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What a Walking Fish Can Teach Us About Human Evolution

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What does a mouse have in common with a cartilaginous fish known as a little skate?

At first glance, you might think not much. One’s fluffy, with big ears and whiskers; the other breathes with gills and ripples its way around the ocean. One is a lab animal or household pest; the other is most likely to be seen in the wild, or the bottom of a shallow pool at an aquarium. But it turns out these two vertebrates have something crucial in common: the ability to walk. And the reason why could change the way we think about the evolution of walking in land animals—including humans.

A new genetic study from scientists at New York University reveals something surprising: Like mice, little skates possess the genetic blueprint that allows for the right-left alternation pattern of locomotion that four-legged land animals use. Those genes were passed down from a common ancestor that lived 420 million years ago, long before the first vertebrates ever crawled from sea to shore.

In other words, some animals may have had the neural pathways necessary for walking even before they lived on land.

Published today in the journal Cell, the new research began with a basic question: how did different motor behaviors evolve or change in various species over time? Author Jeremy Dasen, an associate professor at the NYU Neuroscience Institute, had previously worked on the movement of snakes. He was inspired to look into skates after reading Neil Shubin’s book, Your Inner Fish: A Journey Into the 3.5-Billion-Year History of the Human Body, but didn’t really know where to start.

“I had no idea what a skate looked like,” Dasen says. “I’d eaten it in a restaurant before. So I did what everyone does, I went onto Google to find videos of skates.” One of the first things he found was a Youtube video of a clearnose skate engaging in walking behavior. “I was like, wow, that’s really cool! How does it do that?” he says.

Using skates collected by the Marine Biological Laboratory at Woods Hole, Dasen and others endeavored to find out. First, the basics: Little skates are bottom-dwellers who live all along the East Coast in the Atlantic Ocean. They don’t actually have legs, and their walking doesn’t look like a human going for a stroll. What they use are anterior pelvic fins called “crus,” located under the much bigger diamond-shaped sail-like fin that undulates when they swim.

When they’re feeding, or need to move more slowly, they engage their crus in a left-right alternating movement along the ocean floor. From the bottom, it almost looks like little feet propelling the skate forward.

But Dasen and his team weren’t just interested in the biomechanics; they wanted to identify the genes that controlled the motor neural pathways for skate walking.

When looking at the layout of a vertebrate, geneticists often begin with Hox genes, which play a crucial role in determining an organism’s body plan. If the genes are knocked out or misordered, it can spell disaster for the animal (as in the experiment in which a fly grew legs instead of antennae on its head after scientists intentionally knocked out certain Hox genes).

Dasen and his colleagues also looked at a genetic transcription factor called Foxp1, located at the spinal cord in tetrapods. The simplified explanation is that it works by triggering motor neurons that allow for the walking movement.

“If you knock [Foxp1] out in model organisms like mice, they’ve lost all the ability to coordinate their limb muscles,” Dasen says. “They have a severe type of motor discoordination that prevents them from walking normally.” It’s not that the mice without Foxp1 don’t have the limbs or muscles necessary to walk—they just don’t have their circuitry wired correctly to do so.

That combination of genes in little skates that allows them to step their way across the seafloor in search of dinner goes all the way back to a common ancestor that lived 420 million years ago—a surprise to the researchers, since the ability to walk was thought to come after the transition from sea to land began, not before. The fact that such genetic traits stuck around for so long, and evolved in such unique ways across different species only added to Dasen’s excitement.

“There’s a lot of literature on the evolution of limbs, but it doesn’t really consider the neuronal side of things because it’s much harder to study,” Dasen says. “There’s no fossil record for neurons and nerves. There’s much better ways of studying evolution by looking at bony structures.”

Plenty of researchers have looked to the fossil record for details about the earliest land dwellers. There’s Elginerpeton pancheni, an early tetrapod that lived outside the ocean sometime around 375 million years ago. And then there’s Acanthostega, another ancient vertebrate that scientists recently analyzed to learn about its limb growth patterns and sexual maturity.

Meanwhile, other biologists have gleaned clues by looking at some of the weirdest fish alive today, many of which have ancient lineages. Some have looked at coelacanths and sarcopterygians, or lungfish (the latter use their pelvic fins to move in a motion like walking). Others have investigated bishr movement. The African fish species is equipped with lungs as well as gills, so it can survive out of water—and has a movement similar to walking when forced to live on land, as seen in the 2014 experiment conducted by University of Ottawa biologist Emily Standen and others.

Standen says she greatly admires the new research on little skates. “I would’ve expected that there would’ve been quite a bit of similarity [in the systems behind different animal’s movement], but the fact that it’s as close as it is was a lovely surprise,” she says. “It speaks to what I believe in quite strongly, that the nervous system and how it develops and functions is very flexible.”

That flexibility has clearly been key across evolutionary history. Thanks to that 420-million-year-old ancestor, we now have everything from fish who swim, to snakes that slither, to mice that walk, to skates that use a combination of movements—with the Foxp1 gene expressed or suppressed depending on the animal’s unique body plan and locomotion.

And now that we know a little more what’s controlling that movement in skates, it’s possible that knowledge could have a future use in understanding bipedalism in humans.

“The basic principal by which motor neurons connect to different circuits is not really worked out [in complex organisms], so the skate is a way to look at that in a more simplified system,” Dasen says. But he doesn’t want to get too ahead of himself in predicting what that might mean for the future. Dasen just hopes that upon seeing the research, people will simply think, “Gee whiz, that’s really neat. They can walk!” 

The Meteorite That Killed the Dinosaurs May Have Also Triggered Underwater Volcanoes

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The end of the Cretaceous period 66 million years ago was a rough time to be living on Earth.

Three global catastrophes occurred nearly simultaneously: The Chicxulub meteorite slammed into what is now Mexico’s Yucatan Peninsula, the massive Deccan Traps volcanic province in modern-day India erupted, and some three-quarters of Earth’s plants and animals, including all non-avian dinosaurs, went extinct. The occurrence of these three events at the same time in our planet’s history has fueled a decades-long debate about causal links. Either a large sequence of volcanic eruptions or an extraterrestrial impact could conceivably cause a mass extinction – but were they all somehow connected?

As Earth scientists, we have reason to believe that there may be another event to add to the list. Our new research, published in Science Advances, shows that the Chicxulub impact may have triggered additional volcanic activity far from the Deccan Traps – along tens of thousands of miles of undersea volcanic ridges that lie at the edges of tectonic plates. The meteorite impact caused large seismic waves that traveled around the globe and were apparently capable of flushing magma out of the mantle and into the oceanic crust. This would presumably be more bad news for the dinosaurs and other flora and fauna of the time.

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It is well known that seismic activity can trigger a variety of hydrologic phenomena, and sometimes even volcanic eruptions. In the aftermath of nearby large earthquakes, dry streams can start flowing, well levels can go up or down, and geysers sometimes erupt. Seismicity also sets off volcanic activity, but only when conditions are just right – it’s only about 0.4 percent of explosive volcanic eruptions that might be triggered by large earthquakes.

So could the massive earthquake generated when the Chicxulub meteorite crashed into Earth be related to the ongoing eruptions in the Deccan Traps? This volcanic province covered much of India with lava flows in less than a million years. A University of California, Berkeley-led team of researchers (including one of us, Leif Karlstrom) revisited the possibility of a connection between these two events.

The most recent efforts to date these eruptions have clearly shown that the Deccan Traps began spewing lava before the meteorite impact and the mass extinction occurred. But the Berkeley-led study suggested that the Chicxulub impact triggered a rapid increase in their eruption rate. If true, all three events could conceivably be connected: The impact would be followed by accelerated volcanic activity that could contribute to the mass extinction.

Underwater lava flows ooze out between tectonic plates, as at Axial Seamount, where it lies on top of older lavas. (Bill Chadwick, Oregon State University, and ROV Jason, Woods Hole Oceanographic Institution, CC BY-ND)

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If the triggering-by-impact hypothesis is right, we’d expect that other volcanic systems would have been set off as well.

At any given time, the vast majority of the volcanic activity on Earth isn’t occurring in continent-covering floods of magma or in explosions like at Mount St. Helens. It’s on the seafloor, where the tectonic plates are spreading apart. As the Earth’s crust splits, the mostly solid mantle layer rises to fill the space created. It melts as it decompresses on the way up.

Illustration of a mid-ocean ridge, with magma rising from the mantle and erupting through the crust at the boundary between tectonic plates. (Background, E. Paul Oberlander, WHOI Graphic Services. Inset, Bill Chadwick, Oregon State University, and ROV Jason, Woods Hole Oceanographic Institution. Modified by Joseph Byrnes, CC BY-ND)

This new magma percolates its way to the surface and fuels nearly continuous volcanic activity along what are known as mid-ocean ridges. This process creates practically all of the crust on the bottom of the ocean. Since the ages of the seafloor are relatively well-known, it preserves a record of oceanic volcanic activity stretching back over 100 million years. This remarkable record of volcanic activity creates an opportunity to test the triggering hypothesis.

In our new study, we used publicly available data sets to make a record of the structure of the seafloor stretching back 100 million years. Since better topographic maps exist for Mars and Venus than do for the Earth’s seafloor on a global scale, we were forced to use indirect methods to look for variations in seafloor structures.

Minute variations in the strength of gravity at different locations as measured by satellites provide the requisite mapping tool. Spots that have an excess amount of rock sitting on the seafloor, as you’d expect to result from accelerated volcanic activity, will have a slightly stronger measurement for Earth’s gravitational field.

The time with the most small structural anomalies on the sea floor – indicating 8 percent more mass anomalies than on average – occurs at 66 million years ago and coincides with the age of the Chicxulub meteorite impact. (Byrnes and Karlstrom, Sci. Adv. 2018;4: eaao2994, CC BY-ND)

We then inspected the record of these “gravity anomalies” to look for any changes to the structure of the seafloor that happened quickly. We found an unusual abundance of these small structural anomalies on the seafloor happened within 1 million years of the Chicxulub impact. The gravity anomalies are consistent with roughly 650 foot high piles of excess material lying on 66-million-year-old seafloor in the Indian and Pacific Oceans.

The total volume of excess material is difficult to pin down, because a large amount of magma could have been injected into the lower crust where it would have a weaker gravitational signature. But we estimate that around the time of the Chicxulub impact, on the order of 23,000 to 230,000 cubic miles of magma erupted out of the mid-ocean ridges, all over the globe. This is on par with the largest eruptive events in Earth’s 4.5-billion-year history, including the Deccan Traps.

Dots mark areas on the seafloor that show high rates of spreading at the time of the Chicxulub impact 66 million years ago. Colors indicate the maximum gravity anomaly within 2 degrees. (Byrnes and Karlstrom, Sci. Adv. 2018;4: eaao2994, CC BY-ND)

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Our observations suggest the following sequence of events at the end of the Cretaceous period. Just over 66 million years ago, the Deccan Traps start erupting – likely initiated by a plume of hot rock rising from the Earth’s core, similar in some ways to what’s happening beneath Hawaii or Yellowstone today, that impinged on the side of India’s tectonic plate. The mid-ocean ridges and dinosaurs continue their normal activity.

About 250,000 years later, Chicxulub hits off the coast of what will become Mexico. The impact causes a massive disruption to the Earth’s climate, injecting particles into the atmosphere that will eventually settle into a layer of clay found across the planet. In the aftermath of impact, volcanic activity accelerates for perhaps tens to hundreds of thousands of years. The mid-ocean ridges erupt large volumes of magma, while the Deccan Traps eruptions flood lava across much of the Indian subcontinent. In the end, three-quarters of the Earth’s plant and animal species have disappeared; the only remaining dinosaurs are the feathered, flying variety, normally referred to as birds.

Now, the goal is to further refine our understanding of each event and their interactions. Was there enough mid-ocean ridge activity to contribute to the mass extinction, or was the triggered submarine volcanism merely a symptom of some more significant planetary ailment? Were other volcanic systems triggered by the Chicxulub impact? Which played a larger role in driving the extinction: the volcanism or the meteor?

What is clear is that this new research points to global-scale connections between catastrophes, a good reminder that events happening on the other side of the planet can have effects felt everywhere.