of a scientific controversy.
In a paper published last week in the journal Molecular Biology and Evolution, Prashant Sharma, a professor of biology at the University of Wisconsin-Madison, and his colleagues dispute the idea that horseshoe crabs operate on their own highly specific and individual branch. the tree of life. Instead, they argue that the animals belong right in the middle of the arachnids family tree, the group that includes spiders and scorpions. If their analysis is correct, the roots of the arachnid tree are questioned and it is suggested that arachnids have a stranger, more complicated evolutionary history than scientists realized.
The new paper is the latest salvo in a debate about the rise of arachnids. Scientists have traditionally relied on detailed analysis of the bodies of living and extinct creatures to understand evolution. They examined the tips of arachnid mandibles, leg placement and other features, and tracked traits through evolutionary time. The tree they sketched showed a common ancestor of the entire group that crawled ashore more than 600 million years ago. Since that time, almost all arachnids have lived on land (although they can climb your doorframe to trap flying insects in a web).
But it’s hard to know what really happened on the shores of a younger Earth 600 million years ago. The ancestors of arachnids broke into a bevy of new species very suddenly, and distinguishing which groups diverged first and made their own branches of the tree of life has always been difficult, said Antonis Rokas, a professor of evolutionary biology at Vanderbilt University.
In recent years, with the advent of genome sequencing, another way of building family trees has become possible. If comparing anatomy was like looking at passenger lists on Ellis Island to create a genealogy, this new technique is like evolutionary 23andMe, sorting organisms by the similarities in their genetics. It can be a way to verify what previous methods have found and even make new discoveries.
But, and this is where the debate comes in, the new trees don’t always match the old ones. dr. Sharma and his colleagues were unable to find consistent evidence of that shared common ancestor — the root of the traditional arachnid tree — when they built a genome-based tree for a 2014 paper.
Instead, the tree suggested it was more likely that arachnids diverged from each other even further in the past. And they were not a single, closely related group, but separate clusters of species grouped together by scientists. If that were the case, then the horseshoe crabs, which were thought to be merely neighbors of the arachnids, were actually members of the clan.
The study of Dr. Sharma and his colleagues from 2014 was small, but in the new study, they used genetic data from more than 500 species and anatomical data. The results were the same: arachnids didn’t clump together tightly. As a result, horseshoe crabs nested between them.
“In the end we just had to say that heresy out loud,” said Dr. Sharma.
If the common ancestor of arachnids is actually much deeper in evolutionary history, then their ancestors may have crawled to land more than once. There may have been multiple waves of that startling transition, with gills turning into lungs and limbs taking on new roles.
“We used to think that certain morphological features or ecological transitions, from land to sea or from sea to land, were very rare,” says Dr. Rokas, who is not the author of the article. “But we really don’t know for any line how difficult these transitions are.”
In other words, perhaps radical change is less difficult than we think. In this alternate tale of the early days of the arachnids, horseshoe crabs stayed comfortably in the water while their kin took their chances ashore, at least two and maybe three or four separate times over the eons. And if their bodies look the same, Dr. Sharma and his co-authors, this may be because evolution gave them similar solutions to the problem of getting on dry land, relentlessly grinding into shapes that worked.
The team’s 2014 paper was received critically by researchers who disagreed with its interpretation. A group of paleontologists and molecular biologists followed up with a paper suggesting ways genetic information could build a tree that brought arachnids back together.
The latest results, still based mostly on genetic data, are hard to reconcile with what’s in the fossil record, some paleontologists say. They imply a much more complicated evolutionary path for the arachnids than fossils suggest, said Dr. Bicknell.
Others describe this article as a way station in the slow progress of science towards the truth.
“I personally think it’s an interesting finding,” said Jeffrey Shultz, a professor of entomology at the University of Maryland who studies arachnid evolution, “but experience shows that results can change when the same data is analyzed by different workers, when new data is added to the mix or when new insights into genomic evolution come to light.”
The new results are sure to spark debate, said Hannah Wood, a curator and entomologist at the National Museum of Natural History, Smithsonian Institution.
“But this is how this stuff works,” she said, adding that another group may challenge their hypothesis. “I think we’ll have an answer eventually.”
Where is the horseshoe crab? For now, said Dr. Bicknell, this last idea of their history is another quirk of many.
“They’re weird enough as it is — just throw more fuel on that fire,” he said. “It’s just a case of real, in the family tree, when their weird branch jumped off the main trunk. When did it happen? And why did it happen? It continues that discussion.”